Note: Descriptions are shown in the official language in which they were submitted.
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ELECTRIC SUBMERSIBLE PUMPS
This is a divisional application of Canadian Patent Application Serial No.
2,530,162 filed
on June 21, 2004.
This invention relates to motors and electronic drives for electric
submersible pumps and
compressors, and is concerned more, but not exclusively, with centrifugal
pumps. It
should be understood that the expression "the invention" and the like
encompass the
subject-matter of both the parent and the divisional application.
Submersible pumping is a well-established technique for extracting
hydrocarbons from
deep boreholes, where the natural pressure in the reservoir is insufficient to
lift the fluid
or gas to surface. The technique is also used in water production.
Typically the production requirement is to lift large volumes of liquid
against a pressure
difference related to the depth of the well in which the pump is installed.
For very heavy
crude oils, slow-speed positive displacement pumps are suitable. These are
usually
rotated by a motor at the surface connected to the pump by a long flexible rod
system.
Centrifugal pumps have been found most suitable for normal crude oils, gas and
water.
These pumps are rotated by a submerged motor connected directly to the pump,
with
electric power being delivered from the surface by a long cable. Also, the use
of electric
cables makes installation possible in deep or long horizontal wells which
would otherwise
not be possible with the use of rotating rods.
The electric motors used for driving the centrifugal pumps are very elongated,
sometimes
of a length of more than one hundred times their diameter. The resulting
complexity of
such a device, the difficulty of its manufacture and the quantity of the
degradable
insulation materials it employs all reduce the system reliability.
Electric motor shaft power output is defined as the product of rotation speed
and torque.
For a given physical size and type of motor there is a limit to the level of
torque that can
be produced, typically due to self-heating. A high-speed motor therefore
provides a
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means for obtaining more power from the same length of motor, or the same
power from
a shorter length.
The output of a pump is normally given in terms of its hydraulic power, which
is the
product of flow rate and lifting pressure (in rationalised units). Centrifugal
pump
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technology is characterised by the power output being proportional to the cube
of the
rotational speed. This known relationship, sometimes termed the "affinity
law", means
that a relatively small increase in the rotational speed can give rise to a
substantial
power increase.
Centrifugal pumps are frequently made with hundreds of impellers threaded on a
common shafft, each impeller adding a little to the lifting pressure. Reducing
the
number of impellers by increasing the speed would therefore afford an
improvement in
reliability.
The above demonstrates that a high-speed motor and pump would, by being
shorter for
a given power, present direct advantages in reliability due to reduced
complexity, or
alternatively yield a higher output for a similar size. A large proportion of
boreholes are
deviated from the vertical and commonly even to the horizontal. A much shorter
motor/pump combination would also lead to a reduction in damage caused by
mishandling and bending during deployment through the curved sections of the
borehole. Furthermore, the much-shortened length would allow motor/pump
. combinations to be assembled and tested in ideal conditions at the
manufacturer's plant
prior to being transported to the borehole location.
As will be described more fully below, innate iimitations in the established
motor and
motor controller technology used in the electric submersible pumping industry
have
prevented the objective of higher speed being recognised or addressed.
Historically, electric submersible motors used for centrifugal pumping have
been of the
asynchronous, or induction, type. The stator is made of steel laminations and
copper
windings, and the rotor of steel laminations with copper bars forming the
winding
Jmown as a squirrel cage. The rotor laminations are keyed to a shaft, this
shaft
providing the means of transmitting output torque. The rotor poles are
produced by
induction or transformer action between the stator and the rotor, using a
portion of the
stator current. The stator, in addition, produces a rotating stator field due
to the
alternating current in its windings. Since the transfonner coupling to the
rotor requires
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an alternating field in the rotor, the rotor must tarn at a different (lower)
speed than
synchronous speed, producing a so-called slip frequency for induction.
Electric
submersible motors are made with two poles in order to achieve the maxim.um
rotating
speed from a standard 60Hz utility supply. This speed is typically 3500 rpm,
slightly
less than the unattainable synchronous speed of 60 Hz x 1 pole pair x 60 s/min
= 3600
rpm.
It has become common to use variable speed drives to power these motors,
rather than
direct connection to the utility supply. Variable speed drives first convert
utility AC
power, typically at 60Hz, to DC, and then by electronic switching convert the
DC to a
variable frequency alternating voltage. The use of a variable speed drive
confers
advantages during starting when-it can limit the motor current to a safe
level, and during
production when it can be used to manage flow rates. The latter is important
when the
changing characteristics of a reservoir are considered over its producing
life. Although
variable speed drives, by creating an artificial supply of 70Hz or more, can,
operate the
motor at higher speed than when directly connected to the utility supply, this
is a limited
capability. Firstly the elongated induction motor is not suited to high-speed
operations
due to internal losses and small mechanical clearances, and secondly at the
medium
voltages used (often several thousand volts rms) drive losses become very
high.
Performance is generally limited up to 80Hz or about 4500rpm.
In order to maximise the induced rotor pole strength it is necessary to
minimise the gap
between the rotor and the stator. Unless very hot, the oil in the gap is
sheared by the
rotor turning yet remains in laminar flow. As a result the friction absorbs
several
percent of motor power. Motor efficiencies above 90% are sought, and this is
an
important source of loss in existing motors. The internal heating caused by
these losses,
and the copper losses in the squirrel cage, reduce motor life by aging the
insulation
materials.
The small gap is also a cause of premature failure due to mechanical causes.
The
limited diameter of boreholes is a natural disadvantage to both motors and
pumps, and
as a result their design is very elongated. A pump and induction motor
assembly for
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producing 250Hp may be 20 metres long. This slender assembly is difficult to
handle
and particularly subject to damage when being deployed into deviated or
horizontal
wells, since small deflections of the motor housing can cause the rotor to
impact on the
stator. Rotor vibration due to bearing wear or imbalance also inareases the
chance of
rotor impact.
The requirement for the rotor to be made of laminations and the li,mited
overall motor
diameter act together to constrain the diameter of the inner torque-carrying
shaft. It is
common practice, for example, to couple two 250HP motors of 5.62inch diameter
together so as to make a longer 500HP motor. Shaft strength limitation
prevents this
being increased to 750HP or 1000HP.
To provide a high-speed electric pumping system, it is desirable to increase
the rotor
clearance, and to reduce the internal sources of power loss that increase with
speed. It is
alsonecessary to use a drive technology which remains efficient at high speed
and at the
different operating voltage levels needed for different motor speeds required
during the
life of the well.
A further requirement of any high or low speed electric submersible system
using
variable speed drives is to minimise the deleterious effects of the electrical
switching
used to produce the alternating output voltages. Switching events on the long
cables
used in submersible cable propagate as wave fronts that reflect at connections
and most
particularly at the motor terminals. These reflections cause voltage
transients that can
approach twice the original voltage, and hence destroy insulation to earth.
Commonly
the motor voltage is presumed to be proportionally distributed through the
turns of the
stator winding, and the inter-tum insulation is less than that of the winding
to earth.
However a wave front impinging on a motor terminal must travel through the
winding
turn by tarn before settling to its final value. Therefore there are short
periods in which
one turn of a winding carries the wave front at fall voltage and an adjacent
turn is
unexcited. This internal voltage difference can exceed the inter-turn
insulation rating,
again causing premature failure. Increasing the insulation level to overcome
these
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problems reduces the space available for the copper in the winding and also
reduces the
heat transfer from the copper, so ihat the motor specification is reduced.
An associated consideration for transients is the interference caused to data
transmission
5 systems used to convey data from instrumentation located in the well bore.
The foregoing has emphasised high-speed centrifugal pumping systems. However
the
same principles of reliable motor performance, matching efficient drives and
circumventing the effects of transients on long cables are all applicable to
positive
displacement pumping systems.
Positive displacement pumps have a flow rate essentially detemsined by a
characteristic
volume per revolution multiplied by rotation speed. The torque demand at the
pump
shaft is determined by the back-pressure of the fluid column being lifted.
These pumps
usually operate at low speeds of a few hundred revolutions per minute. Since
shaft
power is the product of rotation speed and torque, it follows that these pumps
are also.
characterised by extremely high torque demand. Where there is sand production
with
the pumped fluid and where the wells are deviated or horizontal, the rod
connection to
the surface has a very short working life. In these cases it is desirable to
use a downhole
motor with the positive displacement pump.
However, induction motors are inherently unsuited to low speed and high torque
(although variable speed drives have improved their capabilities in this
regard). Thus
current installations rely on a gearbox to match the normal motor running
speed and
torque to the pump characteristics. This is also problematic as it is
extremely difficult
to make a reliable high torque gearbox in the small borehole diameter, and it
is also
expensive.
A motor having high torque at any speed including low speed is therefore
preferable.
It is an object of the invention to provide an efficient electric submersible
pump,
comprising a reliable electric submersible motor capable of operating at low,
medium
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and high speeds, and overcoming many of the above-described disadvantages of
existing motors.
It is a yet further objective of the present invention to provide a high power
electrical
submersible pumping system of the order of half the length of conventional
equipment.
According to one aspect of the present invention, there is provided downhole
electric
motor having at least three phases and comprising a permanent magnet rotor and
a stator
bearing phase windings in slots in the stator, each phase winding
incorporating a
plurality of coils each extending through a respective pair of closed stator
slots and
surrounding a respective portion of the stator between said stator slots, and
adjacent
coils of different phases extending through opposite parts of a respective one
of the
stator slots.
According to another aspect of the present invention, there is provided a
downhole
electric motor having a rotor and a stator bearing phase windings in slots in
the stator,
the stator being made up of first and second concentric parts vVhich together
define the
slots in the stator so as to permit the phase windings to be fitted to the
first part prior to
fitting of the second part to enclose the phase windings within the slots.
Preferably a projection is provided on one of the concentric parts for
extending at least
part of the way across one of the slots to separate adjacent windings within
the slot.
Conveniently the slots are substantially 'shaped to conform to the cross-
section of the
phase windings in order to provide close thermal contact and mechanical
support
between the phase windings and the surrounding material of the stator.
In one embodiment of the invention at least one of the concentric parts
comprises a
preassembled stack of laminations to which the other of the concentric parts
is fitted to
enclose the phase windings within the slots.
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In an alternative embodiment of the invention at least one of the concentric
parts is cast
from insulated ferromagnetic powder and is fitted to the other of the
concentric parts to
enclose the phase windings within the slots.
The invention also provides a method of constructing a downhole electric motor
having
a rotor and a stator bearing phase windings in slots in the stator, the method
comprising
the steps of fitting phase windings to a first part of the stator, and
combining the first
part with a second part of=the stator concentric with the first part so as to
enclose the
windings within slots, a projection on one of the concentric parts extending
at least part
= 10 of the way across one of the slots to separate adjacent windings within
the slot when the
first part is combined with the second part.
The invention also provides a method of constructing a downhole electric motor
having
a rotor and a stator bearing phase windings in slots in the stator, the method
comprising
the steps of fitting phase windings preassembled with an outer layer of
insulating
material to a first part of the stator, and combining the first part with a
second part of the
stator concentric with the first part so as to enclose the windings within
slots with the
outer layer of insulating material serving to insulate the windings from the
surrounding
material of the stator.
