JP6668844B2 - Rotating electric machine - Google Patents

Description

  The present invention relates to a rotating electric machine.

  As a rotating electric machine that outputs torque by using a magnetic flux of a permanent magnet, a rotating electric machine that can vary an effective magnetic flux amount by a permanent magnet is known. For example, Patent Literature 1 discloses a rotating electric machine having a stator on which an armature winding is wound and a rotor rotatably provided through a gap between the stator and the stator. A structure is described in which the rotor is divided into two parts, a first rotor and a second rotor, and field magnets having different polarities are alternately arranged in the rotation direction.

  With such a structure, the rotating electric machine described in Patent Literature 1 operates the second rotor in accordance with a change in the torque or the number of revolutions, and determines the polarity of the permanent magnet of the first rotor and the permanent magnet of the second rotor. By changing the positional relationship with the polarity, the effective magnetic flux amount of the permanent magnet can be adjusted. At this time, the rotating electric machine described in Patent Document 1 changes the relative positional relationship between the polarity of the permanent magnet of the first rotor and the polarity of the permanent magnet of the second rotor, by controlling the input to the actuator. The second rotor is controlled to be in a predetermined state by a signal.

  In a state where the second rotor is moved to an arbitrary predetermined position separated from the first rotor in the direction of the rotation axis, a magnetic flux flow generated in the direction of the rotation axis may be cut off by the magnetoresistive layer provided on the stator core. it can. Thereby, iron loss in the high-speed rotation region of the variable magnetic flux type rotating electric machine can be reduced.

JP 2010-246196 A

  However, the rotating electric machine described in Patent Literature 1 requires an actuator and a control device for controlling the actuator in order to position the second rotor in a predetermined state, as described above. Further, precise control is required because the second rotor is mechanically moved so that the first rotor and the second rotor have a predetermined positional relationship. Therefore, the magnetic flux of the permanent magnet cannot be varied with a low-cost configuration.

  The present invention has been made in view of the above circumstances, and has as its object to provide a rotating electric machine that can vary the magnetic flux of a permanent magnet with a low-cost configuration.

In order to achieve the above object, the present invention provides a stator having an annular stator core, a first rotor having a first rotor core facing the stator core on at least one surface side in the axial direction of the stator, A second rotor having a second rotor core facing the stator core on a radially inner surface side of the stator, wherein the stator is disposed on the stator core at predetermined intervals in a circumferential direction. A plurality of stator teeth, and an armature coil that is toroidally wound between adjacent stator teeth, wherein the first rotor is disposed on the first rotor core and faces the stator teeth in the axial direction. And an induced current is induced based on the magnetic flux generated on the rotor side and the stator side wound around the rotor tooth. And guide coil, the wound in the rotor teeth and a magnetic coil field for generating a magnetic field by energization of the induction current, the rotor teeth, so that the induction coil and the field coil is layered in the axial direction The second rotor has a salient pole portion radially opposed to the stator teeth, a permanent magnet is included in the salient pole portion, and the rotor teeth of the first rotor are And the magnetic poles of the permanent magnets of the second rotor are opposite to each other .

  According to the present invention, it is possible to provide a rotating electric machine capable of changing the magnetic flux of a permanent magnet with a low-cost configuration.

FIG. 1 is a perspective view of a rotating electric machine according to one embodiment of the present invention. FIG. 2 is a partial cross-sectional perspective view of the rotating electric machine according to one embodiment of the present invention. FIG. 3 is a partial cross-sectional perspective view showing the rotating electric machine according to one embodiment of the present invention, in which an armature coil, an induction coil, and a field coil are omitted. FIG. 4 is a connection diagram of the rectifier circuit and the induction coil and the field coil in the rotary electric machine according to one embodiment of the present invention. FIG. 5 is a partial cross-sectional perspective view showing the rotary electric machine according to one embodiment of the present invention, showing a state where the first rotor teeth are magnetized to magnetic poles opposite to the magnetic poles of the permanent magnet. is there. FIG. 6 is a perspective view of the rotating electric machine according to one embodiment of the present invention, showing a state where the magnetic flux of the permanent magnet and the magnetic flux of the field coil cancel each other. FIG. 7 is a schematic diagram showing the amount of magnetic flux linking from the rotor to the stator when the rotor of the rotary electric machine according to one embodiment of the present invention is rotating at a high speed. FIG. 8 is a graph showing the amount of magnetic flux linked from the rotor to the stator with respect to the rotation speed of the rotor, in the rotating electric machine according to one embodiment of the present invention.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. 1 to 8 are diagrams illustrating a rotating electric machine according to one embodiment of the present invention.

