Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
Multi-Speed Electric Motor
Cross-Reference to Related Applications
[0001] This application claims priority from U.S. provisional application
number 61/952,463,
filed March 13, 2014.
Technical Field/Field of the Disclosure
[0002] The present disclosure relates generally to electric motors, and
specifically to
selectively operating alternating current electric motors at different speeds.
Background of the Disclosure
[0003] Alternating current (AC) electric motors rely on alternating currents
passed through
induction windings within the stator to cause rotation of the rotor. So-called
three phase AC
motors include three matched sets of windings positioned radially about the
stator. By
supplying sinusoidal AC power to each of the sets of windings such that each
set receives an
alternating current offset by 120 degrees, a largely continuous torque can be
imparted on the
rotor as it rotates.
[0004] Unlike a brushed DC motor, output speed in an AC motor is controlled by
the
frequency of the current sent to the stator windings. In order to control
output torque, and thus
speed, a variable frequency drive (VFD) is used to vary the current fed to the
AC motor.
Because the inductive reactance of the stator windings is proportional to the
frequency applied
to the winding, increased voltage is necessary to maintain a relatively
constant current within
the windings, and thus a relatively constant output torque.
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[0005] In order to properly drive the AC motor, VFD's often operate using a
volts/Hz control
scheme. In volts/Hz control, the VFD varies the output speed of the motor by
supplying AC
power to the stator windings at a particular frequency and voltage. For a
given desired torque,
voltage is proportionally related to the frequency by a so-called "voltage-to-
frequency" or
"volts/Hz" ratio. By using closed-loop feedback, a VFD using volts/Hz can
maintain motor
speed in changing conditions. Depending on the configuration of the stator
windings, the
frequency may result in different output speeds and torques for the rotor.
Summary
[0006] The present disclosure provides for a method for controlling the speed
of an AC motor.
The method may include providing the AC motor. The AC motor may include a
rotor, the rotor
adapted to be rotated by the interaction between an internal induced
reluctance or permanent
magnetic field and an electromagnetic field; and a stator, the stator
including a plurality of
windings, the windings adapted to induce an electromagnetic field to rotate
the rotor, the
windings being grouped into winding phase groups, each winding phase group
corresponding to
and coupled to a phase of AC power supplied to the AC motor, each winding
phase group
including at least two windings, and the windings of each winding phase group
selectively
reconfigurable between a series and a parallel configuration. The method may
include
configuring the windings of each winding phase group in the parallel or series
configuration;
supplying AC power to the windings of the stator at a first volts/Hz ratio,
causing rotation of the
rotor at a first torque ratio and a first drive ratio with a first maximum
rotor speed; reconfiguring
the windings of each winding phase groups from the parallel configuration to
the series
configuration or from the series configuration to the parallel configuration;
and supplying AC
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power to the windings of the stator at a second volts/Hz ratio, causing
rotation of the rotor at a
second torque ratio and a second drive ratio with a second maximum rotor
speed.
[0007] The present disclosure also provides for a method for controlling the
speed of an AC
motor. The method may include providing the AC motor. The AC motor may
include: a rotor,
the rotor adapted to be rotated by the interaction between an internal induced
reluctance or
permanent magnetic field and an electromagnetic field; and a stator, the
stator including a
plurality of windings, the windings adapted to induce an electromagnetic field
to rotate the rotor,
the windings being grouped into winding phase groups, the winding phase groups
selectively
configured in a Wye configuration or a delta configuration, each winding phase
group
corresponding to and coupled to a phase of AC power supplied to the AC motor,
each winding
phase group including at least two windings, and the windings of each winding
phase group
selectively reconfigurable between a series and a parallel configuration. The
method may also
include configuring the windings of each winding phase group in the parallel
or series
configuration; supplying AC power to the windings of the stator at a first
volts/Hz ratio, causing
rotation of the rotor at a first torque ratio and a first drive ratio with a
first maximum rotor speed;
reconfiguring the windings of each winding phase groups from the parallel
configuration to the
series configuration or from the series configuration to the parallel
configuration; and supplying
AC power to the windings of the stator at a second volts/Hz ratio, causing
rotation of the rotor at
a second torque ratio and a second drive ratio with a second maximum rotor
speed.