The invention also provides a downhole electric motor having a first multiple-
phase
section and a second multiple-phase section and separate supply leads for
supplying
said first and second sections with electrical power from the surface.
The invention also provides a method of construoting a downhole electric motor
having
a rotor and a stator bearing phase coils in slots in the stator, the method
comprising the
steps of fitting open ended conductive loops within the slots in the stator,
and closing
the conductive loops to form the phase coils.
The invention also provides a permanent magnet motor having a rotor provided
with
permanent magnet means, and a stator coaxial with the rotor, wherein the
permanent
magnet means is provided with an anti-corrosion coating.
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7a
In one aspect, the invention provides a downhole electric motor having a first
multiple-
phase section and a second multiple-phase section constituting redundant phase
windings
and a respective supply lead for supplying each of the first and second
sections with
electrical power from the surface, wherein the supply leads constitute
redundant sources
of supply for driving the motor such that the motor may be driven by supply of
power to
only one of the sections by the associated power lead in the event of failure
of power
supply to the other section, wherein the supply leads are connected to
separate motor
drives at the surface, and wherein drive cooperating means are arranged to
coordinate
operation of the motor drives.
In one aspect, the invention provides a method of operating a downhole
electric motor
having a first multiple-phase section and a second multiple-phase section
constituting
redundant phase windings and a respective supply lead for supplying each of
the first and
second sections with electrical power from the surface, wherein the supply
leads
constitute redundant sources of supply for driving the motor, the motor being
driven by
electrical power supplied from the surface to the first section by the
associated power lead
in a normal operational mode and being driven by electrical power supplied
from the
surface to the second section by the associated power lead in a fault
operational mode,
wherein the supply leads are connected to separate motor drives at the
surface, and
wherein drive cooperating means coordinate operation of the motor drives.
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For a better understanding of the present invention and in order to show how
the same
may be carried into effect, reference will now be made, by way of example, to
the
accompanying drawings, in which:
Figure 1 schematically illustrates an electric submersible pumping system;
Figure 1A is a block diagram of a variable speed drive;
Figure 2 illustrates an embodiment of a downhole motor in accordance with the
irivention
in cross-section;
Figure 3 illustrates the embodiment of Figure 2 in axial section;
Figure 4 illustrates the means of assembly of an elongated motor;
Figure 5 illustrates a possible construction of a motor stator in a housing;
Figure 6 illustrates an alternative construction of a motor stator in a
housing;
Figure 7 illustrates a possible rotor journal bearing;
Figure 8 illustrates a possible rotor assembly;
Figure 8a illustrates a preferred embodiment of a rotor assembly;
Figure 9 illustrates a bearing that creates internal support pressure;
Figure 10 illustrates a known electrical representation of a permanent magnet
synchronous motor;
Figure 11 shows the electrical waveforms of an idealised permanent magnet
synchronous
motor;
Figure 12 shows a known electrical circuit diagram for the output stage of a
variable
speed drive;
Figure 13 shows the typical electromotive force of a trapezoidal-wound
permanent
magnet synchronous motor;
Figure 14 shows the electrical waveforms of an idealised permanent magnet
synchronous
motor operated as a brushless DC motor;
Figure 15 shows representative waveforms of the variable speed drive of Figure
12 for a
motor when a variable output voltage or current is required;
Figure 16 shows representative waveforms of the variable speed drive of Figure
12 for a
motor when a variable output voltage or current is required, but with a low
switching
frequency;
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Figure 17 shows representative waveforms of the variable speed drive of Figure
12
incorporating practical switches when a high-speed motor is driven and a
variable
output voltage or current is required;
Figure 18 shows the known idealised characteristics of positive displacement
pumps
and centrifugal pumps, turbines and fans;
Figure 19 shows an electrical circuit diagram providing efficient means for.
varying the
speed of positive displacement pumps and cezLtrifugal pumps, turbines and fans
by
varying the intemal voltage of a variable speed drive;
Figure 20 illustrates a means of providing the supply voltages to a variable
speed drive
in accordance with the invention;
Figure 21 illustrates the improvement in efficiency provided by a variable
speed drive in
accordance with the invention;
Figure 22 illustrates a known phasor diagram for the interpretation of the
operation of
an idealised permanent magnet synchronous motor according to Figure 10;
Figure 23 illustrates a means of optimisation of the control of a permanent
magnet
synchronous motor by varying the variable speed drive output voltage in
accordance
with the invention;
Figure 24 illustrates a means of rotor assembly of an elongated permanent
magnet
motor in a final position;
Figure 25 shows a means of rotor assembly of an elongated permanent magnet
motor in
an intermediate position;
Figure 26 shows a means of stator assembly of an elongated permanent magnet
motor;
Figure 27 shows a stator bore cross-section;
Figure 28 shows a mandrel cross-section suited to the manufacture of a stator
assembly
of an elongated permanent magnet motor of the present invention;
Figure 29 shows a bearing outer housing suitable for insertion in an elongated
permanent magnet motor of the present invention;
Figures 30 and 31 show schematic end views of the stator assembly of a motor
in
accordance with the invention, Figure 31A showing a detail within a slot of
the
assembly;
Figure 32 schematically illustrates the assembly of such a stator assembly;
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Figure 33 schematically illustrates an improved pumping system according to
the
present invention;
Figure 34 illustrates a motor for the pumping system of Figure 33;
Figure 35 shows a fiuther electrical circuit diagram for the output stage of a
variable
5 speed drive;
Figure 36 shows an axial key between the stator and the housing for use in a
motor of
the present invention;
Figtue 37 shows a bearing outer ring suitable for use in a motor of the
present
invention;
10 Figure 38 shows a cross-section through a coil suitable for use in a motor
of the present
invention; and
Figures 39a and 39b illustrate a possible means of forming a coil in such a
motor.
With reference to Figure 1, a representative installation of an electric
submersible pump
(ESP) is shown. A borehole 101 drilled in the earth is sealed with respect to
the earth
from the surface to below a reservoir 102 with casing 103. The casing 103 is
perforated
at 104 to allow reservoir fluid to enter the well. A pump 107 is provided to
lift fluid
from the well up tubing 105 to the surface. The tubing 105 is sealed to the
casing 103
by packing 106 so that the reservoir fluid must go through the pump to reach
surface. A
permanent magnet submersible motor 108 (PMSM) is mounted beneath the pump 107.
The connecting shaft of the motor 108 passes through a seal and pump thrust
bearing
assembly 109, often termed a'protector'. The pumped fluid passes over the
motor 108
before entering the pump 107 and thus provides a certain amount of cooling of
the
motor 108.
A power cable 110 for the motor 108 is run up past the pump 107 and alongside
the
tubing 105 until it emerges at the surface wellhead and passes to a variable
speed drive
111. This drive 111 is powered by the utility supply 112 or a generator.
It will be appreciated that other configurations of the installation are
possible, such as
mounting the pump below the motor, and taking the cable up the tubing or
making it an
integral part of the tubing. Arrangements, such as that disclosed in US
6000915, which
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accommodate the pump concentrically within the motor bore will generally be
found to
make poor use of the lunited borehole cross section and are not preferred.
Figure 2 shows a cross-section of an embodiment of PMSM in accordance with the
invention comprising a central rotor and surrounding annular stator within a
housing
202. The rotor has a central shaft 201 for transmitting the output torque; and
a plurality
of magnetically penneable sleeves 203 carrying pennanent magnets 204. The
sleeves
203 are torsionally locked to the shaft 201 by keys, shrinkage or other means
in the art.
It is preferable to make the sleeves 203 separate from the shaft 201 as shown,
for
reasons of mechanical -stability, to facilitate assembly and to permit the
optimum
strength material for the shaft 201 to be chosen independently of the sleeve
material.
The magnets 204 are preferably of a samarium cobalt composition as this gives
the best
economic perfortnance at the temperatures commonly found in deep boreholes
used for
hydrocarbon production. Other materials such as neodymium iron boron may be
used
in appropriate circumstances, or improved materials as they become available.
During high-speed rotation the magnets 204 experience considerable centrifugal
force,
and the adhesive that bonds them to the sleeves 203 may weaken with age. A
retaining
sleeve 205, preferably of metal, provides a durable means of retention. To
avoid the use
of materials which degrade during prolonged operation at high temperature, it
is
preferable to make the sleeve 205 a tight fit by shrinking it on, rather than
depending
upon tape, adhesives and fillers. The sleeve 205 is preferably of one piece
although, for
ease of assembly, several shorter rings may be fitted adjacent to each other
if required.
US 4742259 discloses a technique for fitting a sleeve with axial constraint.
Thi.s
technique requires the fitting of end washers that are pressed to the shaft
to. locate them
without using positive abutments to do so. In a preferred arrangement shown IN
figure
8a, rings 422, preferably made of non-magnetic, non-conducting material, may
be slid
onto the rotor sleeve 203, coming up against abutments 424, and the retaining
sleeve
20S over the magnets 204 may be rolled over the outer faces of the rings 422,
as at 425,
thereby locking the whole assembly in place axially without the need for
adhesives.
Variations on this locking method are possible within the scope of the
invention, such as
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deforming the sleeve 205 with a punch into a detent on the outer surface of
the ring 422,
or using a snap ring and groove as a shoulder in place of machined feature
424.
The assembly so far described is termed the rotor, and the length of motor
delineated by
a sleeve is termed a rotor stage. Figure 3 shows in axial section a single
rotor stage.
The magnets 204 are circumferentially disposed about the sleeve 203, and
altemately
poled in an essentially radial direction to cause a spatially alternating
magnetic flux to
cross the clearance gap 209. Other magnet arrangements will be known to
persons
skilled in the art. The entire motor and hence the gap 209 are filled with a
benign fluid,
such as a highly refined mineral oil, to balance the -inside of the motor
against the
external wellbore pressure.
Preferably the magnets 204 are plated, for example in a vapour deposition
process, with
corrosion-resistant material such as aluminium, so that they may resist
corrosion from
any ingress of moisture into the motor or from other sources, and so that any
small loose
particles of magnet material will be sealed into the magnets and not come free
to
circulate within the motor bearing system.
Contained within the tubular housing 202 is a stack of thin magnetically
permeable
laminations 206 as may be seen more clearly in Figures 3 and 4. Insulated
wire,
preferably made of copper coated with high-integrity insulation such as
polyetheretherketone or polyimide materials, is wound through the slots 207,
and
looped back 214 at the ends of the lamination stack as part of the coil
winding process.
The wound lamination assembly constitutes the stator of the motor.
The PMSM motor constructed as described will have many desirable
characteristics for
submersible pumping, associated with the general nature of permanent magnet
motors.
For example, by providing a rotor flux from permanent magnets, there is no
need to
-30 energise the rotor, unlike the field winding that requires separate power
in an induction
motor. This reduces the motor current by the amount needed for rotor
magnetisa.tion,
which therefore reduces the ohmic loss in the stator windings and the power
cable. It
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also eliminates the rotor cage winding and thus an in.ternal source of
heating. The
copper within the stator is used only for the production of the rotating
stator flux. The
inherent torque output for the motor, which is derived from a product of space
utilisation, rotor flux and stator flux, is very high compared to an induction
motor. This
torque is available at any speed.
