What Are Auxiliary Contacts Used for?

Auxiliary contacts are used to provide memory to control circuits by being an additional motor starter. The motor starter closes the auxiliary contact when energized, causing a closed circuit. They are secondary switching devices, which work in connection with primary switching devices such as relays, contactors and circuit breakers.

Auxiliary contacts are secondary devices.


A contactor is a name designated for when a relay is used to switch large amounts of electrical power through its contacts. The most common use for contactors is the controlling of electric motors. Contactors have multiple contacts which are normally open, and which causes the load to shut off when the coil is de-energized.


The auxiliary contact has a rating much lower than large motor power contacts. It is the lowest contact. The auxiliary contact is used a lot in a relay logic circuit or for some other part of the motor control scheme. Contacts are typically at least 480 volts per horsepower. The auxiliary contact has a lower rating, but possesses the same armature as the power contacts.


There are three devices going to the motor that are called overload heaters. If the temperature of these heaters reaches a critical point, a normally closed switch contact will open. The closed contact is usually connected with the relay coil. When it opens, the relay will de-energize, therefore shutting off the motor.


The overload contact can sense heater temperature with a thermo-mechanical mechanism. This causes it to sense an analogue of the real motor. If there is excessive heating and the overload contact trips, it will mean that the real motor has reached its critical temperature. The heaters are supposed to cool down so an accurate proportion of the motor's thermal condition is indicated. This will not allow power to be re-applied until the motor is ready for start-up.