WO2014024288A1

WO2014024288A1 - Electric motor

Info

Publication number: WO2014024288A1
Application number: PCT/JP2012/070325
Authority: WO
Inventors: 弘文土井
Original assignee: 三菱電機株式会社
Priority date: 2012-08-09
Filing date: 2012-08-09
Publication date: 2014-02-13
Also published as: JP5885846B2; CN204231076U; JPWO2014024288A1

Abstract

A ring-shaped groove (23, 24) that passes through a coil pass-through hole (22) through which a coil (12) of a stator (3) is passed toward a power-distribution plate (14) is formed and thermally cut on the outer side of a shaft hole (21) for accommodating a bearing (8) of a bracket (20), suppressing heat conduction to the bearing (8). Additionally, the area of the bracket (20) surrounding the coil (12) is made into a thin section (25), reducing heat dissipation due to induction heating of the coil (12).

Description

Electric motor

This invention relates to an electric motor having a structure for radiating heat generated by a stator coil and a power distribution board.

A general three-phase synchronous AC motor (see, for example, Patent Document 1) creates a magnetic pole in a rotor by a permanent magnet of a stator, and creates a magnetic pole in a stator tooth by a coil disposed between stator teeth of the stator. . The energization direction of the three-phase coils arranged between the stator teeth is switched by a power distribution plate (bus bar), and is switched between the S pole and the N pole. By sequentially switching the energizing directions of the three-phase coils, the magnetism of each stator tooth rotates and the rotor rotates due to the magnetic action.

In order to drive this motor, current flows through the coil and the power distribution board, and the temperature of each part of the motor rises due to the heat generated. In some cases, the required performance could not be obtained. In such a case, in order to cool the coil and the distribution board as much as possible, a method has been adopted in which the coil and the distribution board are brought into contact with a housing having a relatively low temperature by external cooling via an insulating member to dissipate heat. (For example, refer to Patent Document 2).

JP-A-8-214519 JP 2010-104212 A

However, since the current flowing through the coil and the distribution board is switched as described above, if the housing is too close to the coil and the distribution board, the temperature of the metal housing rises due to induction heating.

On the other hand, since the temperature of the bearing that rotatably holds the rotor also rises during rotation due to self-heating due to heat transfer and friction, the temperature of the bearing can also be radiated to the housing to lower the temperature. Then it is important.

Therefore, in order to dissipate the heat generated by the coil and the distribution board, a means for reducing the distance between the coil and the distribution board and the housing is taken. However, due to the temperature rise of the housing due to induction heating, the heat dissipation effect of the coil and the distribution board is sufficient. In addition, the heat dissipation effect of the bearing is reduced.
On the other hand, in order to suppress the temperature rise of the housing due to induction heating, if the distance from the coil and the power distribution plate is increased, the temperature of the coil and the power distribution plate does not decrease, and the coil penetrates the housing. Due to the large coil penetration portion, there is a problem that a sufficient heat radiation effect cannot be obtained even if the heat generated by the bearing installed on the center side is transmitted to the housing.

This invention was made in order to solve the above problems, and it aims at providing the electric motor which can suppress the temperature rise of a housing and can suppress degradation of components, such as a bearing by heat damage. .

An electric motor according to the present invention holds a coil that generates a magnetic field when energized, forms a magnetic path of the magnetic field generated by the coil, a rotor that rotates by a magnetic attraction repulsive force of the stator, and the center of the stator A bearing that rotatably holds the rotor part on the side, a power distribution plate that distributes power to the coil of the stator part according to the rotational position of the rotor part, and a housing that holds the stator part and the power distribution board side by side in the axial direction, The housing holds the stator portion on one surface, the bracket portion holding the power distribution plate on the opposite surface, the shaft hole formed in the center of the bracket portion to receive the bearing, and the outer peripheral side from the shaft hole of the bracket portion A coil through hole for passing the coil of the stator portion in the axial direction to the power distribution plate side, and the outer periphery of the shaft hole of the bracket portion. Those having an annular groove through.

According to this invention, by forming the annular groove on the outer peripheral side from the shaft hole of the bracket portion, it is possible to suppress heat transfer to the bearing by cutting the heat with this groove. Further, by forming a groove to partially reduce the thickness of the bracket portion, heat generation due to induction heating of the coil can be reduced, and an unnecessary increase in the temperature of the housing can be suppressed. Therefore, it is possible to provide an electric motor capable of suppressing deterioration of components such as a bearing due to heat damage.