Further aspects of the motor construction may be addressed to give reliable
high-speed
performance. Firstly, as mentioned above, a major source of inefficiency in
induction
motors is the frictional drag in the necessarily small rotor-stator gap. Iiz a
normal mass-
produced PMSM the gap is also kept small in order to economise on the amount
of
magnet material required. However, if it is considered that permanently
magnetic
material is not itself significantly magnetically permeable, then for magnetic
purposes
the gap between the stator and the rotor is that between the lamination tip
210 and the
outer surface of the sleeve 203. The mechanical clearance gap 209 is only a
part of tliis.
Thus, if, for example, the magnet thickness was 3mm and the clearance gap was
increased from 0.25 mm to 1.25 mm, or 500%, the magnetic gap would only have
increased 30%. With only a modest increase in the amount of.magnetic material
it is
possible to purposely design the motor to ensure a sufficiently large
mechanical
clearance such that at high speed the fluid in the clearance is turbulent.
Above 5400
rpm for a rotor of diameter more than 50 mm a gap greater than 1.25 mrn is
preferred
for this purpose. A designer may use the known Reynolds number theory to
estimate
the needed gap size for other operating conditions, fluids and motor sizes.
Although the
friction loss is higher in turbulent flow than in laminar flow, turbulent flow
ensures
much more effective heat transfer between the rotor and the stator, so
reducing the
maximum internal temperature. At any speed the large clearance will reduce the
likelihood of inechanical damage to the rotor during installation caused by
bending of
the outer housing, and also provide a measure of tolerance to contaminant
particles.
Furthermore, it will be found that the deliberately large gap reduces the eddy
current
losscs, and hence heating, induced in the retaining sleeve 205 and magnetic
material
according. to their conductivity. These losses increase approximately as the
square of
rotation speed, but diminish with distance from the lamination tips 210. The
inter-
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magnet spaces 213 may be filled but, unless care is taken to seal the cavity,
particles of
filler may dislodge over time and damage the motor. If required the cavities
may be
left unfilled. This is made possible by the sleeve 205, since it presents a
low drag
rotating surface to the clearance gap while malcing an enclosure to .trap. the
fluid in the
cavities. This trapped fluid is limited to bodily rotation or axial flow and
does not
contribute to friction in the clearance gap.
A fitrther reduction in eddy current losses in the rotor can be obtained by
laminating the
magnets axially. Rotor eddy current losses originate from flux harmonics in
the stator,
the eddy currents circulating on the face of the magnets and penetrating
through the
depth of the magnets and then into the steel that the magnets are bonded to.
Most of the
ohmic losses resulting from this current flow are in the magnets, assuming the
retaining
sleeve is non-conducting or very thin, and the current flow increases with the
face area
of the magnets. Accordingly, in the same way that the stator steel is
laminated to
reduce the effect of rotor flux, the magnets can be laminated to reduce the
effect = of
stator flux. For an elongated motor the face area is the width of the magnet
times the
continuous length of the magnet section. Therefore, by using a series of short
magnets
to make a continuous length that are electrically isolated from one another
where they
would otherwise touch, the effect is to produce an axially laminated magnet.
Practically
the magnet ends may be coated with epoxy or varnish during assembly or spacers
used.
An approximately equal length and width of each magnet will be found to gi've
a good
reduction in losses while not unduly complicating manufacture. This method is
unlikely
to work usefully in motors of conventional length to diameter ratios as the
magnet face
area is already relatively small.
Other types of PMSM construction are possible, while maintaining the large
gap. A
slot-less constraction in which the laminations become a stack of rings, or
are replaced
with a magnetically permeable tube, requires much more magnet material and
will
normally be found uneconomic for submersible pumping.
It is also possible to design the PMSM so that the slots are fully closed or
almost fully
closed in the vicinity of the lamination tips 210. This ensures the retention
of the
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winding without use of insulating retaining wedges that may degrade. It also
reduces
the cogging'torque, that is the altemating accelerating and retarding torque
developed as
the magnets come into and out of maximum overlap with the teeth.
5 For the purpose of maximum power output at high efficiency it is necessary
to optimise
the electromagnetic design. Unlike conventional submersible pump induction
motors, -
which invariably have two poles for the reasons given above, it will be found
that the
optimum number of poles is usually six or more for PMSM motors up to
approximately
seven inches.(17.5cm) in diameter. Four poles will give an acceptable output
for smaller
10 motors but even more poles are preferred for larger motors. The higher pole
count
allows the flux density in the stator laminations to be better distributed so
that the
amount of steel in the outer areas 21-1, may be reduced. This permits the area
of the
slots 207, and hence the amount of copper in the windings, to be increased.
When the
high frequency restrictions discussed below on drive output are considered it
will be
15 appreciated that, in larger motor sizes, higher pole counts are more
demanding of the
drive. A limit may be reached where accepting additional stator lamination
outer
material is appropriate to make the drive practical. Conversely as ~ taught in
US 6388353, with drives--and step-up transformers typical of oilfield
induction motor
technology, a high pole count motor permits operation at low speed and high
torque for
-progressive cavity pumps. For example, a ten-pole motor driven at a frequency
of 60Hz
will rotate at 720rpm.
Figure 5 shows a representative cross-section of a PMSM motor of the present
invention
constructed using known technology in the field of submersible induction
motors. A
plurality of rotor assemblies is used to achieve the desired output power, the
assemblies
being rotationally locked to a common shaft 201 running contiuuously through
the
electrical section of the motor. Shaft stability is ensured by bearings 401
between each
rotor assembly. These bearings 401 are commonly made of two concentric rings
running freely one over the other, one keyed axially and rotationally to the
shaft 201 and
the other locked to the stator bore using thermal expansion caused by the
motor's self-
heating, or pegged in some way. An advantageous feature of the motor design is
that
bearings are provided at each end of the stator to improve rotor stability.
Existing
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16
motors having end bearings located towards the ends of the motor and beyond
the stator
end turns present a potential instability vibration issue due to the
unnecessarily long
unsupported rotor section.
US 4513215 and US 4521708 teach means added to the bearing outer ring for
pegging
or gripping the bearing outer ring to prevent rotation during motor start up,
before
thermal expansion has taken effect. However the larger shaft diameter made
possible
with PMSM motors necessarily reduces the bearing outer ring wall thickness so
that
such known methods cannot be used with such motors. Figure 37 shows a bearing
outer
ring 3704 suitable for use with PMSM motors utilizing a spring clip 3702
fitted to the
stator which engages in a shallow axial groove 3701 in the bearing outer ring
3704 (as
may be best appreciated by referring to the inset view showing a section taken
normal to
the plane of the drawing). The spring clip 3702 is preferably an open circular
spring,
such as a commercial circlip or steel wire, since this provides a natural
axial resilient
lead-in as the spring clip 3702 engages the bearing outer ring 3704. When the
bearing
outer ring 3704 is inserted into the stator bore, in all probability the.
groove 3701 will
not be opposite the spring clip 3702. The resilient lead-in allows the spring
clip 3702 to
push back to allow bearing insertion. When the motor starts, the bearing outer
ring
3704 will rotate until the groove 3701 comes opposite the spring clip 3702,
allowing it
to expand and engage the groove 3701, thereby preventing fiarther rotation. A
spring
loaded pin or cantilever may also be used. In normal constraction the stator
is made of
brass -or possibly steel laminations 403 at the bearing sections. , To make
the stator-
mounted spring clip 3702 practical, = these laminations are preferably
replaced by a
single thiclc block 3703, cut as if it were a very thick larnination (slots
not shown in
Figure 37). This may be a casting. The spring clip 3702 is thenmounted in
a,pocket in
the block so that it cannot fall out during assembly. A small peg 3705
prevents the
spring clip 3702 rotating.
An improved method of assembling the rotor illustrated in Figures 7 and 8
simplifies
the bearing assembly and'also the means of affixing the rotor sleeves 203 to
the motor
shaft 201, using a reduced number of parts.
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17
The need to assemble the rotor with long solid sleeves 203 presents a problem
in that a
stable fit to the shaft 201 is necessary, but the required shaft straightiiess
for closely
fitting sleeves 203 to pass smoothly over the shaft 201 during assembly is
very
demanding. The preferred means of assembly is to use support rings 411 as
shown in
Figure 7. These rings 411 are a close fit on the shaft 201 but, being short in
length, will
slide easily over it. Lands 415 provide a concentric fit for the sleeves 203
and shoulders
416 provide an axial abutment. The bore of the sleeve 203 is only a loose fit
on the
shaft 201. As shown in Figure 8, one or more sleeves 203 and support rings 411
may be,
threaded onto the shaft 201, and axially constrained by convenient means such
as snap
rings 414. Each sleeve 203 must be- rotationally fixed to the shaft 201 in
order to
transfer the motor torque, and each ring 411 must be prevented froin rotation
on the
shaft 201 in order to eliminate wear. Referring again to Figure 7, a key 413
may be
provided to accomplish this, in which case, during assembly, the ring 411 is
first slid
onto the shaft 201, then the key 413 is inserted into a groove 419 in the
shaft 201 and
within a locating notch 418 in the ring 411. The. sleeve 203 has an internal
groove 417
so that, when it is slid onto the shaft 201, it becomes rotationally locked to
the shaft 201
by the key 413 and also prevents the key 413 from subsequently falling out.
The use of a relatively short key, such as the key 413, ensures that the
torsional stress in
the sleeve 203 is limited to that caused by the torque generated by the
magnets on the
same sleeve 203. In a long motor, the portion of the shaft 201 under the
sleeve 203
nearest the output end of the motor will carry a high torque accumulated from
all the
other sleeves. Particularly where multiple motors are connected in series to
increase
power -output, this torque can be very high. If the accumulated stress in the
shaft 201
were to be shared with the sleeve 203 by way of a long key, there would be a
risk that
the magnets, being brittle, would fracture. (In submersible induction motors
long keys
are used to maintain all the laminations of the rotor in alignment, as well as
to transfer
torque.)
A further consequence of very high torque is that twist in the shaft 201 may
cause
sleeves 203 at opposite ends of the shaft 201 to come out of alignment with
each other
and hence with the stator, with the result that the sleeves 203 cannot at the
same time
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18
produce maximum torque. US 6388353 suggests mounting the sleeves on the shaft
with
an angular skew relative to one another so. that, when twisted in use, the
sleeves are
brought back into alignment. Alternative methods that can be used with series-
connected motors are (i) to stagger the angle of each shaft to the next in
line, such as by
cutting the splines at the ends of the shaft with asmall angular offset
relative to one
another, or (ii) to connect the housings to one another with a-small angular
offset.
