Note: Descriptions are shown in the official language in which they were submitted.
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TRACTION MOTOR AND DRIVE SYSTEM
Technical field:
The present invention relates to a traction motor and
a drive system, e.g. for railway locomotives and motor
coaches, in which the traction motor and/or other electric
machines included in the system are provided with a magnetic
circuit comprising a magnetic core and at least one winding.
8ackgrouad art:
The magnetic circuit in electric machines usually
comprises a laminated core, e.g. of sheet steel surrounded
and fixed with a welded construction. To provide
ventilation and cooling the core is often divided into
stacks with radial and/or axial ventilation ducts. For
larger machines the laminations are punched out in segments
which are attached to the frame of the machine, the
laminated core being held together by pressure fingers and
pressure rings. The winding of the magnetic circuit is
disposed in slots in the core, the slots generally having a
cross section in the shape of a rectangle or trapezium.
In multi-phase electric machines the windings are made
as either single or double layer windings. With single
layer windings there is only one coil side per slot, whereas
with double layer windings there are two coil sides per
slot. By coil side is meant one or more conductors combined
vertically or horizontally and provided with a common coil
insulation, i.e. an insulation designed to withstand the
rated voltage of the machine to earth.
Double-layer windings are generally made as diamond
windings whereas single layer windings in the present
context can be made as diamond or flat windings. Only one
(possibly two? coil width exists in diamond windings whereas
flat windings are made as concentric windings, i.e. with
widely varying coil width. By coil width is meant the
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distance in arc dimension between two coil sides pertaining
to the same coil.
Normally all large machines are made with double-layer
winding and coils of the same size. Each coil is placed
with one side in one layer and the other side in the other
layer. This means that all coils cross each other in the
coil end. If there are more than two layers these crossings
complicate the winding work and the coil end is less
satisfactory.
Before it became possible to use industrial frequency
(50 or 60 Hz) for traction motors, the first alternating
voltage systems were electrified with low-frequency voltage
(15 to 162/3 or 25 Hz). The traction motor used for a long
time in such systems was a single-phase series commutator
motor, also known as a single-phase traction motor. This
functions almost like a direct current motor except that
both field and rotor current are reversed every half period
since it is supplied with alternating current. For
commutation to take place without damaging arcing at the
commutator, low frequency and motors with low speed had to
be chosen.
The main advantage with alternating systems as opposed
to direct current systems is that the alternating voltage
can be transformed (even though direct voltage can nowadays
be transformed with so-called choppers). It is thus
possible to maintain a relatively high voltage on the
overhead conductor in relation to the voltage with which the
motor operates. Due to the high voltage in the overhead
conductor the current becomes lower, thus giving better
power transmission ability and lower losses in the line
network. Supply stations can be located rather far apart
(30-120 km).
The most commonly used traction motor today is the
three phase asynchronous motor due to its simplicity and
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voltage and frequency which is produced by power
semiconductor circuitry from the line voltage (dc system) or
from the transformer secondary voltage (ac system).
Machines of the above-mentioned type, with conventional
stator winding, cannot be connected to a high-voltage
network at e.g. 15 kv without the use of a transformer to
lower the voltage. The use of a motor in this way,
connected to the high-voltage network via a transformer,
entails a number of drawbacks as compared with if the motor
could be connected directly to the high-voltage network.
The following drawbacks may be noted, among others:
- the transformer is expensive, increases transport costs
and requires space
- the transformer lowers the efficiency of the system
- the transformer consumes reactive power
- a conventional transformer contains oil, with the
associated risks.
Description of the invention:
The object of the present invention is to provide a
motor and a drive system therefor for electric railway
operation and the like, which solves some of the problems
inherent in known systems in this area.
The present invention provides a motor according to
claim 1 and a drive system according to claim 6 or claim 7.
2S The invention is thus based on a special technia_ue for
constructing electric machines, motors, generators,
transformers, etc. in which the electric windings are
produced with insulation other than oil, and preferably dry,
in a special manner. This permits either elimination of the
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transformer and/or the construction of transformers without
the drawbacks inherent in conventional ones that have been
mentioned above.
The invention may naturally also include such special
machines combined with conventional machines.
Thus a machine of the type to which the invention
relates may be a transformer or a traction motor which does
not then need any transformer. The alternatives may of
course be combined.
The drive system and the components according to the
invention can be adapted to the electric supply system of
various railway systems and, with applicable modifications,
is intended for railway systems with external power supply
or with their own power supply system, for railways with
different voltage levels and different frequencies and for
both alternating and direct current systems, as well as for
both synchronous and asynchronous motor operation.
