unbalanced 3phase

[angryman]

Is it possible for an unbalanced 3 phase power supply to damage a properly rated starter/overload? What exactly is the problem that may be caused by an unbalanced power supply?

I have a problem with the supply voltage not only being more than 10% out of the 230 +/- 10% rule but with varying voltages with a range of approx. 30v ph-ph. Apparently an overload is failing on one leg......

any information would help....

THanks,
Mark

 

[dpc]

Unbalanced voltages should not be a problem for the starter, but they are certainly a problem for any three-phase motor.

A 10% voltage imbalance is severe and your motor will draw very imbalanced current. It's possible the unequal current is high enough on one leg to damage the starter/overload, but that seems unlikely. Generally, the motor would fail before the starter.

But the "+/- 10% rule" you mention relates to the voltage level, not the imbalance. If your voltage is 10% low uniformly on all three phases, that will also cause high current flow to the motor.

To determine voltage imbalance, take the average of the three voltages and compare with the phase voltage that is furthest from the average (either above or below). This percentage is the voltage imbalance. It should not exceed about 3%.

 

[jghrist]

Is the overload protection being damaged or is it tripping on overload? As dpc said, the 10% voltage imbalance will cause a very imbalanced current which could very well trip on overload.

 

[angryman]

the overload is supposedly being damaged (automation direct ms25-1600).

They are using a phase adder to a single phase supply then feeding the contactor/overload - motor circuit with that. It appears that they've measured various voltages on each leg (ex: 256,240,273)

 

[angryman]

sorry, that was leg to leg...L1-L2, L1-L3 etc..

 

[dpc]

I don't see how that level of imbalance is going to damage the overload relay, unless it is undersized or improperly applied. I'd be much more concerned about the motor. The overload relay is supposed to protect the motor, so it needs to at least be able to protect itself. Is it possible the overload is operating beyond its voltage rating? What type of damage is occuring?

With those three voltage values, your imbalance is about 6.5% if my math is correct. That's bad. The current imbalance may be 30 to 40%. This can be checked with a clamp-on ammeter. If this is a motor you care about, and it normally operates close to its rating, I'd be concerned.

 

[thewellguy]

I agree with dpc. Check the current imbalance. Then check the amps at the top of the starter Next, move the motor leads one spot to the right (1-2 2-3 3-1) turn the motor back on and check the amps in the same spot. If the amp draws move with the motor leads then the problem is the motor. if no change the problem is the power supply.

 

[jraef]

If the OL relay itself is being damaged, check the type of wire being used to connect the motor, If they used extra flexible hook-up wire, it can overheat faster than standard stranded wire under overload conditions. I have seen it burn off the terminals at the OL relay connection.

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