Patent Description:
Motors for driving used in electric vehicles and hybrid vehicles are required to provide significant power output so that permanent magnet motors including a rare earth element that retains intense energy are generally used. The motors for driving use, from among such permanent magnet motors, embedded-type magnet motors, which can satisfy the requirement to provide a large torque at low speeds and a wide rotation speed range.

Torque fluctuations of a motor are causes of noises and vibrations. In particular, in the case of electric vehicles, there arises the problem that the torque fluctuations make the ride uncomfortable at low speeds. Conventional motors generally adopt a countermeasure to provide skew in order to reduce the torque fluctuations. For example, there is known a motor in which an electromagnetic steel sheet provided with grooves is arranged on the side of outer periphery of a magnet embedded in a rotor and the grooves are arranged as being displaced in a direction along the periphery of the rotary shaft one portion from another. <CIT>relates to an internal permanent magnet type rotating electrical machine, which has an annular stator and a rotor that is arranged inside the stator with an air gap interposed between the stator and the rotor. <CIT>relates to a self-initiated permanent magnet synchronous motor, which has a rotor that has two-pole permanent magnets, as well as magnetic substances in a peripheral direction between the magnetic poles of the permanent magnets.

In the case of the motor described above that is provided with grooves on the side of outer periphery of the magnet, the grooves are arranged at positions where magnetic fluxes flow in each of cases when power is applied and when power is not applied. As a result, a problem arises. For example, if the grooves are provided at positions such that fluctuations when power is on are decreased, cogging torque is increased, and on the other hand, if the grooves are provided at positions such that the cogging torque is reduced, the torque fluctuations when power is applied are increased.

An object of the present invention is to improve the performance (for example, efficiency, reliability, cost performance, or productivity) of a motor.

In particular, it is provided a rotor for a rotating electric machine having the features defined in claim <NUM>. Further, it is provided a rotating electric machine having the features defined in claim <NUM>.

According to the present invention, the performance (for example, efficiency, reliability, cost performance, or productivity) of a motor can be improved.

Hereafter, an embodiment of the present invention is explained referring to the attached drawings.

The rotating electric machine according to the present embodiment can suppress both cogging torque when power is not applied and torque fluctuations when power is applied as will be explained below so that a reduction in size, a reduction in cost and reduction in torque fluctuations can be achieved. As a result, the rotating electric machine according to the present embodiment is suitable as a motor for driving an electric vehicle and an electric vehicle that produces low vibration and low noises and hence giving comfortable ride quality can be provided. The rotating electric machine can be applied to a genuine electric vehicle that is driven only by a rotating electric machine and to a hybrid electric vehicle that is driven by both an engine and a rotating electric machine. Hereafter, explanation is focused on the hybrid electric vehicle.

<FIG> presents a schematic diagram showing the construction of a hybrid electric vehicle having mounted thereon a rotating electric machine according to an embodiment of the present invention. A vehicle <NUM> has mounted thereon an engine <NUM> and a first rotating electric machine <NUM>, a second rotating electric machine <NUM>, and a battery <NUM>. The battery <NUM> supplies direct current power to the rotating electric machines <NUM> and <NUM> when driving forces of the rotating electric machines <NUM> and <NUM> are required and the battery <NUM> receives direct current power from the rotating electric machines <NUM> and <NUM> upon regenerative driving. Transfer of direct current power between the battery <NUM> and the rotating electric machines <NUM> and <NUM> is conducted through a power converter unit <NUM>. Though not shown, the vehicle has mounted thereon a battery that supplies low voltage power (for example, <NUM>-volt power) and supplies direct current power to a control circuit, which is explained hereinbelow.

The rotation torques by the engine <NUM> and the rotating electric machines <NUM> and <NUM> are transmitted to a front wheels <NUM> through a transmission <NUM> and a differential gear <NUM>. The transmission <NUM> is controlled by a transmission control unit <NUM> and the engine <NUM> is controlled by an engine control unit <NUM>. The battery <NUM> is controlled by a battery control unit <NUM>. The transmission control unit <NUM>, the engine control unit <NUM>, the battery control unit <NUM>, the power converter unit <NUM>, and an integrated control unit <NUM> are connected to each other through communication line <NUM>.

The integrated control unit <NUM> receives state information indicating a state of each of the control units from the control devices downstream of the integrated control unit <NUM>, i.e., the transmission control unit <NUM>, the engine control unit <NUM>, the power converter unit <NUM>, and the battery control unit <NUM> through the communication line <NUM>. The integrated control unit <NUM> calculates a control command for each of the control devices based on the state information. The calculated control commands are transmitted to the respective control units through the communication circuit <NUM>.

The battery <NUM>, which is at high voltage, comprises a secondary battery such as a lithium ion battery or a nickel-metal hydride battery and outputs direct current power at high voltage in the range of <NUM> V to <NUM> V or higher. The battery control unit <NUM> outputs information on a state of discharge of the battery <NUM> and information on a state of each unit cell of the battery included in the battery <NUM> to the integrated control unit <NUM> through the communication line <NUM>.

The integrated control unit <NUM> determines whether or not charge of the battery <NUM> is necessary based on the state information from the battery control unit <NUM> and outputs an instruction to perform power-generating operation to the power converter unit <NUM> when the charge of the battery <NUM> is determined to be necessary. The integrated control unit <NUM> in the main performs management of output torques of the engine <NUM> and the rotating electric machines <NUM> and <NUM>, calculation of an integrated torque and a distribution ratios of torques from the output torque of the engine <NUM> and the output torques of the rotating electric machines <NUM> and <NUM>, and transmission of control commands based on results of the calculation to the transmission control unit <NUM>, the engine control unit <NUM>, and the power converter unit <NUM>. The power converter unit <NUM> controls the rotating electric machines <NUM> and <NUM> to generate the torque output or generated power energy as commanded based on the torque command from the integrated control unit <NUM>.

The power converter unit <NUM> is provided with a power semiconductor that constitutes an inverter for driving the rotating electric machines <NUM> and <NUM>. The power converter unit <NUM> controls a switching operation of the power semiconductor based on the command from the integrated control unit <NUM>. The rotating electric machines <NUM> and <NUM> are operated as electric machines or alternators by the switching operation of the power semiconductor.

