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A simple, tacho-less, IM drive with direct torque control down to zero speed

This paper investigates the operation of a sensorless induction motor drive using direct torque control with a constant switching frequency. The constant frequency direct torque controller utilizes a predictive-type current regulator to achieve dead-beat torque and flux control. Satisfactory torque control can also be maintained under transient conditions and when operating in overmodulation conditions or in the "transition region" to six-step operation, with excellent dynamic response. In the steady-state, the stator voltage vector which drives the torque and flux to the reference value is calculated during each fixed switching period. Under transient or overmodulation conditions, an alternative switching algorithm must be used since dead-beat control is no longer possible. A simple method in overmodulation utilizes the voltage reference vector from the direct torque control algorithm. This scheme, although not resulting in dead-beat control, has been shown to provide very satisfactory performance in overmodulation. In this paper, sensorless torque control is maintained down to zero speed and light load using only stator voltage integration to determine the flux, i.e. no complex speed or flux observers are employed. With this approach, excellent dynamic performance in the "normal" or "high speed" range is achieved together with operation down to zero speed.