- the difficulties in troubleshooting a hermetically sealed compressor
- the differences in various compressor manufacturers and HVAC manufacturers
- the commonality of single phase compressors
- the basics of troubleshooting an air conditioner or heat pump compressor
- Compressor capacitors and compressor mechanics
- basic tips for compressor repair
- lots of resource and related links that allows you to go in depth and learn about HVAC compressors
The average compressor in many residential condensing units are hermetically sealed compressors. The only visible working parts on the compressor are the electrical connections and the copper discharge and suction lines.
Air Conditioner Compressor Troubleshooting
This makes it difficult to repair any compressor once a diagnosis is made to the air conditioner or heat compressor. Troubleshooting air conditioning and heat pump compressors is not difficult once you understand the basics of the compressor. It doesn’t matter what kind of residential air conditioner or heat pump compressor you are working on. As long as it is single-phase for use in a residential heat pump or air conditioner. This article will cover the basics of troubleshooting a compressor when the circuit breaker trips continuously.
Air Conditioner Compressor Troubleshooting - Different Air Conditioner or Heat Pump Compressors
Many residential heat pump or air conditioner compressors are black and a few are red depending on the manufacturer of the compressor. Trane is the only OEM in the USA that manufacturers their own compressors. This falls to American Standard because Trane and American Standard are owned by the same company. In any regard the color does not matter as all the components inside the compressor are the same. That is a scroll compressor is a scroll compressor.
There are new scroll compressors out that have the ability to unload. Other electronic scroll and rotary compressors that have the ability to fully modulate. This article does not address these newer compressors. Scroll compressors and the scroll technology were initially owned by Copeland which is owned by Emerson Electric. There are also inverter duty compressors in some split system air conditioner and heat pumps. These are manufactured by Nordyne and are different than the compressors we are talking about. These newer systems require a different compressor troubleshooting analysis and technique than described here. The newer modulating systems can now be found in many different brands of HVAC equipment.
So now that we know this and that it doesn’t matter what color the compressors are or who manufactured the compressor that every residential air conditioner or heat compressor is the same except the air conditioner or heat pump compressors mentioned above.
Air Conditioner Compressor Troubleshooting – Compressor Basics
The Compressor Electrical Basics
Compressor Start & Run Windings Whether it is a scroll compressor or a reciprocating compressor (of which scrolls and reciprocating compressors make up the bulk of residential air conditioner or heat pump compressors in use today) there are two major components to the compressor. Each compressor has an electric motor and an assembly of mechanical components which are driven by the electric motor.
The compressor electric motor has two separate windings inside for the average residential air conditioner or heat pump single-phase compressors. The windings inside the compressor are the run winding and the start winding. When troubleshooting an air conditioner or heat pump compressor one should see three terminals on the outside of the compressor.
These three electrical terminals are called or should be labeled in some manner – Common Terminal or C – Start Terminal or S – and the Run Terminal or R. Somewhere near these terminals there should some labels……….sometimes they say CSR and CSR stands for Common – Start – Run. These are the compressor basics. The other item in the electrical circuit for the compressor that you can troubleshoot with the compressor is the run capacitor.
There are some compressors that also have start capacitors. The capacitors are external to the compressor. The run capacitor is wired with the run winding and start capacitor is wired with the start winding. Always check OEM wiring diagram for correct wiring. The start capacitor will have a relay in the circuit. The start capacitor will only be used in the start circuit for a split second. Then the relay, usually a potential relay and sometimes a current relay, will open the relay contacts. This takes the start capacitor from the start circuit.
The run capacitor remains in the run circuit for the duration of the compressor run time. For electrical compressor troubleshooting you will need:
- an Ohm Meter
- basic knowledge of electrical safety
- and to know the aforementioned information. One word of caution! Capacitors hold a charge of electricity. You can be seriously injured or worse by coming into contact with a charged capacitor. Discharge the capacitor using a properly rated resistor.
When troubleshooting the electrical system of a compressor there are three different problems that can occur with the windings of the compressor.
