The Proper Way to Ohm
a Single-phase Compressor

Joeseph T. Prock - Author

November 2019 • 3 min read

In this vi-minute video, Doug Smiley, Technical Preparation Manager, shows how to read ohms on a compressor.

Have you ever faced the state of affairs where you have a dark-brown plug and need a prophylactic way to measure ohms on a single-stage compressor?

Nosotros recommend the post-obit:

Before you begin any work, ensure your work infinite is safe. Approximately 1 in 20 HVAC technicians suffer injuries that could exist prevented by taking the proper condom precautions. Nosotros're dealing with electricity here; don't mess around!

For rubber'due south sake, the beginning thing you demand to practise is disconnect the power at the unit disconnect or breaker. Call up proper lockout, tag out rules!

Adjacent, locate the 3 wires that ability up the compressor – the common, start and run winding – and remove the wires from the contactor and the capacitor. In one case the wires are asunder, you tin can ohm out the wires, plug and compressor as they all crave an inspection. With the multimeter gear up 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 fix of wires. The lowest resistance volition be found between the run(R) to common(C). The resistance between start(S) to common(C) is typically iii-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 exist due to excessive loads, poor airflow or lack of superheat. The internal overload is wired in serial with both the kickoff and run winding which prevents the compressor from coming on. If it'due south tripped, you volition read infinity (OL) between Southward to C and R to C, while seeing a measurable resistance betwixt R to S. In this case, the compressor may be extremely hot to the touch on. Allowing the compressor to absurd downward should reset the internal overload but yous need to investigate further ensure the reset was indeed completed.

In the instance that any of the tests above neglect, you demand to cheque the compressor posts themselves. Remember to exercise extreme caution considering on a 90-caste 24-hour interval, the force per unit area in a de-energized R410A system tin can be approximately 275 psi and when a compressor has shorted out, the plug tin can overheat and become soft. Therefore, when y'all remove the compressor leads (wires) from the plug to check resistance through the compressor windings, the force per unit area of the Freon tin can launch the plugs with as much as 20,000 pounds of force. Non only can that damage the unit, but it can send the terminal through anything that is in front of information technology – be that your hand or whatsoever other body part.

Remember, always proceed to i side of the unit of measurement when working on unmarried-phase compressors!

Information technology is highly encouraged that the mechanic recover refrigerant in guild to lower the pressure in the organization. If the compressor is shorted, you volition need to recover the refrigerant anyway, so this gives you a head start on the compressor changeout. With the organisation pressure level lowered, it is at present safer to remove the plug and examination the compressor terminals direct. Repeat all the previous steps and tape your findings.

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

Recollect when removing the wires from the compressor, utilise a pair of insulated needle-nose pliers and keep your hands off to the side, not in front of the plug. This manner, if something goes wrong yous will destroy a tool, non your fingers.

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