[0008] The present disclosure also provides for A method for controlling the
torque of an AC
motor. The method may include providing the AC motor. The AC motor may include
a rotor, the
rotor adapted to be rotated by the interaction between an internal induced
reluctance or
permanent magnetic field and an electromagnetic field; and a stator, the
stator including a
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plurality of windings, the windings adapted to induce an electromagnetic field
to rotate the rotor,
the windings being grouped into winding phase groups, each winding phase group
corresponding
to and coupled to a phase of AC power supplied to the AC motor, the winding
phase groups
selectively reconfigurable from a Wye configuration to a delta configuration,
and each winding
phase group including at least two windings, the windings of each winding
phase group
selectively reconfigurable between a series and a parallel configuration. The
method may also
include determining a first torque requirement; configuring the AC motor into
a first
configuration in which the winding phase groups are configured in the Wye or
delta
configuration and the windings of each winding phase group are configured in
the series or
parallel configuration, the first configuration having a first torque ratio;
and supplying AC power
to the AC motor, rotating the rotor.
[0009] The present disclosure also provides for a method for controlling the
speed of an AC
motor. The method may include providing the AC motor. The AC motor may include
a rotor, the
rotor adapted to be rotated by the interaction between an internal induced
reluctance or
permanent magnetic field and an electromagnetic field; and a stator, the
stator including a
plurality of windings, the windings adapted to induce an electromagnetic field
to rotate the rotor,
the windings being grouped into winding phase groups, each winding phase group
corresponding
to and coupled to a phase of AC power supplied to the AC motor, the winding
phase groups
selectively reconfigurable from a Wye configuration to a delta configuration,
and each winding
phase group including at least two windings, the windings of each winding
phase group
selectively reconfigurable between a series and a parallel configuration. The
method may also
include determining a first speed requirement; configuring the AC motor into a
first
configuration in which the winding phase groups are configured in the Wye or
delta
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configuration and the windings of each winding phase group are configured in
the series or
parallel configuration, the first configuration having a drive ratio; and
supplying AC power to the
AC motor, rotating the rotor.
[0010] The present disclosure also provides for a method for controlling the
speed of an AC
motor. The method may include providing the AC motor. The AC motor may include
a rotor, the
rotor adapted to be rotated by the interaction between an internal induced
reluctance or
permanent magnetic field and an electromagnetic field; and a stator, the
stator including a
plurality of windings, the windings adapted to induce an electromagnetic field
to rotate the rotor,
the windings being grouped into winding phase groups, each winding phase group
corresponding
to and coupled to a phase of AC power supplied to the AC motor, the winding
phase groups
selectively reconfigurable from a Wye configuration to a delta configuration,
and each winding
phase group including at least two windings, the windings of each winding
phase group
selectively reconfigurable between a series and a parallel configuration. The
method may also
include configuring the AC motor into a first configuration in which the
winding phase groups
are configured in the Wye or delta configuration and the windings of each
winding phase group
are configured in the series or parallel configuration, the first
configuration having a first
maximum rotor speed; supplying AC power to the AC motor, rotating the rotor;
determining an
optimal terminal voltage for the AC motor; and reconfiguring the AC motor into
a second
configuration in which the winding phase groups are configured in the Wye or
delta
configuration and the windings of each winding phase group are configured in
the series or
parallel configuration, the second configuration being different from the
first configuration, the
second configuration having a volts/Hz ratio capable of rotating the AC motor
at the optimal
terminal voltage.
