Modeling of Induction and Synchronous Machines

Abstract

Induction motor is the most widely used ac motor in the industry. An induction motor like any other rotating machine consists of a stator (the fixed part) and a rotor (the moving part) separated by air gap. The stator contains electrical windings housed in axial slots. The induction machines used in industries are mainly three-phase, except for small power where single phase machines are common. Each phase on the stator has distributed winding, consisting of several coils distributed in number of slots. The distributed winding results in MMF due to the current in the winding to be stepped waveform similar to a sine wave. The MMF wave has its maximum value at the center of the winding. In three-phase machine the three windings have spatial displacement of 120 degrees between them. When balanced three phase currents are applied to these windings, the resultant MMF in the air gap has constant magnitude equal to \({{3}\over{2}}\) times the magnitude of one phase, and rotates at an angular speed of ω electrical radians per second. Here ω is the angular frequency of the supply current. The actual speed of rotation of magnetic field depends on the number of poles in the motor. This speed is known as synchronous speed of the motor and is given by