What Is a Single Phase Motor?

By John K. Taylor

A single phase motor is run from a power source with a singular distribution of alternating current electric power. This means that in the power supply, all of the voltages vary in unison. The typical characteristics for electric motors of single phase are that the current produces a pulsating rather than a rotating magnetic field and that an extra circuit is needed for starting this motion.

The currents in single phase motors reach their peak values simultaneously, making one phase. Three phase systems, on the other hand, reach peak current values sequentially, making three stages. In comparison to three phase systems, these motors do not have the same high efficiency levels, but can last indefinitely with little maintenance.

There are many different types of motors available, each category defined by the method used to start the mechanism. These categories include the split phase motor, capacitor start/ induction run, permanent slip capacitor and the capacitor start/capacitor run.

The split phase motor, also referred to as induction start/ induction run, is one of the simplest electric motor designs of single phase. It features a start and a main winding, with the start winding having a smaller gage wire and fewer turnings. This difference creates resistance in the motor and causes a rotating field. The main winding is heavier and maintains the running of the motor.

The capacitor start/ induction run type is a single phase motor that features a capacitor in the start winding which creates increased starting torque. A switch mechanism disconnects the start winding and capacitor when the speed reaches about 75% of its rated speed. The capacitor start motor also features a lower starting current, making it suitable for a wide range of applications.

The permanent split capacitor features a run capacitor which is connected permanently to the auxiliary start winding. Permanent split capacitors have low starting torque and low starting currents. Unlike many single phase electric motors, permanent split capacitors do not require any additional starting mechanisms.

The Capacitor start/ capacitor run type is a electric motor of single phase that combines elements from the capacitor start/ induction run and permanent split capacitor motors. It features a start type capacitor connected to the auxiliary winding as well as a run type capacitor that becomes active once the start winding is switched off. This allows for high starting torques at low currents.

These motors are generally used for low power applications where it would be inefficient to use more high powered mechanisms. Many single phase motors are ideal for applications with low inertia while others are engineered to meet high starting torque requirements.