In cases when a transformer is deemed necessary, it is
an object of the present invention that the transformer
shall be manufactured using a cable of the same type and in
corresponding manner as for the other electric machines
included in the drive system.
The advantage gained by satisfying the above objects is
the avoidance of an intermediate, oil-filled transformer,
the reactance of which otherwise consumes reactive power.
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To achieve this, the magnetic circuit and its
conductors in at least one of the electric machines included
in the vehicle are produced with threaded permanently
insulated cable the exterior of which is connected to a
selected potential such as earth.
The major and essential difference between known
technology and the embodiment according to the invention is
thus that the latter includes in at least one machine which,
due to the nature of its magnetic circuit can be directly
connected via breakers and isolators to a high supply
voltage, up to between 10 and 800 kV. The magnetic circuit
thus comprises one or more laminated cores with a winding
consisting of a threaded cable having one or more
permanently insulated conductors having a semiconducting
layer both at the conductor and outside the insulation, the
outer semiconducting layer being connected to earth
potential.
To solve the problems arising with direct connection of
electric machines, both rotating and static machines, to all
types of high-voltage power networks, at least one machine
in the drive system according to the invention has a number
of features as mentioned above, which differ distinctly from
known technology. Additional features and further
embodiments are defined in the dependent claims and are
discussed in the following.
The features mentioned above and ether characteristics
of the drive system and at least one of the electric
machines included therein according to the invention,
include the following:
- The winding for the magnetic circuit is produced from
a cable having one or more permanently insulated
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conductors with two semiconducting layers, one
surrounding the strands and one forming a sheath. Some
typical conductors of this type have insulation of
cross-linked polyethylene or ethylene propylene rubber.
For the present purpose the conductors may be further
developed both as regards the strands in the conductor
and the nature of the outer sheath.
- Cables with circular cross section are preferred, but
cables with some other cross section may be used in order to
obtain better packing density, for instance.
- Such a cable allows the laminated core to be designed
according to the invention in a new and optimal way as
regards slots and teeth.
- The winding is preferably manufactured with insulation
in steps for best utilization of the laminated core.
- The winding is preferably manufactured as a multi-
layered, concentric cable winding, thus enabling the number
of coil-end intersections to be reduced.
- The slot design may be suited to the cross section of
the winding cable so that the slots are in the form of a
number of cylindrical openings running axially and/or
radially outside each other and having an open waist running
between the layers of the armature winding.
- The design of the slots may be adjusted to the relevant
cable cross section and to the stepped insulation of the
winding. The stepped insulation allows the magnetic core to
have substantially constant tooth width, irrespective of the
radial extension.
- The above-mentioned further development as regards the
outer sheath entails that at suitable points along the
length of the conductor, the outer sheath is cut off, each
cut partial length being connected directly to earth
potential.
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The use of a cable of the type described above allows
the entire length of the outer semiconducting sheath of the
winding, as well as other parts of the drive system, to be
kept at earth potential. An important advantage is that the
electric field is close to zero within the coil-end region
outside the outer semiconducting layer. With earth
potential on the outer layer the electric field need not be
controlled. This means that no field concentrations will
occur either in the core, in the coil-end regions or in the
transition between them.
The mixture of insulated and/or uninsulated impacted
strands, or transposed strands, results in low stray losses.
The cable for high voltage used in the magnetic circuit
winding is built up of an inner core/conductor with a
plurality of strands, at least one semiconducting layer, the
innermost being surrounded by an insulating layer, which is
in turn surrounded by an outer semiconducting layer having
an outer diameter in the order of 6-250 mm and a conductor
area in the order of 10-3000 mmz.
If at least one of the machines in the plant according
to the invention is constructed in the manner specified,
start and control of the motors) used in the locomotive of
motor coach can be achieved with the start methods, known
per se.
According to a particularly preferred embodiment of the
invention, at least two of these layers, preferably all
three, have the same coefficient of thermal expansion. The
decisive benefit is thus gained that defects, cracks and the
like are avoided during thermal movement in the winding.
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Since the insulation system, suitably permanent, is
designed so that from the thermal and electrical point of
view it is dimensioned for over 10 kV, the system can be
connected to high-voltage power networks without any
intermediate step-down transformer, thereby achieving the
advantages referred to above.
The above-mentioned and other advantageous embodiments
of the invention are defined in the dependent claims.
Brief description of the drawings:
The invention will be described in more detail in the
following description of a preferred embodiment of the
construction of the magnetic circuit of an electric machine,
with reference to the accompanying drawings in which
Figure 1 shows a schematic end view of a sector of the
stator in an electric machine in the plant according to the
invention;
Figure 2 shows an end view, step-stripped, of a cable
used in the winding of the stator according to Figure 1; and
Figures 3 to 5 show traction motor drive systems
according to different embodiments of the invention.