Within a single stator, the simple expedient of twisting the stator will
effect a
compensating correction to a twisted shaft, and is similar to a well-known
technique for
reducing motor cogging torque. The compensation of all these methods has
variable
effectiveness as the skew is fixed at one angle to compensate the angle of
twist at one
level of torque, and cannot therefore be correct at other levels of torque. It
should be
noted that the amounts of twist referred to are very small, typically less
than a degree,
and the problem may not be significant if the motor is designed to maximise
shaft
diameter and hence resistance to torsion. Accordingly it is preferred to
design elongated
motors so as not to suffer from excessive shaft twist.
The sleeve 403 carries, or is integral with, the shaft bearing. A ring 407 of
bearing
quality material, such as that marketed under the tradename Deva Metal, may be
pressed onto the ring 411. The outer ring 420 of the bearing runs on the ring
407, and is
axially captured by thrust washers .421 which themselves are captured between
the
sleeves 203 and the support rings 411. Alternative arrangements for the
bearings, in
which for example the support ring 411 is made entirely of bearing material,
eliminating
the ring'412, are possible within the scope of the invention. Similarly the
outermost
rings 411 may be of modified shape as their outermost.ends do not mate to
rotor
sleeves.
Substantial heat, of the order of 100W, will be generated within the bearing.
This heat
is transferred to the laminations and thence the motor housing by two main
means,
namely conduction through the outer ring 420, and conduction through the
support
ring
.411 and the rotor. In the latter case heat passes through the magnets near
each bearing
and across the oil-filled rotor/stator gap. This second path, though less
direct than the
first, will significantly raise the magnet temperature. A thermal insulator in
this path
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19
between the bearing running surface and the magnet, such as may be provided by
making the support ring 411 of ceramic, will increase the thermal resistance
in this path,
and thus reduce the magnet temperature rise.
In a completed PMSM the rotor is centred by the bearings 401 and so is
magnetically
balanced. Particularly when the motor is installed vertically the bearing
loads will be
very low, and the bearings 401, which necessarily run hydrodynamically for
maximising lifespan, may become unstable, resulting in shaft whirl and other
vibrations.
In the present invention therefore, the bearings 401 must be designed to
create sufficient
internal pressure to remain stable and hydrodynamic at low shaft load. Figure
9 shows
a means of achieving this in which a proportion of the length of each bearing
401 is
etched or machined with spiral grooving 409. The grooving 409 swirls the oil
within
the bearing 401 at interface 407, increasing bearing pressure and enforcing
stability.
The length of grooving 409 is a means of varying the pressure. The grooving
409,
being inherently a miniature pump, controls and also promotes flow of oil
through the
bearing 401, assisting in cooling and cleaning it. Altezuative bearings, such
as known
bearings with non-circular bores, may also be used to achieve stability.
A fiuther means of purging oil through the bearings 401 is to bore the mQtor
shaft 201
for the introduction of oil throughout the length of the motor to cross bores
406 at each
bearing 401 as shown in Figure 7. Utilising an impeller or cross drillings on
the shaft
201, preferably by way of an oil filter thereupon, oil is forced into the
shaft 201, through
the bearings 401 and then returned by way of the rotor-stator clearance 409.
Pilot bores
410, as shown in Figure 7, or grooves in the bearing housings or the stator
bore may be
provided to assist this return path, as will unfilled interstices between the
magnets on
the rotor. The bearing running clearances, being small, resist and thereby
limit the flow
of oil from the shaft 201. This is particularly the 'case for= plain bearings.
Because
spiral groove bearings control their rate of oil intake, it is preferable to
arrange a
copious supply of fresh oil near the inlet end of the bearing 401, such as by
making the
cross bores 406 near and at least partially beyond the inlet end of the
spirals. Then
there is no need to force the oil through the bearings 401. Instead the oil
moves freely
through the bores 406, circulating past the bearing, while the bearing ingests
the portion
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of the oil that it requires. Alternatively or additionally, oil that is
flowing axially
through the rotor-stator annulus would not normally help to lubricate the
bearings 401
as no pressure is developed to force it into them. The spiral groove bearings
willbenefit
as they ingest from the flow. This method thereby separates the general
circulation of
5 substantial rates of fresh and cooling oil from the individual bearing
lubrication process.
It is generally applicable to any type of elongated fluid-filled motor.
If.the stator bore is of constant, carefully controlled diameter, then, the
rotor assembly,
complete with bearings, may be slid into the stator. The stator thereby
provides the
10 outer support for the bearing outer rings 420. However this known
arrangement
necessarily requires the bearing rings to rub the stator bore during
insertion, with the
possible risk of abrasive damage.
Altern.atively the stator bore may be made of smaller diameter 403 in the
axial sections
15 opposite the bearings, such that the reduced bore lies within the stator to
rotor clearance,
as indicated by the broken lines 408 in Figure 7. In the assembled position
shown in
Figure 5, the bearings 401 are shown in contact with these specially reduced
sections
403 of bore.
20 This means of assembly is not immediately suited to PMSMs due to the
extremely high
side magnet forces between the rotor and the stator caused when the rotor
becomes
slightly eccentred in the stator bore. This is well known in small industrial
motors
where external fixtures are used to handle the forces involved during
insertion. In an
elongated motor the problem is very serious since it is not effective to
support the shaft
from each end. It may be found from electromagnetic calculations that a force
of
thousands of Newtons per metre of rotor length and millimetre of deflection
may be
produced. A much smaller force is sufficient to bend, or deflect, a shaft held
only at its
ends. Any deflection increases the side force leading to more deflection. Thus
if the
rotor bearings mate only to a restricted stator bore, then for most of the
insertion process
they will not provide support to- the rotor, and the rotor will deflect until
the bearings
touch the stator bore between the restricted sections. It would be difficult
to slide the
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21
rotor into the motor, and also the bearing outer diameters will now be offset
from the
restricted bore section 403, so that the restrictions become obstnzctions.
Figures 24 to 29 show representative means of the invention for overcoming
these
problems. More particularly Figure 27 is a cross-section of the stator bore in
the
vicinity of the bearing restricted bore section 403. The restriction surface
2101 is
intenupted by three equiangularly spaced cutaways 2103 that take the bore back
to the
normal diameter of the lamination tips 210. Figure 29 is a cross-section of
the bearing
outer ring incorporating three equiangularly spaced rebates 2105 and
intermediate lands
2104 corresponding in position to the cutaways 2103 of the restricted bore
section 403,
the rebated surface 2102 having the same diameter as the restriction surface
2101, and
these surfaces 2101, 2102 mating when the rotor is finally installed and the
bearings are
in the restricted bore section 403, as shown in Figure 24. The outer surfaces
2113 of the
bearing outer ring are a sliding fit with the lamination tips 210. At interim
positions
during installation, as shown in Figure 25, the outer surfaces 2113 provide a
good
degree of centralisation of the shaft 201 between each rotor assembly. This
mechanical
support ensures the rotor side forces remain acceptably low during rotor
insertion, and
make it possible to insert the rotor assembly without damage. The external
fixture will
necessarily constrain the axial movement of the rotor to prevent it being
pulled into the
stator by the magnetic force. It will be apparent to a slcilled person that,
by.suitable
shaping of the leading edges of the rebates 2105, the alignment of the rotor
bearings to
pass through the bore restrictions may be facilitated. However, in a motor
with many
bearings, this will remain troublesome. The larger rotor-stator gap in a PMSM
permits
a means of alignment to be used in which each bearing ring is bored so that a
stout wire,
such as a tempered steel wire, may be threaded through each bearing. Keeping
this, or a
guide strip in a groove cut in the surface 2103, taut will greatly facilitate
the alignment
of all the rebated bearings as the rotor is inserted.
Figure 4 shows a first step in the known, labour intensive method of
manufacturing an
30. electric submersible induction motor stator. The loose laminations 206 are
threaded
onto a mandrel 303 and inserted into the housing, being prevented from escape
by an
i.nternal ring 301. The shoulder 304 is then used to compress the lamination
stack,
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22
which is fastened in place by a second internal ring 302. The housing length
extends
beyond the laminations considerably further than the illustration shows, in
order to
leave space for the stator winding end turns and for mechanical components.
Winding
the stator is laborious as the conductors have to be threaded axially through
the slots
back and forth, turned around, wrapped in additional end insulation and
inspected, all
taking place inside the housing ends. Furthermore, the laminations necessarily
are a
relatively loose fit in the housing in order to be able to slide them in. This
leaves a
significant thermal contact resistance between the laminations and the
housing, which
impedes heat transfer, raising the motor internal temperature and hence
reducing its
reliability. During motor start up, the torque reaction on the stator is
transmitted to the
housing by way of rings 301 and 302, and, when the motor has warmed up, the
stator
expands so that it grips the housing along its length. It is a significant
mode of failure
during start up for the middle section of the stator to twist between the ends
portions,
thereby damaging the windings, and the arrangements described below are
effective in
preventing such failure.
The preferred embodiments of the present invention radically change the method
of
assembly of elongated motors. Firstly the more wnventional approach to making
small
non-submersible motors is adopted, in which the stator is wound before
insertion into its
housing.
A shrink fit of the finished stator is used to ensure high contact pressure
with the
housing, reducing thermal contact resistance and thereby minimising the -
internal
temperature of the motor. However the invention provides for the special
circumstances
of an elongated motor that has intemal bearing surfaces that may be on a
reduced stator
bore diameter. Figures 26 and 28 illustrate a means of aligning the
laminations ready for
winding utilising a rebated mandrel 2107 having outer surfaces 2105 that are a
close but
sliding fit in the stator bore. The rebates 2106 are clear of the restriction
surfaces 2101.
This mandre12107 provides a centring surface for all the laminations,
including those
used for the bearings. A simple nut and shoulder on the mandre12107 is
sufficient to
clamp the laminations ready for winding by threading of the wire through the
slots. The
shoulders of the rebates 2106 may be partly tapered to bring the bearing
laminations
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23
into rotational alignment. However, in order to reduce sliding friction when
the
mandrel 2107 is eventually removed, the surfaces 2105 are preferably reduced
to thin
ribs or have ridges that reduce the contact area. If the laminations are open
to the stator
bore, or if the tips have notches on the intemal bore, such features on the
mandrel 2107
or inserts mounted thereupon may be used to rotationally align all the
laminations.
The laminations may be welded together on their outer diameter so as to
maintain the
close stacking of the laminations, or some other known means may be used.
After
winding and possibly varnish impregnating, the stator may be ground on the
outside
diameter to make it a close fit in the housing. Preferably the housing will be
pre-
expanded in order that, after insertion of the stator,, the housing will relax
to a shrink fit.
The pressure of contact will then greatly reduce the contact thermal
resistance. A
means of expanding the housing for assembly is to pre-heat it. Conversely, to
repair a
stator, or to recover the housing for re-use, it wi11 be necessary to expand
the housing
with the stator in situ. The length of the stator renders the required force
to be too high
for a press tool to be used. A preferred means is to use an induction heater,
which
essentially comprises an electrical coil that slides over the housing, the
=coil being
connected to a power generator of appropriate frequency. It is known from the
theory
of induction heating that, by choosing a frequency such that the skin depth of
radiation
in the housing does not penetrate through the housing, energy may be rapidly
and
selectively imparted to it without penetrating the stator. This provides a
time window in
which the housing will release the stator and the stator may be extracted
before it
becomes heated by diffusion and expands to re-establish the lock. This method
is suited
even to elongated motors.
