Unbalanced Phase Currents in 3~ Motor

In summary, someone said that prolonged running causes increase in resistance of stator winding, and this results in increased current. However, this explanation appears wrong at the very outset because increased resistance should actually decrease the current.
  • #1
QwertyXP
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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?
 
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  • #2
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)?
 
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  • #3
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.
 
  • #4
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?
 
  • #5
My personal feeling is that it was mis-wired, as wirenut suggested.

Of course only the electricians who investigated know for sure.
 
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  • #6
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.
 
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  • #7
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.
 
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Related to Unbalanced Phase Currents in 3~ Motor

1. What causes unbalanced phase currents in a 3~ motor?

Unbalanced phase currents in a 3~ motor are usually caused by unequal loads on the motor's three phases. This can be due to variations in the mechanical load on different parts of the motor, or differences in the electrical characteristics of the connected equipment.

2. How do unbalanced phase currents affect the performance of a 3~ motor?

Unbalanced phase currents can lead to decreased motor efficiency and increased wear and tear on the motor's components. They can also cause the motor to overheat and potentially fail, leading to costly repairs or replacements.

3. Can unbalanced phase currents be detected and measured?

Yes, unbalanced phase currents can be detected and measured using a clamp meter or other current sensing device. This allows for the identification and correction of any imbalances in the motor's phases.

4. How can unbalanced phase currents be corrected?

Unbalanced phase currents can be corrected by adjusting the motor's load distribution or by using a phase balancing device. In some cases, it may also be necessary to replace or repair damaged components within the motor.

5. What are the consequences of not addressing unbalanced phase currents in a 3~ motor?

If left unaddressed, unbalanced phase currents can lead to reduced motor efficiency, increased energy consumption, and potential motor failure. This can result in costly downtime and repairs, as well as negative impacts on the overall productivity of the system in which the motor is used.

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