Hi,

I`m actually commisionning a new motor and the imbalance at no load is around 25 % ( 10 A,14 A,10 A)which looks very high. The voltage is normal.Under the loading condition, we are observing a very high slip condition.How much imbalance is normally tolerated ?

Bob

Hi Bob

This level of imbalance is not too unusual with an unloaded motor. You will find that the imbalance will reduce as load is applied to the motor.
The imbalance occurs because there is always an imbalance in the supply voltage and/or phase angle, and there is also an imbalance in the magnetic circuit of the motor.
At no load, you can consider the motor as acting as a generator connected to the supply where the current drawn is the difference between the supply voltage and the voltage generated. This amplifies the percentage variation and increases the imbalance.

For the supply to be perfectly balanced, you would need to get identical voltage readings from each phase to neutral, and also between phases. There is always a difference, although some times smaller than others.
The motor is wound on to a core that is made up from a stack of laminations. The steel used in the laminations has a magnetic grain and so there is a bias in the inductance of the core unless the steel has been rotated as it has been punched to reduce the overal grain orientation.

Rolling the phases around on the motor terminals will usually show one connection that has an exaggerated imbalance and one connection that has a reduced imbalance. As the load is applied, the "load" current will be better balanced and so the imbalance will reduce with increasing load.
The important issue is what the imbalance is at full load. The motor can tolerate a large imbalance at no load, but is very intolerant of imbalance at full load.

Best regards,
Mark.

dear Mark,

your explanation for unbalance in no load currents is very illuminating and true. my observation is that the said un balance is more in case of two pole motors than in case motors with higher no. of poles.Can it be explained why so?

with regards
subrao

Hello Subrao

Yes, I have observed the same but do not have a good explanation.
I suspect that the physical positioning of the windings possibly causes the effect of the grain orientation in the steel to have more affect on a two pole motor than a four pole motor.

Best regards,
Mark.