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[0011] The present disclosure also provides for a method for controlling the
holding torque of an
AC motor. The method may include providing the AC motor. The AC motor may
include a
rotor, the rotor adapted to be rotated by the interaction between an internal
induced reluctance or
permanent magnetic field and an electromagnetic field; and a stator, the
stator including a
plurality of windings, the windings adapted to induce an electromagnetic field
to rotate the rotor,
the windings being grouped into winding phase groups, each winding phase group
corresponding
to and coupled to a phase of AC power supplied to the AC motor, the winding
phase groups
selectively reconfigurable from a Wye configuration to a delta configuration,
and each winding
phase group including at least two windings, the windings of each winding
phase group
selectively reconfigurable between a series and a parallel configuration. The
method may also
include determining a first holding torque requirement; and configuring the AC
motor into a first
configuration in which the winding phase groups are configured in the Wye or
delta
configuration and the windings of each winding phase group are configured in
the series or
parallel configuration, the first configuration having a first torque ratio.
[0012] The present disclosure also provides for a method for controlling the
speed of a
drawworks. The method may include providing a drawworks. The drawworks may
include a
drum driven by an AC motor. The AC motor may include a rotor, the rotor
adapted to rotate the
drum, the rotor adapted to be rotated by the interaction between an internal
induced reluctance or
permanent magnetic field and an electromagnetic field; and a stator, the
stator including a
plurality of windings, the windings adapted to induce an electromagnetic field
to rotate the rotor,
the windings being grouped into winding phase groups, each winding phase group
corresponding
to and coupled to a phase of AC power supplied to the AC motor, the winding
phase groups
selectively reconfigurable from a Wye configuration to a delta configuration,
and each winding
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phase group including at least two windings, the windings of each winding
phase group
selectively reconfigurable between a series and a parallel configuration. The
method may also
include determining a torque requirement, the torque requirement based on the
weight of the load
on the drawworks; configuring the AC motor into a first configuration in which
the winding
phase groups are configured in the Wye or delta configuration and the windings
of each winding
phase group are configured in the series or parallel configuration, the first
configuration having a
first torque ratio; supplying AC power to the AC motor; and rotating the
drawworks
[0013] The present disclosure also provides for a method for controlling the
speed of a
drawworks. The method may include providing the drawworks. The drawworks may
include a
drum driven by an AC motor. The AC motor may include a rotor, the rotor
adapted to rotate the
drum, the rotor adapted to be rotated by the interaction between an internal
induced reluctance or
permanent magnetic field and an electromagnetic field; and a stator, the
stator including a
plurality of windings, the windings adapted to induce an electromagnetic field
to rotate the rotor,
the windings being grouped into winding phase groups, each winding phase group
corresponding
to and coupled to a phase of AC power supplied to the AC motor, the winding
phase groups
selectively reconfigurable from a Wye configuration to a delta configuration,
and each winding
phase group including at least two windings, the windings of each winding
phase group
selectively reconfigurable between a series and a parallel configuration. The
method may also
include providing a control system, wherein the control system is adapted to
reconfigure the
winding phase groups; measuring the weight of the load on the drawworks;
determining a torque
requirement using the control system, the torque requirement based on the
weight of the load on
the drawworks; configuring the AC motor into a first configuration in which
the winding phase
groups are configured in the Wye or delta configuration and the windings of
each winding phase
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group are configured in the series or parallel configuration, the first
configuration having a first
torque ratio using the control system; supplying AC power to the AC motor; and
rotating the
drawworks.
Brief Description of the Drawings
[0014] The present disclosure is best understood from the following detailed
description when
read with the accompanying figures. It is emphasized that, in accordance with
the standard
practice in the industry, various features are not drawn to scale. In fact,
the dimensions of the
various features may be arbitrarily increased or reduced for clarity of
discussion.
[0015] FIG. 1 depicts a schematic view of an AC motor control system
consistent with
embodiments of the present disclosure.
[0016] FIG. 2 depicts a schematic view of an AC motor control system
consistent with
embodiments of the present disclosure.
[0017] FIGS. 3a-3d depict schematic views of four stator coil configurations
consistent with
embodiments of the present disclosure.
