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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
I’d like to know if this is a common issue or not. My HVAC contractor, for my commercial building, is telling me that all 4 of my HVAC units (all 3.5 years old or less) need Compressor contactors replaced due to them being worn. I think this is a bunch of bologna. In my experience, these would RARELY need replacement unless there is a loose connection causing chattering. I’m being told this is part of regular Preventative Maintenance. Could you please provide your input and opinion? Thank you. Jill
Hello Jill and thank you for visiting my site!!! I’ve not seen an issue like this. However, if they are cheap as dirt parts (Made in (name a place known for low quality)) then it is possible. I recently went to a large apartment building condo were most of tenants were sold on these new generic mini-splits. We’re talking over 50 units and within 3 months every single one of them failed for one reason or another. Talk about job security for a repairman!!! It was nothing but made in somewhere-istan (big country in Asia) junk.
Anyhow, most contactors that I have replaced have bad had bad contacts and the measured rated voltage was not being passed across the contacts. In other words, test the voltage across the contacts from one side of the contacts (T1 and T2 to L1 and L2) and if there is a difference then it is likely the something is wrong with the contacts. In some cases, depending on the size of the system, the contacts can be replaced. In other cases the entire contactor needs to be replaced. That is only of there is a measured difference from one side of the contactors to the other side as noted above.
When a contactor is pulled in there is an arc of electricity. This arc can create very high temperatures and this is why the contacts get pitted and scared and eventually, need to be replaced. Replacing all 4 seems kind of fishy to me although not impossible as noted above. I worked for a company for several years as a technician and when we were not busy we were doing preventive maintenance calls. Part of my PM was testing volts and amps across contactors and for the compressor as a whole. I kept a log for each piece of equipment I PMed and when something was out of whack on my readings then I knew something was wrong somewhere. On occasion it turned out to be the compressor contactor. Over a five year span I replaced approximately 7 or 8 contactors if my memory serves me correctly.
A few contactors on larger equipment I repaired for reasons you describe above or because the contacts were pitted and scarred too badly. However, most contactors on residential and light commercial equipment are throw-aways and get a direct replacement. I would be suspicious and ask some serious questions about replacing 4 contactors in one day on all the units. Someone could be trying to boost their revenue by offering their techs incentives and that leads the techs to doing things like this. Not saying that is the case but it is possible. It is the reason why I’m against spiffs. Spiffs is what the industry calls a commission to the tech for up-selling the customer. Good Luck!!
BTW, always ask for the bad parts when they say they are replacing something that is bad.
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Thanks for your input. The landlord of the building is an electrical contractor so I doubt they bought anything cheap, but good to know. I have also heard worn contacts can happen if the voltage isn’t rated properly as well. What do you know about this?
My father is a retired union pipe fitter, retired for 20 years now, but he is extremely doubtful the contacts are worn. In his 30+ yrs of experience he said maybe once or twice he had to replace contactors.
Thanks, Jill
A power problem (low voltage) that would cause any issue with the contacts would create more heat however you would have bigger problems if it was too severe. You would lose motors because they would likely operate above their performance factors for amp draws. If you have a 240 volt (or any number for voltage) feed everything should be fine as long as the voltage is +/- 10%. That should be recorded in the maintenance log also. In most cases, when there is a power problem, it is a problem the power company needs to fix as it likely originates from their infrastructure. The only other thing I could think of for voltage problems is if you were running off of a backup generator and overloaded the generator. However, most modern generators will trip the electrical circuit if it is overloaded. Besides, unless you have a very large wattage generator you shouldn’t be running HVAC equipment off of it unless it is specifically designed for it.