Inverter control apparatus and control method thereof

1. An inverter control apparatus that controls an inverter having a DC-link voltage pulsating at double a system frequency, the inverter control apparatus comprising:

a current sensor to sense an output current of the inverter;

a voltage sensor to sense a DC-link voltage of the inverter; and

a controller configured to:

generate a q-axis current reference in a sine squared form;

generate a q-axis current boundary value on a basis of the output current and the DC-link voltage, the q-axis current boundary value representing limit of the q-axis current reference due to variation of the DC-link voltage;

generate an average of the q-axis current boundary value by averaging the q-axis current boundary value for a half cycle of the system frequency;

generate a d-axis current reference based on the q-axis current reference and the average of the q-axis current boundary value; and

drive a three-phase motor based on the d-axis current reference and the q-axis current reference.

2. The inverter control apparatus according to claim 1 , wherein the controller comprises:

a velocity controller to generate an output torque command value on the basis of the output current; and

a current reference generator to generate the current reference corresponding to the output torque command value.

3. The inverter control apparatus according to claim 2 , wherein the current reference generator comprises a system angle estimator to estimate at least one of a system angle, a system frequency, a double system angle and a double system frequency on the basis of the DC-link voltage.

4. The inverter control apparatus according to claim 3 , wherein the current reference generator further comprises a q-axis current reference generator to generate a q-axis current reference in a sine squared form, which is synchronized with the system angle, on the basis of the system angle.

5. The inverter control apparatus according to claim 4 , wherein the current reference generator further comprises a d-axis current reference generator to calculate the average of the periodically varying q-axis current boundary value and to generate a d-axis current reference on the basis of the average of the rotor based q-axis current boundary value and the q-axis current reference.

6. The inverter control apparatus according to claim 5 , wherein the d-axis current reference generator comprises:

a current margin calculator to calculate the average of the periodically varying q-axis current boundary value on the basis of the DC-link voltage and to generate a q-axis current margin value on the basis of the average of the rotor based q-axis current boundary value;

a current margin reference unit to generate a q-axis current margin reference on the basis of the output torque command value; and

a first adder to generate an error value by subtracting the q-axis current margin reference from the q-axis current margin value.

7. The inverter control apparatus according to claim 6 , wherein the current margin calculator comprises:

a d-axis voltage boundary value calculator to generate a d-axis voltage boundary value corresponding to a maximum instantaneous voltage that can be applied to a d axis on the basis of the DC-link voltage;

a unit gain calculator to generate a unit gain that changes a voltage value into a current value;

a q-axis current boundary converter to generate a q-axis current boundary value on the basis of the d-axis voltage boundary value and the unit gain; and

a q-axis current margin calculator to subtract the q-axis current reference from the average of the rotor based q-axis current boundary value to generate a q-axis margin value.

8. The inverter control apparatus according to claim 6 , wherein the current margin reference unit sets the d-axis current reference to a positive value such that the d-axis current reference is set to a negative value when a generated torque is insufficient due to current limitation.

9. The inverter control apparatus according to claim 6 , wherein the current margin reference unit sets the q-axis current margin reference to “0” in a steady state or when current is not limited.

10. The inverter control apparatus according to claim 4 , wherein the q-axis current reference generator comprises:

a sine square calculator to generate a unit sine square waveform having the system angle;

a q-axis current reference converter to generate a q-axis current reference corresponding to the output torque command value;

a first multiplier to multiply the unit sine square waveform by the q-axis current reference to generate a q-axis current reference in a sine squared form;

a current gain setting unit to generate a current gain that makes the average of the output torque command value equal to the average of a current reference modified torque generated according to the q-axis current reference; and

a second multiplier to multiply the q-axis current reference in a sine squared form by the current gain to generate a q-axis current reference.

11. The inverter control apparatus according to claim 10 , wherein the current gain setting unit sets the current gain to “2”.

12. The inverter control apparatus according to claim 3 , wherein the system angle estimator comprises:

a DC-link voltage square calculator to square the DC-link voltage to generate a DC-link voltage square;

a band pass filter to generate a double system frequency component value having a frequency twice the system frequency on the basis of the DC-link voltage square and the double system frequency;

a phase retarder to generate a 90° phase-retarded value having a phase retarded by 90° from the double system frequency component value;

a fifth frame converter to generate a synchronous reference frame based d-axis virtual voltage and a synchronous reference frame based q-axis virtual voltage, which are constants and have a phase difference therebetween, on the basis of the 90°-phase-retarded value, a value obtained by multiplying the double system frequency component value by “−1”, and the double system angle; and

a phase lock unit to generate at least one of the system angle, the system frequency, the double system angle, and the double system frequency on the synchronous reference frame based d-axis virtual voltage and constant “0”.

13. The inverter control apparatus according to claim 12 , wherein the phase lock unit comprises an all-pass filter or a secondary general integrator to retard a phase.

