Question

Explain the principle of AC motor starting and speed control

Starting a motor by simply connecting it to the supply is called DOL starting and is the simplest, most economical method of starting.

Special reduced voltage starter [star / delta and autotransformer] are used when excessively large starting current may cause severe dip [voltage dip] on the supply which could affect the operation of other loads.

Large and small motors intended for connection to the emergency generator use reduced voltage starter. Starting current does not significantly decreases from standstill to half speed and only reduces to a reasonable level at 80% speed. Motor should accelerate quickly up to speed to prevent excessively long run-up times causing over temperature in the stator winding insulation and long duration voltage dips on the supply. Run up time depends on the starting torque developed by the motor. T α ( V )2

Load on motor will also have an effect on eh run up time.

Size of starting current is not increased, when motor is started against load. But it is only determined by the value of supply voltage and standstill impedance of stator winding. Some drives such as centrifuges and large fans may require special high torque motors, if acceptable run up times are to be achieved [over 10 seconds]. Operating speed of an induction motor is determined by the speed of the rotating magnetic field. This in turn is determined by the stator supply frequency and number of pole pairs in the flux pattern of the rotating magnetic field.

Supply voltage variation have no appreciable effect on motor speed apart from some difference created by the associated change of driving torque.

Marine system use dual wound or pole change machine for multispeed application and ward Leonard or inverter fed motor for variable speed operation.