ECN Forum Forums General Topics of the Trade General Discussion Area What are the heaters for in a motor starter?

The 'heater' differs from a fuse in that it will never, ever react to a short circuit. And, no, they do not 'burn out' when they trip.

First off, they are brand-specific to the type of starter they're in. You can't use Square D heaters in an Allen Bradley starter.

The older style used various mechanical means - springs, solder pots, etc- to react to heat build-up. When they got hot enough, they would open an internal contact, which would cause the starter coil to lose power and open the power contacts. You would have to wait for them to cool before you could press the 'reset' and then start up everything again.

The newer ones are electronic.

Whatever the type, you can set them / get them in settings very close to the actual operating current of the equipment. That is, a motor that normally drew 7.3 amps might have heaters set at 7.4 amps. The idea was to protect the equipment as best you could. (That same motor might have 50A fuses protecting it, and an 8FLA on the nameplate).

Another way to look at it: Fuses protect the wires, heaters protect the motor.