Refrigeration Compressors Humming?
Quick! Open That Disconnect!

Refrigeration compressors should never hum! No, I'm not talking about a properly operating compressor humming merrily along!!!

I mean a compressor that is not running at all. When you apply power to the condensing unit, all the compressor does is hum!

What's going on??? What could be the problem???

That depends on the type of electricity being used.

Single phase refrigeration compressors and three phase compressors hum for many of the same reasons. But they are also very different.


1. Mechanical: If the compressor is seized up internally, it will hum when trying to start.

Reasons? Check for busted pistons, a broken crankshaft, lack of proper oil level, or a defective oil pump.

2. Electrical: Low voltage will cause the compressor to hum.

Reasons? Check the voltage from the power company. Maybe the incoming voltage for all the equipment is bad!!!

Check for a damaged disconnect, bad breaker, burnt wire, loose connections, bad contactor or starter, or wire gauge too small for the amperage.


1. Mechanical: There are no mechanical differences between a single phase compressor and a three phase. Only the motor is different.

2. Electrical: We will start with single phase compressors

Single phase compressors in commercial refrigeration condensing units all use start components.

The potential relay's job is the allow the start capacitor to temporarily remain in the circuit at start-up.

The start capacitor gives the compressor an extra kick to get going.

A damaged potential relay or start capacitor always results in a humming compressor.

Also check for burnt wires or bad connections on the start components.

A defective run capacitor will also keep the unit from starting.

The best rule of thumb... change all three. They are so dependent, a problem with one means possible damage to the others.

Three phase compressors in commercial and industrial refrigeration are simple.

If they hum and don't start, most likely you have a single-phase condition. One leg of power is missing.

Once again, check the disconnect, breaker, or starter. Also look for burnt wires.

One more thing. Have you ohmed the motor windings? Internal motor winding damage can result in the terrible hum!!!

Whew!!! An awful lot of info. Sorry about that!!!

Sometimes understanding refrigeration compressors is not so simple!!!

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