A UNIFIED METHOD OF MODELING 3-PHASE INDUCTION MOTORS

Many textbooks on electro-mechanical machines introduce the induction motor using a transformer circuit model connected to a fictitious load resistor, the value of which changes as a function of the slip speed. The method is essentially a power based analysis, which yields some neat, quick results. The developed torque is calculated in terms of the power developed in that fictitious resistor. Although clever in its simplicity, the method fails to provide much physical insight into how the developed torque comes about. One is just expected to trust the circuit model. The approach described in this paper is based on theory of Lorentz force and Faraday's Law used successfully to explain the operation of DC commutator machines. The method is readily understood and it provides direct insight into how the torque is developed. The paper will introduce a torque angle, which can be useful in visualizing how the machines work. It will also be shown that the traditional transformer model of the motor can be derived from the new analysis.