What are the causes of motor current imbalance?

- Oct 16, 2019-

The motor current is a bit unbalanced, indicating that the motor current and current are similar, but the difference is not large. In practical applications, it is normal for the three-phase current of the motor to deviate; however, if the three-phase current deviation is large, the cause should be further identified. The national standard document GB 8680.1-1998 stipulates that the three-phase motor current imbalance shall not exceed 10%.


From the point of view, the motor current is not balanced, there may be two reasons:


Is the three-phase voltage balanced?


In the daily work process, the motor current is not balanced, we should first consider whether the power supply voltage is normal. For example, whether the three-phase voltage deviation is too large or not.


If the power supply voltage is unbalanced, it will directly affect the three-phase motor current imbalance. When the motor is working normally, the three-phase voltage unbalance of the motor end must not exceed 5%, that is, 380*5%=19 volts.


Note: When measuring whether the three-phase voltage is balanced, the motor must be disconnected first to avoid affecting the measurement results.


Short circuit between turns


Another possibility is that the turn-to-turn is short-circuited. For some reason (such as the insulation is broken down somewhere in the coil), some coils in the middle of the winding are short-circuited. This will become a short-circuited one-phase coil with a small number of turns, a normal one-phase number of turns, a large resistance, and an unbalanced current.


To determine if it is a turn-to-turn short circuit, you can use a bridge to measure the DC resistance of each winding. The bridge is as shown:


image