How Voltage Imbalance Affects Three-Phase Motor Performance

What Motor Performances Deteriorate When the Power Supply Voltage is Unbalanced?

For single-phase motors, it's usually sufficient to consider the magnitude and relative stability of the power supply voltage. However, for three-phase motors, the balance of the power supply voltage must also be taken into account. Voltage imbalance can negatively affect various motor parameters to varying degrees, including current, torque, speed, temperature rise, vibration, and noise.

Changes in Current and Temperature Rise:
When the three-phase supply voltage is unbalanced, the most noticeable effect is on the motor's current. Especially under rated voltage and rated speed, the imbalance in current is much more significant than the voltage imbalance itself. Unbalanced voltage creates a reverse magnetic field in the air gap between the stator and rotor, opposite to the motor’s rotating magnetic field. This alters the current in the stator windings and the frequency of the rotor current—especially affecting the rotor, where losses increase significantly. The rise in current and associated additional losses directly lead to increased heat loss in the motor, which in turn results in a higher temperature, indicating a deterioration in the motor’s thermal performance.

Changes in Speed and Torque:
To better understand the relationship between speed and voltage, we can analyze the extreme case of a phase loss. When a motor loses a phase, it exhibits increased vibration and temperature, decreased speed, increased current, and abnormal noise during operation. It often cannot start properly and emits loud noise during startup. After phase loss, the motor operates weakly with reduced torque, and the stator windings may burn out quickly. When the power supply voltage is unbalanced, the symptoms are similar to those of phase loss, though typically less severe.

Changes in Vibration and Noise:
Motor vibration and noise can be categorized into mechanical and electromagnetic types. Under unbalanced voltage conditions, electromagnetic vibration and noise become especially prominent. The vibration and noise observed during phase loss in a three-phase motor represent the extreme case of voltage imbalance.