Introduction

Single-phase motors are used mainly in domestic applications for the operation of vacuum cleaners, washing machines, ventilating fans, compressors, and the like. About 90% of all motors manufactured are of the single-phase type. Because of their low efficiency and power factor, their current rating, depending on the kW rating of the motor, may be relatively high.

Single-phase motors have low-power ratings and are generally noisy because the instantaneous power drawn from a single-phase system (the only kind available in an ordinary home) is pulsating in nature (it has a positive and negative value over a complete voltage cycle). In contrast, three-phase motors are less noisy because the instantaneous power drawn from a balanced three-phase system is constant. Nevertheless, more expensive single-phase motors, such as those used for residential room ventilators, incorporate circuits that minimize the negative component of the pulsating power.

The most commonly used types of motors that may be operated from a single-phase supply are as follows:

• Induction motors, which are, internally, two-phase motors, at least during starting. This class of motors includes the so-called split-phase motor, the capacitor-start motor, the capacitor-run motor, and the capacitor-start-capacitor-run motor.
• Shaded-pole motors.
• Single-phase synchronous motors.
• Series ac/dc motors.