14. The inverter control apparatus according to claim 12 , wherein the phase lock unit comprises a phase lock loop to lock the phase of a received signal and keep the frequency of an output signal uniform.

15. A method to control an inverter control apparatus that includes a current sensor, a voltage sensor and a controller and controls an inverter having a DC-link voltage pulsating at double a system frequency, the method comprising:

generating a q-axis current reference in a sine squared form;

generating a q-axis current boundary value on a basis of the output current and the DC-link voltage, the q-axis current boundary value representing limit of the q-axis current reference due to variation of the DC-link voltage;

generating an average of the q-axis current boundary value by averaging the q-axis current boundary value for a half cycle of the system frequency;

generating a d-axis current reference based on the q-axis current reference and the average of the q-axis current boundary value; and

driving a three-phase motor based on the d-axis current reference and the q-axis current reference.

16. The method according to claim 15 , wherein generating the q-axis current reference comprises estimating at least one of a system angle, a system frequency, a double system angle and a double system frequency on the basis of the DC-link voltage.

17. The method according to claim 16 , wherein the generating the q-axis current reference further comprises generating a q-axis current reference in a sine squared form, which is synchronized with the system angle, on the basis of the system angle.

18. The method according to claim 17 , wherein generating the q-axis current reference comprises:

generating a unit sine square waveform having the system angle;

generating a q-axis current reference corresponding to the output torque command value;

multiplying the unit sine square waveform by the q-axis current reference to generate a q-axis current reference in a sine squared form;

generating a current gain that makes the average of the output torque command value equal to the average of a current reference modified torque generated according to the q-axis current reference; and

multiplying the q-axis current reference in a sine squared form by the current gain to generate a q-axis current reference.

19. The method according to claim 18 , wherein the current gain setting unit sets the current gain to “2”.

20. The method according to claim 16 , wherein estimating at least one of the system angle, the system frequency, the double system angle and the double system frequency comprises:

squaring the DC-link voltage to generate a DC-link voltage square;

generating a double system frequency component value having a frequency twice the system frequency on the basis of the DC-link voltage square and the double system frequency;

generating a 90° phase-retarded value having a phase retarded by 90° from the double system frequency component value;

generating a synchronous reference frame based d-axis virtual voltage and a synchronous reference frame based q-axis virtual voltage, which are constants and have a phase difference therebetween, on the basis of the 90°-phase-retarded value, a value obtained by multiplying the double system frequency component value by “−1”, and the double system angle; and

generating at least one of the system angle, the system frequency, the double system angle, and the double system frequency on the synchronous reference frame based d-axis virtual voltage and constant “0”.

21. The method according to claim 15 , wherein generating the d-axis current reference comprises:

generating a d-axis voltage boundary value corresponding to a maximum instantaneous voltage that can be applied to a d axis on the basis of the DC-link voltage;

generating a unit gain that changes a voltage value into a current value;

generating a q-axis current boundary value on the basis of the d-axis voltage boundary value and the unit gain;

subtracting the q-axis current reference from the average of the q-axis current boundary value to generate a q-axis margin value;

generating a q-axis current margin reference on the basis of the output torque command value;

generating an error value by subtracting the q-axis current margin reference from the q-axis current margin value; and

sequentially applying the error value to a low pass filter and a proportional integrator to generate a d-axis current reference.

22. The method according to claim 21 , wherein generating the d-axis current reference comprises setting the d-axis current reference to a positive value such that the d-axis current reference is set to a negative value when generated torque is insufficient due to current limitation.

23. The method according to claim 21 , wherein apparatus generating the d-axis current reference comprises setting the q-axis current margin reference to “0” in a steady state or when current is not limited.

24. A controller to control an apparatus comprising:

a current sensor to sense an output current of the apparatus;

a voltage sensor to sense a DC-link voltage of the apparatus; and

a processor configured to:

generate a q-axis current reference in a sine squared form;

generate a q-axis current boundary value on a basis of the output current and the DC-link voltage, the q-axis current boundary value representing limit of the q-axis current reference due to variation of the DC-link voltage;

generate an average of the q-axis current boundary value by averaging the q-axis current boundary value for a half cycle of the system frequency; and

generate a d-axis current reference based on the q-axis current reference and the average of the q-axis current boundary value.

25. A method to control an inverter, the method comprising:

sensing an output current and a DC-link voltage of the inverter;

generating a q-axis current reference in a sine squared form;

generating a q-axis current boundary value on a basis of the output current and the DC-link voltage, the q-axis current boundary value representing limit of the q-axis current reference due to variation of the DC-link voltage;

generating an average of the q-axis current boundary value by averaging the q-axis current boundary value for a half cycle of the system frequency; and

generating a d-axis current reference based on the q-axis current reference and the average of the q-axis current boundary value.