Inter-turnshort circuits in windings: Specific manifestations of current imbalance in three-phase motors

Motor windings are mostly wound from multiple turns of wire; a turn-to-turn short circuit occurs when the insulation between adjacent turns within the same winding breaks, causing them to become electrically connected. A short circuit results in abnormal current flow through the windings and localised overheating, leading to the motor becoming hot, emitting unusual noises and experiencing a drop in output; in severe cases, it can burn out the windings. An inter-turn short circuit in a motor’s windings is a typical fault that causes three-phase current imbalance originating from the motor itself. The resulting changes in current exhibit a highly regular pattern, which is distinctly different from the imbalances caused by power supply or line faults, making it extremely easy to identify on site. The most characteristic manifestation of this fault is that the current in the short-circuited phase is abnormally high, whilst the currents in the other two phases are slightly lower and of roughly equal magnitude; the three-phase current imbalance far exceeds the standard limit of 10%.

In the event of an inter-turn short circuit, the effective number of turns in the affected phase winding is reduced, resulting in a significant decrease in the winding’s resistance and reactance; provided the three-phase supply voltage remains balanced, the current in the faulty phase rises sharply. The greater the number of short-circuited turns in the faulty phase winding, the more pronounced the increase in current and the more severe the fault; a minor inter-turn short-circuit is equivalent to overloading, whilst a severe one can cause instantaneous localised overheating and a fire. In a motor with inter-turn damage, the windings of two phases remain intact, whilst one phase is completely carbonised and blackened, exhibiting the typical current characteristic of ‘one phase extremely high, two phases balanced but relatively low’. Accompanied by current imbalance, the motor will exhibit marked abnormal operating conditions: due to severe asymmetry in the three-phase magnetic fields, motor torque ripple intensifies, whilst operational vibration and noise increase significantly. At the same time, the winding of the faulty phase will heat up rapidly, causing a sudden rise in temperature, and will become too hot to touch within a short period. If operation continues, overheating will accelerate insulation ageing and breakdown, gradually progressing to inter-phase short circuits, earth faults, and ultimately the burning out of the motor windings. Furthermore, whilst a slight turn-to-turn short circuit may not cause a noticeable imbalance in no-load current, once the motor is operated under load, the three-phase current imbalance is rapidly amplified, making it highly likely to trigger the motor’s overload protection and cause a trip.