A magnetic starter is an electromagnetically operated device, that starts and stops a connected motor load.

 

Magnetic starters consist of: An electrical contactor, and an overload switch, which provides protection in case of a sudden loss of power. 

 

The overload switch, commonly referred to as a "heater", is designed to protect the motor from damage in the event of a short circuit, or from being over-loaded and overheating.  

 

The simplest overload relay is activated by heat from high current flowing through the overload, over a bimetallic strip.

 

A bimetallic strip is a band of two different metals attached to each other, where each metal has a different coefficient of thermal expansion. As this bimetallic strip heats up, one metal will expand faster than the other, and cause the assembly to curve.

When it gets too hot, the curvature will be enough for the contacts in the overload to become separated.

 

Since the overload has a contact wired into the control circuit of the contactor, this effectively breaks the circuit and de-energizes the system. Once the bimetallic strip cools, it will straighten out and allow the circuit to close again.  If you're in need of a new magnetic starter or overload heaters, stop in or give us a call.  We carry starters for both single, and three phase power.