Detailed Description
[0018] It is to be understood that the following disclosure provides many
different embodiments,
or examples, for implementing different features of various embodiments.
Specific examples of
components and arrangements are described below to simplify the present
disclosure. These are,
of course, merely examples and are not intended to be limiting. In addition,
the present
disclosure may repeat reference numerals and/or letters in the various
examples. This repetition
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is for the purpose of simplicity and clarity and does not in itself dictate a
relationship between
the various embodiments and/or configurations discussed.
[0019] FIG. 1 depicts an exemplary AC motor control system 200 consistent with
embodiments
of the present disclosure. AC motor 202, as depicted, may be a three-phase AC
motor. As
discussed below, AC motor 202 may include a rotor and stator, the stator
including a plurality of
stator windings. The stator windings may be grouped into three matched sets of
windings
positioned radially about the stator. By supplying sinusoidal AC power to each
of the sets of
windings such that each set generally receives an alternating current offset
by 120 degrees, a
torque can be imparted on the rotor as it rotates through interaction between
the induced
magnetic fields of the stator windings and the rotor. As understood in the
art, torque produced by
an AC motor is generally related to the current supplied to the AC motor and,
in some instances,
may be linearly related thereto. As a permanent magnet motor increases in
speed, the back EMF
induced by the permanent magnet motors into the stator fields also increases.
The back EMF
reduces the voltage available to create current and thus reduces torque. For
high speed rotation,
for example, the voltage supplied to the permanent magnet motor must be higher
than the voltage
supplied at a low speed. The ratio between motor speed and voltage supplied
may thus be
expressed as a volts/Hz ratio. By changing winding configurations, as will be
discussed herein
below, the torque output of the motor may be controlled by selecting winding
configurations
with different volts/Hz ratios.
[0020] In some embodiments, AC motor 202 may be a permanent magnet motor. As
understood
in the art, a permanent magnet AC motor includes a stator which includes
windings as previously
described. The rotor, which is positioned in close proximity to the stator,
includes a plurality of
permanent magnets positioned about its periphery. The interaction between the
varying
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orientation of the magnetic field induced by the stator and the permanent
magnet field of the
permanent magnets of the rotor thus rotates the rotor.
[0021] In some embodiments, AC motor 202 may be coupled to speed switching
device 204 by a
plurality of conductors. For each winding phase group, as will be discussed
below, at least two
conductors may be used to supply AC power. Although depicted and discussed as
three
conductors 206a-c, one having ordinary skill in the art with the benefit of
this disclosure will
understand that the three conductors discussed and illustrated are meant to
represent the phases
of AC power supplied to AC motor 202. In some embodiments, more than 3
conductors may be
utilized. For example, the number of conductors may be a multiple of two times
the number of
winding groups in AC motor 202.
[0022] Additionally, in some embodiments, one or more neutral conductors 208
may be included
to couple the star or Wye point as discussed below to the rest of the power
system. In some
embodiments, a neutral conductor 208 may be omitted entirely.
[0023] Conductors 206a-c, as will be further discussed herein below, supply
three-phase AC
power from speed switching device 204 to the winding groups of AC motor 202.
Although
depicted as a single unit, one having ordinary skill in the art with the
benefit of this disclosure
will understand that speed switching device 204 need not be contained in a
single unit, nor does
it need to be positioned outside the housing or apparatus of AC motor 202.
[0024] Speed switching device 204 may be coupled to VFD system 222 via three-
phase
conductors 214, 216, 218. VFD system 222 may, in some embodiments, be
positioned to supply
modulated three-phase AC power to AC motor 202 via speed switching device 204.
As
understood in the art, VFD system 222 may modulate the three-phase AC power
according to,
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for example, a volts/Hz control scheme to, for example, allow AC motor 202 to
operate at a
continuously variable speed for a given stator configuration. VFD system 222
may be coupled to
three-phase AC supply lines 212. In some embodiments, contactor 220 may be
positioned to
selectively connect or disconnect AC motor control system 200 from supply
lines 212. In some
embodiments, DC link 224 may provide power to VFD system 222.
