Three Major Electrical Motor Winding Faults

Motor winding faults are the primary cause of equipment shutdowns, mainly including inter-turn short circuit, inter-phase short circuit, and ground insulation fault. Many operation and maintenance personnel cannot distinguish the differences between the three types of faults, resulting in time-consuming troubleshooting, misjudgment and delayed maintenance. In fact, without precision instruments, the types of winding electrical faults can be quickly judged only through external abnormalities such as motor current, vibration, abnormal noise and heat generation, which greatly improves the maintenance efficiency and accurately locates the root causes.

1. Inter-turn Short Circuit: Slight Current Imbalance and Local High Temperature

An inter-turn short circuit occurs when the insulation of internal wires in the phase winding is damaged and the coil wires are connected to each other, which is a minor and concealed fault. It has typical abnormal characteristics: when a small number of turns are short-circuited, there is a slight difference in the three-phase current without sharp fluctuations, and the overall current imbalance is mild; the motor operates with a dull slight abnormal noise and slightly increased vibration. The most obvious feature is rapid local heating of the winding and uneven temperature distribution of the motor body, with the temperature at the fault point much higher than other positions. The equipment generally will not trip immediately, but long-term operation will expand the short-circuit range and eventually burn out the winding, evolving into a more serious motor winding faults.

2. Inter-phase Short Circuit: Severe Current Imbalance and Severe Fault Response

An inter-phase short circuit is formed by the insulation breakdown and direct connection between two or three-phase windings. It is a sudden severe fault and also the most harmful type of motor winding fault. Once the fault occurs, the current of the two short-circuited phases rises sharply and far exceeds the rated current, while the current of the third phase is disordered, resulting in an extremely severe imbalance of the three-phase current. Meanwhile, the motor suffers from violent vibration, harsh and dull noise, and rapid overall temperature rise. The load capacity is greatly reduced and the rotating speed is unstable, which will most likely trigger an over-current protection trip. If the motor is not shut down in time, the winding will be burned out in a short time.

3. Ground Insulation Fault: Concealed Current Abnormality Without Obvious Overall Abnormality

A ground fault occurs when the damaged winding insulation contacts grounded parts such as the iron core and motor frame, which is the most easily overlooked fault. In the case of slight grounding, the three-phase load current is basically balanced without obvious fluctuation, temperature rise or abnormal noise, and the only feature is continuous ground leakage current. When a direct grounding fault occurs, the current of the faulty phase rises slightly, the three-phase current is slightly unbalanced, and the zero-sequence current increases significantly, causing frequent early warning trips of leakage and zero-sequence protection. This fault is highly concealed with no obvious external abnormalities. However, the continuous leakage current will burn the insulation continuously, and eventually induce inter-phase or inter-turn short circuit faults.

4. Quick Fault Judgment Summary

(1) Local heating and slight current imbalance – Most likely inter-turn short circuit; (2) Severe current difference, violent vibration and instant high-temperature trip – Basically inter-phase short circuit; (3) Normal external operation with leakage warning and continuous leakage current – Winding ground insulation fault. In conclusion, accurate identification and advance troubleshooting of various hidden dangers can effectively avoid equipment shutdown, winding burnout and other major equipment problems caused by motor winding faults.