Introduction of Single Phase Motor
An AC motor is an electric motor driven by an alternating current (AC).It commonly consists of two basic parts, an outside stationary stator having coils supplied with alternating current to produce a rotating magnetic field, and an inside rotor attached to the output shaft that is given a torque by the rotating field.There are two main types of AC motors, depending on the type of rotor used. The first type is the induction motor or asynchronous motor; this type relies on a small difference in speed between the rotating magnetic field and the rotor to induce rotor current. The second type is the synchronous motor, which does not rely on induction and as a result, can rotate exactly at the supply frequency or a sub-multiple of the supply frequency.
Split Phase Induction Motor
The split-phase motor, also called an induction-start/induction-run motor, is probably the simplest single-phase motor made for industrial use, though somewhat limited. It has two windings: a start and a main winding. The start winding is made with smaller gage wire and fewer turns relative to the main winding to create more resistance, thus putting the start winding’s field at a different electrical angle than that of the main winding, and causing the motor to rotate. The main winding, of heavier wire, keeps the motor running the rest of the time.
A split-phase motor uses a switching mechanism that disconnects the start winding from the main winding when the motor comes up to about 75% of rated speed. In most cases, it is a centrifugal switch on the motor shaft.
The split-phase motor’s simple design makes it typically less expensive than other single-phase motor types for industrial use. However, it also limits performance. Starting torque is low, typically 100 to 175% of rated load. Also, the motor develops high starting current, approximately 700 to 1,000% of rated. Consequently, prolonged starting times cause the start winding to overheat and fail; so don’t use this motor if you need high starting torque.
Other split-phase motor characteristics: Maximum running torque ranges from 250 to 350% of normal. Plus, thermal protection is difficult because the high locked-rotor current relative to running current makes it tricky to find a protector with trip time fast enough to prevent start-winding burnout. And, these motors usually are designed for single voltage, limiting application flexibility.