It is sectional drawing which shows the structure of the electric motor which concerns on Embodiment 1 of this invention. It is a figure explaining rotation operation of the electric motor concerning Embodiment 1. FIG. The structure of the housing of the electric motor which concerns on Embodiment 1 is shown, Fig.3 (a) is the top view seen from the power distribution board side, FIG.3 (b) is sectional drawing cut | disconnected along the AA line. FIG. 4A is a plan view seen from the power distribution plate side, and FIG. 4B is a cross-sectional view cut along the line BB. It is sectional drawing explaining the positional relationship of the thin part in an electric motor which concerns on Embodiment 1, and a power distribution board. It is a housing modification of the electric motor which concerns on Embodiment 1, Comprising: It is sectional drawing explaining the positional relationship of a thin part and a power distribution board. FIG. 7A shows a structure of a housing of an electric motor according to Embodiment 2 of the present invention, FIG. 7A is a plan view seen from a power distribution board side, and FIG. 7B is a cross-sectional view cut along a CC line. is there. FIG. 8A is a plan view of the electric motor according to the second embodiment, and FIG. 8A is a plan view seen from the power distribution plate side, and FIG. 8B is a cross-sectional view cut along the line EE. It is the top view seen from the power distribution board side which shows the housing modification of the electric motor which concerns on Embodiment 2. FIG.

Hereinafter, in order to explain the present invention in more detail, modes for carrying out the present invention will be described with reference to the accompanying drawings.
Embodiment 1 FIG.
An electric motor 1 shown in FIG. 1 constitutes a three-phase AC synchronous motor, and mainly includes a cylindrical housing 2, a stator portion 3 fixed inside the housing 2, and a rotor portion 5 that rotates a shaft 4. And a power distribution plate (bus bar) 14 disposed on one end face side of the stator portion 3. In FIG. 2, the top view of the stator part 3 and the rotor part 5 seen from the power distribution board 14 side is shown. However, the housing 2 and the coil 12 are not shown.

The rotor part 5 is configured by laminating electromagnetic steel plates, and is formed with two protrusions protruding outward in the circumferential direction at intervals of 180 degrees, and the protrusions are shifted by 90 degrees in the axial direction of the shaft 4. (

Protrusions

5a, 5b). The shaft 4 is rotatably supported by bearings 7 and 8 fixed to the housing 2. The rotor part 5 is fixed to the shaft 4, and the rotational force generated in the rotor part 5 is externally output by rotating the shaft 4 integrally with the rotor part 5.

The stator unit 3 includes two

stator cores

9 and 10 and a magnet 11 disposed between the

stator cores

9 and 10. The

stator cores

9 and 10 are configured by laminating electromagnetic steel plates in the axial direction of the shaft 4. Each of the

stator cores

9 and 10 is formed with a plurality of

teeth

9a and 10a protruding from the outside toward the central shaft 4 side, and one U-shaped pair of

teeth

9a and 10a overlapping in the axial direction of the shaft 4 is formed. The coil 12 is attached. The stator portion 3 is press-fitted from the opening end portion of the housing 2 and is sandwiched between a cover 19 that closes the opening end portion and a bracket portion 20 (details will be described later) protruding from the inner peripheral surface of the housing 2. It is fixed in the state.

The distal end portion of each coil 12 mounted on each

tooth

9a, 10a passes through a coil through hole 22 formed in the bracket portion 20 of the housing 2 and a through hole (not shown) formed in the insulating member 6. It protrudes to the inverter board 13 side, and this protruding portion is connected to the power distribution plate 14 (U phase, V phase, W phase) by welding or the like. The power distribution plate 14 is a conductive member that is integrally formed with the insulating member 6 such as an insulating resin and is insulated from the metal housing 2 such as aluminum, and is arranged in an annular shape along the circumferential direction of the shaft 4 and has one end portion. The other end of the inverter board 13 is connected to the coil 12. The insulating member 6 including the power distribution plate 14 is press-fitted from the opening end portion of the housing 2 (the side opposite to the press-fitting side of the stator portion 3), and is attached to the cover housing 17 and the bracket portion 20 attached to the opening end portion. It is fixed in a clamped state.