When the rotating electric machines <NUM> and <NUM> are operated as electric machines, direct current power from the high voltage battery <NUM> is supplied to direct current terminals of the inverter in the power converter unit <NUM>. The power converter unit <NUM> converts supplied direct current power into three-phase alternating current power by controlling the switching operation of the power semiconductor and supplies the obtained alternating current power to the rotating electric machines <NUM> and <NUM>. On the other hand, when the rotating electric machines <NUM> and <NUM> are operated as alternators, the rotors of the rotating electric machines <NUM> and <NUM> are driven and rotated by rotating torque applied from outside to generate three-phase alternating current power in stator windings of the rotating electric machines <NUM> and <NUM>. The generated three-phase alternating current power is converted into direct current power by the power converter unit <NUM>. The obtained direct current power is supplied to the high voltage battery <NUM> to effect charging.

<FIG> presents a circuit diagram of the power converter unit <NUM> shown in <FIG>. The power converter unit <NUM> is provided with a first inverter unit for the rotating electric machine <NUM> and a second inverter unit for the rotating electric machine <NUM>. The first inverter unit includes a power module <NUM>, a first drive circuit <NUM> that controls the switching operation of each power semiconductor 21in the power module <NUM>, and a current sensor <NUM> that detects current in the rotating electric machine <NUM>. The drive circuit <NUM> is provided on a drive circuit board <NUM>. On the other hand, the second inverter unit includes a power module <NUM>, a second drive circuit <NUM> that controls the switching operation of each power semiconductor <NUM> in the power module <NUM>, and a current sensor <NUM> that detects current in the rotating electric machine <NUM>. The drive circuit <NUM> is provided on a drive circuit board <NUM>. A control circuit <NUM> provided on a control circuit board <NUM>, a capacitor module <NUM>, and a transmitting and receiving circuit <NUM> implemented in a connector board <NUM> are used in common by the first and the second inverter units.

The power modules <NUM> and <NUM> operate in response to corresponding drive signals output from the drive circuits <NUM> and <NUM>, respectively. The power modules <NUM> and <NUM> convert direct current power supplied from the battery <NUM> into three-phase alternating current power and supplies the obtained power to stator coils, which are armature coils of the corresponding rotating electric machines <NUM> and <NUM>, respectively. The power modules <NUM> and <NUM> convert the alternating current power induced in the stator coils of the rotating electric machines <NUM> and <NUM> into direct current power and then supply the resultant direct current power to the high voltage battery <NUM>.

The power modules <NUM> and <NUM> include a three-phase bridge circuit as shown in <FIG>. Series circuits corresponding to the three-phases are each electrically connected in parallel between the positive electrode side and the negative electrode side of the battery <NUM>. Each of the series circuits includes a power semiconductor <NUM> constituting an upper arm and a power semiconductor <NUM> constituting a lower arm. The power semiconductors <NUM> are connected to each other in series. The power module <NUM> and the power module <NUM> have substantially the same circuit construction as shown in <FIG>. Here, the power module <NUM> is explained on behalf of the both.

In the present embodiment, IGBT (Insulated Gate Bipolar Transistor) <NUM> is used as the power semiconductor for switching. IGBT <NUM> includes three electrodes, i.e., a collector electrode, an emitter electrode, and a gate electrode. Between the collector electrode and the emitter electrode of IGBT <NUM> is electrically connected a diode <NUM>. The diode <NUM> includes two electrodes, i.e., a cathode and an anode. The cathode and anode are electrically connected to the collector electrode and emitter electrode, respectively, of IGBT <NUM> so that a direction of from the emitter electrode to the collector electrode of the IGBT <NUM> is a forward direction.

Also, MOSFET (Metal Oxide Semiconductor Field-Effect Transistor) may be used as the power semiconductor for switching. MOSFET includes three electrodes, i.e., a drain electrode, a source electrode, and a gate electrode. Since MOSFET includes a parasite diode between the source electrode and the drain electrode so that a direction of from the drain electrode to the source electrode is a forward direction, it is not necessary that MOSFET includes the diode <NUM> as shown in <FIG>.

The arms for respective phases each include the source electrode of IGBT <NUM> and the drain electrode of IGBT <NUM> electrically connected to each other in series. In the present embodiment, only a single IGBT is shown for each of the upper and lower arms for each phase. In actuality, a plurality of IGBTs is electrically connected in parallel since current capacity to be controlled is huge. Hereafter, a single power semiconductor is described in order to make explanation simpler.

In the example shown in <FIG>, each of the upper and lower arms for each phase includes three IGBTs. The drain electrode of IGBT <NUM> in each upper arm for each phase and the source electrode of IGBT <NUM> in each lower arm for each phase are electrically connected to the positive electrode side and the negative electrode side, respectively, of the battery <NUM>. Middle points of respective arms for each phase (a connection part between the source electrode of the upper arm side IGBT and the drain electrode of the lower arm side IGBT) are electrically connected to the armature coils (stator coils) of the corresponding phase of the corresponding rotating electric machines <NUM> and <NUM>.

The drive circuits <NUM> and <NUM> constitute respective drive units for controlling the corresponding power modules <NUM> and <NUM> and generate drive signals for driving IGBTs <NUM> based on the control signals output from the control circuit <NUM>. The drive signals generated in the drive circuits <NUM> and <NUM> are output to the gate of each power semiconductor in the power modules <NUM> and <NUM>. The drive circuits <NUM> and <NUM> are each provided with six integrated circuits that generate drive signals supplied to the respective gates of the upper and lower arms for each phase. The six integrated circuits are formed as one block.

The control circuit <NUM> constitutes the control unit in each of the power modules <NUM> and <NUM>. The control circuit <NUM> comprises a microcomputer that calculates control signals (control values) for operating (turning on or off) the plurality of power semiconductors for switching. Torque command signals (torque command values) from a superordinate control unit, sensor outputs from the current sensors <NUM> and <NUM>, and sensor outputs from the rotation sensors mounted on the rotating electric machines <NUM> and <NUM> are input to the control circuit <NUM>. The control circuit <NUM> calculates control values based on the input signals and outputs control signals for controlling the switching timing to the drive circuits <NUM> and <NUM>.

The transmitting and receiving circuit <NUM> implemented on the connector board <NUM> is to connect the power converter unit <NUM> and an outer control unit, and transmits and receives information with other units through the communication line <NUM> as shown in <FIG>. The capacitor module <NUM> constitutes a smoothing circuit for suppressing a fluctuation in direct current voltage generated by the switching operation of IGBT <NUM> and is electrically connected in parallel to the terminal on the direct current side in the first power module <NUM> and the second power module <NUM>.