A further method with great advantages in terms of manufa.cturing and repair
cost, as
well as in terms of reliably preventing.the stator from rotating within its
housing, is to
mechanically Iock the stator to the housing with an anti-rotation device or
devices. One
possibility is to provide an axial key 3603 between the stator and the housing
202, as
illustrated in Figure 36. The housing 202 is formed with an axial groove 3601,
such as
by milling or broaching, and the stator is formed with a corresponding axial
groove
3602. The axial key 3603 or series of keys fits in both grooves 3601, 3602 so
as to
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24
prevent the stator from tartung relative to the housing 202. This technique
avoids the
need to either press fit the stator with great force or to shrink fit the
housing over it, both
of which are inevitably time consuming in manufacturing terms and require
specialist
equipment. With the keyed housing there is no possibility for breaking free.
It will be
apparent that there are many other possible arrangements that can be adopted
using
keying, such as leaving an integral raised feature on the circumference of the
lamination
which locates in the groove 3601. Where the laminations are pre-bonded into
shorter
lengths the anti-rotation device may be applied on a per length basis.
The embodiment disclosed has the further advantage over conventional
construction of
induction submersible motors that, by providing easy access to the end-
windings, the
highest quality winding procedure may be followed and the results easily
inspected.
This technique is applicable to all types of elongated motor.
Figure 30 shows an improved reliability and improved performance method of
winding
a PMSM. The winding is a known short-pitched concentric winding, which is not
norrnally suited to industrial PMSMs or induction motors because the back-emf
waveform is far from sinu$oidal. It may however be driven by a variable speed
drive in
which the back-emf waveform is taken into account, and it is particularly
suited to the
variable speed drive embodiments of the present invention that are disclosed
below. In
the example shown the motor has three phases A, B, C. The laminations 2201
have six
slots for an eight-pole rotor. For phase A, coi12202 is wound through adjacent
slots so
there is a single tooth 2210 separation between the coil sides. Identical
coils 2203 and
2204 are wound ozn alternate teeth for the other -phases B and C, where the
hatching
emphasises the extent of the completed coils. The slots are shown closed. at
the stator
bore as this is preferable.
One advantage for reliability is that there are only three coils, one per
phase.
Consequently there is no crossing of phase windings at the end turns. The end
turns
2205 possess further advantages. The end turns 2205 fall naturally within the
radial
limits of the slots and are short. This minimises risks with insulation
chafing, provides
a short path back to the stator for conduction cooling, and has minimal energy
waste in
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unproductive copper. This in direct contrast to double-layer lap windings,
as=used in
induction submersible motors. In these the end turns need to expand beyond the
radial
limit of the slots and/or become very long in order to accommodate the wire
crossings
between layers and phases. Since the winding area is constrained by the
housing
5 internal diameter and the bearing/rotor outer diameter, the problem is
severe.
A winding with six slots and eight poles as described above provides phase
separation
in the slots. However, at high speed, the high pole count makes heavy demands
of the
variable speed drive as described. below. Furthermore the self-inductance of
each
10 winding is high, requiring more drive voltage to overcome its reactanee for
a given
motor current, which is manifest as a poor power factor. A compromise
embodiment of
six poles and nine slots for a motor having three phases A, B and C, as shown
in Figore
31, is satisfactory and preferred. In this embodiment three series-connected
coils are
provided for each phase, and each slot accommodates the coils of two phases.
15 However, since these coils are wound around separate teeth, they are
naturally spaced
apart by a gap 3106 as indicated in Figure 31 and can be well insulated from
each.other.
A preferred improvement to the laminations where two coils are adjacent in the
same
slot is to introduce partial teeth 3110 shown representatively in Figure 31A.
These teeth
20 have little effect on the motor magnetic ci.rouit as they do not form a
closed loop aroun.d
the coils. However they form an intermediate path for heat transfer to the
outside of the
motor, and, if the motor body is held near the potential of the neutral point,
there is less
strain on the insulation between each of the coils and the tooth 3110 than
between the
coils of different phases.
A fu.rther, preferred feature, that may be used where the coils are pre-
fabricated and
accommodated in a common slot including partiat teeth 3110 for separating
adjacent
coils as shown in Figure 31A, is the provision of slots that are substantially
shaped to
conform to the cross-section of the coils in order to provide close thermal
contact and
mechanical support between the coils and the surrounding laminations. Such an
arrangement is shown in Figure 3 8 in which only one of the coils is shown
within the
slot, and the coil comprises four coil sections 3801 preformed from
rectangular wire,
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26
each individual coil section being encapsulated within a respective layer of
insulation
3803 and the coil sections being fitted together to form a rectangular bar
which is itself
encapsulated within an overall layer of insulation 3802.
A further, and preferred, aspect of the present invention is shown in Figure
32, which,
for convenience in drawing, is again shown based on six-slot laminations., In
this case
the laminations are made in two parts, as if split in the vicinity of the root
of the slots so
that a multi-tooth inner part 2207 and a circular outer part 2208 are
obtained, as shown
separately on the right hand side of the figure and fitted together on the
left hand side of
the figure. These parts 2207, 2208 are preferably made from the same piece of
material
and are thus of closely similar thickness. In the final assembly this will
reduce bridging
of the tooth tips between laininations due to thickness mismatches. In the
assembly
process the lamination parts 2207 are assembled onto a mandrel, and separately
wound
and formed coils, preferably vacuum pressure impregnated and over-wound with
protective insulation, are then simply slid over the lamination teeth 2209 of
the parts
2207. Sometimes varnish is considered unsuitable in the face of hydrolysis
from
moisture ingress into the motor. In these cases, wire insulation - such as
polyetheretherketone may be used in loose coils, whilst still being wrapped to
resist
chafing in the electromagnetic fields of the motor, Conveniently stacks of
outer
laminations 2208 bonded together or welded along their exterior are slid over
the wound
core to complete the stator. While it is possible to heat-shrink these onto
the vvound
core, the housing shrink fit disclosed above will also apply the light
compression
necessary to ensure good mechanical stability of the stator.
The outer lamination stacks may instead be made as magnetically permeable
tubes of
cast insulated iron powder, with the advantages of offering a smooth surface
where the
inner and outer parts of the stator come together, and of economy of
materials. Each
tube may be made by combining smaller arcuate segments, to reduce the size and
cost
of the casting, such an arcuate construction being unfeasible with laminations
which
would effectively be small steel fragments. T'he partial teeth disclosed above
may be
incorporated into the outer ring of the split lamination. The disclosed multi-
part stator
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27
must also be multipart or open-slotted outwards at the bearing sections 202 to
permit
loading of the winding from the outside.
This method of assembly translates the known advantages of form-wound coils
used in
physically large industrial motors to the difficult elongated small diameter
geometry of
submersible motors.
Form-wound coils for lap wound large motors are manufactured separately from
the
laminations and are then inserted into rectangular slots open to the stator
bore. In
submersible motors there is insufficient working space in the bore .of the
laminations to
load the formed-coils ready to insert into the slots, and for high speed
motors the open
slots would cause substantial losses.
By opening the lamination slots from the outside, the invention permits formed
coils to
be used while not incurring these problems. The particular advantage of the
concentrated winding is that the formed coils are very simple and that the
large winding
slots for the small size of motor facilitates the use of semi-rigid
rectangular or wedge-
shaped copper wire.
With formed coils the wire is bent once at any position as it is wrapped to
form the coil,
unlike the. conventional process for winding elongated motors in which the
wire is
threaded back and forth through the slots. Consequently a much more rigid wire
may
be used, as there is no work hardening and insulation damage that would occur
if it was
attempted to wind conventionally, with repeated bending. Rigidity solves the
known
problem in elongated motors of wires crossing within a coil deep inside the
stator. It
provides a non-rabbing and stiff end turn assembly. Round wire is known to
give a
very poor copper fill factor in a slot compared to rectangular wire,
essentially because
the latter packs together better. Typically the thermal conductivity from the
copper
through its insulation back to the lamination is improved also. With a high
copper fill
the motor will have much reduced internai heating compared to a conventionally
round
wire wound motor. This is a source of improved reliability, or alternatively
of higher
torque for the same temperature rise.
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28
While rectangalar wire is preferred, it will be appreciated that formed coils
made from
round' wire will nevertheless be superior to round wire conventionally wound
by
threading through a stator. For a lower pole count motor, such as, the
commonly used
two-pole induction motor, the winding pitch is necessarily substantially half
the
circumference of the motor. This means that many end turn crossings are
unavoidable.
The flexibility of round wires is beneficial in this case, whilst retaining
the key
advantage disclosed above of prefabricating the coils.
10. Formed coils that are fully encased in insulation over the portions that
enter the stator
slots do not require insalating slot-liners. FurFhermore it is not necessary
to impregnate
the coils after insertion to complete the insulation and secure them into the
slots,
provided that the insulation is impregnated or encapsulated prior to insertion
and the
slots are shaped to retain the coils as disclosed above. The means that the
present
invention not only allows the stator to be removed from the motor housing, but
permits
the winding to be disassembled from the stator. Axial movement of the coils
can be
prevented by inserting insulating blocks between the end turn loops and the
stator.
Despite the many advantages of the split lamination construction, it does
require careful
design and attention to manufa.cturing to ensure satisfactory engagement of
the stator
parts. All the aforementioned advantages for windings may be obtained when the
number of coil turns is not too large, and especially for short-pitched coils,
by using
one-part laminations as will now be described with reference to Figures 39a
and 39b. A
four-turn coil is made first from four U-shaped coil sections 3901 bound
together by a
layer of insulating materia13902 over at least the straight parts of the coil
sections 3901
as shown in Figure 39a, -each such coil section being termed a hairpin coil.
During
manufacture the open ends of the U-shaped coil sections 3901 are inserted
directly into
the laminations and bearing sections from one end without requiring them to be
divided
into two or more parts. Once this has been done the coil loops may be
completed by
joining appropriate ends together 3903 as shown in Figure 39b, for example by
brazing
directly or using bridging pieces. These joints may then be covered with
insulation,
such as insulating tape, and impregnated or encapsulated. It will be
appreciated that the
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29
details of the b.airpin structure may be varied in a number of different ways
within the
scope of the invention. It is not necessary in any of the coils for the start
and finish of a
coil to be at the same end of the structure.
Referring again to Figure 5, the disclosure of the described embodiments has
been
based on a single wound stator. Commonly, for higher power, multiple housed
motors
are combined in series . Figure 6 illustrates an embodiment in which two or
more stator
sections are accommodated within a single housing 202, in this case with one
stator
section per rotor section. The corresponding phase windings 214 of each stator
section
.10 are connected in series while =integrity of synchronicity of the sections
is obtained by
rotationally aligning the stator and rotor sections. The rotor sections are
easily aligned
on the shaft by keys. Disadvantageously; in a lap wound motor, the end turns
will
consume a large proportion of the overall motor length and the reliability
will diminish
in proportion to the extra end turns. Also, for a high speed motor, the
distance between
bearings will become large and possibly necessitate a larger number for
shorter stages to
maintain mechanical stability of the rotor. However, in the preferred
embodiments
disclosed aboVe in which concentrated windings are used, the penalty for
additional end
turns is much reduced and the motor is feasible from reliability and
performance points
of view. In this case the practical advantages are the possibility of
manufacturing a
large variety of motor powers from a basic stator length and the relative ease
of winding
shorter stators.