The inverter board 13 converts an external power source (not shown) into an alternating current, and sequentially switches the U phase, V phase, and W phase of the power distribution plate 14 based on the position signal input from the sensor 15 to the power distribution plate. Current is passed through 14. The inverter board 13 is attached to the inside of the cover housing 17 and is covered with a cover 18.

The sensor 15 is installed in the inner circumferential space of the insulating member 6, detects the rotational position of the shaft 4 by detecting the position of the sensor target 16 that rotates integrally with the shaft 4, and sends a position signal to the inverter board 13. Output.

Here, an outline of the operation of the electric motor 1 will be described.
The magnetic flux generated by the magnet 11 magnetized in the axial direction flows out from the teeth 9a of the stator core 9 arranged on the N-pole side of the magnet 11 to the protrusion 5a of the rotor portion 5, and advances through the rotor portion 5 in the axial direction. It becomes a field magnetic flux that comes out of the protruding portion 5 b on the pole side and flows into the teeth 10 a of the stator core 10 disposed on the S pole side of the rotor portion 5. Thus, the magnetic field magnetomotive force of the magnet 11 acts on the rotor portion 5, so that the protrusion 5 a of the rotor portion 5 facing the N pole side of the magnet 11 is magnetized to the N pole, and the S pole of the magnet 11. The protrusion 5b of the rotor part 5 facing the side is magnetized to the S pole. When a current flows through the U-shaped coil 12 via the power distribution plate 14, the

teeth

9a and 10a of the

stator cores

9 and 10 are magnetized according to the direction of the flowing current to generate a rotating magnetic field and generate torque. . By sequentially switching the direction of the current flowing through the coil 12, the NS polarities of the

teeth

9a and 10a are rotated and moved as shown in FIGS. 2 (a) to 2 (c). 5 rotates.

Next, the shape of the bracket part 20 of the housing 2 will be described.
3A and 3B show the structure of the housing 2 of the electric motor 1 according to the first embodiment. FIG. 3A is a plan view of the housing 2 viewed from the power distribution plate 14 side, and FIG. It is sectional drawing cut | disconnected along the line.
A shaft hole 21 is formed in the center of the bracket portion 20 and the bearing 8 is installed. Moreover, the coil through-hole 22 which the coil 12 penetrates in the bracket part 20 is formed for the number of terminals of the coil 12, and the coil 12 is penetrated from the stator part 3 side to the power distribution board 14 side. In this electric motor 1, since the

teeth

9a and 10a of the stator part 3 are provided in six poles, six U-shaped coils 12 are required and six coil through holes 22 are also required. Then, as shown by a two-dot chain line in FIG. 3A, the tips of adjacent coils 12 are inserted through one coil through hole 22.

Further, an annular groove 23 is formed on the surface of the bracket portion 20 facing the power distribution plate 14 so as to pass through the coil through hole 22. Similarly, an annular groove 24 is formed on the surface of the bracket portion 20 facing the stator portion 3 so as to pass through the coil through hole 22. Thereby, compared with the axial direction thickness D1 of the bracket part 20, the thin part 25 in which the thickness D2 of the part which formed the

grooves

23 and 24 is thin is formed.

Hereinafter, the difference in the heat dissipation effect when the thin portion 25 is formed and when it is not formed will be described.
FIG. 4 shows a housing 102 having a conventional structure in which the thin portion 25 is not formed. 4A is a plan view of the housing 102 viewed from the power distribution plate 14 side, and FIG. 4B is a cross-sectional view of the housing 102 cut along the line BB.

Since the coil 12 installed in the stator part 3 needs to be electrically joined to the power distribution plate 14, the bracket part 20 of the

housings

2 and 102 must be penetrated, and the bracket part 20 has a coil through hole 22. Must be formed and passed through the coil 12. Therefore, heat generated by the coil 12 due to the current flowing is transmitted to the bracket portion 20 and the housings 2 and 102 (arrow H1 in FIGS. 3B and 4B).

On the other hand, in order to transmit the heat generated in the coil 12 and the distribution board 14 due to the current flow to the bracket portion 20 and the

housings

2 and 102 to dissipate heat, the distance between the inner wall surface of the coil through hole 22 and the coil 12, and the distribution board 14 and the bracket part 20 must be as close as possible.

