Compressor Motor Check-Out

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Unidentified compressor terminals

Once in a while you will come across a compressor whose terminal markings are not clear or missing altogether.  To identify the Common, Start, and Run terminals, the only tool you’ll need is an ohmmeter.  Use a good, dedicated ohmmeter.  Do not attempt this with the ohm scale offered on your clamp-on ammeter, for instance, it’s not precise enough.

First, assign one unidentified terminal as “A”, another as “B” and the third as “C”, as shown in figure 1, for example.  Pull the disconnect and remove all wires from the compressor terminals.  Now, whip out that old Simpson meter and by using the Rx1 scale, you can identify these terminals.  Place your ohmmeter between each set of terminals and record the readings.

Let’s say you read the following:

A to B = 2.5 Ω

B to C = 3.0 Ω

A to C = 0.5 Ω

Here are the guidelines:

The pair with highest ohm reading is not Common.

The next highest pair is the Start winding.

The remaining terminal is the Run terminal.

Now, apply the guidelines.  Which pair has the highest ohm reading? “B” to “C”, correct?  Therefore, Common is our assigned terminal “A”.  Next, what is the next highest pair?  “A” to “B”, correct?  Therefore, knowing that “A” is Common, the Start winding is between “A” and “B”, making “B” the Start terminal.  That leaves our remaining terminal “C” as the Run terminal.  Easy peasy.

This relationship is important to know because the terminals of a good compressor motor will always have the following relationship:

Common to Start + Common to Run = Run to Start

Our example followed that relationship perfectly, as it should:

C to S = 2.5 Ω

C to R = 0.5 Ω

R to S = 3.0 Ω

Looking at the internal wiring of a compressor (as in Figure 2) may help to solidify that point in your mind.  Notice the thinner wire used in the Start winding with many turns.  That will create a higher ohm reading.  Also, notice the heavier wire used in the Run winding with fewer turns.  This will result in a lower ohm reading.  Also, when you’re reading from Run to Start, you are reading both windings in series, hence the highest ohm reading.

To check for an open internal overload (OL), first allow the compressor to cool off.  If the internal overload is open, it can take many hours to reset.  You can speed this process up by dumping liquid refrigerant into the low side at the service valve.  The heat in the compressor will be drawn to the cool refrigerant as it evaporates in the compressor shell.

This method is preferred to duct taping a garden hose to the compressor dome.  Line voltage and water simply don’t mix.  Don’t be afraid to dump liquid refrigerant into the system.  Remember it was a lack of refrigerant that most likely caused the internal overload to open in the first place.  It’s a thermal overload, and it opens in response to a high temperature.

Now, place your ohmmeter between Common and Run, and then between Common and Start.  If the overload is open, then both readings will show infinity (ꝏ), or OL on a digital ohmmeter.

Short to ground

Checking for a short to ground is fairly simple.  Place your ohmmeter between the Start terminal and ground, then between the Run terminal and ground.  A short would be indicated by any reading of continuity.

If your compressor has a locked rotor, you can reverse the direction that the motor turns by reversing the Start and Run windings.  Reversing the rotation of the motor will usually free up a locked rotor.  This procedure is not a cure for a compressor that is failing but it can buy you a little time.  Be sure to reinstall the wires on the proper terminals once the rotor is freed up.

What we have discussed here is relevant to single-phase compressors.  For three-phase compressor motors, all windings should have the same resistance.  Each winding has its own thermal overload (OL), or they sometimes share a common overload.  You can reverse the rotation of a three-phase motor by interchanging any two leads.

These checks can be performed as needed during troubleshooting. They can also be employed as part of a spring PM and/or a fall PM for heat pumps.

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