Description of preferred embodiments:
In the schematic end view of a sector of the stator 1
according to Figure 1, pertaining to an electric machine of
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rotating type included in the plant according to the
invention, the rotor 2 of the machine is also indicated.
The stator 1 is composed of a laminated core. Figure 1
shows a sector of the machine corresponding to one pole
pitch. A number of teeth 4 extend radially in from a yoke
part 3 of the core towards the rotor 2 and are separated by
slots 5 in which the stator winding is arranged. Cables 6
forming this stator winding are high-voltage cables which
may be of substantially the same type as those used for
power distribution, e.g. PEX cables. One difference is that
the outer, mechanically-protective sheath, and the metal
screen normally surrounding such power distribution cables
are eliminated so that the cable for the present application
comprises only the conductor and at least one semiconducting
layer on each side of an insulating layer. Thus, the
semiconducting layer lies naked on the surface of the cable.
The cables 6 are illustrated schematically in Figure 1,
only the conducting central part of each cable part or coil
side being drawn in. As can be seen, each slot 5 has
varying cross section with alternating wide parts 7 and
narrow waist parts 8. The wide parts 7 are substantially
circular and surround the cabling. The waist parts 8 serve
to radially fix the position of each cable. The cross
section of the slot 5 also narrows radially inwards. This
is because.the voltage on the cable parts is lower the
closer to the radially inner part of the stator 1 they are
situated. Slimmer cabling can therefore be used towards the
inside, whereas coarser cabling is necessary further out.
In the example illustrated cables of three different
dimensions are used, arranged in three correspondingly
dimensioned sections 51, 52, 53 of slots 5.
The above description of the magnetic circuit for a
rotating electric machine built up with the cable 6 is also
applicable to static electric machines such as transformers,
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reactor windings and the like. The following important
advantages are obtained both from the design and the
manufacturing point of view:
- the windings of the transformer can be constructed
without consideration to any electric field distribution and
the problematical transposition of parts in known technology
is thus unnecessary,
- the transformer core can be designed without taking
into consideration any electric field distribution,
- no oil is required for electric insulation of cable and
winding and instead the cable and winding can be surrounded
by air or by a non-flammable or slowly burning liquid,
- in many applications no special bushing is required as
is the case for oil-filled transformers, for electrical
communication between the outer connections of the
transformer and the coils/windings located therein,
- the lack of oil greatly reduces the risk of fire and
explosion in a transformer of the invention,
- the transformer can be made rigid than a conventional
transformer, increasing its ability to withstand short
circuits,
- the transformer is less noisy, cleaner and requires
less maintenance, and
- the manufacturing and testing technology required for
a dry transformer with magnetic circuit as described above,
is considerably simpler than that required for conventional
transformers/reactors.
Figure 2 shows a step-wise stripped end view of a high-
voltage cable for use in an electric machine included in the
plant according to the present invention. The high-voltage
cable 6 comprises one or more conductors 31, each of which
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comprises a number of strands 36 which together give a
circular cross section of copper (Cu), for instance. These
conductors 31 are arranged in the middle of the high-voltage
cable 6 and are surrounded in the embodiment shown by a part
insulation 35. However, it is feasible for the part
insulation 35 to be omitted on one of the conductors 31. In
the present embodiment of the invention the conductors 31
are together surrounded by a first semiconducting layer 32.
Around this first semiconducting layer 32 is an insulating
layer 33, e.g. PEX insulation, which is in turn surrounded
by a second semiconducting layer 34. Thus the high-voltage
cable need nvt include any metallic screen or outer sheath
of the type that normally surrounds such a cable for power
distribution. As traction equipment often becomes very
warm, the insulating layer 33 can comprise heat resistant
polymers, e.g. silicone rubber or fluorinated polymers. The
semiconducting layers 32, 34 may comprise similar material
to the insulating layer but with conducting particles, such
as carbon black, soot or metallic particles, embedded
therein. Generally it has been found that a particular
insulating material has similar mechanical properties when
containing no, or some, carbon particles.
The use of electric machines provided with magnetic
circuits of the type described above enables the electric
supply of traction motors, as well as the traction motors
themselves, to be greatly simplified and made more
efficient. In railway operation with alternating voltage
the supply voltages currently used are generally 15 kV, 162/3
Hz, 11 kV 25 Hz or 25 kv, 50/60 Hz in the supply line 104
from which the current collector 112 of the locomotive
supplies one or more traction motors 114, as shown in
Figures 3 to 5.