However, if the coil 12 and the bracket portion 20 are too close to each other, induction heating is generated by the alternating current flowing through the coil 12 and the bracket portion 20 and the

housings

2 and 102 are heated, so that the coil 12 and the power distribution plate 14 are sufficient. It becomes impossible to dissipate heat.
Further, heat generated in the bracket portion 20 and the

housings

2 and 102 due to induction heating is transmitted (arrow H2 in FIGS. 3B and 4B), and the temperature of the bearing 8 installed in the shaft hole 21 is unnecessarily increased. End up.

Therefore, in the housing 2 of the first embodiment, as shown in FIG. 3,

annular grooves

23 and 24 that pass through the coil through holes 22 are provided on the outer peripheral side of the shaft hole 21 of the bracket portion 20, so that the thin portion 25 is provided. Form and reduce the portion that generates heat by induction heating. As a result, at least heat generation by induction heating of the coil 12 can be reduced, and an unnecessary temperature rise of the bracket portion 20 and the housing 2 can be suppressed. Therefore, heat transfer to the bearing 8 (arrow H2 in FIG. 3B) can be suppressed, and the bearing 8 can be prevented from being unnecessarily heated, and self-heating due to friction of the bearing 8 can be prevented from the bracket portion. 20 and the housing 2 can be radiated to lower the temperature. Further, by forming the

grooves

23 and 24, heat transfer from the bracket portion 20 to the bearing 8 can be cut off. Therefore, deterioration of the bearing 8 due to heat damage can be suppressed.

FIG. 5 is a cross-sectional view illustrating the positional relationship between the thin portion 25 and the power distribution plate 14 in the electric motor 1 according to the first embodiment. In the figure, the stator portion 3, the coil 12, etc. are omitted.
The power distribution plate 14 is brought into contact with one side of the bracket portion 20 via the insulating member 6, and heat generated by the power distribution plate 14 due to current flowing is transmitted to the bracket portion 20 and the housing 2 to dissipate heat, thereby increasing the temperature. Suppressed. At this time, by disposing the power distribution plate 14 on the outer peripheral side of the annular groove 23 of the bracket portion 20 via the insulating member 6, an air layer is formed by the groove 23 to be heat-cut, Suppresses heat transfer. Further, the heat generated by the power distribution plate 14 is transmitted from the contact surface 26 between the power distribution plate 14 and the bracket portion 20 to the bearing 8 through the thin portion 25, and a heat dissipation path indicated by an arrow H3 is formed. Therefore, it is possible to prevent the bearing 8 from being heated unnecessarily.

Further, although the temperature of the stator portion 3 also rises due to the heat generated by the coil 12, heat transfer to the bearing 8 can be suppressed by forming the groove 24 in the bracket portion 20 in contact with the stator portion 3.

3 and 5, the

annular grooves

23 and 24 are formed on both surfaces of the bracket portion 20, but either one may be used. Since the temperature of the insulating member 6 is higher than that of the stator portion 3, it is desirable to form the groove 23 at least on the power distribution plate 14 side.
Furthermore, the groove 23 may be deepened so that heat transfer from the power distribution plate 14 to the bearing 8 can be further suppressed. An example of this configuration is shown in FIG.

FIG. 6 is a cross-sectional view for explaining a modification of the bracket portion 20 in the electric motor 1 according to the first embodiment. In the figure, the stator portion 3, the coil 12, etc. are omitted.
In this modification, a deep groove 23 is formed on the surface of the bracket portion 20 facing the power distribution plate 14 to increase the axial distance from the power distribution plate 14 to the thin portion 25. Therefore, the heat dissipation path indicated by the arrow H3 in FIG. 6 is extended from the heat dissipation path indicated by the arrow H3 in FIG. 5, and heat dissipation from the outer peripheral surface of the bracket portion 20 and the housing 2 increases, and from the power distribution plate 14 to the bearing 8. It is possible to further suppress the heat transfer.

Furthermore, in FIG. 3, FIG. 5 and FIG. 6, a heat radiating member such as a heat sink may be installed on the outer peripheral surface of the housing 2 to enhance the heat radiating effect.

