How to Check a Single Phase Compressor

How to Check a Single Phase Compressor

Service Advisor: Peter Kirtschej

When approaching a condenser that has a fan motor running but the compressor is not, the first thing I would check is to verify that the compressor is getting voltage.

On single phase compressors, 208/230 volt units, check between the Run and Start terminals.  You should see between 198 and 258 volts. If you do not, trace the wires back to where you may have lost power.  There is possibly a bad connection or faulty wire.

Once voltage has been determined, turn power off.  Next, I suggest checking the capacitor.  Usually the capacitor is swollen or has actually exploded.  At this point it is obvious what needs to be replaced.

If the capacitor is not swollen then we need to check the capacitance.  Disconnect all the terminals from the top of the capacitor.  Let's take, for example, a 45/10/370 + or - 6% capacitor. What this means, is that the 45 microfarads is for the compressor start winding, the 10 microfarads is for the condenser fan motor.  The 6% means that as long as you are within 6% of the 45 and the 10 microfarads, the capacitor tests good. The 370 means this the maximum amount of voltage that can be applied to the capacitor before it shorts out. If you were to remove a 370 volt capacitor, and install a 480 volt capacitor, that would be okay. How ever if you replaced a 480 volt capacitor with a 370 volt capacitor it would almost instantly burn out. 

Set your multi meter to MFD, (microfarads).  Take your leads and check between the C (common) and S (start) terminal.  You should see within 6% of 45 microfarads.  If you do not, then the capacitor should be replaced.  Also check between the F (fan) and C (common) terminal.  You should be within 6 % of 10 microfarads.  If you are not, replace the capacitor. A weak capacitor will put too much stress on a motor winding, eventually burning that winding out.

Once you have verified the capacitor is good, check the windings on the compressor. Pull all the connections off the compressor, making sure the wires are labeled or marked down as to where they go.  Check for continuity between clean ground and each winding to verify we are not shorted to ground. The resistance between any winding and ground should exceed 1,000,000 ohms (1 mega ohm).

If you are not shorted to ground, then continuity between the motor windings needs to be checked.  Measure from Run terminal to Common terminal.  This will be the lowest ohm value of the three measurements, between 1.0 to 2 ohms.  Next, measure from Start terminal to Common terminal.  This reading is the middle ohm reading.  The ohms value should be between 2 and 3 ohms. The third reading should be the highest, Start to Run, approximately the sum of the first two readings.

Here are a few examples:


Run to Common - 1.5 ohms

Start to Common - 3 ohms

Start to Run - 4.5 ohms

This is a good compressor.

Example #2

Run to Common - infinite ohms

Start to Common - infinite ohms

Start to run - 4 to 5 ohms

Open compressor or internal overload protection.

Let the compressor cool down or cool it down with a bag of ice, garden hose. I have seen in the past were a compressor was considered condemned and the following day, was rechecked and ohmed out okay.  Make sure you give ample time for the compressor to cool down. Once the compressor is cooled down recheck the ohm readings. Replace the compressor if the overload never resets.

Example #3

Run to Common - 1 ohm

Start to Common - infinite    

Start to Run - infinite

This is an open compressor start winding.  The compressor would need to be replaced.