<FIG> presents a cross-sectional view of the rotating electric machine <NUM> or <NUM> shown in <FIG>. The rotating electric machines <NUM> and <NUM> have substantially the same structures. Hereafter, explanation is made taking the structure of the rotating electric machine <NUM> as a representative example. The structure explained hereafter does not have to be adopted in both of the rotating electric machines <NUM> and <NUM> but it will be sufficient if it is adopted at least one of them.

Inside a housing <NUM>, there is held the stator <NUM>. The stator <NUM> includes the stator core <NUM> and the stator coil <NUM>. The rotor <NUM> is rotatably held inside the stator core <NUM> with an air gap <NUM>. The rotor <NUM> includes the rotor core <NUM>, permanent magnets <NUM>, and nonmagnetic wear plates <NUM>. The rotor core <NUM> is fixed to a shaft <NUM>. The housing <NUM> has a pair of end brackets <NUM> each provided with a bearing <NUM>. The shaft <NUM> is rotatably held by these bearings <NUM>.

As shown in <FIG>, the shaft <NUM> is provided with a resolver <NUM> that detects the positions of the poles of the rotor <NUM> and rotation speed of the rotor <NUM>. An output from the resolver <NUM> is introduced into the control circuit <NUM> shown in <FIG>. The control circuit <NUM> outputs the control signals to the drive circuit <NUM> based on the introduced output. The drive circuit <NUM> outputs the drive signals to the power module <NUM> based on the control signals. The power module <NUM> performs switching operation based on the control signals to convert the direct current power supplied from the battery <NUM> into three-phase alternating current power. The three-phase alternating current power is supplied to the stator coil <NUM> and a rotating magnetic field is generated in the stator <NUM>. The frequency of the three-phase alternating current is controlled based on the detected value by the resolver <NUM>. Also, the phases of the three-phase alternating current are controlled based on the detected value by the resolver <NUM>.

<FIG> presents a perspective view of the rotor core <NUM> of the rotor <NUM>. The rotor core <NUM> includes two cores <NUM> and <NUM> as shown in <FIG>. The length H2 of the core <NUM> along its axial direction is set to be substantially the same as the length H1 of the core <NUM> along its axial direction. <FIG> and <FIG> show the stator <NUM> and the rotor <NUM> in cross-section. <FIG> presents a cross-sectional view along the A-A line passing through a part of the core <NUM> (see, <FIG>). <FIG> presents a cross-sectional view along the B-B line passing through a part of the core <NUM> (see, <FIG>). In <FIG> and <FIG>, depiction of the housing <NUM>, the shaft <NUM>, and the stator coil <NUM> is omitted.

On the inner periphery side of the stator core <NUM>, there are uniformly arranged a number of slots <NUM> and teeth <NUM> all around. In <FIG> and <FIG>, not all of the slots and teeth have been allotted reference numerals but only some of the teeth and slots have been allotted reference numerals on behalf of the whole. In the slot <NUM>, a slot insulator (not shown) is provided and a plurality of phase winding wires of u-phase to w-phase is fitted. In the present embodiment, distributed winding is adopted as the method of winding the stator coil <NUM>.

The distributed winding is a method of winding a coil wire by which the wire is wound around the stator core <NUM> such that the phase winding wire is accommodated in two slots that are remotely arranged over a plurality of slots <NUM> intervening therebetween. In the present embodiment, the distributed winding is adopted as the method of wire winding, so that the formed distribution of magnetic flux is nearly sinusoidal, with the result that reluctance torque can be easily obtained. Therefore, control of the rotation speed over a wide range of the number of rotations ranging from low rotation speed to high rotation speed can be achieved by utilizing field weakening control and reluctance torque. The distributed winding is suitable for obtaining motor characteristics adapted for electric vehicles.

Each of the cores <NUM> and <NUM> of the rotor core <NUM> is provided with holes <NUM> in each of which a rectangular magnet is to be inserted. The permanent magnets <NUM> are introduced into the holes <NUM> and fixed thereto with an adhesive or the like. The widths of the holes <NUM> in the circumferential direction are set to be larger than the widths of the permanent magnets <NUM> in the circumferential direction. On both sides of the permanent magnets <NUM> are formed magnetic air gaps <NUM>. The magnetic air gaps <NUM> may be filled with the adhesive. Alternatively, the magnetic air gaps <NUM> may be filled with forming resins together with the permanent magnets <NUM>, which will then be integrally fixed. The permanent magnets <NUM> operate as field poles of the rotor <NUM>.

The directions of magnetization of the permanent magnets <NUM> are set along the radial direction of the rotor core <NUM> and reversed every field pole. That is, assuming that the surface of a permanent magnet 254a on the stator side is an N pole and a surface of the permanent magnet 254a on the axis side is an S pole, a surface of an adjacent permanent magnet 254b on the stator side is an S pole and a surface of the permanent magnet 254b on the axis side is an N pole. The permanent magnets 254a and 254b are arranged alternately in the circumferential direction. In the present embodiment, twelve of such permanent magnets <NUM> are arranged at regular intervals. Thus, the rotor <NUM> has twelve poles.

The permanent magnets <NUM> may either be embedded in the rotor core <NUM> after magnetization or be inserted in the rotor core <NUM> before magnetization and then magnetized by applying thereto a strong magnetic field. Since the permanent magnets <NUM> after the magnetization are strong magnets, if the permanent magnets <NUM> are magnetized before they are fixed to the rotor <NUM>, strong attractive forces are generated between the rotor core <NUM> and the permanent magnets <NUM> when the permanent magnets <NUM> are fixed and the resulting centripetal forces prevent the operation for producing the rotor. In addition, dust such as iron powder may adhere to the permanent magnets <NUM> due to the strong attractive forces. Therefore, the method in which magnetization is performed after the permanent magnets <NUM> have been inserted into the rotor core <NUM> is more productive than otherwise.

The permanent magnets <NUM> may include sintered magnets containing neodymium or samarium, ferrite magnets, bond magnets containing neodymium, and so on. The permanent magnets <NUM> have a residual magnetic flux density of approximately <NUM> to <NUM> T.