  As shown in FIGS. 1 and 2, the rotating electric machine 1 includes a stator 10 having three-phase armature coils 11 of W-phase, V-phase, and U-phase that generate a magnetic flux when energized, and a magnetic flux generated by the stator 10. And a rotor 20 that rotates by passing therethrough.

(Stator)
The stator 10 includes an annular stator core 12 made of a magnetic material having a high magnetic permeability, and an armature coil 11 wound around the stator core 12. Stator 10 is fixed to a motor case (not shown) in a magnetically isolated state via a connecting piece (not shown) made of a non-magnetic material provided on the outer peripheral surface of stator core 12. Thereby, for example, generation of a leakage magnetic flux is suppressed.

  As shown in FIGS. 2 and 3, the stator core 12 is formed with a plurality of stator teeth 13 projecting from the stator core 12 on both sides in the axial direction and on the inner surface side in the radial direction at predetermined intervals in the circumferential direction. Slots 14 that are groove-shaped spaces are formed between the stator teeth 13 that are adjacent in the circumferential direction. Here, the axial direction indicates a direction in which the rotation shaft 2 (see FIG. 7) of the rotor 20 extends. The radial direction is a direction orthogonal to the direction in which the rotation axis 2 of the rotor 20 extends, and is indicated in the radial direction with the rotation axis 2 as the center. The radially inner side indicates a side closer to the rotating shaft 2 of the rotor 20 in the radial direction, and the radially outer side indicates a side farther from the rotating shaft 2 of the rotor 20 in the radial direction. The circumferential direction indicates a circumferential direction around the rotation shaft 2 of the rotor 20.

  The armature coil 11 is toroidally wound around a slot 14 formed between adjacent stator teeth 13 in the circumferential direction of the stator core 12. The W-phase, V-phase, and U-phase armature coils 11 are wound around the slots 14 by concentrated winding. The toroidal winding is a method in which the winding of the armature coil 11 is wound around the stator core 12 by alternately passing the inside and outside of the ring of the stator core 12.

  The armature coil 11 is formed of a rectangular wire having a rectangular cross section, and is wound around the slot 14 in a toroidal winding state by edgewise winding. The edgewise winding is a method of winding the rectangular wire vertically with the short side of the rectangular wire facing the inside and outside in the radial direction of the rotary electric machine 1 with respect to the slot 14.

  Thereby, the rectangular wires adjacent to each other in the winding pitch direction are in surface contact with each other on the long side, so that the number of windings can be increased while maintaining the cross-sectional area according to the current. Therefore, the space factor of the armature coil 11 can be improved, and the magnetomotive force of the stator 10 can be increased.

  The stator teeth 13 have side surfaces 13 a on one side and the other side in the axial direction of the stator core 12, and a radial inner surface 13 b of the stator core 12. A first rotor tooth 212, which will be described later, faces the side surface portion 13 a of the stator tooth 13 in the axial direction. A second rotor tooth 222, which will be described later, faces the inner surface 13b of the stator tooth 13 in the radial direction.

  The stator 10 generates a rotating magnetic field that rotates in the circumferential direction when the three-phase alternating current is supplied to the armature coil 11. The magnetic flux generated in the stator 10 (hereinafter, this magnetic flux is referred to as “main magnetic flux”) is linked to the rotor 20. Thereby, the stator 10 can rotate the rotor 20.

  Specifically, the armature coils 11 are arranged on both sides in the circumferential direction of the stator teeth 13, and the pair of armature coils 11 includes a magnetic flux generated from one armature coil 11 and a magnetic flux generated from the other armature coil. The winding direction and the energizing direction are set such that the direction of the magnetic flux generated by the coil 11 is opposite to the circumferential direction.

  Thus, for example, when one armature coil 11 is in the V + phase and the other armature coil 11 is in the V− phase, the magnetic flux generated from the pair of armature coils 11 is sandwiched by the pair of armature coils 11. It occurs toward the stator teeth 13 so as to collide with the stator teeth 13. The magnetic flux generated by the stator teeth 13 changes its direction in a direction orthogonal to the circumferential direction of the stator core 12 and travels from the stator teeth 13 to the rotor 20.