The stator sections may be carried and aligned on a common mandrel for
insertion in
the motor housing 202, similarly to the foregoing descriptions for a single
stator. The
stator bore restrictions in which to house the bearings 401 are replaced by
housings 404
concentric with the individual stator sections. Concentricity is maintained by
the motor
housing 202 when the entire assembly of bearing housings 404 and stator
sections are
inserted. The series connection 405 of the windings 214 of the stator sections
is
preferably achieved by permanent means such as brazing. The use of connectors,
while
possible, reduces reliability. It is a feature of the invention that winding
before
insertion permits these connections to be made and inspected beforehand.
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High speed multi-pole PMSMs present a variable speed drive problem that the
present
invention addresses as described below. The origin of the problem is that the
base
electrical frequency that the drive must generate is the product of the number
of motor
pole pairs and the number of shaft revolutions per second.. A. standard
induction motor
5 having two poles and tnrnin.g at 3600 rpm therefore has an electrical
frequency of 60Hz.
A PMSM in accordance with the invention rotating above 4500 rpm has a much
higher'
frequency. At 7200 rpm and six poles, the electrical frequency is 360 Hz. This
six-fold
increase is a step change in operating conditions for electric submersible
pumping..
systems and well beyond the range of general industrial drives.
Figure 10 shows a known electrical representation of a balanced PMSM with
three
phases a, b, c and isolated neutral. Referring to phase a, reference numeral
701 is a
motor terminal to which the voltage Va is applied. Current Ia indicated by the
reference
numeral 702 flows into the motor winding which has resistance R indicated at
703 and
an effective inductance L indicated at 704. The effect of the permanent
magnets
rotating past the stator winding is to induce an electromotive force (EMF) Ea
indicated
at 705. The other phases b and c may be described in the same way with
appropriate
substitution of indices. The three phases are joined together at the neutral
point N
indicated by the reference numera1706.
It will be appreciated that multiple motors, or stators, may be connected
electrically in
series so that the resistances, inductances. and EMFs add to make a single
equivalent
larger motor with a common shaft. Placing the terminals in parallel is also
possible but
poses difficulties in controlling currents between all the windings. More
realistic motor
models in which for example the EMF source and inductance are lumped together
as an
element that calculates the time rate of change of the internal flux linkage,
and in which
magnetic saturation is taken into account, are all refinements which do not
affect the
present invention. The number of phases, three, is well suited to the task of
electric
submersible pumping motors, but is not limiting.
An idealised PMSM as described with reference to Figure 10 produces sinusoidal
EMF,
with each phase 120 degrees apart, and is driven by a three-phase sinusoidal
voltage
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source. Figure 11 shows graphically how a sinusoidal voltage V, 802, applied
to the
motor with suitable amplitude and in the presence of the motor EMF E, 801,
will result
in a phase current I, 803. The source may also be current-controlled in which
case V is
the consequence of I and E.
The sinusoidal nature of the electrical quantities is ideally suited to the
task of electric
submersible pumping. This is because the smoothly varying waveforms do not
cause
damaging transients at the motor terminals, and because the motor torque can
be shown
to be constant with rotation, which reduces the likelihood of torsional
vibration.
The construction of such a motor requires careful attention to the
distribution of the
turns of the windings within the stator slots. To produce a sinusoidal EMF
with a
reasonable number of slots cut in the laminations requires the turns from
different
phases to share slots and to be distributed among many slots. This immediately
causes
a reduction in reliability due to the potential for insulation failure in the
many end-turn
crossings and due to the mixed phases. There is also the loss of useful copper
due to
increased insulation between the phases.
When the windings are made so that the phases are kept in separate slots, the
back EMF
will be more similar in form to B in Figure 13. This is often referred to as a
trapezoidal
EMF. If the motor is driven with sinusoidal voltage or current the performance
will not
be as good as the ideal sinusoidal PMSM made with the same amount of copper in
the
windingg.
Figure 14 shows how a motor with trapezoidal EMF. is driven, -compared to the
sinusoidal motor waveforms of Figure 11. The key feature is that voltage is
applied to
the motor across two phases only at a time whereas in a sinusoidally driven
motor
voltage is applied to three phases at a-time. The two-phase driven trapezoidal
wound
permanent magnet motor is commonly termed a brushless DC motor. The two phases
are changed cyclically, as in AB, BC, CA, AB... Whenever the phase pair is
changed,
one phase is electrically disconnected. Since there will be current in the
phase winding,
the terminal voltage exhibits a voltage flyback spike 1002, known as a
commutation
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32
spike. These spikes occur twice per electrical cycle, on each phase. They
present a
serious limitation for the successful use of brushless DC motors in electric
submersible
pumping, since the voltage spikes lead to damaging electrical conditions on
long cables,
as hereinbefore described. The electric submersible pumping system of the
present
invention drives all three motor phases continuously such that damaging
transients will
not arise, without requiring the motor emf to be sinusoidal. * It is
convenient
nevertheless to explain the principles in terms of sinusoidal waveforms, as
the
fandamental frequency component of the drive and motor electrical quantities
predominate in a detailed analysis.
Figure lA shows a block diagram of a drive circuit 111 comprising an
adjustable
voltage converter 113 and an inverter 114 for supplying drive currents =at
output
terminals 901 at the surface for supplying the three phases A, B and C of the
motor via
the power cable extending down the borehole. The inverter 114 is supplied with
an
upper voltage at 904 and a lower voltage at 905, the difference between the
upper and
lower voltages being commonly termed the link voltage.
Figure 12 shows a schematic circuit diagram for the inverter 114 which is well
known.
For each phase output there is an upper switch and a lower switch,
representatively
shown for terminal AA at 906 as 902 and 903 respectively. By alternately
turning on
these switches, upper switch 902 on and lower switch 903 off or vice versa,
this
terminal may be sensibly connected to either the upper voltage 904 or the
lower voltage
905. This arrangement is termed a two-level inverter. It will be appreciated
by. one
versed in the art of inverter design that multi-level inverters may be made in
which the
terminals may be switched to voltage levels intermediate between the upper
voltage and
the lower voltage, such multi-level inverters being usable in alternative
embodiments in
accordance with the present invention. A filter is connected between the
switch
tenminals AA, BB and CC and the drive terminals A, B, C at 901,
representatively
comprising inductors 907 and capacitors 908. The purpose of the filter is to
smooth out
the rapid switchin.g transitions, and thereby present a smooth voltage to
terminals A, B,
C. It will be appreciated that other filters, for example for the removal of
radio
interference, may be added.
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By contrast, in a brushless DC motor inverter, the filter is not present and
the motor is
connected directly to the terminals AA, BB and CC. Only two phases are
active,.that is
only one switch is tarned on; at a time, whilst the switches for the third
phase both
remain turned off as described above.
In driving of the motor in accordance with the present invention all three
phase outputs
are active at all times. In a sinusoidal variable speed drive it is necessary
to use.pulse-
width modulation.(PWM) or other switching modulation scheme known in the art,
e.g.
hysteretic, space vector, switching table, to create the effect of a
sinusoidal output
current. In the following description PWM drive is referred to by way of non-
limiting
illustration.
Figure 15 shows one phase of the output of a PWM drive according to which the
upper
and lower switches of a phase leg are alternated with a variable mark-space
ratio. The
voltage curve shows the switching at terminal AA, whereas the superimposed
phase
current curve is seen be sinusoidal with only a little ripple. Fourier
analysis of the
voltage would show it to have a predominant. fundamental component at the
phase
frequency: Filter 907, 908 filters the voltage output of the drive circuit so
that only the
fundamental smooth voltage is passed to the power cable and thence to the
downhole
motor. This is therefore a suitable transient-free approach, in principle, for
the PMSM
submersible pump application
However, to produce a high-power high-speed variable speed drive with
sinusoidal
output presents severe difficulties, as will now be described. The method is
best suited
to trapezoidal or similar EMF but is also applicable to sinusoidal driving,
the difference
being in the harmonic content of the waveforms and hence the best use of
available
power capacity.
The majority of variable speed drive circuits operate at typical utility
supply voltages of
380 V AC - 690 V AC, since the power semiconductors that they use for switches
are
well proven and efficient. However, just as in utility.power transmission, for
efficient
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34
motor operation using long power cables, it is necessary to use Medium
Voltages,
commonly in the range 1000 V AC - 4000 V AC. Such voltages reduce the motor
current and hence the obmic losses in the cable. The majority of variable
speed drive
circuits for use with submersible pumps are therefore installed with a step-up
transforxner on the output. These transformers are a source of additional
power loss,
direct cost, and are often large and oil-filled, requiring special
environmental
precautions and substantial space. A wide speed range requires expensive core
material
for high speed but also a very large core to prevent magnetic saturation if
operation at
low speed is also required. They are in addition to input transformers,
commonly
required as described below to reduce harmonic distortion of the supply and to
match to
the available supply voltage.
A Medium Voltage drive cireuit operates from a supply voltage directly at the
voltage
which is required for the motors.. It therefore eliminates the undesirable
output step-up
transformer but has certain limitations for the purpose of high speed pumping.
.
Medium Voltage power semiconductors when used for the switches 902 and 903 of
the
drive circuit of Figure 12 have large switching losses, i.e. unlike ideal
switches they
carry both current and voltage during the time it takes to open or close the
path to
current. The losses are inherently proportional to the number of switching
operations
per second. As an example, to turn on a switch at 3000VD.C assuming a current
of
200A might cause a loss of 1 J (Joule). If repeated 1000 times per second, the
heat
created would be 1000 W. It is easy to see that, once accumulated across all
the
switches of the drive, there would be a substantiai- cooling problem and. loss
of.,
efficiency.
To produce the quality sinusoidal waveform in Figure 15, thirty switching
cycles per
fundamental motor frequency cycle were used. Figure 16 shows the effect of
reducing
this to ten switching cycles per fundamental motor frequency cycle. The
waveform is
already of poor quality and difficult to filter.
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With.high speed multi-pole motors the switching speed becomes too high to be
economic with Medium Voltage semiconductors. For example, a high speed motor
with six poles operating at 7200 rpm has a fundamental frequency of 360 Hz, so
that the
drive should operate with a switching frequency of at least 3600 Hz just to
achieve the
5 quality of the response of Figure 16, and preferably at least twice that.
The normal
range for Medium Voltage power semiconductors is 500 - 1000 Hz. This is why
Medium Voltage drives for two-pole induction. motors, which need a fundamental
of 60
Hz at 3450 rpm, are typically specified at an upper ftdamental of less than
90. Hz, far
short of that needed for the high speed motors referred to above. If a lower
voltage
10 drive is considered as an alternative, then despite more efficient
semiconductors,. it too
will reach a switching limit at high power. Moreover the step-up transformer
has to be
a more costly design as mentioned above.
