This old compressor contactor serves a compressor and a condenser fan motor.

Compressor Contactors for Air Conditioners and Heat Pumps – Compressor contactors are are simply heavy-duty switches that allow it to carry extra amperage that is used by the compressor while it is running. The contactor is made up of a coil and typically two contacts for a double contactor and 1 for a single pole contactor.

Three phase commercial units have three contactors and are rated at a much higher amperage than double pole or single pole contactors. The contacts that make the switch on a call for heating or cooling (depending on whether it is a heat pump or air conditioner) are made of silver oxide. The contacts are typically normally open and make when there is a call for cooling or heating.

Compressor contactors are considered an electro-mechanical control. Since they are mechanical and electrical, contactors are subject to failure for various reasons.

Compressor Contactors for Air Conditioners and Heat Pumps - Troubleshooting

When the thermostat calls for heating or cooling the system goes through a sequence of operations. The thermostat is the main control switch and closes the control circuit. In air conditioners and heat pumps, the “Y” circuit is closed and this energizes the compressor contactor. That, in turn, turns on the compressor and the condenser fan motor because the contacts on the compressor contactor close.

When the contacts close sometimes there is arcing of electricity from the fixed contact and contact that moves to the fixed contact. This arcing of electricity creates heat and the heat, depending on the amperage, can slightly melt or scar the contacts. Both the moving contacts and the fixed contacts can have this occur. Over time this damages the contacts and this can result in the failure of the contactor. It can also lead to the compressor overheating under the right conditions.

In residential and light commercial HVAC systems the compressor contactor is replaced. In heavy commercial systems, the contacts are replaced. Residential and light commercial compressor contactors are not made to be disabled and repaired while heavy commercial contactors are made to be disassembled and repaired.

How A Contactor Works | Compressor Contactors for Air Conditioners and Heat Pumps

3-pole Contactor with Auxiliary Contacts

The compressor contactor has an armature that slides down when the coil of the contactor is energized. Sometimes a loud humming can occur in the contactor when the contactor is energized. This noise can be annoying. In residential and light commercial systems the contactor can be sprayed with oil. Be careful not to spray too much. Additionally, make sure the system is de-energized before spraying the oil into the armature.

Oil can be flammable so use a little bit and only on the armature. If any oil or an excess of oil gets on the contacts make sure you clean it up. Clean it before energizing and running the equipment. If the contactor can be disassembled you can disassemble it and use sandpaper to sand the armature. This should stop the nuisance humming coming from the contactor. If it doesn’t then replace the contactor especially if it annoys someone or there are complaints.

Another problem that can occur with a compressor contactor is the failure of the coil. In most residential and light commercial systems the coil is powered by 24 volts. In heavy commercial systems, the coil voltage can be 120 volts and up. Some compressor contactor coils have a range of voltage(s) depending on the manufacturer and based on the available control voltage. In the case of a failed coil, the contactor needs to be replaced.

Air Conditioner Compressor Contactor Diagram

Compressor Contactors for Air Conditioners and Heat Pumps - Maintenance and Replacement

Compressor contactors are needed for heavy-duty switching where the amperage is higher than normal. It important that the contactor is properly rated for the amperage of the system. If not it will not be long before the contacts are burned up, pitted, or scarred in normal operation. Many residential systems will require a 20 to 30 amp contactor for normal duty. Light and heavy commercial will require higher-rated contactors.

Always check with the manufacturer for proper ratings of compressor contactors. If you replace a contactor it needs to be replaced with an exact replacement. Some single-pole controls are single-pole to allow electricity to pass to other necessary components. In many cases, there is a single-pole contactor in an air conditioner or heat pump. This is as designed. T2 is allowing current to be passed through the closed side of the contactor. This is necessary for a crankcase heater to protect the compressor in the colder months.

Compressor Contactors for Air Conditioners and Heat Pumps - AC Compressor Contactor Common Problems

• One of the biggest complaints with a compressor contactor is people hear a humming noise from the air conditioner. It simply sounds like an electrical humming noise or vibration. This is not uncommon. If the noise becomes too annoying the solution is to replace the compressor contactor with a new one (for residential systems).
• Insects can get into the control panel and get under the switching contacts. If the switch enables the insects are smashed in the contacts. This prevents the wrong amount of voltage or no voltage at all from passing through the contacts. That will cause a malfunction and need to be addressed by a professional.
• As mentioned above, the contacts get pitted and scared resulting in a bad electrical connection. Replacement is recommended for residential contactors.
• A dead short or surge can cause the low voltage coil in the contactor to not function as it should. Replacement is recommended for all contactors.

Compressor Contactors for Air Conditioners and Heat Pumps

Technical Resource: Refrigeration and Air Conditioning Technology