Magnetic Starter

a low-voltage electrical device for remote control (starting, stopping, and reversing direction) and protection of low-power and medium-power squirrel-cage induction motors. There are nonreversible and reversible magnetic starters, as well as starters for switching the windings of multi-speed electric motor drives. A magnetic starter consists of a contactor, a push-button panel, and a thermal relay. The contactor has three main contact systems (for connection to a three-phase power supply) and one to five auxiliary contacts.

The operation of a nonreversible AC magnetic starter is as follows. When the “start” button is pushed, voltage is supplied to the contactor winding, and the contactor operates, thus closing the main contacts and the auxiliary contacts. The auxiliary contacts shunt the contacts of the depressed push button, so that it can be released after starting the motor. When the “stop” button is pushed, the power circuit for the contactor winding is broken and the main contacts are opened. When the intake current demand increases drastically because of overloading or a fault in the motor, the thermal relay operates and opens the contacts in the power circuit of the contactor winding. The rated actuating current of the thermal relay may be 0.2-200 amperes (A).

Reversible magnetic starters have two contactors with mechanical and electrical interlocks so that only one contactor can be switched on at a time. When the contactors are switched on alternately, the phases of the power supply are shifted and the direction of the motor is reversed.

The magnetic starters in common use are made for AC voltages of 127, 220, 380, and 500 volts (V); the current ratings of the main contacts range from 6 to 400 A, and the current ratings of the auxiliary contacts range from 6 to 10 A. Under normal operating conditions the starters can withstand 3-5 million (sometimes up to 10 million) on-off cycles. They can operate at rates of 150-1,200 connections per hour, and low-power starters can operate up to 3,000 times per hour. They are manufactured in ordinary, enclosed, or explosion-proof types.