The present invention overcomes these problems by deliberately over-modulating
(non-
15 linearly modulating) the output stage of Figure 12 in conjunction with a
variable voltage
source such as shown in Figure 19. Normally the PWM waveform in Figure 15 may
be
used to produce a good sinusoidal waveform until the peak value exceeds 4/h
times the
internal drive voltage. If the depth of modulation is increased beyond this
the.PWM
output will become distorted, that is the modulation will become non-linear,
as shown in
20 Figure 17. This non-linearity is characteristic of any modulation scheme
used for
driving a motor in accordance with the present invention where the output
voltage is
unable to follow the peak of the sine wave or other waveshape that is
demanded, in
simple proportion. A particular case of over-modulation is to consider the
peak voltage
as fixed to the upper and lower levels of the internal drive voltage for=
substantial parts
25 of each cycle, with pulse width or other modulation used to progressively
vary the
output between the upper and lower levels of the intern.al drive voltage for
the
remaining parts of the cycle. It is also possible, within the scope of the
invention, to
generate the waveform within the linear. range of modulation, particularly at
lower
power levels.
The distorted switching waveform shown in Figare 17 has several features.
There are
far fewer switchin.g cycles than at lower modulation, and these are at the
lowest-current
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36
intervals of the fundamental cycle. Switching losses will therefore be much
reduced
even if the switching frequency is kept high to facilitate flltering. The
filtered output
voltage, obtained, for example, by filtering the single phase in isolation, is
quite similar
to trapezoidal, and is transient free as required. When the filter is as shown
in Figure
12, it becomes a three-phase filter and the phase-phase voltage applied to the
motor will
be found to be even more smooth. The output is usable for sinusoidal motors,
with
some unwanted harmonics, and is well adapted to the non-sinusoidal windings of
the
preferred motors of the present invention.
Figure 21 shows the benefits of over-modulation more clearly. Horizontal axis
1702 is
modulation depth normalised to 4/ a, and vertical axis 1701 is the heat loss
of a typical
switch. Curve 1703 shows the switch conduction loss, which is typically low
for a
submersible motor running at 100A. Curve 1704 shows the switching loss. It is
high
for normal modulation levels, but reduces rapidly by a factor of three as over-
modulation is increased. This represents a dramatic improvement and makes
Medium
Voltage drives of the present invention suited to high speed motors.
Since the amplitude of the drive voltage is fixed once over-modulation is
employed, the
only way of varying the motor voltage and hence the speed is to vary the
internal drive
voltage Vlj~,k applied between the terminals 904 and 905 in Figure 12 by means
of the
adjustable voltage converter 113. There are many known circuits to do this,
including
phase-controlled rectifiers and choppers.
However, the present invention seeks to make use of the special
characteristics of the
pumps it is powering, in order to further improve drive performance. In
accordance
with the power characteristic of centrifu.gal pumps as mentioned earlier and
as depicted
in Figure 18,, the power output at half speed is only 12.5%o or so of the full
speed power,
and therefore of little interest in the well for which the motor and pump are
specified.
Similarly, though less dramatic, in a positive displacement pumping system the
power
will be proportional to speed and more than half power is normally required.
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37
Therefore a properly specified drive can be assumed to be run most of the time
above
half speed.
A first embodiment of adjustable voltage converter 113 particular to the
present
invention incorporates a specially adapted variable voltage chopper source
shown in
Figure 19 to provides an efficient means of regulating the intern:al drive
voltage, and
hence the motor speed, over the power range of interest.
In this circuit a first fixed supply voltage source 1401 is connected in
series with a
second fixed supply voltage source 1402, and a chopper, comprising a switch
1403, a
diode 1404, an inductor 1405 and a capacitor 1406, is connected across the
source 1402.
By varying the duty cycle of the switch 1403, the voltage across the capacitor
1406 may
be varied between zero and the fixed voltage of the source 1402. Since the
voltage
across the capacitor 1406 is in series with the fixed voltage of the source
1401, the
voltage across the output terminals 904 and 905 may be varied from the fixed
voltage of
the source 1401 to the sum of the voltages of the sources 1401 and 1402.
When the motor is operating at low speed, as when starting and stopping, the
power
level and motor frequency will be low. Consequently conventional pulse width
modulation by the drive autput may be used with little penalty, with the
chopper turned
off, leaving the drive voltage fixed at the level of 1401. At full power
output the
chopper may be left permanently on. Therefore it has no switching losses in
either case.
The switching losses of a chopper are proportional to switching frequency,
input voltage
and output current. The advantages of the arrangement shown are that the input
voltage
is only half that of the full supply, and tha.t- the frequency of chopping may
be set
independently of the motor speed since it is used to produce the link voltage
and not the
modulated drive output to the motor. For example, with a pump load and high
speed
corresponding to. high power, the chopper might be operated at 500 Hz to limit
switch
losses, whereas the output. stage in Figure 12 may be switching, except when
saturated,
at 3600 Hz or more to produce a fundamental frequency of 360 Hz. If the
conventional
modulated PWM approach with a fixed internal supply were used, the output
might
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38
have to be limited to 500 Hz, that is there would not even be one pulse per
half cycle,
resulting in an ineffective drive.
If a chopper were used across the full available supply voltage, the losses
would.be
doubled as the switching voltage would be doubled while the switched current
remained
the same. This may be acceptable for lower power low cost drives where the
dual fixed
voltage supply 1401, 1402 is not implemented.
A further feature to optimise the drive, based on the characteristics of the
electric
submersible pump. is to vary the chopper frequency. Inductor 1405 is heavy,
costly and
has losses proportional to ripple current and average output current. As such
it is an
undesirable addition by comparison with conventional drives.
The inductance value is usually chosen to limit the chopper ripple current to
a
reasonable level. The ripple current is at a maximum when the chopper output
is half its
input voltage (50% duty cycle). At the same time, because of the nature of the
pump
load, the output power will be significantly reduced. Therefore at this
condition the
chopper frequency can be relatively high, permitting a lower inductance value.
As the
voltage increases the output power will increase, and the chopper frequency
must be
reduced to limit the switching losses. Since' the ripple reduces as the
output. voltage
increases (higher duty cycle) but increases as the frequency- is reduced, it
can be seen
that a compromise profile of frequency versus power output can be found which
allows
a much smaller value of inductance than would otherwise be the case, reducing
the
adverse factors of weight, cost and power loss; It is quite reasonable to
reduce the value
by a factor of two, or four if the chopper is connected across the full supply
and not a
portion of it. Thus variation of the internal frequency of the adjustable
voltage
converter with output serves to irnprove efficiency and/or reduce the size of
components.
Figure 20 shows a suitable circuit for the fixed voltage sources 1401 and
1402. In this
circuit the utility supply is first transformed by a transformer 1501 having
two
secondary windings 1502, the output of each of which is fed to a three-phase
rectifier
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39
and smoothing capacitor. The resulting DC supplies are connected in series. By
altering the relative turns ratios so as to change the relative sizes of the
voltages of the
sources 1401 and 1402, the variable speed range of the supply may be adapted
to
particular requirements.
A particularly beneficial choice is when one secondary winding is wye-
connected and
the other secondary winding is delta-connected, and the turns. ratios are
adjusted such
that the rectified outputs are equal. In this case the current pulses taken
from the supply
from one capacitor are displaced in time with respect to the current pulses
taken from
.the other capacitor. This known arrangement is beneficial to the supply as
the current
pulses taken from the supply by the assembly occur twelve times per supply
cycle and
not six as when using a single rectifier. This substantially reduces the
harrnonic
distortion imposed by the drive circuit on the utility supply.
A further -embodiment of adjustable voltage converter 113 in accordance with
the
present invention incorporates a three-phase boost converter as shown in
Figure 35.
This arrangement uses a two-level iriverter as in Figure 9, but operated so as
- to absorb
power from the input terminals 3501, U, V, W which in this case are connected
to the
utility supply or a generator, and produce a voltage output between the
tertninals 905
aind 904. This circuit arrangement and variations thereof can use a greater or
lesser
number of switches. Their use in industrial drives is mainly for four
purposes, namely
(a) to draw a sinusoidal current from the utility supply which im.pro.ves the
harmonic
content, (b) to adjust the utility supply 'power factor, (c) to permit the
regeneration of
power from an over-running motor load to be return.ed to the utility supply,
and (d) to
provide a constant link voltage just above the maximum that would normally be
obtained from an ordinary rectifier. The latter is also the minimum link
voltage that can
be used for the circuit to function with these purposes. In this embodiinent
the three-
phase boost converter is used to produce a variable voltage between the
minimum
voltage level and a maximum voltage level that is above twice this level, and
therefore
provides a suitable adjustable range covering the main power levels of
interest. A
particular benefit for smaller submersible motors is that ordinary 380 - 480
VAC utility
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supplies can be boosted into the low Medium Voltage range, and therefore the
Medium
Voittage drives of this invention may be made without the cost of input
transformers.
Other arrangements for efficient variable voltage are possible. If the fixed
voltage
5 sources 1401 and 1402 are kept separate and a chopper is placed across each,
and the
chopper outputs are connected in series, one chopper may be kept fully on or
fully off
which the other chopper varies its output. In this way the entire voltage
range may be
covered with only one chopper operating at less than the sum of the available
supply
voltage. It is also possible to connect the chopper across the fixed voltage
source 1401
10 and to connect the fixed voltage source 1402 separately to the chopper
output so as to
add to it. Where the interference to the utility could be tolerated, one or
both of the
rectifiers in Figure 20 could be rendered controllable by means of a thyristor
bridge.
Three-phase chopper circuit arrangements are known and can also be used.
15 It is desirable to dynamically control the drive, cable and motor
arrangements to realise
optimum efficiency. Unlike an induction motor a PMSM must be driven
synehronously
with its shaft rotation. With a brushless DC motor, for which two phases are
driven at
a time as hereinbefore described, there are numerous schemes to determine the
shaft
rotation without the use of rotation sensors (such as "Sensorless Vector and
Direct
20 Torque ControP", 1998, P Vas, Oxford University Press) based on observing
the effect
of the voltage on the undriven phase. With a PMSM, because all three phases
are
continuously driven, the shaft rotation must be determined in another way. In
a factory,
a means is required to directly measure the instantatieous angular position of
the rotor
using a shaft sensor, such as a resolver or encoder, and -to use the result to
control the
25 phase of the voltage or current output of the variable speed drive circuit.
However,
apart from the uncertain reliability of such transducers, the additional
cabling or other
means needed to transmit the position information from deep in the borehole to
the
variable speed drive circuit at the surface makes a sensorless technique
almost certain to
be required.
It is possible to control AC PMSM motors without sensors utilising a
computerised or
discrete component model of the motor, based on an electrical equivalent
circuit as in
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41
Figure 10 or a physics-based description, incorporating intimate knowledge of
the
motor's electromagnetic design. The model is kept supplied with terminal
voltage,
current and frequency information, which allows it to estimate the motor's
internal
variables such as rotor position. In turn these allow the control algorithm to
decide how
to adjust the drive output. These methods depend on an accurate model.