  Then, a part of the magnetic flux toward the rotor 20 passes through a first rotor core 211 and a second rotor core 221 to be described later, and then the stator teeth sandwiched by the pair of W + and W− phase armature coils 11. Go to 13. Further, a part of the magnetic flux toward the rotor 20 passes through a first rotor core 211 and a second rotor core 221 to be described later, and then the stator teeth sandwiched by the pair of U + phase and U− phase armature coils 11. Go to 13.

  Thus, on the surface where the stator teeth 13 and the rotor 20 face each other, a magnetic circuit of the magnetic flux generated by the armature coil 11 is formed. The rotating electric machine 1 rotates the rotor 20 using a surface where the stator teeth 13 and the rotor 20 face each other as a torque generating surface.

  As described above, the armature coil 11 of the stator 10 is toroidally wound and concentratedly wound. For this reason, when a three-phase alternating current is supplied to the armature coil 11, in addition to a rotating magnetic field that rotates in synchronization with the rotation of the rotor 20, a harmonic rotating magnetic field that is asynchronous with the rotation of the rotor 20 is generated in the stator 10. I do. The harmonic rotating magnetic field includes a second spatial harmonic in the stationary coordinate system (a third time harmonic in the synchronous rotating coordinate system). Therefore, a harmonic component is superimposed on the magnetic flux generated in the stator 10.

(Rotor)
As shown in FIGS. 1, 2, and 3, the rotor 20 includes a pair of axial gap rotors 210 arranged so as to sandwich the stator 10 in the axial direction, and a radially arranged radially inner side of the stator core 12. The gap rotor 220 is included.

  The pair of axial gap rotors 210 and the radial gap rotor 220 are integrally rotatably fixed to the rotating shaft 2 (see FIG. 7) so as to rotate in synchronization with each other. The pair of axial gap rotors 210 and the radial gap rotor 220 may be integrated.

  Each of the pair of axial gap rotors 210 includes an annular first rotor core 211 made of a magnetic material having a high magnetic permeability, an induction coil 215, and a field coil 216. The first rotor core 211 includes a plurality of first rotor teeth 212 protruding from the first rotor core 211 toward the stator 10 in the axial direction at predetermined intervals along the circumferential direction of the first rotor core 211. Is formed.

  The first rotor teeth 212 face the stator teeth 13 on both axial sides of the stator core 12, that is, on one side and the other side in the axial direction of the stator core 12.

  An induction coil 215 and a field coil 216 are wound around the first rotor teeth 212 so as to form layers in the axial direction. Each of the induction coil 215 and the field coil 216 is formed of a winding covered with an insulating material.

  The induction coil 215 is disposed closer to the stator 10 than the field coil 216. The induction coil 215 generates an induction current based on a harmonic component superimposed on the magnetic flux generated on the stator 10 side.

  Specifically, when a three-phase alternating current is supplied to the armature coil 11 and a rotating magnetic field is generated in the stator 10, the magnetic flux of the harmonic component generated on the stator 10 is linked to the induction coil 215. Thereby, the induction coil 215 induces an induced current.

  The field coil 216 is disposed closer to the first rotor core 211 than the induction coil 215 is. The field coil 216 generates a magnetic field by rectifying and supplying an induced current generated by the induction coil 215.

  Thereby, the first rotor teeth 212 can function as an electromagnet, and the surface of the stator teeth 13 and the first rotor teeth 212 facing each other can function as a torque generating surface.

  The radial gap rotor 220 has a second rotor core 221 made of a magnetic material having high magnetic permeability and fixed so as to be able to rotate integrally with the rotating shaft 2 (see FIG. 7), and a permanent magnet 223.

  A plurality of second rotor teeth 222 protruding radially outward from the second rotor core 221 are formed on the second rotor core 221 at predetermined intervals along the circumferential direction of the second rotor core 221. Have been.

  The second rotor teeth 222 face the stator teeth 13 on the radially inner side of the stator core 12. The second rotor teeth 222 constitute a salient pole in the present invention. A permanent magnet 223 is arranged on the second rotor tooth 222.

