Starting
A single phase induction motor is not self-starting; thus, it is necessary to provide a starting circuit and associated start windings to give the initial rotation in a single phase induction motor. The normal running windings within such a motor can cause the rotor to turn in either direction, so the starting circuit determines the operating direction.
A polyphase induction motor is self-starting and produces torque even at standstill. The four methods of starting an induction motor are direct on-line, reactor, auto-transformer and star-delta. Unlike a wound-rotor motor, the rotor circuit is inaccessible and it is not feasible to introduce extra resistance for starting or speed control.
For small single-phase shaded-pole motor of a few watts, starting is done by a shaded pole, with a turn of copper wire around part of the pole. The current induced in this turn lags behind the supply current, creating a delayed magnetic field around the shaded part of the pole face. This imparts sufficient rotational character to start the motor. These motors are typically used in applications such as desk fans and record players, as the starting torque is very low and low efficiency is not objectionable.
Larger single phase motors have a second stator winding fed with out-of-phase current; such currents may be created by feeding the winding through a capacitor or having it have different values of inductance and resistance from the main winding. In some designs, the second winding is disconnected once the motor is up to speed, usually either by a centrifugal switch acting on weights on the motor shaft or a thermistor which heats up and increases its resistance, reducing the current through the second winding to an insignificant level. Other designs keep the second winding on when running, improving torque.
Polyphase motors have rotor bars shaped to give different speed/torque characteristics. The current distribution within the rotor bars varies depending on the frequency of the induced current. At standstill, the rotor current is the same frequency as the stator current, and tends to travel at the outermost parts of the squirrel-cage rotor bars (the skin effect). The different bar shapes can give usefully different speed/torque characteristics as well as some control over the inrush current at startup. Polyphase motors can generate torque from standstill, so no extra mechanism is required to initiate rotation.
In a wound rotor motor, slip rings are provided and external resistance can be inserted in the rotor circuit, allowing the speed/torque characteristic to be changed for purposes of acceleration control and speed control. Generally, maximum torque is delivered when the reactance of the rotor circuit is equal to its resistance.
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—Henry David Thoreau (1817–1862)