Drive circuit for electric motors

1. A method of operating a drive circuit for parallel electric motors, at least one of which is a permanent magnet (PM) motor, said method comprising:

receiving measurements of stator phase currents in the parallel electric motors;

selecting a target PM motor, from among the parallel electric motors, that generates a largest torque output;

executing a vector control algorithm to generate a complex command voltage vector for the target PM motor;

generating and transmitting a pulse width modulation (PWM) signal based on the complex command voltage vector for controlling an inverter; and

operating the inverter according to the PWM signal to supply three-phase alternating current (AC) power to the parallel electric motors.

2. The method of claim 1 , wherein selecting a target PM motor comprises:

comparing respective stator phase currents for the parallel electric motors; and

determining which of the parallel electric motors is drawing the greatest torque producing current through its stator windings.

3. The method of claim 1 , wherein operating the inverter further comprises supplying the three-phase AC power to at least one parallel PM motor other than the target PM motor.

4. The method of claim 3 , wherein supplying the three-phase AC power to the at least one parallel PM motor further comprises dissipating excess current in the form of an additional flux current.

5. The method of claim 1 , wherein operating the inverter further comprises supplying the three-phase AC power to at least one parallel induction motor.

6. The method of claim 1 further comprising:

receiving at least one stator phase current measurement; and

computing at least one stator phase current measurement based on the at least one received stator phase current measurement.

7. A drive circuit for parallel electric motors, at least one of which is a permanent magnet (PM) motor, said drive circuit comprising:

an inverter coupled to the parallel electric motors and configured to supply an alternating current (AC) signal to stator windings thereof based on a PWM signal; and

a digital signal processor (DSP) coupled to said inverter and configured to:

receive respective stator phase current measurements for the parallel electric motors;

select a target PM motor, of the parallel electric motors, having a largest torque output among the parallel electric motors;

execute a vector control algorithm to generate a complex command voltage vector for the target PM motor;

generate the PWM signal based on the complex command voltage vector for the target PM motor; and

transmit the PWM signal to said inverter to operate said inverter and supply the AC signal to the stator windings of the parallel electric motors.

8. The drive circuit of claim 7 , wherein said DSP is further configured to select the target PM motor based on the respective stator phase current measurements for the target PM motor exceeding the respective stator phase current measurements for each other PM motor among the parallel electric motors.

9. The drive circuit of claim 7 , wherein said DSP is further configured to:

receive at least one stator phase current measurement for a given electric motor among the parallel electric motors; and

compute at least one other stator phase current measurement for the given electric motor.

10. The drive circuit of claim 7 , wherein said inverter comprises three phase legs for generating a three-phase AC output power to be delivered to the parallel electric motors.

11. The drive circuit of claim 10 , wherein said DSP is further configured to generate respective PWM signals for controlling each of said three phase legs.

12. The drive circuit of claim 7 , wherein said DSP, when executing the vector control algorithm, is further configured to:

determine a rotor position for the target PM motor;

determine a rotor angle for the target PM motor based on the rotor position, respective stator phase currents for the target PM motor, and an electrical parameter of the stator windings of the target PM motor;

transform the respective stator phase currents for the target PM motor into a flux-torque coordinate system in a rotating rotor reference frame based on the rotor angle;

compute commanded flux and torque voltage components based on the transformed respective stator phase currents for the target PM motor; and

compute the complex command voltage vector based on the commanded flux and torque current components.

13. The drive circuit of claim 7 , wherein said inverter is coupled to at least one induction motor among the parallel electric motors, and wherein said inverter is further configured to supply the AC signal to the stator windings thereof based on the PWM signal.

14. The drive circuit of claim 7 , wherein said inverter, in supplying the AC signal to the stator windings of the parallel electric motors other than the target PM motor, is further configured to dissipate excess current in the stator windings in the form of an additional flux current.

15. The drive circuit of claim 13 , wherein said DSP is further configured to selectively operate at least one induction motor in parallel with the target PM motor through said inverter.

16. The drive circuit of claim 15 , wherein said DSP is further configured to selectively operate the at least one induction motor using line frequency power.

17. A system, comprising:

a first three-phase motor configured to drive a load;

an inverter coupled to said first three-phase motor and configured to supply a three-phase output power thereto;

an induction motor configured to drive the load, and further configured to be selectively coupled in parallel to said first three-phase motor and said inverter;

a three-phase AC line-frequency bus configured to be selectively coupled to said induction motor; and

a digital signal processor (DSP) coupled to said inverter and configured to:

selectively, when in a direct operating mode, couple said induction motor to said three-phase AC line-frequency bus, and decouple said induction motor from said first three-phase motor and said inverter;

selectively, when in a parallel operating mode, couple said induction motor to said inverter, and decouple said induction motor from said three-phase AC line-frequency bus;

control said inverter to supply the three-phase output power from said inverter to said first three-phase motor; and

control said inverter to supply the three-phase output power from said inverter to said induction motor when in the parallel operating mode.

18. The system of claim 17 , wherein said DSP is further configured to select one of the direct operating mode and the parallel operating mode based on a quantification of the load.

19. The system of claim 17 , wherein said first three-phase motor comprises a permanent magnet motor.

20. The system of claim 17 , wherein said first three-phase motor comprises a second induction motor.

21. The system of claim 20 , wherein said DSP is further configured to selectively, when in the direct operating mode, couple said second induction motor to said three-phase AC line-frequency bus.

22. The system of claim 21 further comprising a third induction motor configured to drive the load, and a fourth induction motor configured to drive the load, the third and fourth induction motors configured to be selectively coupled in parallel to said three-phase AC line-frequency bus.

23. The system of claim 22 , wherein said inverter has an operating capacity that is less than the aggregate operating capacity of all motors configured to drive the load.

24. A method of controlling fluid flow generated by fluid-moving equipment driven by a plurality of motors, said method comprising:

receiving a fluid flow demand signal indicating an amount of fluid flow commanded to be output by the fluid-moving equipment driven by the plurality of motors;

determining a state of operation, of a plurality of predefined states of operation, in which to operate a drive circuit coupled to the plurality of motors to generate the commanded amount of fluid flow;

adjusting at least one relay associated with at least one motor of the plurality of motors to activate and selectively couple the at least one motor to an inverter or an AC line-frequency power according to the determined state of operation; and

adjusting operation of the inverter to generate the commanded amount of fluid flow.