[0025] In some embodiments, AC motor control system 200 may include resistive
load bank
226. In such embodiments, motor disconnect switch 234 may be positioned across
three-phase
conductors 214, 216, 218 to selectively couple three-phase conductors 214,
216, 218 to dynamic
braking conductors 228, 230, 232, and dynamic braking switch 238 may be
coupled to
selectively couple dynamic braking conductors 228, 230, 232 to resistive load
bank 226. Control
device 236 may be positioned to couple motor disconnect switch 234 and dynamic
braking
switch 238 to allow switches 234, 238 to be operated in concert to, for
example, allow resistive
load bank 226 to provide dynamic braking to AC motor 202 when AC motor 202 is
a permanent
magnet motor. In some embodiments, control device 236 may selectively
electrically couple
resistive load bank 226' to DC link 224 to provide dynamic braking. As
understood in the art,
power regenerated by the dynamic braking of AC motor 202 may be utilized to
run other
electrical equipment coupled to the power supply. In some embodiments, only
surplus power
from DC link 224 may be passed to load bank 226'.
[0026] FIG. 2 depicts an exemplary embodiment of a switching mechanism of
speed switching
device 204. Ac motor 202 may include three phase winding groups u, v, w, each
having two or
more windings. FIG. 2 depicts each winding group u, v, w, having corresponding
windings lu,
2u, lv, 2v, 1w, 2w. One having ordinary skill in the art with the benefit of
this disclosure will
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understand that other configurations of winding groups u, v, w having more
than two windings
may be used without deviating from the scope of this disclosure.
[0027] For the purposes of this illustration, windings lu, 2u, lv, 2v, lw, 2w
are shown adjacent
to switches uAl, uA2, uB, vAl, vA2, vB, wAl, wA2, wB. One having ordinary
skill in the art
with the benefit of this disclosure will understand that switches uAl, uA2,
uB, vAl, vA2, vB,
wAl, wA2, wB may be located apart from AC motor 202. Switches uAl, uA2, uB,
vAl, vA2,
vB, wAl, wA2, wB are positioned to selectively reconfigure the connections
between windings
lu, 2u, lv, 2v, 1w, 2w and three-phase conductors 214, 216, 218, and the
interconnections of
windings lu, 2u, lv, 2v, lw, 2w.
[0028] Switches uAl and uA2 are positioned to, by switching at the same time,
change windings
lu and 2u from a parallel configuration (as shown) to a series configuration.
Likewise, switches
vAl and vA2 and switches wAl and wA2 are positioned to likewise transition
between parallel
and series configurations for windings lv, 2v, and windings lw, 2w
respectively. Switches uAl,
uA2, vAl, vA2, wAl, and wA2 are positioned to be switched simultaneously to
transition each
of winding groups u, v, and w between the parallel and series configurations
simultaneously.
FIG. 2 depicts a "parallel delta" configuration as will be discussed below.
[0029] Switches uB, vB, and wB are positioned to switch between a delta
winding configuration
(as shown) and a Wye configuration. As understood in the art, in a delta
winding configuration,
winding groups are coupled to three-phase conductors 214, 216, 218 such that
the ends of each
winding group are connected between two of three-phase conductors 214, 216,
218. Specifically,
in the embodiment depicted in FIG. 2, when in the delta configuration, winding
group u connects
between three-phase conductors 214 and 216, winding group v between conductors
216 and 218,
12
and winding group w between three-phase conductors 218 and 214. In the Wye
configuration,
each winding group connects between one of the three-phase conductors and the
Wye or star
point or, as depicted, optional neutral line 208. Switches uB, vB, and wB are
positioned to be
switched simultaneously. Both the switch between parallel/series and delta/Wye
may, in some
embodiments, be controlled by VFD system 222.
