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2 % Diff In Voltage - 40 % Diiff In Current


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I have (seen) a 3 ph induction 380 V motor running with current difference of about 40 % caused by voltage difference of about 2 % across phases. This because taps on the supply transformer were set wrongly. I doubt the phenomena could be explained but does anyone else has encountered a similar case - insignificant diff in V supply leading to huge diff in A drawn ? The amps (if I remember good) were some 90/120/140 A.
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Hello Yuri


This is not uncommon and occurs primarily at light load.


If you consider the induction motor as being a resistive load and a generator, then you can see how this can happen.

The motor is driven by the supply with a rotating magnetic field in the stator resulting form the applied voltage.

The rotating magnetic field in the stator causes a magnetic field in the rotor which is spinning at the same speed as the stator field.

The rotor field causes a voltage to be generated in the stator in the same manner as a DC motor and this subtracts from the supply voltage. The generated voltage is not balanced in voltage and phase and subtracting this from the unbalanced supply voltage can exaggerate the difference or reduce the difference. Rotating the connections to the motor will alter the current balance with a particular connection minimising the imbalance.


The basic imbalance in the motor is due to the imbalance in the iron inductance and is primarily due to the grain orientation of the steel used for the stator laminations.


As the resistive component of current increases, the measured current imbalance will reduce, so the imbalance reduces with load.


Best regards,


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Hello Mark


I have "rotated" the leads to the motor as my first idea was unbelief that such small inbalance in V could cause such huge curr inbalance and I suspected the motor. But the inbalance just transferred - precisely the same readings now on the other leads. Other motors (less powerfull) fed from that transformer have a less conspicuous difference in currents between phases.


Best regards



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