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Tech Tip: Compressor Common – Run – Start

Have you ever been in a situation where the compressor you were working on was not properly labeled and you were unsure of which terminal was “Common”, “Run”, or “Start”?  There is a way to determine what leads are by measuring ohms.

Safety First!  Remove all power to unit and properly lock out.  It is recommended not to work directly in front of the compressor terminals.  There is a possibility that the terminals pins have been damaged to the point where the seal that holds them in place can break and release the refrigerant.

Discharge all capacitors and remove plug or wires from the contactor.  Inspect the terminal posts to ensure that they are not burnt or corroded.  This could cause inaccurate readings.  Measure and record the resistance between terminals.  Clean as needed and measure resistance.

The resistance (ohms) between Run and Common will be the lowest reading.  The resistance (ohms) between Start and Common will be the middle reading.  The resistance (ohms) between Start and Run will be the highest reading.

Most compressors have an internal overload that will open if the compressor gets to hot.  The overload is wired in the common leg and will prevent the compressor from running if tripped.  When tripped, you will read infinity between start to common and run to common, but you will read resistance between run and start.  You will have to let the compressor cool and the overload to should reset before further testing.  Compressors can take hours to cool down and the overload to reset.

Formula: CS + CR = SR
The run winding has the least resistance and carry the most current = CR
The start winding have the medium resistance and carry least current = CS
The total of the run and start windings is the high windings = SR

S—Medium ohms—C—Small ohms—R
S—————High ohms————–R




CS = 11
CR = 4
RS = 15
S—11 ohms—C—4 ohms—R
S————15 ohms————R
This same formula can be used on motors when the data tag cannot be read.