Air Conditioner Compressor Troubleshooting - Compressor Troubleshooting – Ohming the Windings
None of these tests require any knowledge of Ohms Law.
1. Shorted Windings – This is when the windings touch one another. This usually occurs because the insulation on each winding has dissolved allowing the copper windings to touch each other. Your Ohm Meter will likely read zero in this instance. The compressor is bad if you get this reading.
2. Open Winding – This occurs when a winding is open or broken. Your Ohm Meter will read infinity. If you get this reading the compressor is bad.
3. Grounded Winding – This nearly the same as an open winding except the winding is touching the casing of the compressor. Your Ohm Meter reading will be zero from the terminal to the casing of the compressor. Touch one lead of the meter to the copper pipe going into the compressor (discharge or suction). And then to one of the CSR terminals.
If any of those conditions exist the circuit breaker for the condensing unit will trip when the compressor tries to start. If you perform this test and everything seems normal and the circuit breaker still trips you likely have a mechanical issue and we will address this next.
Air Conditioner Compressor Troubleshooting - The Math
What are normal readings on the Ohm Meter you ask? A good example of normal reading would be resistance of say 7 Ohms on the start winding and 4 Ohms on the run winding. Your compressor may show a different measurement of Ohms but as long as you get resistance it is a good sign. If you measure between the run winding and the start winding you should get the sum of the other two readings. Example – C -S = 7 Ohms. C – R = 4 Ohms. S – R = 11 Ohms.
It is imperative that the compressor capacitors are functioning properly and are not bad. To properly check a capacitor to make sure it is good you need a special meter that reads microfarads. Some multi-meters have the ability to read microfarads. The reading from the meter should be plus or minus 10 percent of the rating on the capacitor.
Related Link: Learn more about troubleshooting compressor overheating (opens in a new window)
Air Conditioner Compressor Troubleshooting - The Compressor Mechanical Basics
Of the several types of compressors out on the market and that are installed in the average residential air conditioner and heat pump condenser we will only focus on the two types of basic compressors. These compressors currently make up approximately 95% of compressors installed in the average residential air conditioner or heat pump.
These two types of compressors are the scroll compressor and the reciprocating compressor. The scroll compressor Has outpaced the reciprocating compressor in volume of installations. This is in the average residential air conditioner or heat pump condenser or package unit.
To properly troubleshoot any compressor mechanically whether it is a reciprocating or a scroll you need to eliminate any electrical problems with the compressor. There is not much one can do if the mechanically or electrically broke except to replace the compressor but we must determine if there is a major uncorrectable problem with the compressor.
Air Conditioner Compressor Troubleshooting | Additional Test
One other test that can be done with the mechanical system will be an oil test. A special kit is required for this function. With the oil test kit, a sample of the compressor oil is taken. Then it is sent to a lab to determine that the oil contains all normal properties required of refrigerant oil. The compressor oil test kit can usually be purchased at an HVACR supply store.
The oil test will only be necessary if you find the windings have shorted. That indicates the compressor is a burn out. If the compressor burned out it will need to be replaced. If you are replacing a burnout then the old oil will have to be flushed. Flush the entire refrigeration circuit before putting the new compressor into service.
Air Conditioner Compressor Troubleshooting | The Mechanical Components
After you have checked out all the electrical components and eliminated any problems with the electrical circuit and you continue to trip the circuit breaker for the condensing unit the next step is to try to eliminate any mechanical seizure of the compressor mechanical components. After I have checked the compressor electrical system and eliminated any problems with that I get an age old tool that every man loves. The hammer. This is true for reciprocating compressors but I cannot say I have found this works for any scroll compressors.
If you have used this repair technique on a scroll and it worked for you please leave a comment below. We want to hear from you. I take a good hammer and hit the compressor a few good times and then restore power. Approximately fifty percent of the time the compressor comes to life and lives out a normal life. If that happens I always make sure there is a time delay in the compressor circuit. Mechanical lock-ups are usually the result of short cycling.
Air Conditioner Compressor Troubleshooting - Resources
For more information on air conditioners click here.
Air Conditioner Compressor Troubleshooting
Air Conditioner Compressor Troubleshooting