When the power supply voltage is unbalanced, which performances of the motor will deteriorate?

For single-phase motors, only the magnitude and relative stability of the power supply voltage need to be considered. However, for three-phase motors, the balance of the power supply voltage must also be taken into account. The imbalance of the power supply will cause negative impacts of varying degrees on parameters such as the current, torque, speed, temperature rise, vibration and noise of the motor.

1.The changes in current and temperature rise

When the three-phase voltage of the power supply is unbalanced, the most obvious impact is on the change of current.  Especially when the motor operates at the rated voltage and rated speed, the unevenness of the current is very large, and the unevenness of the current is greater than that of the voltage.  Unbalanced voltage will generate a magnetic field in the air gap between the stator and rotor of the motor, which is opposite to the direction of the motor's rotating magnetic field.  This will change the current size in the stator winding of the motor and the current frequency in the rotor part, especially having a significant impact on the rotor part, and the rotor loss will also increase significantly as a result.

Due to the increase in current and the resulting increase in losses, the thermal loss of the motor directly increases, which is intuitively manifested as a rise in motor temperature, that is, the temperature rise performance of the motor deteriorates.

To better understand the relationship between rotational speed and voltage, we can analyze it based on this extreme state of motor phase loss. When a motor is missing a phase, its vibration increases during operation, the temperature rises, the speed drops, the current increases, and it makes abnormal sounds. When starting, there will be a lot of noise and it cannot start successfully. When a motor is missing a phase, it operates weakly, that is, the torque decreases, and the stator winding of the motor will burn out very quickly. When the power supply voltage is unbalanced, the symptoms it presents are the same as those in a phase loss state, but the severity is slightly milder.

2.Changes in motor vibration and noise

The vibration and noise of motors can be classified into mechanical and electromagnetic types. When the power supply voltage is unbalanced, electromagnetic vibration and noise will be particularly obvious.The vibration and noise manifestation of a three-phase motor with a phase loss is the limit state of voltage imbalance.