Unbalanced Phase Currents in 3~ Motor

[QwertyXP]

A 3-phase induction motor in our factory had unequal phase currents. One of the currents was nearly normal while the other two were about twice the usual value. Somebody said that prolonged running causes increase in resistance of stator winding, and this results in increased current. But this explanation appears wrong at the very outset, because increased resistance should actually decrease the current.

What might the reason be?

 

[wirenut]

Was this taken out of service and re-installed at any time?
Was it running ok and then a problem developed?
My first thought was that a winding was connected backwards, but that would only happen if the connections were removed and no markings existed on them.
The explanation you got should affect all 3 currents, not just 2.
You could also have 2 shorted windings, only a very sensitive meter will tell you that, or a very complex troubleshooting routine. Do all your windings have the same resistance (within 5-10%)?
Are any shorted to ground (motor housing)?

 

[jim hardy]

A 3-phase induction motor in our factory had unequal phase currents. One of the currents was nearly normal while the other two were about twice the usual value.

I'm guessing from your use of past tense that it burnt up ?

If so, wirennut is probably right.

 

[QwertyXP]

Our electrical workshop received a complaint that the motor's overload relay was tripping (motor didn't burn up though). Upon inspection, the electrician discovered that two of the phases had higher currents. I'm assuming that currents were normal when the motor was installed because electricians are required to measure them after installation. I didn't get the chance to make any measurements myself, but I was just wondering what the cause might be. I guess measuring the stator winding resistance should have given me the clue.

So a possible conclusion is that prolonged running actually decreased the resistance by weakening the insulation and thus shorting the windings. Increase in winding resistance can never increase the current. Does this look like a valid explanation?

 

[jim hardy]

My personal feeling is that it was mis-wired, as wirenut suggested.

Of course only the electricians who investigated know for sure.

 

[Okefenokee]

Many 3~ motors have multiple taps and you can actually change the operating voltage (and consequently the current) by connecting the appropriate leads together. I was giving a print with a tap diagram once and was asked to wire a motor for a certain voltage. They gave me the wrong print. Luckily I'm mechanically savvy and I know what a motor should sound like. I ran it for a second and realized that it was wrong. A little research later and I found the right print.

Does your building have one or more wild legs? Some services purposely have a higher voltage in one or more legs. It's a terrible idea but they do it for flourescent lighting. When you use the higher leg for flourescent lights they draw less current and then you can have more lights on one breaker circuit. If your building is fairly new you probably won't see this.

 

[Baluncore]

When running in delta configuration, misplaced tap link or a shorted turn in one winding will cause a higher current in the two phases that drive that winding.