The Proper Way to Ohm a Single-phase Compressor


When I have a blown plug, what's the safest way to ohm a single-phase compressor? - Roxanne George, Minneapolis, Minnesota

Hi Roxanne,

Tom Mallinos from HVAC Learning Solutions here. Glad you're taking the time to make sure you're taking all the necessary safety precautions. Did you know that about one in 20 HVAC technicians are injured by not taking the proper steps - and those injuries can be career-ending or even life-threatening?

When a compressor has shorted out, the plug can overheat and get soft. When you're removing 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 it damage the unit, but it can send the terminal through anything that's in front of it - including your hand or any other body part. So remember, always keep to one side of the unit when working on a single-phase compressor. Here are a few other tips.

For safety's sake, the first thing you need to do is disconnect the power. Locate the three wires that power up the compressor - the common, start, and run winding. Next, discharge the capacitor and disconnect the wires from the compressor's three-prong plug.

Now you're ready to take your readings. With the multimeter set to R x 1 scale (ohms), measure the resistance between the run and start winding, start and common winding, and run to common winding. There should be a measurable resistance through each set of windings. The lowest resistance will be found between 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, and 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. These open on a rise in temperature due to excessive loads, poor airflow, or lack of superheat, which prevents the compressor from coming on. The internal overload is wired in series with both the start and run winding. If it's tripped, you'll read infinity (OL) between S to C and R to C, but will see a measurable resistance between R to S. In this case the compressor maybe extremely hot to the touch. Allowing the compressor to cool down should reset the internal overload. You will have to investigate further to find out.

With the multimeter set to R x 10,000 (10K), each winding should be checked 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's both, you'll need to replace the compressor.

Remember, if you have to remove the wires from the compressor, use a pair of insulated needle-nose pliers and make sure your hands are off to the side, not in front of the plug. That way, if something goes wrong, you'll destroy a tool - not your fingers.

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Tom Mallinos is a Technical Trainer for HVAC Learning Solutions.

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