Indirect speed sensor for asynchronous machine

This paper presents a new and simple approach for sensorless speed control of asynchronous machine. It shows a self-contained electronic module which determines the speed of the machine with only two phase current measurements. Thus, the speed calculation is independent on the machine parameters and on control laws; this allows to design the control by considering the speed as an input. The principle of the method is based on rotor slot harmonics without high frequency injection. In fact, the rotor slot harmonics depend on rotor speed. These signals appear in the stator's spectrum with very low amplitude, in particular for a motor with skewed rotor slots. Moreover, the harmonics move with stator frequency. An analog adaptive band pass filter follows and amplifies this harmonics. As the response time is high at low speed, a digital algorithm is included in the microcontroller in order to decrease the response time. The proposed indirect speed sensor requires only some classical analog components and a low cost microcontroller. A short time calibration is necessary for the first start time of the inverter and the motor. Experimental results on a testing bench with different speed benchmarks are presented.