Substantial
effort is devoted to measuring the model parameters for a given motor before
use, or by
various monitoring means during operation. In the case of submersible motors,
or in
other applications where long cables are required, the cable resistance a.nd
reactance
parameters must be incorporated into the model. Furthermore cable and motor
parameters are subject to change with operating temperature and age. In the
present
invention it is shown how qualitative knowledge of the motor load
characteristics may
be introduced so as to refine the rotor position estimate for a PMSM without
having to
measure these uncertain system parameters. A general purpose drive is designed
to
cover a wide range of loads and dynamically varying conditions, as in a servo,
and
cannot assume particular properties of the load.
The characteristic of the load that is required for =the feature of the
invention riow to be
disclosed is that its power be steady for a steady speed. By averaging over a
sufficiently
long period random or short term load fluctua.tions can be accommodated. A
submersible centrifugal pump, or a turbine, meets this condition.
Therefore, if the PMSM motor control is changed while keeping the frequency,
and
hence synchronous speed, constant, the load power will remain unchanged. The
optimum control condition will be when the drive output power, which is
measurable, is
muiimised. For example i~ a pump is turning at a fixed speed and takes 300kW
from
the drive at one control condition, and 298kW at another, then the second
condition is
more efficient as there is less power supplied, regardless of what the motor
and cable
parameters are thought to be and regardless of what the actual pump power is,
since it
has not changed.
Suitable means of effecting these control changes to find the most efficient
system
operating point are now disclosed. At constant speed the internal mechanical
friction
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42
losses will be fixed, so that the dominant variable loss that needs to be
reduced to a
minimum by the control is the ohmic loss in the windings.
Figure 22 shows the phasor diagram for the PMSM schematic shown in Figure 10
and
corresponding to the waveforms shown in Figure 11. This diagratn, which is
known to
those skilled in the art, refers to one phase of an ideal balanced motor with
isolated
neutral. The motor EMF amplitude E, denoted by the reference numeral 1801, is
taken
as reference. The phase current I lags behind this by an angle ~. The voltage
drop due
to the winding resistance 1802 and the voltage across the internal inductance
1804 sum
as vectors to equal the driving voltage V denoted by the reference numeral
1805. The
motor power output, ignoring internal mechanical and iron losses, is given by:
--P = 3/2 E I cosM
The EMF E depends to a good approximation only ori the motor speed, so that,
for
fixed speed operation and since the output power is fixed by the load, the
quantity
I cos(~) will be constant. The broken line 1806 shows the locus of constant
output
power. It is evident that minimum current, and hence least loss in the
resistance of the
copper winding, z 12R, occurs when ~ is zero.
In open-loop operation of the PMSM motor a given three-phase voltage or
current is
applied at a given frequency. The motor operates in accordance with the phasor
diagram at an angle ~, satisfying the relationships between the parameters.
Operation is
at some non-zero ~, and as a result the motor is never optimally efficient. If
~ increases
beyond approximately 45 degrees, the motor operation becomes unreliab1e since
fluctuations in the load may increase the angle to the point where there is
insufficient
current available from the drive circuit to maintain the output power. At this
point the
motor will lose synchronism and stall. If the conditions are as shown in
Figure 22, then
increasing the motor voltage will force ~ to reduce and hence reduce the
current and
ohmic losses. It will eventaally reach zero, the most efficient point, and
then become
negativc, when the current and ohmic losses will increase again.
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43
Figure 23 shows this in terms of the input power 1902 of a representative
motor plotted
against terrninal voltage 1901, normalised to the most efficient point of
operation. The
curve 1903 represents the average (real) input power and is the sum of the
fixed motor
output power and the copper losses. The curve 1904 represents the volt-ampere
power,
which includes a power factor. It can be seen that, by varying the input
voltage, a point
may be found which minimises whichever power quantity is desired. Increasing
the
normalised input voltage from below unity changes t from positive through zero
to
negative.
Therefore the optimum operating point of the system at a given speed may be-
found by
varying the input voltage independently of particular knowledge of the motor
or cable
parameters or actual load power. Since the motor output power demanded by the
pump
for constant pump speed and fluid type is constant, the control observable is
the input
power measurable at the surface. Paramaters corresponding to the age or nature
of the
motor and/or cable are not required.
This method has broader applicability. For example, if a current-controlled
drive is
used, then current may be used as the control variable and the drive voltage
and output
power will vary as a consequence. .Alternatively, for a fixed amplitude of
voltage or
current, the phase of this quantity relative to the estimated rotor augle
could be slightly
changed. In this case the optimum condition will be when the speed is
maximised,
since, from the characteristics of the load, increased speed corresponds to
increased
output power, and the maximum output power always occurs when ~ reaches zero.
(In
Figure 22, changing speed changes the reactance X and the emf E in
proportion.)
Practically the above method is applicable to PMSMs without the simplified
assumptions of Figure 10, since the goal is simply least drive. power for a
fixed output
power. It is preferably implemented as a long time average correction to an
established
closed-loop control, or as a slow adjustment for open-loop control, ensuring
that load
fluctuations do not cause false corrections. It will be evident to those
skilled in the art
of control theory that it is possible to merge the input power minimisation
into an inner
control loop, by weighting its importance to other error terms or using it as
a constraint.
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44
In this way the control loop keeps primary control over the range of ~ that is
permitted,
while accepting a safe level of correction from the power-optimised control of
the
present invention. It should also be noted that, for real non-sinusoidal
motors and
drives, the issue of torque ripple can be important, and a lesser or greater
amount of
correction might be empirically added to keep this to an acceptable level.
Such an
arrangznent is applicable to any synchronous motor with' suitable load and
thereby
includes brushless DC motors and drives.
The improved winding and construction methods described above make it possible
to
further extend the reliability of the entire pumping system by means of
cooperative
duplication of failure prone electrical elements. The cost of replacing a
failed system,
and the loss of fluid production until the repair is complete, will far exceed
the cost of
the duplicate parts to be described.
Figure 33 shows a variant installation according,to which a single submerged
motor 108
is connected to two electric power cables 110 and 110', each connected at the
surface to
a corresponding motor drive 111 or 111'. Figure 34 diagrammatically shows the
construction of the motor in the installation of Figure 33 in which two sets
of motor
windings are provided, namely a first winding set 3 001 connected to the cable
110 and a
second winding set 3001' connected to the cable 110'. The motor is wound as a
six-
phase motor, divided into two sets 3001 and 3001' of three phases, each with
its own
neutral connection. The motor may either be driven as a six-phase motor to its
full
power capability, or alternatively as a three-phase motor using either set of
windings at,
reduced power.
The advantage of such an arrangement is that failure of the cable 110 or its
splices or
connections; or of the drive circuit 111 or of any of the coils of the winding
set 3001
will not affect any of the corresponding parts associated with the winding set
3001'. By
using concentrated windings the windings of the phase sets may more easily be
kept
well insulated from one another than with lap windings.. In the simplest case
one coil
per phase is wound over alternate teeth in twelve slots, the corresponding
phases of each
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set being adjacent to one another. In this way the six motor leads exit the
motor directly
from their coils without crossing.
The above arrangement of six phases split into two sets of three phases,
though having
5 practical advantages, is not limiting. However two-phase motors still
require three
conductors in a cable, whereas motors with a larger number of phases require
further
cable conductors, which is undesirable.
A six-phase drive output circuit may be constructed by adding three extra
pairs of
10 switches to Figure 9. The phases operate at a 60-degree separation, rather
than 120
degrees. However, such a drive sold as a unit is complex and is substantially
only
useful for a fault tolerant application. The preferred embodiment uses two
adapted
three-phase drive circuits 111 and 111', with the adaptation being made by
means of
suitable signal and power connections 3002 between the drive circuits. The
signal
15 connections must ensure that the corresponding output phases of the drive
circuits are
60 degrees apart. One drive -circuit makes the rotor angle calculation to
produce the
master phase signal used by both drive circuits. To ensure smooth running
across'all of
the phases, the amplitude of the drive output voltage must be the same on each
phase.
In the case of the high-speed drives disclosed in the present description, the
terminals
20 904 and 905 of the drive circuits may be connected together so that their
voltages are
the same. One of many known power supply sharing methods may be used to
equalise
the power supplied by each chopper circuit.
In an alternative embodiment of the fault-tolerant pumping system, two motors
are
25 connected mechanically in series on a common shaft, but powered by separate
cables
and drives as before. The two motors may be operated individually or
simultaneously.
In the latter case the drive circuits must again be arranged to cooperate.
This method is
not applicable in the majority of wells as usually' the motor diameter is the -
largest that
can be fitted within the casing 103, and the motor cable 110 is fed past the
pump and
30 into the top of the motor 108 immediately below it. In such cases it is not
possible to
pass a second cable past this first motor to a farther motor arranged at a
deeper level.
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The foregoing has assumed two duplicate motor sections that are identical, as
that is the
simplest fault tolerant arrangement. However, with suitable changes to the
control and
drive levels, a plurality of motor sections cables and drives of different
characteristics
may be used within the scope- of the invention, so long as they are controlled
to the
same shaft speed.
The electric submersible pump system of the present invention has broad
application,
particularly in the field of downhole wellbore operations. Drilling for
wellbore fluid at
large depths is typically restricted to relatively narrow boreholes, so the
facility of the
present invention to provide the same motor power in a smaller overall package
is
immediately advantageous.
A further application of the present invention is to compress wellbore fluid
in situ. It
may sometimes not be required to immediately transport the wellbore fluid to
the
surface from its underground reservoir, but to compress it either for later
recovery or
merely to facilitate further exploration. Alternatively it may be required to
transport the
wellbore fluid from a first subterranean location to a second subterranean
location, for
the above reasons amongst others.
A recent development in mining operations is the application of multi-lateral
wellbore
systems in which a number of small diameter wellbores are drilled
substantially
horizontally from a central subterranean sump. Currently known pumping systems
have
significant difficulties in pumping from lateral wellbores, whereas the pump
of the
present invention can still maintain a high output in such environments. In
this case,
wellbore fluid is transported from the multiple lateral wellbores to the
central sump,
where it may be recovered to the surface or compressed as described above.
As hereinbefore described, an objective of the present invention is to provide
a high-
speed electric submersible pump, capable of operating at speeds above the
current
maximum of approximately 4,000 rpm. The standard operating speed of
embodiments
of the invention intended for the above applications is above 4,500 rpm, and
an optimal
CA 02619583 2008-02-20
47
speed, providing a marked improvement over current systems, is approximately
7,200
rpm=
The present invention discloses a permanent magnet synchronous motor
submersible
pumping system. It will be appreciated by the person skilled in the art that
various
modifications may be made to the above embodiments without deparlin:g from the
scope
of the invention.
Reference should also be made to " rhe Technology of Artificial Lift Methods",
Vol.
2b, K.E. Brown, Penwell Publishing 1980.
