Concept and implementation of a simplified speed control strategy for survivable induction motor drives

Induction machine is one of the most widely used machines in residential and industrial applications. Traditional drive methods for induction machines such as vector control usually require complex control routines, fast processing unit and multiple system status feedbacks. The complexity of these methods greatly reduces the reliability of the system since the failure of the sensor or even the drift of system parameters could potentially result in system malfunction. This creates the need for a simple, cost-effective and reliable control strategy as a backup to continue operation of the system in case of failure of current and voltage feedback sensors. In this paper, an effective, yet simple and low-cost state switching control technique is proposed and implemented for a three phase squirrel cage induction machine system. This state switching control operates at two specific duty cycles of PWM which produces phase voltages with different magnitudes across the phase windings. At the same time, the frequency of phase voltages is derived from the reference speed. By switching between these two states, precise speed regulation can be achieved. This new control method makes the controller extremely simple in design and implementation for induction machine. Simulation and experimental results are included in this paper to validate our claims.