A contactor is defined as an electrical relay used to control the flow of power in a circuit. Many people also refer to them as a relay. A relay is actually defined as a device that responds to a small current or voltage change by activating switches or other devices in an electric circuit. In the real world of heating and air conditioning, what this means is a contactor is capable of handling a higher current than a relay. Relays are usually reserved for small pilot duty applications. Most heating and air conditioning service technicians will not call a contactor a relay.
We will be discussing a typical single pole or double pole contactor as would be found in a modern home using a split system gas furnace with air conditioning or a heat pump. Single poles have only one set of contacts while double poles have two. Single poles are used by many manufacturers of equipment. Single poles only break half the power to the fan motor and compressor while double poles break it all. We feel most heating and air conditioning service technicians probably like breaking both sides, at least we do.
Contactors actually have at least two separate circuits in them. (Some commercial contactors have many circuits). One circuit is the low voltage coil circuit and the other is the load circuit.
The coil circuit in MOST home air conditioning or heat pump systems uses a 24 volt coil, however, there are some systems that use a 230 volt coil in conjunction with a small relay that switches the contactor on and off. These 230 volt coil systems are predominantly on older systems or commercial units these days. Before changing any contactor be sure the correct coil voltage is used or a burnt coil may result. The coil voltage will be marked on a small tag on the side of the contactor. The line / load circuit in the home contactor is used to supply 230 volts to the load side of the equipment from the line side. In other words, the main 230 volt power coming from the power source of the home flows across the contacts in the contactor to the line side (compressor, fan motor and any other component) when the coil is energized by a call from the home thermostat for cooling or in a heat pump for cooling or heating. When the thermostat isn't calling for the unit to be running, the coil releases and the contacts open causing the compressor and fan motor to stop.
Thousands of on and off cycles however, cause a couple of problems. The most obvious problem is that the contacts themselves get burnt from constantly arching as they open and close the 230 volt connection. Contacts can actually weld themselves closed after a period of time. These thousands of cycles also cause the plastic sliding mechanism to wear. This wear often causes the contactor contacts to stick closed even though the thermostat has stopped sending an electrical signal to the coil. When this happens it usually goes unnoticed by the homeowner and can quickly lead to a frozen evaporator coil and or compressor. When a frozen condition exists the oil in the compressor is also frozen and can't lubricate the internal parts inside the compressor. This conditions amounts to the same thing as running your vehicle with no oil! The other issue with burnt contacts is that there may be a voltage drop and added heat sent to the compressor and fan motor since the contacts are burnt.
To put things in perspective, changing a contactor is very cheap protection against having to replace a $ 1,000- $ 1,500 compressor. Contacts should be examined every year before operating the unit and usually should be replace every five years or so.