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

This paper describes an indirect sensing, or sensorless, method for rotor flux position for brushless permanent magnet (BPM) motors operating over a wide speed range, while keeping maximum torque per ampere and/or maximum efficiency capabilities. The method described is particularly applicable to trapezoidal back emf type of BPM motors, The typical trapezoidal waveform of the motor internal voltages (or back emf) contains a 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 motor speed and load condition, and is practically free of noise that can be introduced by the inverter switching, making this a robust sensing method, In contrast with indirect sensing methods based on detection of the back-emf signal that require heavy filtering, the third harmonic signal needs only a small amount of filtering to eliminate the switching frequency and its side bands, As a result, the method described here is not sensitive to filtering delays, allowing the motor to achieve a good performance over a wide speed range, Motor starting is also superior with this method since the third harmonic signal can be detected and processed at lower speeds than for the conventional method of back-emf sensing, Moreover, an alternative way to acquire the third harmonic signal without the need to access the stator neutral terminal is discussed, This is particularly interesting when the motor neutral connection is not available or expensive to have access, The third harmonic indirect sensing scheme is implemented in the laboratory and compared to a conventional back-emf sensing method, The results of this comparison show that it is possible to drive a BPM motor with high efficiency and torque per ampere over a wide speed range when using the third harmonic sensing and control methods.