The slight imbalance in the motor current of the low-voltage distribution cabinet indicates that the motor currents in the high-voltage distribution cabinet are roughly the same, with only minor differences in the American-style transformers. In practical applications of distribution cabinets, some deviation in the three-phase current of the JP cabinet motor is normal; however, if the deviation is significant, the cause needs to be investigated. The national standard GB 8680.1-1998 stipulates that the three-phase motor current imbalance must not exceed 10% .
There are two possible reasons for the unbalanced motor current in a box-type transformer:
Are the three-phase voltages balanced?
In daily operation, if the motor current is unbalanced, we should first consider whether the power supply voltage is normal. For example, is the three-phase voltage deviation too large, or is there a missing phase?
If the power supply voltage is unbalanced, it will directly affect the current imbalance of the three-phase motor. When the motor is working normally, the three-phase voltage imbalance at the motor terminals must not exceed 5% , which 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.
Inter-turn short circuit
Another possibility is an inter-turn short circuit. Due to some reason (such as insulation breakdown somewhere in the coil), some coils in the middle of the winding are short-circuited. This will result in the short-circuited phase having fewer turns and the normal phase having more turns, leading to different resistances and an unbalanced current.