Known traction motors for alternating voltage are
normally driven by voltages of up to 1 kV and the locomotive
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must therefore be equipped with a transformer and with
speed-control equipment, the latter constituting thyristors
in modern locomotives.
The transformers used in the known locomotive are oil-
s filled and have a number of mechanical and electrical
drawbacks, as well as incurring environmental problems. The
rotating machines and used for converting and operation in
the known locomotive have various problems, both mechanical
and electrical, that can be dealt with to a more or less
satisfactory extent.
The above-mentioned problems can be eliminated or
minimized by designing the magnetic circuits in at least one
of the electric machines of the system in accordance with
the present invention.
Figures 3 to 5 show a 3-phase asynchronous motor 114
providing the mechanical power for the locomotive and having
a winding formed from a high-voltage cable as exemplified in
Figure 2. The winding of the motor 114 has the advantages
which have been described above.
Figure 3 shows a drive system for the motor, comprising
a transformer 122 and a thyristor bridge 123, connected via
a smoothing and filtering circuit 124 to a dc/3-phase ac
converter 125 which supplies the 3-phase motor 114. The
transformer 122 has a winding formed from a cable such as is
shown in Figure 2. This transformer therefore has the
advantages listed above and is also lighter and less bulky
than a known oil-filled transformer.
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Figure 4a shows a drive system including a rotating
converter 130 comprising a motor M supplied directly from
the current collector 112 and a generator G which supplies
the 3-phase motor 114 via a regulator device 131. Tap
connections 132a, 132b can be used to control the voltage
supplied to the motor 114 and the number of poles connected
for coarse speed control.
Figure 4b shoes an alternative system in which the
rotating converter 130, which preferably generates
multiphase, e.g. six-phase ac, is connected to a rectifier
bridge 133 which supplies the motor 114 via a dc/3-phase ac
converter 125. Figure 4c shows a further alternative system
in which the supply from the rotating converter 130 to the
motor 114 is via an ac/ac frequency converter 134.
In the systems shown in Figures 4a, 4b and 4c, either
or both of the motor M and generator G are wound using a
cable as exemplified in Figure 2. The motor and generator
may be separate machines sharing a common shaft, or
alternatively the rotating converter may comprise a single
unit as described, for example, in German Patents 372390,
386561 and 406371. The rotating converter may also be a
phase converter as described in "Das Handbuch der
Lokomotiven", pp. 254-255, "Electrischer Bahnen" eb, 85.
Jahrgang, Heft 12/1987, pp. 388-389, or Lueger, "Lexicon der
Technik", p.395.
Figure 5 shows a system in which the motor 114 is a
high voltage motor which is supplied by a regulator device
135 connected to the current collector 112. The regulator
device is preferably a direct semiconductor ac/ac converter.
Since the motor 114 is supplied with a high voltage, no
transformer or other voltage changing means is required and
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the drive system has the advantage of being compact and
light.
Although certain voltage values have been noted above,
these shall only be considered as examples. Similarly,
various combinations of conventionally designed electric
machines and electric machines provided with the magnetic
circuit according to the invention are feasible. The
invention shall not therefore be deemed as restricted to the
systems described with reference to the drawings, but covers
all feasible systems defined in the appended claims.
Although it is preferred that the electrical insulation
should be extruded in position, it is possible to build up
an electrical insulation system from tightly wound,
overlapping layers of film or sheet-like material. Both the
semiconducting layers and the electrically insulating layer
can be formed in this manner. An insulation system can be
made of an all-synthetic film with inner and outer
semiconducting layers or portions made of polymeric thin
film of, for example, PP, PET, LDPE or HDPE with embedded
conducting particles, such as carbon black or metallic
particles and with an insulating layer or portion between
the semiconducting layers or portions.
For the lapped concept a sufficiently thin film will
have butt gaps smaller than the so-called Paschen minima,
thus rendering liquid impregnation unnecessary. A dry,
wound multilayer thin film insulation has also good thermal
properties.
Another example of an electrical insulation system is
similar to a conventional cellulose based cable, where a
thin cellulose based or synthetic paper or non-wover.
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material is lap wound around a conductor. In this case the
semiconducting layers, on either side of an insulating
layer, can be made of cellulose paper or non-woven material
made from fibres of insulating material and with conducting
particles embedded. The insulating layer can be made from
the same base material or another material can be used.
Another example of an insulation system is obtained by
combining film and fibrous insulating material, either as a
laminate or as co-lapped. An example of this insulation
system is the commercially available so-called paper
polypropylene laminate, PPLP, but several other combinations
of film and fibrous parts are possible. In these systems
various impregnations such as mineral oil can be used.
