TABLE OF CONTENTS In the preceding chapter we shed light on the operation of a three-phase induction motor. It employs an ingenious scheme of placing three identical phase windings 120 electrical in space phase with respect to each other. When these windings are excited by a balanced three-phase power source, they set up a uniform magnetic field that revolves around the rotor periphery at synchronous speed. The flux-cutting action induces an electromotive force (emf) and thereby a current in the rotor conductors. The interaction of the rotor current and the revolving magnetic field causes the rotor to rotate at a speed somewhat lower than the synchronous speed of the motor.
What happens when one of the three phases of the source is not connected to the motor? Is the rotor able to start from its standstill position and take up some mechanical load? In fact, we have already answered this question in Chapter 3. The motor behaves more like a two-phase motor and produces a revolving field that induces an emf and the current in the rotor conductors. The motor develops a starting torque and forces the rotor to rotate.
Let us go one step further. What happens if only one of the three phases is excited? At this time you may be surprised to know that if the rotor is already rotating, it continues to rotate in that direction. However, if the motor is at rest and then one of the three phases of the motor is excited by a...
