HVAC Contactors and Control Relays
Most heat pumps and central air conditioning systems have a high-voltage switching device in the condensing (outdoor) unit called a contactor. Most contactors are controlled by a low-voltage electrical circuit which magnetically energizes a coil causing the metal contacts to engage (close). This supplies high voltage to the compressor, fan motor and any other high-voltage components. Standard contactors have one, two or three sets of contacts (poles) depending on the system's electrical requirements. A double pole contactor has two sets of contacts and both need to be engaged (closed) for the unit to work properly. A single pole contactor allows one leg of the high-voltage circuit to constantly supply power to a component. You will find these mostly in heat pumps. The heat pump's compressor has a heater that warms its oil in cold conditions. If you find that an outdoor condensing unit is not running and the poles are open, do not assume it's a defective contactor. TECH NOTE: Remember, just because the contacts show open, there is still high voltage supplying the contactor! Turn the power off to the condensing (outdoor) unit before handling a contactor. There are so many reasons why a contactor would show open. Check for low voltage current supplying the magnetic coil. If you are getting proper low voltage readings, check the contactor for defects like burnt or pitted metal contacts. Make sure there is nothing from causing the contacts not to close like bugs, nests or dirt. If the high-voltage electrical connections on the contactor shows burnt wires or even melting, there is a high voltage problem in a component causing this. Do not replace a contactor assuming this will remedy the problem. The component or reason this occurred (burnt wires or melting) needs to be repaired. If you are not getting a low voltage current to the contactor's magnetic coil and the metal contacts look clean, there is a problem with the system's low-voltage circuit wiring or a low-voltage sensor or device (ie. thermostat) that is causing the problem. TECH NOTE: If you find that the contacts are closed and you are getting the proper voltage coming to and from the electrcal connections on both sides of a contactor, you have a defective high voltage component. See Capacitors. If you find that the contactor is defective, replace it with the same type (poles), voltage and ampere (amp) rating. The easiest way to replace a contactor is wire-to-wire from old to new. TECH NOTE: When removing a contactor or contactor's wires, turn off power to BOTH indoor and outdoor units (ie. condenser and air-handler / condenser and furnace).