[0030] By selectively actuating switches uA 1, uA2, uB, vA 1, vA2, vB, wA 1,
wA2, and wR, AC
motor 202 may be selectively switched between so called series Wye (as
depicted in FIG. 3a),
series Delta (FIG. 3b), parallel Wye (FIG. 3c), and parallel delta (FIG. 3d)
winding
configurations. By reconfiguring between these configurations, the theoretical
torque and
rotation speed of the rotor of AC motor 202 may be varied to extend the
theoretical ranges
available to VFll system 222
[0031] For example, in the exemplary configuration described above, when in a
series Wye
configuration as depicted in FIG. 3a, the nominal torque may be calculated as:
6.4. Pp. (13w
Trq ¨ ________
2
where In is the nominal drive current, Pp is the number of motor pole-pairs,
and Ow is the
nominal flux per winding. As understood in the art, the nominal torque
represents the theoretical
maximum torque on the rotor of AC motor 202. Similarly, the no-load speed at
maximum
voltage for the rotor of AC motor 202 may be calculated as:
VI
LI) = _______
2,5 irk; Pp ,
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where Vi is the nominal drive output voltage. As understood in the art, the no-
load speed at
maximum voltage may be interpreted as the theoretical maximum rotor speed of
AC motor 202
in a given configuration.
[0032] In the exemplary configuration described above, when in a series delta
configuration as
depicted in FIG. 3b, the nominal torque may be calculated as:
Trq ¨ ________
2
and the no-load speed at maximum voltage may be calculated as:
V1
2 Ow Pp
Thus, the series delta configuration has a torque ratio of approximately 0.58
times that of the
series Wye configuration. As used herein, the torque ratio approximately
represents the
theoretical amount of torque available in the corresponding configuration
relative to another
configuration, here the series Wye configuration. The series delta
configuration has a no-load
speed at maximum voltage of 1.73 times that of the series Wye configuration.
The ratio of no-
load speed at maximum voltage between configurations is referred to herein as
a drive ratio.
[0033] In the exemplary configuration described above, when in a parallel Wye
configuration as
depicted in FIG. 3c, the nominal torque may be calculated as:
6.10. P.
Trq _________
4
and the no-load speed at maximum voltage may be calculated as:
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171
w,fl ¨ ____
cl)õ, Pp
Thus, the parallel Wye configuration has a torque ratio of .5 and drive ratio
of 2 compared to the
series Wye configuration.
[0034] In the exemplary configuration described above, when in a parallel
delta configuration as
depicted in FIG. 3b, the nominal torque may be calculated as:
6.1n. Pp.4),
Trq ¨ _______
and the no-load speed at maximum voltage may be calculated as:
tow, ¨ __
11),Pp
Thus, the parallel delta configuration has a torque ratio of 0.29 and drive
ratio of 3.46 compared
to the series Wye configuration.
[0035] Although only six windings are described above, one having ordinary
skill in the art with
the benefit of this disclosure will understand that more than 6 may be
utilized. For example, each
winding group u, v, w may have three or more windings each. Furthermore, in
some
embodiments, subgroups in each of winding group u, v, w may be connected in
series or in
parallel to further increase the number of winding configurations available to
AC motor 202.
[0036] In some embodiments of the present disclosure, AC motor 202 may be used
as the motor
in a piece of wellsite equipment. For example, and without limitation, AC
motor 202 may be
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used to drive a top drive, draw works, rotary table, mud pump, winch, etc. In
some embodiments,
AC motor 202 may be used to drive a thruster or other propulsion device.
[0037] In order to assist with understanding of the operation of AC motor 202
in accordance
with embodiments of the present disclosure, an exemplary operation will now be
described in
which AC motor 202 drives a drawworks. A winding configuration for AC motor
202 may be
selected to have a nominal torque output matched to the load to be lifted. For
example, when
lifting a load which requires the full torque output capability of AC motor
202 to be applied, a
winding configuration having a nominal torque output may be selected. For
example, of the
configurations previously discussed, a series Wye configuration may thus be
selected as its
torque output is the highest of the available configurations. The drawworks
may thus operate in a
"high torque, low speed" configuration.