As described above, according to the first embodiment, the electric motor 1 holds the coil 12 that generates a magnetic field by energization, and forms the magnetic path of the magnetic field generated by the coil 12 and the magnetic attraction of the stator unit 3. The rotor portion 5 that rotates by the repulsive force, the bearing 8 that rotatably holds the shaft 4 fixed to the rotor portion 5 on the center side of the stator portion 3, and the rotation of the rotor portion 5 to the coil 12 of the stator portion 3. A power distribution plate 14 that distributes power in accordance with the position; and a housing 2 that holds the stator portion 3 and the power distribution plate 14 side by side in the axial direction. The housing 2 holds the stator portion 3 on one side, The bracket portion 20 that holds the power distribution plate 14 on the surface, the shaft hole 21 that is formed at the center of the bracket portion 20 and accommodates the bearing 8, and is formed on the outer peripheral side from the shaft hole 21 of the bracket portion 20. A coil through hole 22 that allows the coil 12 of the portion 3 to pass through in the axial direction to the power distribution plate 14 side, and an annular shape that is formed on the outer peripheral side of the shaft hole 21 of the bracket portion 20 over the entire circumference and passes through the coil through hole 22. The

grooves

23 and 24 are configured. For this reason, heat transfer to the bearing 8 can be suppressed by cutting the heat with the

annular grooves

23 and 24 on the outer peripheral side of the bearing 8 of the bracket portion 20. Further, by forming the

grooves

23 and 24 and partially reducing the thickness of the bracket portion 20, heat generation due to induction heating of the coil 12 can be reduced, and an unnecessary increase in the temperature of the housing 2 can be suppressed. The heat radiation from 8 to the housing 2 is not hindered. Therefore, deterioration of components, such as the bearing 8 by heat damage, can be suppressed.

Further, according to the first embodiment, since the annular groove 23 is formed on the surface of the bracket portion 20 that holds the power distribution plate 14, the heat generated by the power distribution plate 14 is cut off by the groove 23 and the power distribution is performed. Heat transfer from the plate 14 to the bearing 8 can be suppressed.

Further, according to the first embodiment, the power distribution plate 14 is brought into contact with the outer peripheral side of the annular groove 23 of the bracket portion 20 via the insulating member 6. A heat dissipation path is formed from the contact surface 26 to the bearing 8 through the outer peripheral side of the bracket portion 20 and the thin portion 25. For this reason, the heat generated by the power distribution plate 14 can be efficiently radiated by the bracket portion 20 and the housing 2.

Embodiment 2. FIG.
FIG. 7 shows the configuration of the housing 2 in the electric motor 1 according to the second embodiment. FIG. 7A is a plan view of the housing 2 viewed from the power distribution board 14 side, and FIG. It is sectional drawing which cut | disconnected along CC line. In addition, in the electric motor 1 of this Embodiment 2, since structures other than the bracket part 20 are the same as that of FIG. 1, FIG. 1 is used below.

As shown in FIG. 6, when the annular groove 23 is formed deeply in the bracket portion 20 of the housing 2, the strength around the shaft hole 21 decreases. Therefore, in the second embodiment, as shown in FIG. 7, a plurality of ribs 30 are installed in the groove 23 to improve the strength of the bracket portion 20. Further, by reducing the thickness of the rib 30, the temperature rise of the rib 30 due to induction heating is suppressed, and the bearing 8 is prevented from being heated unnecessarily. Further, the end surface of the rib 30 is made one step lower than the contact surface 26 with the insulating member 6, and a gap D 3 is opened between the rib 30 and the insulating member 6. Thereby, the heat generated by the power distribution plate 14 is prevented from being transmitted to the bearings 8 via the ribs 30.

Also, as shown in FIG. 5, the ribs 30 can be installed even when the

grooves

23 and 24 are formed on both surfaces of the bracket portion 20. As an example, FIG. 8A shows a plan view of the housing 2 viewed from the power distribution plate 14 side, and FIG. 8B shows a cross-sectional view of the housing 2 cut along the line EE. In the example of FIG. 8, the ribs 30 are formed by forming the grooves 23 on the surface of the bracket portion 20 facing the power distribution plate 14, and the ribs 24 are formed by forming the grooves 24 on the surface of the bracket portion 20 facing the stator portion 3. Install. Thereby, the intensity | strength of the bracket part 20 is improved. Also in this configuration, a gap D3 is formed between the rib 30 and the insulating member 6 to prevent heat transfer from the power distribution plate 14. It is also possible to form a gap between the rib 31 and the stator part 3 to prevent heat transfer from the stator part 3. It is also possible to install only one of the

ribs

30 and 31.

