Speed-sensorless induction machine control for zero speed and frequency

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.