Rotor Position Estimation Using Field Current Response to Phase-shifted PWM for Synchronous Homopolar Motor

The accurate estimation of a rotor position at low speeds is required for the sensorless field-oriented control of the AC motors. At low and zero speeds the back-EMF methods are not applicable, whereas for regulated field synchronous machine self-sensing control is possible. This paper describes the method of high-frequency injection to the stator voltage of a synchronous homopolar motor. The field winding of this motor is magnetically coupled with the stator winding, so the injection into phase winding creates the response in the field current. The proposed method uses a phase shifted PWM to produce the injection and simple algorithms of field current response analyzes. The accuracy of the rotor position estimation was studied and an approach with changing the PWM phase shift was suggested. It helps to achieve maximum accuracy over entire electrical revolution of the motor.