<FIG> presents an enlarged view of a part of the cross-sectional view shown in <FIG>. The core <NUM> of the rotor core <NUM> is provided with grooves that constitute magnetic air gaps <NUM> on a surface of the rotor <NUM> in addition to the magnetic air gaps <NUM> formed on both the sides of the permanent magnets <NUM>. The magnetic air gaps <NUM> are provided to reduce cogging torque and the magnetic air gaps <NUM> are provided to reduce torque fluctuations when power is applied. Assuming that as seen from the inner periphery of the rotor <NUM>, a central axis between the permanent magnet 254a and a next magnet on the left side of the permanent magnet 254a is named q-axis a and a central axis between the permanent magnet 254b and a next magnet on the left side of the permanent magnet 254b is named q-axis b, a magnetic air gap 258a is arranged offset to the right with respect to the q-axis a and a magnetic air gap 258b is arranged offset to the left with respect to the q-axis b. The magnetic air gap 258a and the magnetic air gap 258b are arranged symmetric with respect to a d-axis, which is a central axis of magnetic poles.

On the other hand, <FIG> is an enlarged view of a part of the cross-sectional view shown in <FIG>. The core <NUM> of the rotor core <NUM> is formed of magnetic air gaps 258c and 258d instead of the magnetic air gaps 258a and 258b. As seen from the inner periphery of the rotor <NUM>, the magnetic air gap 258c is arranged offset to the left with respect to the q-axis a and the magnetic air gap 258d is arranged offset to the right with respect to the q-axis b. From <FIG>, <FIG>, <FIG>, and <FIG>, it can be seen that the cross-sectional shapes of the cores <NUM> and <NUM> are the same except that the positions at which the magnetic air gaps 258a and 258b and the magnetic air gaps 258c and 258d are different, respectively.

The magnetic air gaps 258a and 258d are arranged at positions offset from each other by <NUM> degrees in electric angle and the magnetic air gaps 258b and 258c are arranged at positions offset from each other by <NUM> degrees in electric angle. That is, the core <NUM> can be formed by rotating the core <NUM> by one pitch of magnetic poles. As a result, the core <NUM> and the core <NUM> can be produced using the same mold so that their production cost can be decreased. The circumferential positions of the holes <NUM> of the cores <NUM> and <NUM> correspond to each other without any offset. As a result, the permanent magnet <NUM> fitted in each hole <NUM> constitute an integrated magnet penetrating each of the cores <NUM> and <NUM> without being divided in the axial direction. Of course, a plurality of divided magnets <NUM> may be arranged as being stacked in the axial direction of the hole <NUM>.

When a rotating magnetic field is generated in the stator <NUM> by the three-phase alternating current, the rotating magnetic field interacts with the permanent magnets 254a and 254b of the rotor <NUM> to generate a magnet torque. The rotor <NUM> is affected by a reluctance torque in addition to the magnet torque.

<FIG> presents a diagram illustrating a reluctance torque. Generally, an axis along which magnetic flux passes through the center of a magnet is called a d-axis and an axis along which magnetic flux passes one interpolar position to another interpolar position is called a q-axis. The part of the core that is present at the center between the poles of the magnet is called an assisted salient pole member <NUM>. The permeability of the permanent magnet <NUM> provided in the rotor <NUM> is approximately the same as that of air, so that when viewed from the side of the stator, the d-axis member is magnetically concave and the q-axis member is magnetically convex. Therefore, the part of the core in the q-axis part is called salient pole. The reluctance torque is generated by a difference in readiness of transmission of magnetic flux along the axis between the d-axis and the q-axis, i.e., by a salient pole ratio.

As mentioned above, the rotating electric machine to which the present embodiment is applied is one that utilizes both a magnet torque and an assisted salient pole reluctance torque. Both the magnet torque and the reluctance torque each generate torque fluctuations. The torque fluctuations include a fluctuation component that is generated when power is not applied and a fluctuation component that is generated when power is applied. The fluctuation component that is generated when power is not applied is generally called cogging torque. When the rotating electric machine is actually used in a loaded state, there are generated combined torque fluctuations consisting of the cogging torque and the fluctuation component when power is applied.

Most conventional methods for reducing the torque fluctuations of such a rotating electric machine relate to a reduction in cogging torque only but disclose nothing about a reduction in torque fluctuations occurring when power is applied. However, in many cases, noises of the rotating electric machine occur not in an unloaded state but in a loaded state. That is, it is important to reduce torque fluctuations in a loaded state in order to reduce noises of the rotating electric machine. Any countermeasure that relates to cope with the cogging torque only is insufficient.

Now, the method of reducing torque fluctuations according to the present embodiment is explained.

First, influence of the magnetic air gap <NUM> when power is not applied. <FIG> shows a result of simulation of distribution of magnetic flux when current is not flown in the stator coil <NUM>, that is, distribution of magnetic flux by the permanent magnet <NUM>. <FIG> shows two poles, i.e., a region <NUM> constituted by the permanent magnet 254a and a region <NUM> constituted by the permanent magnet 254b. That is, the above-mentioned result is a result of simulation of the rotating electric machine in which the region <NUM> and the region <NUM> are arranged alternately in the circumferential direction, showing an A-A cross-section. Since the rotating electric machine according to the present embodiment includes <NUM> poles, the regions <NUM> and <NUM> each include <NUM> poles, which are alternately arranged in the circumferential direction. For each pole, the magnetic air gaps 258a and 258b are in the assisted salient pole member <NUM> in the region <NUM> but the assisted salient pole member <NUM> in the region <NUM> includes no magnetic air gap <NUM>.

When power is applied, the magnetic flux by the permanent magnet <NUM> is short-circuiting the magnet ends. Therefore, no magnetic flux at all passes along the q-axis. It can be seen that substantially no magnetic flux passes through portions of the magnetic air gaps 258a and 258b provided at positions slightly offset from the magnetic air gaps <NUM> in the magnet ends. The magnetic flux passing the stator core <NUM> passes a part of the core on the side of the stator in the permanent magnet <NUM> to reach the teeth <NUM>. As a result, the magnetic air gaps 258a and 258b give substantially no influence on the magnetic flux when power is not applied that relates to cogging torque. From this, it follows that the magnetic air gaps 258a and 258b give no influence on the cogging torque.

<FIG> shows the result of simulation on the region <NUM> only and <FIG> shows the result of simulation on the region <NUM> only. <FIG> shows a rotating electric machine that includes twelve poles each consisting of the region 401only arranged in the circumferential direction and is constructed such that the direction of magnetization of the permanent magnet <NUM> of each pole is reversed pole by pole. <FIG> shows a rotating electric machine that includes twelve poles each consisting of the region <NUM> only arranged in the circumferential direction and is constructed such that the direction of magnetization of the permanent magnet <NUM> of each pole is reversed pole by pole. <FIG> and <FIG> each show similar magnetic flux distribution to that shown in <FIG>, with no magnetic flux passing along the q-axis.