  The permanent magnet 223 is made of, for example, a neodymium magnet (Nd-Fe-B magnet), and is included in the second rotor teeth 222.

(Rectifier circuit)
The rotating electric machine 1 further includes a rectifier circuit 30 that rectifies the induced current generated in the induction coil 215 and supplies the rectified current to the field coil 216.

  As shown in FIG. 4, the rectifier circuit 30 includes two diodes D1 and D2 as rectifier elements, and is configured as a closed circuit in which the diodes D1 and D2 are connected to two induction coils 215 and two field coils 216. Have been. The rectifier circuit 30 is provided on one side and the other side in the axial direction of the rotor 20 so as to correspond to the induction coil 215 and the field coil 216 on one side and the other side in the axial direction of the rotor 20, respectively.

  The two induction coils 215 in the rectifier circuit 30 are induction coils 215 that are circumferentially adjacent to the axial gap rotor 210. The two field coils 216 are field coils 216 that are adjacent in the circumferential direction of the radial gap rotor 220.

  The diodes D1 and D2 are provided on the axial gap rotor 210 or the radial gap rotor 220, for example, in a state housed in a diode case (not shown). The diodes D1 and D2 may be mounted inside the axial gap rotor 210 or the radial gap rotor 220.

  In the rectifier circuit 30, the alternating current induced by the two induction coils 215 is rectified by the diodes D1 and D2, and the rectified DC current is supplied to the two field coils 216 connected in series as field currents. Supplied. The two field coils 216 generate an induced magnetic flux when supplied with a direct current.

  In the present embodiment, the two field coils 216 generate induced magnetic fluxes in opposite directions by a DC current. Specifically, the first rotor tooth 212 wound with one field coil 216 is magnetized to the N pole, and the first rotor tooth 212 wound with the other field coil 216 is magnetized to the S pole. The winding direction of the field coil 216 is set so as to be magnetized. Also, as shown in FIG. 5, the magnetic pole of the first rotor tooth 212 and the magnetic pole of the permanent magnet 223 located at the same position in the circumferential direction as the first rotor tooth 212 are formed in opposite magnetic poles. , The winding direction of the field coil 216 is set.

(Operation of rotating electric machine)
Next, an operation of the rotating electric machine 1 according to the present embodiment will be described with reference to FIGS.

  As described above, the rotating electric machine 1 according to the present embodiment is a permanent magnet type synchronous motor that includes the permanent magnet 223 on the rotor 20 and outputs torque using the magnetic flux of the permanent magnet 223.

  In the conventional permanent magnet type synchronous motor, since the magnetic flux of the permanent magnet is constant, the back electromotive force generated in the armature coil of the stator by the magnetic flux of the permanent magnet increases as the rotation speed of the rotor increases. When the rotation speed of the rotor reaches a certain rotation speed, the back electromotive force generated in the armature coil becomes equal to the power supply voltage of the permanent magnet type synchronous motor. As a result, no more current can flow through the permanent magnet type synchronous motor. As a result, the rotation speed of the rotor cannot be increased.

  Conventionally, in order to solve such a problem, field weakening control has been performed in which a back electromotive force generated in the armature coil is equivalently reduced by flowing a current through the armature coil of the stator to cancel the magnetic flux generated by the permanent magnet.

  However, this field-weakening control generates a magnetic flux that does not contribute to the torque because a current is applied to generate a magnetic flux in a direction to cancel the magnetic flux of the permanent magnet. For this reason, useless energy is consumed for the output, and the efficiency is reduced.

  Further, in the field-weakening control, since a harmonic magnetic flux is generated, there is a possibility that iron loss and electromagnetic vibration of the permanent magnet type synchronous motor increase due to the harmonic magnetic flux. Further, in the field-weakening control, a magnetic flux in the opposite direction to the magnetic flux of the permanent magnet is generated to suppress the magnetic flux of the permanent magnet, so that irreversible demagnetization of the permanent magnet may occur. For this reason, it is necessary to use a permanent magnet having a relatively high coercive force, which increases the cost.

  When a neodymium magnet is used as the permanent magnet, an eddy current is generated in the permanent magnet due to a change in the external magnetic field due to the field weakening control, and the permanent magnet generates heat. This heat may cause irreversible demagnetization of the permanent magnet. Therefore, it is necessary to add a material such as a rare earth having high heat resistance to the permanent magnet. However, in this case, since the added material such as rare earth becomes an impurity for the permanent magnet, there is a possibility that the intrinsic performance of the permanent magnet cannot be exhibited.

