How to Check a Single Phase Compressor

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SAFETY IS ALWAYS OF THE UTMOST CONCERN. BE MINDFUL THAT YOU WILL BE EXPOSED TO HIGH VOLTAGE AND HIGH AMPERAGE WHILE TROUBLESHOOTING COMPRESSOR CIRCUITS.

There are several reasons as to why a compressor fails to operate

The most common are as follows:

  • Contactor not pulling in/ Contactor Failure
  • Open winding(s)/ Open internal Overload
  • Shorted winding(s)
  • Locked rotor

Contactor not pulling in/ Contactor Failure:

Check for 24vac across Y and C at the contactor coil. If 24vac not present there is an open or short circuit in the 24vac or 120vac wiring. Further investigation is required. If 24vac is present at Y and C and the contactor will not pull in replacement is necessary.

Open windings/Open internal overload

Verifying the integrity of the windings is a simple task and requires a DMM (digital multi meter). Remove disconnect to break line voltage to the condensing unit (verify with meter). Take photo of compressor wiring for future reference. Remove 3 wires (Common, Start, Run). Set meter to lowest ohm value (Ω). Record resistance values for the following C-S, C-R, S-R. On an operational compressor you may see readings as follows C-S 3Ω, C-R 1Ω, S-R 4Ω.. Common- Start will always have a higher resistance value than Common - Run as the start windings are heavier to deal with the in rush current required to start the compressor. Start-Run will always be the added value of C-S and C-R.

If any ONE of these windings are open (OL) the compressor needs to be replaced. If you find C-S and C-R are both open the compressor is off on internal overload. The overload is an internal safety device and is in series with Common This condition generally happens when the compressor is exposed to excessive heat/amp draw. Once the compressor cools the overload

should close, if not, the compressor needs to be replaced. If this is the case the technician still needs to find the reason(s) why it opened. Some common reasons are failed outdoor fan, plugged condenser, under-overcharge, locked rotor…etc…

Shorted windings:

This condition happens when the compressor motor insulation has failed and the winding physically is in contact with the shell of the compressor or another winding.. Generally the circuit breaker for the condensing unit will be tripped and cannot be reset. To verify you will need to scratch the paint off the side of the compressor to bare metal. Set your DMM to the highest ohm value (Ω). Place one meter lead on the bare metal and the other across C, S and R. If there is a short there will be very little resistance to ground, typically 1-2 ohms. If the compressor is shorted it will need to be replaced.

Locked rotor:

This condition exists when the rotor of the motor is physically locked in the stator. When the compressor tries to start it will hum , draw LRA (locked rotor amps) and either trip the circuit breaker or open on internal overload. You can verify with a clamp type ammeter . Exercise caution as the amp draw can be temporarily quite high. The locked rotor rating is on the compressor data plate. If the compressor is locked it will need to be replaced.

Conclusion:

All of the aforementioned conditions require investigation as to why there was a failure in the first place. To simply replace the compressor without doing so would be a disservice to the customer.

ABR Wholesalers, Inc.

Tools used in this post:  Multimeter and Amp Meter

ABR Wholesalres, Inc. is a wholesale distributor of HVAC and Hydronics serving contractors and businesses for over 50 years.