Synchronous electric motor system

1. A synchronous motor drive system comprising:

three-phase inverters each configured to convert DC power to three-phase AC power;

a control circuit configured to control operations of the three-phase inverters; and

a synchronous motor configured to include three-phase coils supplied with three-phase AC power from the three-phase inverters, wherein,

the three-phase inverters include first and second three-phase inverters, and the control circuit controls the operations of the three-phase inverters by causing the first and the second three-phase inverters to use carrier signals at different frequencies from each other to generate three-phase AC power,

each of the first and the second three-phase inverters supplies a different one of the three-phase coils with three-phase AC power,

the synchronous motor has a stator in which stator coils are arranged along a rotation direction of the synchronous motor,

each of the stator coils constitutes one phase of one of the three-phase coils, and

the stator coils include a first stator coil and a second stator coil arranged adjacent to each other, the first stator coil constituting one phase of one of the three-phase stator coils that is supplied with three-phase AC power from the first three-phase inverter, the second stator coil constituting said one phase of another one of the three-phase stator coils that is supplied with three-phase AC power from the second three-phase inverter.

2. The synchronous motor drive system of claim 1 ,

wherein the synchronous motor further includes a rotor including magnetic poles arranged along the rotation direction at equal intervals,

each of the stator coils is wound by concentrated winding, and every consecutive m of the stator coils belong to one of stator coil groups arranged equiangularly, where m denotes an integer that is no less than 2, and

at least two consecutive stator coils in each of the stator coil groups are each included in a different one of the three-phase coils and individually supplied with three-phase AC power from the first three-phase inverter and the second three-phase inverter.

3. The synchronous motor drive system of claim 2 , wherein

the number of the three-phase inverters is three, and in is 3,

the three-phase inverters further include a third three-phase inverter,

two at both ends of every consecutive three of the stator coils are individually supplied with three-phase AC power from the third three-phase inverter and the first three-phase inverter,

one in the middle of every consecutive three of the stator coils is supplied with three-phase AC power from the second three-phase inverter, and

a frequency difference between a carrier signal for the first three-phase inverter and a carrier signal for the second three-phase inverter, and a frequency difference between a carrier signal for the second three-phase inverter and a carrier signal for the third three-phase inverter are greater than a frequency difference between a carrier signal for the first three-phase inverter and a carrier signal for the third three-phase inverter.

4. The synchronous motor drive system of claim 3 , wherein

the control circuit causes the first three-phase inverter and the third three-phase inverter to use carrier signals at a same frequency to generate three-phase AC power, and

the frequency of the carrier signals used by the first three-phase inverter and the third three-phase inverter is lower than a frequency of a carrier signal used by the second three-phase inverter.

5. The synchronous motor drive system of claim 2 , wherein

m is an odd number,

the three-phase inverters further include a third three-phase inverter,

one in the middle of every consecutive in of the stator coils is supplied with three-phase AC power from the second three-phase inverter,

two of every consecutive in of the stator coils, both adjacent to the one in the middle, are individually supplied with three-phase AC power from the third three-phase inverter and the first three-phase inverter, and

a frequency difference between a carrier signal for the first three-phase inverter and a carrier signal for the second three-phase inverter, and a frequency difference between a carrier signal for the second three-phase, inverter and a carrier signal for the third three-phase inverter are greater than a frequency difference between a carrier signal for the first three-phase inverter and a carrier signal for the third three-phase inverter.

6. The synchronous motor drive system of claim 5 , Wherein

the control circuit causes the first three-phase inverter and the third three-phase inverter to use carrier signals at a same frequency to generate three-phase AC power, and the frequency of the carrier signals used by the first three-phase inverter and the third three-phase inverter is lower than a frequency of a carrier signal used by the second three-phase inverter.

7. The synchronous motor drive system of claim 1 , wherein

the synchronous motor further includes a rotor including magnetic poles arranged along the rotation direction at equal intervals,

the stator includes stator teeth arranged along the rotation direction,

every consecutive m of the stator teeth belong to one of stator teeth groups arranged equiangularly, where m denotes an integer that is no less than 2,

each of the stator teeth groups include a first stator tooth, a second stator tooth and a third stator tooth,

the first stator tooth is wound with part of the first stator coil,

the third stator tooth is wound with part of the second stator coil,

the second stator tooth is wound with the rest of the first stator coil and the rest of the second stator coil, and

the first stator coil and the second stator coil are supplied with three-phase AC power from the first three-phase inverter and the second three-phase inverter, respectively.