  Therefore, the rotating electric machine 1 according to the present embodiment has a configuration in which the amount of magnetic flux linked from the permanent magnet 223 to the stator 10 can be adjusted by the operation of the field coil 216 described above without performing the field-weakening control. Thereby, the rotating electric machine 1 according to the present embodiment can solve the problem caused by the field weakening control as described above.

(At low rotor speed)
In the rotating electric machine 1 according to the present embodiment, when the rotation speed of rotor 20 is low, the magnetic flux of the harmonic component is not generated in stator 10, or even if generated, the amount is small. For this reason, the field coil 216 does not generate an induced magnetic flux, or generates a very small amount even if it does.

  For this reason, the first rotor teeth 212 are not magnetized in either the N pole or the S pole, or a very small amount is magnetized. Therefore, all of the magnetic flux of the permanent magnet 223 links with the stator 10.

  As described above, when the rotation speed of the rotor 20 is low, the amount of magnetic flux linked to the stator 10 from the permanent magnet 223 can be increased as compared with when the rotation speed of the rotor 20 is high.

(At high rotor speed)
On the other hand, in the rotating electric machine 1 according to the present embodiment, when the rotation speed of the rotor 20 is high, a magnetic flux of a harmonic component is generated in the stator 10. The amount of magnetic flux of the harmonic component increases as the rotation speed of the rotor 20 increases.

  Thus, an induced current is induced in the induction coil 215 of the axial gap rotor 210, and the induced current is rectified by the rectifier circuit 30 and supplied to the field coil 216 as a DC current.

  The field coil 216 supplied with the DC current generates a magnetic flux in a direction for magnetizing the first rotor teeth 212 so as to be a magnetic pole opposite to the magnetic pole of the permanent magnet 223 corresponding to the same position in the circumferential direction. That is, as shown in FIG. 5, the magnetic pole of the first rotor tooth 212 and the magnetic pole of the permanent magnet 223 are formed in opposite magnetic poles.

  For this reason, as shown in FIG. 6, the magnetic path of the magnetic flux by the field coil 216 (indicated by a white arrow) is the magnetic path of the magnetic flux by the permanent magnet 223 (in black) in both the axial direction and the radial direction. (Indicated by an arrow). This reverse magnetic path cancels out in the stator core 12 of the stator 10.

  Therefore, as shown in FIG. 7, of the magnetic flux of the permanent magnet 223, the magnetic flux that is not canceled by the magnetic flux of the field coil 216 links with the stator 10. In addition, a part of the magnetic flux of the permanent magnet 223 flows toward the first rotor teeth 212 through the gap (gap) at the axial end of the stator core 12. As a result, the amount of magnetic flux linked from the permanent magnet 223 to the stator 10 is suppressed.

  In FIG. 8, in a region smaller than the rotation speed R <b> 1, no magnetic pole is formed in the first rotor teeth 212, and the magnetic flux of the permanent magnet 223 links with the stator 10 in the radial direction. Further, in a region higher than the rotation speed R1, the magnetic poles of the first rotor teeth 212 are formed on the magnetic poles opposite to the magnetic poles of the permanent magnets 223, so that the magnetic flux of the permanent magnets 223 is applied to the stator 10 in the radial and axial directions. The amount of magnetic flux linked to the stator 10 from the permanent magnet 223 is suppressed. Further, as the rotation speed of the rotor 20 increases, the induced current induced in the induction coil 215 increases, and the amount of magnetic flux linked from the permanent magnet 223 to the stator 10 decreases.

  Therefore, even if the rotation speed of the rotor 20 is high, the field weakening control can be unnecessary. For this reason, it is possible to prevent iron loss and electromagnetic vibration caused by harmonic magnetic flux generated by the field weakening control.

  Further, since the field-weakening control is not required, there is no need to use a permanent magnet having a high coercive force, and it is not necessary to add a material such as a rare earth having high heat resistance to the permanent magnet. Thereby, the cost of the rotary electric machine 1 can be reduced.