<FIG> shows the waveform of cogging torque. <FIG> shows a waveform of induced line voltage that occurs on the side of the stator when the rotor <NUM> rotates. The horizontal axis shows the rotation angle of the rotor in electric angle. Line L11 shows the case of the rotor shown in <FIG> in which the region <NUM> having the magnetic air gaps <NUM> and the region <NUM> having no magnetic air gap <NUM> are alternately arranged. Line <NUM> shows the rotating electric machine shown in <FIG> in which only the region <NUM> having the magnetic air gaps <NUM> is arranged. Line <NUM> shows the case of the rotating electric machine shown in <FIG> in which only the region <NUM> having no magnetic air gap <NUM> is arranged. The result shown in <FIG> indicates that presence or absence of the magnetic air gaps <NUM> gives substantially no influence on the cogging torque.

The induced voltage is a voltage generated when the magnetic flux of the rotating rotor <NUM> forms flux linkage with the stator coil <NUM>. As shown in <FIG>, it is understood that the induced voltage waveform is not influenced by the presence or absence of the magnetic air gaps <NUM>. The induced voltage indicates reflection of the magnetic flux of a magnet in the result of simulations shown in <FIG>, <FIG>, and <FIG>. That the induced voltage is not changed means that the magnetic air gaps <NUM> give substantially no influence on the magnetic flux of the magnet.

Now, influences of the magnetic air gap <NUM> when power is applied are explained. <FIG>, <FIG>, and <FIG> each show the result of simulation of magnetic flux distribution when power is applied to the stator coil <NUM>. <FIG> shows the result of simulation on the rotating electric machine similar to one shown in <FIG>. <FIG> shows the result of simulation on the rotating electric machine similar to one shown in <FIG>. <FIG> shows the result of simulation on the rotating electric machine similar to one shown in <FIG>. The rotating electric machine according to the present embodiment is a motor including <NUM> slots per pole. A coil <NUM> of the stator coil <NUM> provided in the slot <NUM> of the stator coil <NUM> is branched into two layers in the direction of the depth of the slot. The coil <NUM> arranged on the bottom side of the slot is a short pitch winding that is inserted into the rotor side of the slot <NUM> skipping over six slots consisting of first to fifth slots assuming that the next slot is taken as first slot. The sort pitch winding is featured in that it can reduce harmonics in the magnetomotive force of the stator, shorten the coil end, and reduce copper loss. The winding for reducing harmonics can minimize sixth-order torque fluctuations specific to three-phase motors and substantially only nearly twelfth components remain.

Referring to <FIG>, <FIG> and <FIG>, the magnetic flux flows along the q-axis in any of the simulation results. This is because the current in the stator <NUM> forms a magnetic flux in the q-axis. Comparing <FIG> and <FIG> with <FIG> in which no magnetic air gap <NUM> is present, it can be seen that in <FIG> and <FIG>, the magnetic air gap <NUM> changes the flow of magnetic flux of the assisted salient pole member <NUM>. Therefore, the magnetic air gap <NUM> that is present in the assisted salient pole member <NUM> gives magnetic influences only when power is applied.

<FIG> shows the torque waveform when power is applied and <FIG> shows the waveform of line voltage when power is applied. The horizontal axis indicates the rotation angle of the rotor in electric angle. Line L21 indicates the case of the rotor shown in <FIG> in which the region <NUM> having the magnetic air gaps <NUM> and the region <NUM> having no magnetic air gap <NUM> are alternately arranged. Line <NUM> shows the rotating electric machine shown in <FIG> in which only the region <NUM> having the magnetic air gaps <NUM> is arranged. Line <NUM> shows the case of the rotating electric machine shown in <FIG> in which only the region <NUM> having no magnetic air gap <NUM> is arranged.

<FIG> indicates that in the rotating electric machine according to the present embodiment, twelfth-order torque fluctuation component, i.e., component of <NUM> degrees period in electric angle is dominant but sixth-order component is almost null. Both L21 and L22 have changed waveforms of torque fluctuations as compared with the torque fluctuations L23 in the case where the magnetic air gap <NUM> is not formed, that is only the region <NUM> is present. This indicates that the magnetic flux when power is applied is influenced by the magnetic air gap <NUM>. Further, the torque fluctuations L22 of the rotating electric machine including only the region <NUM> and the torque fluctuations L23 of the rotating electric machine including only the region <NUM> are approximately opposite in phase to each other. As shown in <FIG>, the rotating electric machine according to the present embodiment has a construction in which the region <NUM> and the region <NUM> are alternately arranged and as indicated by the torque fluctuations L21, sum of the torque fluctuations that is received by the rotor in whole is a mean value of the torque fluctuations L22 and the torque fluctuations L23.

As mentioned above, in the present embodiment, provision of the magnetic air gaps 258a and 258b enables reduction of torque fluctuations when power is applied. To obtain such an effect, it is preferred that the width angles (angles in the circumferential direction) of the grooves that constitute the magnetic air gaps <NUM> are set to be within the range of <NUM>/<NUM> to <NUM>/<NUM> of the pitch angle of the teeth <NUM>. Two or more types of the magnetic air gaps <NUM> may be used to form the assisted salient pole member <NUM>. Thereby, it is becomes more freely to reduce torque fluctuations so that reduction of fluctuations can be performed more precisely.

A further feature is that as the torque is not decreased more than the case where no magnetic air gap is provided. In the case of the structure called "skew" conventionally adopted to reduce torque fluctuations, skewing results in a decrease in torque, which prevents size reduction. However, the present embodiment is featured that not only it is possible to reduce the torque fluctuations when power is applied independently of the cogging torque but also the torque itself is not decreased. This is because the torque fluctuations in the case of the original groove-less rotor dominantly include the twelfth-order component. It is effective that the stator coil is made of a short pitch winding.

Also, it can be seen that the voltage when power is applied is influenced by presence or absence of the magnetic air gap <NUM> as shown in <FIG>. In this case, there occurs a potential difference between the winding of each phase of the stator coil <NUM> facing the rotor <NUM> in the region <NUM> and the winding of each phase of the stator coil <NUM> facing the rotor <NUM> in the region <NUM>, so that when the windings separately for each phase are connected in parallel, circulation current flows to increase loss. As shown in <FIG>, the rotating electric machine according to the present embodiment has the core <NUM> formed by rotating the core <NUM> by one pitch of magnetic pole and the axial lengths of the cores <NUM> and <NUM> are set to substantially the same as shown in <FIG>. As a result, the voltage that occurs in the winding of each phase of the stator coil <NUM> facing each pole can be made substantially the same, so that substantially no circulation current flows. However, when windings of respective phases of the stator coil <NUM> facing the rotor <NUM> in the regions <NUM> and <NUM> are connected to each other in series, substantially no circulation current flows, so that a construction with only the core <NUM> or <NUM> may also be adopted.

