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.

I. Introduction

For the field-oriented control it is necessary to know the precise position of a rotor at every instance of time. Sensorless drives are preferable due to smaller cost, size advantages and reliability. There are three main sensorless methods of rotor position estimation: methods based on the back-EMF estimation, methods which detect magnet anisotropy of the motor and rotary transformer methods applicable to electrically excited synchronous machines. For the first type of observers, the accuracy of the position estimation depends on the value of the back-EMF, which is very small or absent at low and zero speeds. This limits the operating range of back-EMF methods [1]–[3].