8. The synchronous motor drive system of claim 1 , wherein

a ratio of a frequency of a carrier signal used by the second three-phase inverter and a frequency of a carrier signal used by the first three-phase inverter is no greater than 2.

9. The synchronous motor drive system of claim 1 , wherein

the control circuit interchanges a frequency of a carrier signal used by the first three-phase inverter with a frequency of a carrier signal used by the second three-phase inverter, according to a predetermined condition, to generate three-phase AC power.

10. The synchronous motor drive system of claim 9 , wherein

each of the three-phase inverters includes a gate drive circuit, a power circuit corresponding to the gate drive circuit, and a temperature sensor for measuring a temperature of the power circuit,

the control circuit causes the first three-phase inverter or the second three-phase inverter, whichever includes the power circuit whose temperature measured by the temperature sensor is higher, to use a carrier signal at a lower frequency than a carrier signal for the other to generate three-phase AC power.

11. The synchronous motor drive system of claim 10 , wherein

the gate drive circuit included in the first three-phase inverter or the second three-phase inverter, whichever uses a carrier signal at a higher frequency to generate three-phase AC power, drives the power circuit with a higher switching speed than the other.

12. The synchronous motor drive system of claim 1 , wherein

each of the three-phase inverters includes a gate drive circuit and a power circuit corresponding to the gate drive circuit, and

the gate drive circuit included in the first three-phase inverter or the second three-phase inverter, whichever uses a carrier signal a higher frequency to generate three-phase AC power, drives the power circuit with a higher switching speed than the other.

13. The synchronous motor drive system of claim 1 further comprising:

a current detector configured to detect a value of alternating current output from the first three-phase inverter or the second three-phase inverter, whichever uses a carrier signal at a lower frequency to generate three-phase AC power, and

the synchronous motor further includes a rotor including magnetic poles, and the control circuit estimates positions of the magnetic poles based on the value detected by the current detector and controls the operations of the three-phase inverters according to the positions.

14. The synchronous motor drive system of claim 1 , wherein

the control circuit changes, for each of the first and the second three-phase inverters, a frequency of a carrier signal to be used for generation of three-phase AC power, according to a drive state of the synchronous motor.

15. The synchronous motor drive system of claim 1 , wherein

a plurality of DC power sources are provided for supplying the three-phase inverters with DC power, and

each of the three-phase inverters is supplied DC power from a different one of the DC power sources.

16. The synchronous motor drive system of claim 15 , wherein

at least two of the DC power sources have different overvoltage tolerances, and

the first three-phase inverter or the second three-phase inverter, whichever uses a carrier signal at a lower frequency to generate three-phase AC power, is supplied with DC power from one of the power sources that has a low overvoltage tolerance.

17. The synchronous motor drive system of claim 15 , wherein

at least two of the DC power sources supply DC power with different voltages, and

the first three-phase inverter or the second three-phase inverter, whichever uses a carrier signal at a higher frequency to generate three-phase AC power, is supplied with DC power from one of the power sources that supplies DC power with a low voltage.

18. The synchronous motor drive system of claim 1 , wherein

each of the first three-phase inverter and the second three-phase inverter includes a switching device of a different type, and

the switching device included in the first three-phase inverter or the second three-phase inverter, whichever uses a carrier signal at a higher frequency to generate three-phase AC power, is a unipolar device.

19. The synchronous motor drive system of claim 1 , wherein

each of the first three-phase inverter and the second three-phase inverter includes a switching device of a different type,

the switching device included in the first three-phase inverter or the second three-phase inverter, whichever uses a carrier signal at a lower carrier frequency to generate three-phase AC power, is a Si semiconductor, and

the switching device included in the first three-phase inverter or the second three-phase inverter, whichever uses a carrier signal at a higher frequency to generate three-phase AC power, is a wide-bandgap semiconductor with a wider bandgap than the Si semiconductor.

20. The synchronous motor drive system of claim 1 , wherein

the three-phase inverters include switching devices, and the switching devices are included in a single module.

21. The synchronous motor drive system of claim 20 , wherein

the module includes three three-phase inverters aligned, and

one in the middle of the three three-phase inverters includes, as a switching device, a wide-bandgap semiconductor with a wider bandgap than a Si semiconductor.