As mentioned above, if the magnetic air gaps 258a and 258b are formed, this does not give any influence on the cogging torque when power is applied. Therefore, the cogging torque can be reduced separately from the reduction of the torque fluctuations when power is applied, by applying a method of reducing the cogging torque as conventionally used. In the present embodiment, reduction of cogging torque is achieved by adopting the following construction.

<FIG> and <FIG> present diagrams illustrating the method of reducing cogging torques. <FIG> presents a cross-sectional view showing the rotor <NUM> and a part of the stator core <NUM>. In <FIG>, τp indicates pole pitch of the permanent magnet <NUM> and τm indicates width angle of the permanent magnet <NUM>. On the other hand, τg indicates an angle for the permanent magnet <NUM> and the magnetic air gaps <NUM> on both sides thereof, i.e., a width angle of the hole <NUM> shown in <FIG>. By adjusting ratios of these angles τm/τp and τg/τp, cogging torques can be reduced. In the present embodiment, τm/τp is called magnet pole radian and τg/τp is called magnet hole pole radian.

<FIG> presents a diagram showing relationship between the ratio of τm/τp and cogging torque. The result shown in <FIG> relates to the case where τm=τg and the permanent magnet <NUM> and the magnetic air gap <NUM> are in the form of arc concentric to the outer periphery of the rotor <NUM>. In the case where rectangular magnets are used as in the present embodiment, optimum values are somewhat varied. However, needless to say, the same idea is used. In <FIG>, the horizontal axis indicates amplitude of cogging torque and the horizontal axis indicates rotation angle of the rotor <NUM> in electric angle. The magnitude of amplitude of fluctuations varies depending on the magnitude of the ratio τm/τp. When τm=τg, selecting τm/τp at about <NUM>, the cogging torque can be reduced. The tendency that the cogging torque is not changed by the magnetic air gaps <NUM> shown in <FIG> makes it possible to apply ratio of the width of magnet to the pitch of pole τm/τp, to any where similarly. As a result, by designing the shape of the rotor <NUM> to be one shown in <FIG> under the above-mentioned conditions, both the cogging torque and the torque fluctuations when power is applied can be reduced.

In the example shown in <FIG>, explanation has been made assuming τm=τg. However, to efficiently utilize the reluctance torque which is an effect of the assisted salient pole member <NUM>, the magnet hole pole radian τg/τp may advantageously be set to about <NUM> to about <NUM>, preferably about <NUM> to about <NUM>.

<FIG> is an example of calculation of maximum torque when the magnet pole radian τm/τp and the magnet hole pole radian τg/τp are varied. Similarly to <FIG>, the permanent magnet <NUM> and the magnetic air gap <NUM> are in the form of a sector concentric to the outer periphery of the rotor <NUM>. The horizontal axis indicates the magnet hole pole radian τg/τp. That this value is <NUM> indicates that the ratio of the assisted salient pole member <NUM> to the interpolar pitch is <NUM>. Here, the magnet width τm cannot be made larger than the opening angle τm of the magnet hole, and hence, there is obtained: τg≥τm. An increase in τm results in an increase in width of the permanent magnet <NUM>, so that torque increases accordingly. On the other hand, when τm is constant, τg has an optimal value; when τg/τp is about <NUM> to about <NUM>, the maximum torque is largest. This is because the size of the assisted salient pole member <NUM> has an appropriate value and if τg is made too large or too small as compared with that value, reluctance torque becomes too small. When τm is larger than <NUM>, τm=τg is desirable so that the assisted salient pole member <NUM> can be as large as possible.

As mentioned above, the reluctance torque can be most efficiently utilized when τg/τp is set to about <NUM> to about <NUM> and the permanent magnet <NUM> can be made smaller. When a rare earth sintered magnet is used as the permanent magnet <NUM>, it is required to use a most efficient amount of magnet since such a magnet is very expensive as compared with other materials. Since the permanent magnet <NUM> is reduced in size, the induced voltage by the magnetic flux of the permanent magnet <NUM> can be reduced, so that the rotating electric machine can be rotated at higher speeds. Therefore, the rotating electric machine that utilizes reluctance torque as in the present embodiment is generally used in electric vehicles.

<FIG> and <FIG> show a rotor according to another embodiment of the present invention. The present embodiment is the same as the first embodiment excepting what is explained hereafter.

<FIG> shows a rotor of the surface magnet type and <FIG> shows a rotor in which a plurality of magnets is arranged in a V-shape. In either type of the rotor, the assisted salient pole member <NUM> is between any two adjacent permanent magnets <NUM> and the magnetic air gap <NUM> is arranged in the assisted salient pole member <NUM>. Assuming that as seen from the inner periphery of the rotor <NUM>, a central axis between the permanent magnet 254a and a next magnet on the left side of the permanent magnet 254a is named q-axis a and a central axis between the permanent magnet 254b and a next magnet on the left side of the permanent magnet 254b is named q-axis b, the magnetic air gap 258a is arranged offset to the right with respect to the q-axis a and the magnetic air gap 258b is arranged offset to the left with respect to the q-axis b. The magnetic air gap 258a and the magnetic air gap 258b are arranged symmetric with respect to a d-axis, which is a central axis of the magnetic pole. <FIG> and <FIG> show A-A cross-sections of the rotor. Similarly to the above-mentioned embodiment, the B-B cross-section has a shape formed by rotating the shape of the A-A cross-section by one pitch of magnetic pole. As explained above referring to <FIG>, <FIG>, and <FIG>, the reduction of the torque fluctuations in the present embodiment is not affected by the magnetic flux of the magnet, so that it does not depend on the shape of the magnet.

<FIG> illustrates achievement of reduction of torque fluctuations by providing two magnetic air gaps <NUM> for each assisted salient pole member <NUM> according to the present embodiment.

