Contactors aka Electrical Relays

A contactor is an electrical relay that controls the flow of electricity to different components in the furnace or air conditioning system. For HVAC products, the contactor sends voltage to the air conditioning condenser or heat pump compressor, crankcase heater or outdoor fan motor. Voltage from the thermostat or heat pump is transmitted to the contactor’s side terminals. Power is applied to a contactor’s lower terminals, while leads from the compressor and fan motor are connected to the contactor’s top terminals. When the thermostat calls for either heating or cooling, magnetic action forces a connection from the contactor’s 230-volt side (known as the line side) to the side where the compressor and fan motor are connected (the load side). When the thermostat stops calling for action, the contactor opens and stops all power to the equipment.

Types Of Contactors

Contactors come in a variety of sizes and ratings and include single-, two-, three- and four-pole models. The more poles involved, the more voltage can be dispersed through the contactor.

Replacing A Contactor

Several things can go wrong with a contactor, including:

  • Arching from high voltage: Because of the voltage moving across the contacts, the contactor is always arching. As a result, within five years the contacts are likely to become burnt and pitted. When this occurs, the full power doesn’t get to the compressor or fan motor, referred to industrially as a voltage drop. This drop causes excess heat, which can burn wires and ultimately burn out the device’s motor or compressor.
  • The sliding mechanism is shot: The plastic part that the contacts slide in wear after years of use, which can lead to the contactor sticking shut. If this happens, evaporator coils can freeze or the compressor can burn out.
  • Coil voltage error: Be sure to check the coil voltages on the contactor you’re replacing. Installing a 24-volt coil in a 230-volt application will immediately blow the new contactor coil.

First, the power supply should be shut off before attempting any replacement. Simply shut off the circuit breaker from the main panel. Next, the terminals of the contactor to be replaced should be shorted out to bare metal, using an insulated tool, such as a screwdriver. The capacitors can be discharged at their own terminals in the same fashion. Usually there are two capacitors, as mentioned above: one for the compressor and the other for the fan motor. Label your wires so you can tell which connects where. Then unscrew the old contactor, replace the new one, rewire and turn the main circuit breaker on. To test if it’s working appropriately, have the thermostat set for either a heating or cooling action. If the call is answered by your system, the replacement was successful.

Who Makes Contactors?

The following are among the companies who make contactor products:

  • Advance Controls Inc.
  • CHAC Technology Co.
  • Hampton Controls
  • Moeller Electric Co.
  • Onesto Electric Co.
  • Relay Specialties
  • Shanghai Lemore Co.

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