# Voltage imbalance problem



## electricguy483 (May 21, 2009)

I recently had an experience with one of my industrial clients that demonstrated the value of taking power quality measurements before and after new equipment installation. The client called me in because a large three-phase motor was failing a couple of times a year. I connected a Fluke 434 power quality analyzer to the circuit supplying energy to the motor and confirmed that the connections were proper and power type was three-phase Delta. I looked at waveforms and numeric values for all three phases and noticed that excessive loading on one voltage phase caused a phase current value that exceeded the motor’s full load amperage rating. After doing some additional investigation, I discovered that all of the internal single-phase loads on a set of equipment installed a few years ago were connected to the same phase. Balancing the internal single-phase loads between the three phases reduced the overall voltage imbalance which in turn reduced the current imbalance at the motor.


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## micromind (Aug 11, 2007)

Usually, larger motors (over 10 HP) are delta wound. Smaller ones are wye. 

A delta connection (motor, transformer, etc.) will try to balance voltage at the expense of current. This is especially bad if there's a large single phase transformer somewhere in the system. Then you have two phases low, and one phase high. The motor will draw more current from the high voltage phase in an attempt to balance voltage. Imbalanced current on a motor is always bad. 

I've seen this scenario a few times, sometimes simply changing the phases the single phase load is connected to will help, sometimes the single phase load needs to be changed to 3 phase. That gets expensive. Not only must the transformer be changed, but the panel it feeds as well. 

Sometimes I seriously question the value of electrical engineers that design systems such as the above. Other times I know they're worthless! Occasionally, they do a fine job. 

Rob


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