This shape is as follows. Assuming that as seen from the inner periphery of the rotor <NUM>, a central axis between the permanent magnet 254a and a next magnet on the left side of the permanent magnet 254a is named q-axis a and a central axis between the permanent magnet 254b and a next magnet on the left side of the permanent magnet 254b is named q-axis b, the magnetic air gap 258a on the right side with respect to the q-axis a is larger and the magnetic air gap 258e on the left side with respect to the q-axis b is smaller. The magnetic air gap 258b on the right side with respect to the q-axis b is larger and the magnetic air gap 258f on the left side with respect to the q-axis b is smaller. The magnetic air gaps 258a and 258b and the magnetic air gaps 258e and 258f are arranged symmetric with respect to a d-axis, which is a central axis of the magnetic pole. <FIG> shows an A-A cross-section of the rotor. Similarly to the above-mentioned embodiment, the B-B cross-section has a shape formed by rotating the shape of the A-A cross-section by one pitch of magnetic pole. Other details than the above-mentioned are the same as the first embodiment.

In the examples shown in <FIG>, <FIG>, <FIG>, <FIG>, and <FIG>, the magnetic air gap <NUM> is constituted by a groove provided in an outer periphery of the rotor <NUM>. However, the magnetic air gap <NUM> may be constituted by a hole in the assisted salient pole member <NUM> as shown in <FIG>. The magnetic air gap <NUM> and the magnetic air gap <NUM> may be integrated as shown in <FIG>, according to an example outside the scope of the present invention. The magnetic air gap <NUM> may be achieved by providing the assisted salient pole member <NUM> with a region that has a different permeability than the rest as shown in <FIG>. In <FIG>, the permeability of the assisted salient pole member 259a is set to be lower than that of the assisted salient pole member 259b. Other details than the above-mentioned are the same as the first embodiment.

<FIG> illustrates the case where the stator coil <NUM> shown in <FIG> and <FIG> is made of the concentrated winding type. The torque fluctuations in the present embodiment depends on the shape of the rotor <NUM> and hence the torque fluctuations can be reduced in the case of the concentrated winding type, which is a different winding method on the stator side, similarly to what is described above. Other details than the above-mentioned are the same as the first embodiment.

<FIG> presents a perspective view showing the rotor core <NUM> of the rotor <NUM> according to another embodiment. Other details than the above-mentioned are the same as the first embodiment.

The rotor core <NUM> includes two cores <NUM> and <NUM> as shown in <FIG>. The length H2 of the core <NUM> in the axial direction is set to be approximately the same as the length H1 of the core <NUM> in the axial direction. <FIG> and <FIG> each present a cross-sectional view of the stator <NUM> and the rotor <NUM>. <FIG> presents an A-A cross-sectional view passing a part of the core <NUM> (see <FIG>), and <FIG> presents an B-B cross-sectional view passing a part of the core <NUM> (see <FIG>). In <FIG> and <FIG>, depiction of the housing <NUM>, the shaft <NUM>, and the stator coil <NUM> is omitted.

On the inner periphery side of the stator core <NUM>, there are uniformly arranged a number of slots <NUM> and teeth <NUM> all around. In <FIG>, not all the slots and teeth are allotted reference numerals but only some of the teeth and slots are allotted reference numerals on behalf of the whole. In the slot <NUM>, a slot insulator (not shown) is provided and a plurality of phase winding wires of u-phase to w-phase is fitted. In the present embodiment, distributed winding is adopted as the method of winding the stator coil <NUM>.

Each of the cores <NUM> and <NUM> of the rotor core <NUM> is provided with holes <NUM> in each of which a rectangular magnet is to be inserted. The permanent magnets <NUM> are introduced into the holes <NUM> and fixed thereto with an adhesive or the like. The widths of the holes <NUM> in the circumferential direction are set to be larger than the widths of the permanent magnets <NUM> in the circumferential direction. On both sides of the permanent magnets <NUM> are formed magnetic air gaps <NUM>. The magnetic air gaps <NUM> may be filled with the adhesive. Alternatively, the magnetic air gaps <NUM> may be filled with forming resins together with the permanent magnets <NUM>, which will then be integrally fixed. The permanent magnets <NUM> operates as a field pole of the rotor <NUM>.

<FIG> presents an enlarged view of a part of the cross-sectional view shown in <FIG>. The core <NUM> of the rotor core <NUM> is provided with grooves that constitute magnetic air gaps <NUM> on a surface of the rotor <NUM> in addition to the magnetic air gaps <NUM> formed on both the sides of the permanent magnets <NUM>. The magnetic air gaps <NUM> are provided to reduce cogging torque and the magnetic air gaps <NUM> are provided to reduce torque fluctuations when power is applied. Assuming that as seen from the inner periphery of the rotor <NUM>, a central axis between the permanent magnet 254a and a next magnet on the left side of the permanent magnet 254a is named q-axis a and a central axis between the permanent magnet 254b and a next magnet on the left side of the permanent magnet 254b is named q-axis b, a magnetic air gap 258a is arranged offset to the right with respect to the q-axis a and a magnetic air gap 258b is arranged offset to the left with respect to the q-axis b. There is provided no magnetic air gap on both sides of the q-axis b. The magnetic air gap 258a and the magnetic air gap 258b are arranged symmetric with respect to a d-axis, which is a central axis of magnetic poles.

On the other hand, <FIG> is an enlarged view of a part of the cross-sectional view shown in <FIG>. In case of the core <NUM> of the rotor core <NUM>, magnetic air gaps 258c and 258d are formed instead of the magnetic air gaps 258a and 258b. As seen from the inner periphery of the rotor <NUM>, the magnetic air gap 258c is arranged offset to the left with respect to the q-axis a and the magnetic air gap 258d is arranged offset to the right with respect to the q-axis b. There is no magnetic air gap on both sides of the q-axis a. From <FIG>, <FIG>, <FIG>, and <FIG>, it can be seen that the cross-sectional shapes of the cores <NUM> and <NUM> are the same except that the positions at which the magnetic air gaps 258a and 258b and the magnetic air gaps 258c and 258d are different, respectively.

The rotating electric machine shown in <FIG> has a construction such that a region <NUM> and a region <NUM> are arranged alternately. The region <NUM> in <FIG> is equivalent to the region <NUM> in <FIG> and the region <NUM> in <FIG> is equivalent to the region <NUM> in <FIG>. The rotating electric machine according to the present embodiment shown in <FIG> can be said to be electrically and magnetically equivalent to the rotating electric machine according to the embodiment shown in <FIG> although positions at which the magnetic air gaps <NUM> are different between the embodiments. That is, also in the present embodiment, different torque fluctuations occur between the regions <NUM> and <NUM> and they act so as to cancel each other, so that torque fluctuations can be reduced. Similarly to the first embodiment, the magnetic air gap <NUM> is formed at the assisted salient pole member <NUM>, it gives substantially no influence on cogging torque. That is, by providing the magnetic air gap <NUM>, the influence of the cogging torque to the fluctuation of torque can be suppressed and torque fluctuations when power is applied can be reduced substantially independently of the cogging torque.

