What is a “phase unbalance fault” in an induction motor?

Definition of Unbalanced Phase Fault

In an induction motor, "phase unbalance fault" usually refers to the situation where the amplitude of three-phase currents (or voltages) is inconsistent, and the difference in amplitude exceeds the specified range. Ideally, the size of the three-phase voltage vectors should be equal and arranged in the order of A, B, C, with an angle of 2n/3 between each pair. However, due to various factors affecting actual operation, this balanced state may be broken, leading to phase unbalance.

The cause of unbalanced phase failure

Unbalanced Three-Phase Voltage: If the three-phase voltage is unbalanced, there will be reverse current and reverse magnetic field in the motor, generating a large reverse torque, causing an imbalance in the distribution of three-phase currents in the motor and increasing the current in one phase winding.

Overload: When operating under an overload condition, especially during startup, the current in the stator and rotor of the motor increases, causing heat generation. If this persists for a slightly longer period, it is very likely to result in an imbalance of winding currents.

Stator and Rotor Winding Faults of the Motor: When the stator winding has inter-turn short circuit, local grounding, or open circuit, it will cause excessive current in one phase or two roots of the rotor winding, resulting in severe imbalance of three-phase current.

Improper operation and maintenance by personnel: Operators fail to regularly inspect and maintain electrical equipment, leading to human-induced leakage or phase loss in electrical equipment.

The hazards of unbalanced phase faults

Increased Line Losses of Electric Energy: In a three-phase four-wire power supply network, when current flows through the line conductors, electric energy losses are inevitable due to the presence of impedance, and these losses are directly proportional to the square of the current flowing. When the low-voltage grid supplies power in a three-phase four-wire configuration, imbalances in three-phase loads are unavoidable due to the presence of single-phase loads. During unbalanced operation of three-phase loads, the neutral line carries current as well. This not only results in losses in the phase conductors but also causes losses in the neutral line, thereby increasing the line losses in the power grid.

Increased energy loss in distribution transformers: Distribution transformers are the main power supply equipment in the low-voltage grid. When operating under unbalanced three-phase load conditions, it will lead to an increase in distribution transformer losses.

Reduction in Distribution Transformer Capacity : When designing a distribution transformer, its winding structure is designed for balanced load operation conditions, with winding performance being generally consistent and equal rated capacities for each phase. The maximum allowable capacity of the distribution transformer is limited by the rated capacity of each phase. If the distribution transformer operates under an unbalanced three-phase load condition, the lighter loaded phase will have surplus capacity, resulting in a reduction in the transformer's output. The extent of this reduction is related to the degree of imbalance in the three-phase load. The greater the imbalance in the three-phase load, the more the capacity of the distribution transformer is reduced.

Zero-sequence current generated by distribution transformer: When the distribution transformer operates under unbalanced three-phase load conditions, it will generate zero-sequence current, which will vary with the degree of unbalance in the three-phase load. The greater the unbalance, the larger the zero-sequence current.

Conclusion

The "phase imbalance fault" in induction motors is a complex issue involving multiple factors. Understanding its causes and hazards is crucial for ensuring the normal operation of the motor and prolonging its service life. By implementing reasonable maintenance and appropriate protective measures, the occurrence of phase imbalance faults can be effectively reduced.