[0038] If a load requiring less than the full torque output capability of AC
motor 202 is to be
applied, a different winding configuration having a lower torque output but
higher theoretical
maximum speed may be selected. Such a "low torque, high speed" configuration
may allow for
the load to be lifted, for example, more quickly or with a more efficient
voltage supplied to AC
motor 202, known as terminal voltage. Because each winding configuration
allows for AC motor
202 to operate at a different volts/Hz ratio, by selecting the winding
configuration based on the
expected load, in this case the weight of the load to be lifted, the terminal
voltage supplied to AC
motor 202 may be optimized for the given expected load.
[0039] In some embodiments, an operator may manually select which wiring
configuration to
use before the drawworks is engaged to lift the load. In some embodiments, an
automated control
system may operate to select the optimal winding configuration for a given
load. In some
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embodiments, the automated control system may utilize a memory table to
associate optimal
winding configurations to, for example, predefined hoisting operations, load
weights, required
load travel speed, etc. In some embodiments, the automated control system may
be able to
override the manual selection of the operator by, for example, detecting a
suboptimal winding
configuration selection or operating conditions.
[0040] As understood in the art, applications other than a drawworks may
include different
functions that likewise require such "high torque. low speed" and "low torque,
high speed"
configurations. Additionally, in some embodiments, accurate position control
or position holding
may be required. For example, a drawworks may need to hold a load at a certain
height above the
drill floor. Depending on the load, a winding configuration
[0041] In some embodiments, AC motor 202 may also be utilized for regenerative
braking. For
example, while lowering a load, the drawworks may operate as a regenerative
brake and
transform mechanical rotation power from the lowering of the load into
electrical power. Again,
because the volts/Hz ratio for each winding configuration is different, an
optimal winding
configuration may be selected to, for example, optimize the terminal voltage
of AC motor 202
while regeneratively braking. Additionally, in some embodiments, accurate
position control or
position holding may be required. For example, a drawworks may need to hold
the load at a
certain height above the drill floor. Depending on the load, a winding
configuration with
sufficient torque may be selected to, for example, optimize the holding
capability of AC motor
202 for the given load.
[0042] In another exemplary operation, AC motor 202 may drive a piece of
equipment which
undergoes relatively continuous operation such as a top drive. While the top
drive and AC motor
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202 are stopped, a winding configuration having the highest nominal torque may
be selected to,
for example, allow for maximum available torque to overcome the static
friction involved in
beginning rotation of the top drive. Of the configurations previously
discussed, the series Wye
configuration has the highest nominal torque value. AC power is supplied to AC
motor 202 by,
for example, VFD system 222 at a volts/Hz ratio corresponding to the series
Wye configuration.
AC motor 202 is capable of continuously variable speed by varying the voltage
and frequency of
AC power supplied to AC motor 202. As previously discussed, while in the
series Wye
configuration, AC motor 202 is capable of driving rotation up to approximately
the no-load
speed at maximum voltage with a relatively constant torque output. In
practice, the maximum
speed of AC motor 202 will be less than the no-load speed at maximum voltage
owing to, for
example, friction, voltage drops in the motor and elsewhere, losses to heat,
etc.
[0043] Once the top drive has begun to rotate, a lower amount of torque may be
required to
sustain its rotation. As such, it may be more efficient to operate AC motor
202 in a different
configuration for the given speed. Likewise, an operator may wish the top
drive to rotate at a
speed higher than AC motor 202 is capable of providing in the series Wye
configuration. The
operator, whether manually or automatically, may then reconfigure AC motor 202
into a
different configuration depending on the desired torque and speed
requirements. For example, of
the configurations previously described, the series delta configuration has
the next higher drive
ratio and the next lower torque ratio. As AC motor 202 is reconfigured, VFD
system 222
changes the voltage and frequency of its AC power output to correspond with
the voltage and
frequency ratio in the series delta configuration. As understood in the art,
when changing from
the Wye configuration to the Delta configuration, the frequency would remain
the same, but a
lower voltage would be required to maintain rotation at the same speed. Again,
AC motor 202 is
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capable of continuously variable speed by varying the voltage and frequency of
AC power
supplied to AC motor 202 up to approximately the no-load speed at maximum
voltage for the
series delta configuration.
