Q. Explain working of Single-Phase Induction Motors?
For reasons of simplicity and cost, a single-phase power supply is universally preferred for fractional-horsepower motors. It is also used widely for motors up to about 5 hp. Single-phase induction motors are usually two-pole or four-pole, rated at 2 hp or less, while slower and larger motors can be manufactured for special purposes. Single-phase induction motors are widely used in domestic appliances and for a very large number of low-power drives in industry. The single - phase induction machine resembles a small, three-phase, squirrel-cage motor, except that at full speed only a single winding on the stator is usually excited.
The single-phase stator winding is distributed in slots so as to produce an approximately sinusoidal space distribution of mmf. Such a motor inherently has no starting torque. it must be started by an auxiliary winding, by being displaced in phase position from the main winding, or by some similar device. Once started by auxiliary means, the motor will continue to run. Thus, nearly all single-phase induction motors are actually two-phase motors, with the main winding in the direct axis adapted to carry most or all of the current in operation, and an auxiliary winding in the quadrature axis with a different number of turns adapted to provide the necessary starting torque.
Since the power input in a single-phase circuit pulsates at twice the line frequency, all single - phasemotors have a double-frequency torque component,which causes slight oscillations in rotor speed and imparts vibration to the motor supports. The design must provide a means to prevent this vibration from causing objectionable noise.