Acceptable current unbalance in three-phase motor 1

[lucaspenalva]

Hello there,

What are acceptable ranges of current unbalance in three phase motor?

Considering:
current unbalance = (max deviation from average current) / average currrent

I have this motor that powers a pump:
R=87 A
S=91 A
T=88 A
AVERAGE = 88,7 A
MAX DEVIATION FROM AVERAGE = 2,3 A
CURRENT UNBALANCE = 2,3 / 88,7 = 2,6%

Is this normal?

 

[FacEngrPE]

Motor current imbalance can be strongly influenced by the wiring between the source and the motor (assume the source is ideal and unpolluted sign-wave). Example intermittent resistance - like a bad contactor, with one of three contacts not making good connection. Supply and load voltage readings can look good, but current will be imbalanced.

Example from a milling machine. Traverse was motor would start - was a bit slow to get to running speed (motor is very lightly loaded most of the time in this application). Motor ran hot, and occasionally tripped the motor overloads.
Troubleshooting required maintenance electrician several return trips before bad contact was found.
Physics - The motor was acting like a 3 phase rotary converter. The bad phase was able to provide just enough rotation torque for the motor to start. Then the motor supported the sign-wave on the load side of the bad contactor, making the voltage difference across the bad contact small. Phase current imbalance was large. Easy to find once the electrician remembered to bring his clamp ampmeter.

I don't want to think about how this would be calculated.

Fred