Zero-speed tacholess IM torque control: simply a matter of stator voltage integration

Most industry experts agree that the next generation of commercial drives will include some sort of sensorless torque control. Achieving a modest level of control in the very-low-speed range greatly increases the competitive value of the drive and expands its range of applications. After reviewing many of the previously presented sensorless control methods, this paper shows that the simple method of stator-flux orientation can provide zero-speed torque control equally as well as more complex approaches, which rely on elaborate mathematical models to improve the operating range. This paper experimentally demonstrates that, by using only a slight amount of low-pass filtering in the integration of the stator voltage, a reasonable flux estimate can be obtained for stator-flux-field-oriented torque control. With this simple method, adequate torque control has been demonstrated over a wide range of speed and load conditions, and even at zero rotor speed with moderate or heavy load torque.