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The zero-frequency vector control of an induction motor without a speed sensor has proven to be a very difficult task. Uncertainty and variation in the electrical parameters of the motor deteriorate the performance of fundamental-wave flux observers as the frequency approaches zero, and observers based on high-frequency signal injection rely on motor-specific phenomena. A new method has recently been proposed, where a low-frequency current signal is injected in the estimated rotor flux direction. If there is an error in the orientation, the signal gives rise to a torque oscillation and affects the back electromotive force (EMF) through the mechanical system. The phenomenon makes it possible to reach sensorless zero-frequency operation. This paper shows that the method is insensitive to the variation of the motor parameters. Experiments confirm the result. Index Terms--Induction motor drives, sensorless control.

A recently introduced speed-sensorless controller structure for tracking the rotor-flux orientation of an induction motor was implemented in a laboratory setup. The controller is based on superimposing a low-frequency ac test signal on the stator current of the motor. The response in the stator voltage to the signal depends on the orientation of the signal relative to that of the rotor flux. The dependency is due to the reaction of the mechanical system, and it is used to control the orientation error to zero. In the experiments, the controller operated successfully under nominal torque both at zero speed and at zero frequency. Slow and fast speed reversals under nominal torque were also successful. In addition, the system was capable of withstanding both motoring and generating load torque steps up to 50% of nominal torque. Index Terms--Induction motor drives, sensorless control.