Sensorless control of a motor by variable frequency signal injection

1. A method for sensorless control of an electric motor implemented in a variable speed drive, the method comprising:

determining a control voltage to be applied to the motor;

injecting a high frequency signal to the control voltage to obtain an excitation voltage, wherein one or more frequencies of the high frequency signal varies with time;

applying the excitation voltage to the motor;

measuring a current signal induced in the motor by the excitation voltage, wherein the current signal comprises a fundamental current, induced by the control voltage, and a disturbance current, induced by the high frequency signal; and

demodulating the current signal, wherein demodulating the current signal comprises:

applying a first finite impulse response filter to the current signal to extract the fundamental current, wherein a duration of a response from the first finite impulse response filter varies according to the one or more frequencies of the high frequency signal;

subtracting the fundamental current from the current signal to extract the disturbance current; and

determining an amplitude of a ripple of the disturbance current, the amplitude of the ripple depending on a state of the motor.

2. The method according to claim 1 , wherein determining the amplitude of the ripple comprises calculating a zero-mean primitive of the high frequency signal and multiplying the disturbance current by the zero-mean primitive.

3. The method according to claim 2 , wherein determining the amplitude of the ripple further comprises applying a second finite impulse response filter, wherein a duration of a response from the second finite impulse response filter varies according to the one or more frequencies of the high frequency signal.

4. The method according to claim 1 , wherein the one or more frequencies of the high frequency signal vary randomly with time.

5. The method according to claim 1 , wherein the high frequency signal is a sinusoidal signal.

6. The method according to claim 1 , wherein the high frequency signal is a square wave signal.

7. The method according to claim 1 , wherein determining the control voltage comprises using a current controller, applying the excitation voltage to the motor comprises converting the excitation voltage to a pulse width modulation voltage using a pulse width modulation carrier, and a fundamental frequency of the high frequency signal is within an interval defined by a frequency bandwidth of the current controller and a frequency of the pulse width modulation carrier.

8. The method according to claim 7 , wherein the fundamental frequency of the high frequency signal varies between 250 Hz and 1 kHz.

9. The method according to claim 1 , wherein the control voltage is determined in an estimated rotor reference frame of the motor, and the high frequency signal is injected into the estimated rotor reference frame.

10. The method according to claim 1 , further comprising forming a feedback loop by updating the control voltage with the amplitude of the ripple and the fundamental current.

11. A variable speed drive of an electric motor comprising a processor and a memory, the processor being configured to operate according to the method of claim 1 .

12. An electric drive assembly comprising the variable speed drive according to claim 11 and an electric motor controlled by said variable speed drive.

13. A non-transitory computer-readable storage medium comprising instructions which, when executed by a processor, cause the processor to carry out the method of claim 1 .