Indirect sensing for rotor flux position of permanent magnet AC motors operating in a wide speed range

This paper describes an indirect sensing, or sensorless, method for the rotor flux position in permanent magnet (PM) AC motors operating in a wide speed range, while keeping maximum torque per current and/or maximum efficiency capabilities. The method of indirect sensing the rotor flux position proposed here is particularly applicable to trapezoidal back EMF type of PM AC motors. For these types of motors, the open circuit stator phase voltages, also called back EMF, have a trapezoidal shape that contain fundamental and higher order frequency harmonics. In particular, the third harmonic component is extracted from the stator phase voltages while the fundamental and other polyphase components are eliminated via a simple summation of the three phase voltages. The resulting third harmonic signal keeps a constant phase relationship with the rotor flux for any speed and load condition, and is practically free of noises that can be introduced by the inverter switching, making this a very robust sensing method.<>