[0044] If an output speed higher than the series delta configuration is
capable of outputting is
desired, AC motor 202 may be again reconfigured in the same manner as
previously discussed to
a configuration having an even higher drive ratio. Likewise, it may be more
efficient to operate
AC motor 202 in a configuration having a lower torque ratio, meaning lower
voltage for a given
rotor speed. For example, the parallel Wye configuration has the next higher
drive ratio and the
next lower torque ratio. The reconfiguration may occur precisely as discussed
with respect to the
series delta configuration. As understood in the art, when switching from a
series configuration
to a parallel configuration, the frequency of the AC power supplied to AC
motor 202 would be
halved to maintain the rotor speed. VFD system 222 may make this adjustment to
frequency as
well as changing the voltage of AC power supplied to AC motor 202 as the
reconfiguration
occurs.
[0045] If even higher speed or more efficient operation is desired, the
parallel delta configuration
may be selected as, of the configurations previously discussed, it has the
highest drive ratio. The
reconfiguration into the parallel delta configuration may be accomplished as
previously
discussed.
[0046] Alternatively, once the top drive and AC motor 202 are rotating, a
torque above that
available to AC motor in the configuration in which it is currently configured
may be required.
For example, if, while drilling, a relatively hard subsurface formation is
encountered after a
relatively softer layer, more torque may be required to maintain rotation of a
drill bit. In such an
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event, AC motor 202 may be reconfigured into a winding configuration having a
higher torque
ratio. For example, if AC motor 202 is in the parallel delta configuration, as
previously
discussed, any of the series Wye, series delta, and parallel Wye
configurations for AC motor 202
is capable of providing more torque output. In some embodiments, the speed of
AC motor 202
may be reduced to less than the no load speed at maximum voltage for the new
configuration
before AC motor 202 is reconfigured. This deceleration may occur naturally, or
may be
controlled by VFD system 222. Again, VFD system 222 may supply AC power at a
volts/Hz
ratio for the new configuration once AC motor 202 is reconfigured.
[0047] One having ordinary skill in the art with the benefit of this
disclosure will understand that
an operator needs not progress from a first configuration to the configuration
having the next
higher or lower torque ratio or drive ratio. Instead, any configuration may be
reconfigured into
any other configuration depending on, for example, the desired torque output
or desired
maximum speed. An operator, manually or automatically, may, for example,
switch directly
from the series Wye to the parallel delta configuration as long as AC motor
202 is capable of
outputting sufficient torque at the new configuration. Likewise, an operator,
manually or
automatically, may, for example, switch directly from the parallel delta to
the series Wye
configuration.
[0048] Additionally, although described as using a volts/Hz control scheme,
VFD system 222
may supply AC power to AC motor 202 by any control scheme including, for
example and
without limitation, volts/Hz, direct torque control (DTC), flux vector
control, any open loop (also
known as "Encoderless") variable frequency control, or any closed loop control
without
deviating from the scope of this disclosure.
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[0049] The foregoing outlines features of several embodiments so that a person
of ordinary skill
in the art may better understand the aspects of the present disclosure. Such
features may be
replaced by any one of numerous equivalent alternatives, only some of which
are disclosed
herein. One of ordinary skill in the art should appreciate that they may
readily use the present
disclosure as a basis for designing or modifying other processes and
structures for carrying out
the same purposes and/or achieving the same advantages of the embodiments
introduced herein.
One of ordinary skill in the art should also realize that such equivalent
constructions do not
depart from the spirit and scope of the present disclosure and that they may
make various
changes, substitutions, and alterations herein without departing from the
spirit and scope of the
present disclosure.
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