Furthermore, it is desirable that the

ribs

30 and 31 be an integral multiple of the number of coils 12, that is, an integral multiple of the number of poles of the stator portion 3, and be arranged uniformly in a circumferential shape. In the case of FIGS. 7 and 8, since the

teeth

9a and 10a of the stator portion 3 are 6 poles, the

ribs

30 and 31 are equally arranged at 6 locations as an integral multiple of the

teeth

9a and 10a so ing. Further, for example, as shown in the plan view of the bracket portion 20 shown in FIG. 9, the ribs 30 may be installed at twice the number of poles, that is, at 12 locations.

As described above, according to the second embodiment, since the electric motor 1 is configured to install the

ribs

30 and 31 in the

annular grooves

23 and 24, the bracket portion 20 while maintaining the heat cutting effect of the

grooves

23 and 24. The strength of can be improved. Further, the induction heating is not increased by making the

ribs

30 and 31 thinner.

Further, according to the second embodiment, the end surface of the rib 30 is made lower than the surface of the bracket portion 20 that holds the power distribution plate 14, so that heat transfer from the power distribution plate 14 to the bearing 8 through the rib 30 is performed. Can be prevented. Similarly, since the end surface of the rib 31 is made lower than the surface that holds the stator portion 3 of the bracket portion 20, heat transfer from the stator portion 3 to the bearing 8 through the rib 30 can be prevented.

Further, according to the second embodiment, since the number of

ribs

30 and 31 is set to be an integral multiple of the number of poles of the stator portion 3, the

ribs

30 and 31 can be installed so as to have a uniform stress distribution in a circumferential shape. The strength of the bracket part 20 can be improved.

In the present invention, within the scope of the invention, any combination of the embodiments, or any modification of any component in each embodiment, or omission of any component in each embodiment is possible. .

As described above, in the electric motor according to the present invention, the bracket portion of the housing has a shape that can reduce heat generation by induction heating while considering heat cutting and heat dissipation, so that a turbine such as an electric compressor and an electric assist turbo can be operated at high speed. It is suitable for use in an electric motor that is driven by rotation.

1 electric motor, 2 housing, 3 stator part, 4 shaft, 5 rotor part, 5a, 5b protrusion, 6 insulating member, 7, 8 bearing, 9, 10 stator core, 9a, 10a teeth, 11 magnet, 12 coil, 13 inverter Board, 14 Distribution board, 15 Sensor, 16 Sensor target, 17 Cover housing, 18, 19 Cover, 20 Bracket part, 21 Shaft hole, 22 Coil through hole, 23, 24 groove, 25 Thin part, 26 Contact surface, 30, 31 ribs.

Claims

A stator that holds a coil that generates a magnetic field when energized, and that forms a magnetic path of the magnetic field generated by the coil;
A rotor portion that is rotated by a magnetic attractive repulsive force of the stator portion;
A bearing that rotatably holds the rotor portion on the center side of the stator portion;
A power distribution plate that distributes power to the coil of the stator portion according to the rotational position of the rotor portion;
A housing for holding the stator part and the power distribution plate side by side in the axial direction;
The housing is
A bracket portion that holds the stator portion on one surface and the power distribution plate on the opposite surface;
A shaft hole that is formed at the center of the bracket portion and accommodates the bearing;
A coil through hole that is formed on the outer peripheral side of the shaft hole of the bracket portion, and passes the coil of the stator portion in the axial direction toward the power distribution plate;
An electric motor having an annular groove formed over the entire circumference on the outer peripheral side of the shaft hole of the bracket portion and passing through the coil through hole.
2. The electric motor according to claim 1, wherein the annular groove is formed on a surface of the bracket portion that holds the power distribution plate.
3. The electric motor according to claim 2, wherein the power distribution plate is in contact with an outer peripheral side of the annular groove of the bracket portion via an insulating member.
The electric motor according to claim 1, wherein a rib is installed in the annular groove.
5. The electric motor according to claim 4, wherein an end surface of the rib is lower than a surface of the bracket portion that holds the power distribution plate or a surface that holds the stator portion.
5. The electric motor according to claim 4, wherein the number of ribs installed is an integral multiple of the number of poles of the stator portion.