Single-phase induction motors are ubiquitous in domestic and light industrial applications due to their reliance on a single-phase power supply. Unlike their three-phase counterparts, they require special starting mechanisms to overcome the inherent pulsating nature of a single-phase magnetic field, which does not produce a rotating torque on its own. This section delves into the various types of single-phase induction motors, each designed with a unique starting method to achieve self-starting capability.

A single-phase AC supply applied to a stator winding produces a pulsating magnetic field. This field can be resolved into two rotating magnetic fields of equal magnitude but opposite directions. At standstill, these fields produce equal and opposite torques, resulting in a net starting torque of zero. Therefore, a method to create a phase difference between the currents in different stator windings is essential for initiating rotation.

Single-phase induction motors are classified based on their starting mechanisms, which typically involve an auxiliary winding with a phase-shifting element (like a capacitor or inductor) to create a rotating magnetic field at start-up. Once the motor reaches a certain speed, this auxiliary winding is often disconnected.

These motors use an auxiliary starting winding (or starting winding) placed in series with a centrifugal switch. The starting winding has a higher resistance and lower reactance compared to the main winding, creating a phase difference. The centrifugal switch disconnects the starting winding once the motor reaches about 75-80% of its rated speed.

Similar to split-phase motors, capacitor-start motors also use a starting winding and a centrifugal switch. However, a capacitor is connected in series with the starting winding. This capacitor creates a larger phase angle between the currents in the main and starting windings, resulting in higher starting torque and better efficiency compared to split-phase motors. The capacitor and starting winding are disconnected by the centrifugal switch.

These motors employ two capacitors: a starting capacitor (larger capacitance) and a running capacitor (smaller capacitance). The starting capacitor is in series with the starting winding and is disconnected by a centrifugal switch at about 75-80% speed. The running capacitor remains in the circuit permanently, connected in series with the auxiliary winding, to improve running performance, power factor, and efficiency. This configuration provides high starting torque and excellent running characteristics.

In PSC motors, a capacitor is permanently connected in series with the auxiliary winding. There is no centrifugal switch. The auxiliary winding and capacitor are always in the circuit, acting as a secondary winding that helps create a more uniform rotating magnetic field. This results in smooth operation, good efficiency, and quiet running, but with a lower starting torque compared to capacitor-start motors. They are commonly used in fans and blowers.

These motors use a capacitor in series with the auxiliary winding, which remains connected during operation. However, unlike PSC motors, the auxiliary winding in a capacitor-run motor is designed to operate efficiently with the capacitor during the running phase, but it provides very low starting torque. They are less common than PSC motors.

Shaded-pole motors are the simplest and cheapest type. They have a main winding and a short-circuited copper ring (shading coil) around a portion of each pole. The shading coil causes a phase lag in the magnetic flux in the shaded portion of the pole, creating a weak rotating magnetic field. They have low starting torque, low efficiency, and are typically used in small fans, timers, and other low-power applications.