Friday, June 19, 2015

Grounded Compressor Behavior

Typically, a grounded compressor does not really do anything – that is the compressor does nothing. The breaker, or fuse does plenty. Normally the breaker trips and/or the fuse blows as soon as the compressor is energized. In the case of many units with single pole contactors, the breaker may trip and/or the fuse blow as soon as power is turned on to the unit – even if the contactor is not closed. This is because power from one leg feeds the compressor and condenser fan motor all the time. If that leg of power is grounded, the breaker will normally trip immediately.

Occasionally a grounded compressor can cause the condenser fan motor to run slowly all the time. The fan gets one leg of power all the time. If the leg of the compressor that is fed by the normally open pole of the contactor is grounded, that provides a path for the current, causing the fan motor to receive close to 120 volts. With these units, the breaker does not trip until the contactor closes. I have even seen compressors which had a high resistance ground operate, even though they were grounded. If the ground is hundreds, or thousands of ohms, the current passing through to ground will not be enough to blow fuses or trip breakers. This type of ground is most often caused by system contamination creating a path inside the compressor shell.

The other day I experienced a type of grounded compressor which I had not seen before. When I arrived, the compressor would try to start for several minutes, and then the breaker would trip. The fan would run and the compressor would hum. Immediately I thought “capacitor.” But the capacitor checked out. So did the wiring. I then decided to check the amp draw to the compressor. On L1 the amp draw was 0, yet I could hear the compressor humming. I looked at my meter to make sure I had it set correctly and that it was not on hold. Then I decided to check the amps on L2, and read 65 amps. This is a single phase motor. Where are the amps going? Finally, I checked the amp draw on the bonding ground wire and read 24 amps. Current was travelling from L2 to ground – indicating a grounded compressor. The fact that not all of the current was travelling through the bonding ground concerned me, so I turned off the breaker. I then verified that the compressor was grounded by ohming it out. I was there for a second opinion, and I verified that the compressor was dead. I also advised the owner against trying to operate the unit any more, explaining that current traveling through ground is dangerous.

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