# 3 phase compressor



## Chris Kennedy (Nov 19, 2007)

If its a new unit it will have a phase lose/rotation monitor so you would be correct.


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## Bbsound (Dec 16, 2011)

If it is not a newer unit with phase lose/rotation monitor it would burn up, so you may not be correct.


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## etb (Sep 8, 2010)

If your lucky, the OCP will trip. If less lucky, the internal overload will cycle and it might be caught in time. If least lucky, it will fry and contaminate your refrigerant.


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## Hotlegs (Oct 9, 2011)

He said its an r12 unit so I'd say it's 30 years old. Does the motor try to start with only the 2 legs of power and burn itself up? Guess I need to get out some electrical theory books of mine and knock the dust off of them .


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## McClary’s Electrical (Feb 21, 2009)

Hotlegs said:


> He said its an r12 unit so I'd say it's 30 years old. Does the motor try to start with only the 2 legs of power and burn itself up?


Yes. Its called single phasing and will definately burn up motors.


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## brian john (Mar 11, 2007)

If you lose one phase of a 3 phase system and try to start the motor it will burn up

If you lose one phase of a 3 phase system and the motor is running it is possible that motor will continue to run,

If you have one phase of a 3 phase system, nothing will happen.


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## nolabama (Oct 3, 2007)

If you lose one leg the motor turns into a very loud buzzer machine indicating to you that it's single phasing. As stated it ain't good for em.


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## JRaef (Mar 23, 2009)

Assuming nothing that protects against phase loss:

If the motor is NOT running and you try to start it with only 2 phases, it either will not start at all, or it will start, but run in an unpredictable direction because there is no relative rotation. That can be VERY bad for a refrigeration compressor. If it does happen to start, see below. Usually though, there is insufficient initial torque to start it turning and you are in a "Locked Rotor" condition, which trips the CB or blows a fuse.

If the motor IS running and a phase is lost, the motor might CONTINUE to run, but it has severely limited torque. What usually happens is that the motor will not be able to maintain full speed, the slip increases, and the current increases, which EVENTUALLY may trip the overload protection. But it may not, it depends on the load on the compressor at the time the phase is lost. If not fully loaded, the compressor may continue to run because the current that it is pulling is still BELOW the trip threshold of the OL device. If that happens, there is another danger.

When you single phase a 3 phase motor, you create a SEVERE current imbalance. Unbalanced current in a 3 phase winding creates what is called "Negative Sequence Current", which in turn creates Negative Torque in the rotor. So in essence, the motor begins "fighting itself" and for every amp consumed, the heating effect is GREATER than it would normally be. So even though the OL protection device is not detecting that the current is higher than it should be, the EFFECT of that current on the heating of the motor is much higher. So what can happen is that the motor burns up WITHOUT ever tripping the OL relay (or breaker, or fuse, or whatever).


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