The Proper Way to Ohm
a Single-phase Compressor

Mark Parliament

November 2019 • 3 min read

Have you ever faced the situation where you have a brown plug and need a safe way to measure ohms on a single-phase compressor?

We recommend the following:

Before you begin any work, ensure your work space is safe. Approximately one in 20 HVAC technicians suffer injuries that could be prevented by taking the proper safety precautions. We’re dealing with electricity here; don’t mess around!

For safety’s sake, the first thing you need to do is disconnect the power at the unit disconnect or breaker. Remember proper lockout, tag out rules!

Next, locate the three wires that power up the compressor – the common, start and run winding – and remove the wires from the contactor and the capacitor. Once the wires are disconnected, you can ohm out the wires, plug and compressor as they all require an inspection. With the multimeter set to R x 1 scale (ohms), measure the resistance between the run and start wires; start and common wires; and run to common wires.

There should be a measurable resistance through each set of wires. The lowest resistance will be found between the run(R) to common(C). The resistance between start(S) to common(C) is typically three-to-five times higher than run to common winding. Finally, the resistance between run and start should be the sum of S to C and R to C. Remember, there should be a measurable resistance through each winding.

Compressors are equipped with internal overload devices which open on a rise in temperature. This can be due to excessive loads, poor airflow or lack of superheat. The internal overload is wired in series with both the start and run winding which prevents the compressor from coming on. If it’s tripped, you will read infinity (OL) between S to C and R to C, while seeing a measurable resistance between R to S. In this case, the compressor may be extremely hot to the touch. Allowing the compressor to cool down should reset the internal overload but you need to investigate further ensure the reset was indeed completed.

In the case that any of the tests above fail, you need to check the compressor posts themselves. Remember to exercise extreme caution because on a 90-degree day, the pressure in a de-energized R410A system can be approximately 275 psi and when a compressor has shorted out, the plug can overheat and become soft. Therefore, when you remove the compressor leads (wires) from the plug to check resistance through the compressor windings, the pressure of the Freon can launch the plugs with as much as 20,000 pounds of force. Not only can that damage the unit, but it can send the terminal through anything that is in front of it – be that your hand or any other body part.

Remember, always keep to one side of the unit when working on single-phase compressors!

It is highly encouraged that the mechanic recover refrigerant in order to lower the pressure in the system. If the compressor is shorted, you will need to recover the refrigerant anyway, so this gives you a head start on the compressor changeout. With the system pressure lowered, it is now safer to remove the plug and test the compressor terminals directly. Repeat all the previous steps and record your findings.

If the compressor passes the above tests, take a multimeter set to R x 10,000 (10K) and check each winding to ground. You should have an infinite OL resistance to ground. A shorted motor will read 0 ohms across windings, to ground, or both. If it is both, you will need to replace the compressor.

Remember when removing the wires from the compressor, use a pair of insulated needle-nose pliers and keep your hands off to the side, not in front of the plug.

Want to learn more about dealing with compressors and other air conditioning and heat pump topics? Check out our one-day technical class,
Advanced AC & Heat Pump Diagnostics.