  As described above, in the rotating electric machine 1 according to the present embodiment, the amount of magnetic flux linked from the permanent magnet 223 to the stator 10 can be adjusted without performing the field-weakening control, so that the rotation speed of the rotor 20 is high. However, a decrease in efficiency can be prevented. Further, when the rotation speed of the rotor 20 is low, the output can be improved.

  As described above, according to the rotating electric machine 1 of the present embodiment, the induction current is generated in the induction coil 215 of the axial gap rotor 210 based on the harmonic component superimposed on the magnetic flux generated on the stator 10 side. The induced current is rectified by the rectifier circuit 30 and supplied to the field coil 216. Thus, the amount of leakage magnetic flux of the permanent magnet 223 that short-circuits the inside of the magnetic path member 225 can be adjusted.

  The harmonic component superimposed on the magnetic flux generated on the stator 10 side is obtained by supplying a three-phase alternating current to the armature coil 11 which is wound in a toroidal manner on the stator 10 and concentrated. Therefore, a special device such as a DC / DC converter is not required to generate the DC current supplied to the field coil 216.

  Thus, the rotating electric machine 1 of the present embodiment adjusts the amount of magnetic flux linked to the stator 10 from the permanent magnet 223 with a simple configuration without using a special device such as a DC / DC converter. be able to. As a result, the rotating electric machine 1 of the present embodiment can change the magnetic flux of the permanent magnet 223 with a low-cost configuration.

  The rotating electric machine 1 can be suitably adopted, for example, as a motor for a vehicle, a generator for a wind power generator, or a motor for a machine tool.

  In the present embodiment, the first rotor teeth 212 are provided so as to face both axial sides of the stator core 12. However, the present invention is not limited to this, and, for example, one side surface of the stator core 12 in the axial direction is provided. Alternatively, it may be provided so as to face the side surface on the other side. That is, the first rotor teeth 212 may be provided only on one of the one side surface and the other side surface of the stator core 12 in the axial direction. In this case, since only the axial gap rotor 210 on one side in the axial direction is employed, the size of the rotating electric machine 1 can be reduced.

  The rotating electric machine 1 of the present embodiment is also applicable to a double-rotor-type rotating electric machine including two rotors, an outer rotor and an inner rotor, inside the stator in the radial direction. In this case, the armature coil is concentratedly wound around the stator, but no toroidal winding is employed. In this double-rotor type rotating electric machine, an induction coil is arranged on an outer rotor, and a permanent magnet, a magnetic path member and a variable field coil are arranged on an inner rotor. The configuration of the outer rotor and the inner rotor may be reversed.

  While embodiments of the present invention have been disclosed, it will be apparent that modifications may be made by those skilled in the art without departing from the scope of the invention. All such modifications and equivalents are intended to be included in the following claims.

REFERENCE SIGNS LIST 1 rotating electric machine 2 rotating shaft 10 stator 11 armature coil 12 stator core 13 stator teeth 13a side surface 13b inner surface 20 rotor 30 rectifying circuit 210 axial gap rotor 211 first rotor core 212 first rotor tooth 215 induction coil 216 field coil 220 Radial gap rotor 221 Second rotor core 222 Second rotor tooth (salient pole portion)
223 Permanent magnet D1, D2 Diode

Claims (2)

A stator having an annular stator core;
A first rotor having a first rotor core facing the stator core on at least one side in the axial direction of the stator;
A second rotor having a second rotor core facing the stator core on a radially inner surface side of the stator,
The stator is
A plurality of stator teeth arranged at predetermined intervals in a circumferential direction on the stator core;
Having an armature coil that is toroidally wound between the adjacent stator teeth,
The first rotor includes:
Rotor teeth disposed on the first rotor core and axially opposed to the stator teeth;
An induction coil wound on the rotor teeth to induce an induction current based on a magnetic flux generated on the stator side;
A field coil that is wound around the rotor teeth and generates a magnetic field by passing the induced current,
On the rotor teeth, the induction coil and the field coil are wound in layers in the axial direction,
The second rotor,
It has a salient pole portion radially opposed to the stator teeth, and a permanent magnet is included in the salient pole portion,
A rotating electric machine , wherein a magnetic pole of the rotor teeth of the first rotor and a magnetic pole of the permanent magnet of the second rotor are formed in opposite magnetic poles . The rotating electric machine according to claim 1 , wherein the induction coil and the field coil are wound around the rotor teeth so as to form a layer in an axial direction .

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