As shown in <FIG> and <FIG>, the rotating electric machine according to the present embodiment includes the core <NUM> formed by rotating the core <NUM> by one pitch of magnetic pole and the axial lengths of the cores <NUM> and <NUM> are set to substantially the same as shown in <FIG>, so that voltages generated in respective phase windings of the stator coil <NUM> facing each pole can be made approximately equal to each other. As a result, substantially no circulation current flows. However, substantially no circulation current flows when the windings of respective phases of the stator coil <NUM> facing the rotor <NUM> in the regions <NUM> and <NUM> are connected to each other in series. Accordingly, it is no problem to use only the core <NUM> or only the core <NUM>.

<FIG> and <FIG> show a rotor according to anther embodiment. Other details than the above-mentioned are the same as the above-mentioned embodiments.

<FIG> shows a rotor of the surface magnet type and <FIG> shows a rotor of the type in which a plurality of magnets is arranged in a V-shape. In either type of the rotor, the assisted salient pole member <NUM> is between any two adjacent permanent magnets <NUM> and the magnetic air gap <NUM> is arranged in the assisted salient pole member <NUM>. Assuming that as seen from the inner periphery of the rotor <NUM>, a central axis between the permanent magnet 254a and a next magnet on the left side of the permanent magnet 254a is named q-axis a and a central axis between the permanent magnet 254b and a next magnet on the left side of the permanent magnet 254b is named q-axis b, the magnetic air gap 258a is arranged offset to the right with respect to the q-axis a and the magnetic air gap 258b is arranged offset to the left with respect to the q-axis b. There is no magnetic air gap on both sides of the q-axis b. The magnetic air gap 258a and the magnetic air gap 258b are arranged symmetric with respect to a d-axis, which is a central axis of the magnetic pole. <FIG> and <FIG> show A-A cross-sections of the rotor. Similarly to the above-mentioned embodiment, the B-B cross-section has a shape formed by rotating the shape of the A-A cross-section by one pitch of magnetic pole. As explained above referring to <FIG>, <FIG>, and <FIG>, the reduction of the torque fluctuations in the present embodiment is not affected by the magnetic flux of the magnet, so that it does not depend on the shape of the magnet.

<FIG> illustrates achievement of reduction of torque fluctuations by providing two magnetic air gaps <NUM> for each assisted salient pole member <NUM> according to the present embodiment. This shape is as follows. Assuming that as seen from the inner periphery of the rotor <NUM>, a central axis between the permanent magnet 254a and a next magnet on the left side of the permanent magnet 254a is named q-axis a and a central axis between the permanent magnet 254b and a next magnet on the left side of the permanent magnet 254b is named q-axis b, the magnetic air gap 258a on the right side with respect to the q-axis a is larger and the magnetic air gap 258e on the left side with respect to the q-axis b is smaller. The magnetic air gap 258b on the right side with respect to the q-axis b is larger and the magnetic air gap 258f on the left side with respect to the q-axis b is smaller. The magnetic air gaps 258a and 258b and the magnetic air gaps 258e and 258f are arranged symmetric with respect to a d-axis, which is a central axis of the magnetic pole. <FIG> shows an A-A cross-section of the rotor. Similarly to the above-mentioned embodiment, the B-B cross-section has a shape formed by rotating the shape of the A-A cross-section by one pitch of magnetic pole.

In the examples shown in <FIG>, <FIG>, <FIG>, <FIG>, and <FIG>, the magnetic air gap <NUM> is constituted by a groove provided in an outer periphery of the rotor <NUM>. However, the magnetic air gap <NUM> may be constituted by a hole in the assisted salient pole member <NUM> as shown in <FIG>. The magnetic air gap <NUM> and the magnetic air gap <NUM> may be integrated as shown in <FIG>. The magnetic air gap <NUM> may be achieved by providing the assisted salient pole member <NUM> with a region that has a different permeability than the rest as shown in <FIG>. In <FIG>, the permeability of the assisted salient pole member 259a is set to be lower than that of the assisted salient pole member 259b.

<FIG> illustrates the case where the stator coil <NUM> shown in <FIG> is made of the concentrated winding type. The torque fluctuations in the present embodiment depends on the shape of the rotor <NUM> and hence the torque fluctuations can be reduced in the case of the concentrated winding type, which is a different winding method on the stator side, similarly to what is described above.

Various embodiments mentioned above have the following advantageous effects.

According to the above-mentioned embodiments, it is possible to achieve a reduction in cogging torque and a reduction in torque fluctuations when power is applied. The reduction in torque fluctuations can be achieved by making the offset amount of the region of which the magnetoresistance has been varied differ for each magnetically-assisted salient pole member such that the torque fluctuations when power is applied due to the region of which the magnetoresistance has been varied cancel each other.

Claim 1:
A rotor for a rotating electric machine comprising:
a plurality of magnets (<NUM>) and a plurality of magnetically-assisted salient pole members (<NUM>) each provided at a part of a core (<NUM>; <NUM>) between poles of the plurality of magnets (<NUM>), wherein:
the rotor (<NUM>) includes insertion holes (<NUM>) for the plurality of magnets (<NUM>) where an end of a magnet (<NUM>) and an insertion hole (<NUM>) form a first magnetic air gap (<NUM>);
the rotor (<NUM>) includes second magnetic air gaps (<NUM>), each second magnetic air gap (<NUM>) is provided in a magnetically-assisted salient pole member (<NUM>) along an axial direction of the rotor at a position offset in a circumferential direction from a q-axis passing through a salient pole center of the magnetically-assisted salient pole member (<NUM>) and offset from the first magnetic air gap (<NUM>) in a circumferential direction towards the salient pole center of the magnetically-assisted salient pole member (<NUM>) to an extent that it is spaced apart from the respective first magnetic air gap (<NUM>);
the second magnetic air gaps (<NUM>) are formed independently from the first magnetic air gaps (<NUM>), characterized in that adjacent second magnetic air gaps (<NUM>) are arranged asymmetrically with respect to any a q-axis passing through any salient pole center between them and symmetrically with respect to a respective d-axis passing through a magnetic pole center of the magnet (<NUM>) between them.