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In this HVACR Training article, we are using electrical resistance measurements on the PSC compressor terminals in order to determine which tab is Common, Start, and Run. This is used when the tabs are not labeled, there is no plug for the terminals, or there is no electrical wiring diagram available for the wires feeding from the terminals.
The steps listed below can be used to test all single speed PSC motors (i.e., compressors, blower motors). Whenever possible, first measure the electrical resistance via the wires connected to the compressor tabs. This can be done by first removing the compressor wires from the contactor and the capacitor and testing the wires there in the electrical compartment. If that is not giving a proper resistance value, then move to the compressor itself and disconnect the wires from the compressor tabs to take the measurements there. Take precaution any time the wires need to be removed from the compressor terminals because refrigerant pressure is being held back via the Fusite glass and terminals.
To test these motors, we will be using a multimeter. To get started, set the multimeter to ohms (Ω). This setting allows us to test for electrical resistance. Test your meter for accuracy before taking measurements. When the meter probes are not touching, the meter should display OL. When the meter probes are touching, the meter should display 0.0 ohms. Before testing the compressor motor, make sure the power is off to the unit and compressor.
Step 1: Check Resistance Values
A) Connect the alligator clips to terminal pairs in order to check the resistance value of each pair.
B) Gather resistance values and mark which pairs provided each reading.
In our example above, we can see that the resistance measurement between the top and bottom right terminal is 1.5 ohms. The measurement between the bottom two terminals is 7.5 ohms. The measurement between the top and bottom left terminal is 6 ohms. The highest measurement should equal the sum of both of the two lesser measurements.
7.5 = 6 +1.5
(Drawing a diagram for yourself, such as the one above, will make identifying each tab easier in the next step.)
Step 2: Identify Each Terminal
A) Identify the pair with the lowest resistance reading. This pair is always the Common and Run. This pair measured 1.5 ohms. This means that the other remaining tab is Start.
B) Identify the second highest resistance reading. This pair is always the Common and Start. This pair measured 6 ohms. This means that the other remaining tab is Run.
C) Identify the highest resistance reading. This pair is always the Start and Run. This pair measured 7.5 ohms. This means that the other remaining tab is Common.
In this way, you can always determine which tab is Common, Start, and Run. Just find the pair with the lowest resistance measurement. The remaining tab that is not in the pair is Start. You could also just find the pair with the highest resistance measurement. The remaining tab outside of the pair is the Common.
1.5 ohms (Common to Run) + 6 ohms (Common to Start) = 7.5 ohms (Start to Run)
Step 3: Determine if the Compressor has been Shorted to Ground.
When the three windings match up, as they do in our example, this would indicate there is no short. However, it is still a good idea to complete the following step.
A) Clip one alligator clip to one of the terminals.
B) Press the other probe onto a cleaned section of the copper tubing or unpainted steel on the compressor.
C) If the compressor is NOT shorted to ground, the multimeter will read OL.
At this point, you can be confident that the compressor is not shorted to ground and you know what terminals are what. If you need to do a deeper dive into the windings, a megger test would be needed.
If you are looking for a video to better help understand this topic, check out our "How to Find Common, Start, and Run on a PSC Compressor Motor!" video below!
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Published: 04/28/2021 Author: Craig Migliaccio
About the Author: Craig is the owner of AC Service Tech LLC and the Author of the book “Refrigerant Charging and Service Procedures for Air Conditioning”. Craig is a licensed Teacher of HVACR, Sheet Metal, and Building Maintenance in the State of New Jersey of the USA. He is also an HVACR Contracting Business owner of 16 years and holds an NJ HVACR Master License. Craig creates educational HVACR articles and videos which are posted at https://www.acservicetech.com & https://www.youtube.com/acservicetechchannel & https://www.facebook.com/acservicetech/
Hi Craig,
A refrigerator 1/7 hp compressor was not starting. I got resistances 7.7, 4.4, 3.3. The start and run seemed swapped, because it didn't make sense when viewing the relay housing terminals.
I found a spec sheet:: (Embraco Model EMD55CLT 115V)
Start Winding Resistance 5.41 Ω at 25°C
Run Winding Resistance 4.26 Ω at 25°C
I suspect that the 3.3 ohm measured, should be 5.41 ohms, meaning that the start winding are short-circuited.
It's a good thing that Embraco published the winding specs, otherwise I would have never figured this out.
Thanks for nice tips
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