From the perspective of determining the responsibility, motor winding failures can be divided into two categories: manufacturing quality and usage problems. Winding turn-to-turn, phase-to-phase and ground faults can basically be attributed to manufacturing quality; however, insulation problems caused by water ingress, moisture and other factors are not easy to identify unless there is obvious trace evidence . Winding overheating, overload, phase loss and other problems are more often due to usage responsibility.

After the motor winding breaks down, the possibility of local repair is very small, and in most cases the winding needs to be reprocessed. Disassembly inspection shows that the burned part of the winding is dark black and has a burnt smell. When the fault is serious, there are also burnt copper beads and burnt insulation material powder. According to the characteristics of the winding burnout, the cause of the burnout can be basically determined:

(1) If only a few turns of wire in one coil are burned, it is an inter-turn short circuit. For inter-turn faults, we can observe from both ends of the winding. Generally, inter-turn faults have the characteristics of "flowing", that is, there will be burn marks of varying degrees at both ends of the winding, but the degree is not the same. Inter-turn faults are generally limited to one coil of one phase of the winding. Even if other phases also have inter-turn faults, they are also irregular. It is unlikely that two or three phases of the winding will have inter-turn faults, unless the winding is damaged during the production process.

(2) For three-phase windings, regular winding burnout occurs, which people usually call petal shape. If one phase burns out, it should be a phase loss in the triangle-connected winding; but if two phases burn out, it is a phase loss in the star-connected winding. Phase loss problems usually occur during use, and are mostly connection problems, but it is not ruled out that the phase loss is caused by the manufacturing quality of the motor itself.

If a phase loss occurs during motor operation, the motor speed will drop suddenly, and the phase current of the energized phase (the phase that has not been disconnected) will increase a lot, but the motor will not stagnate, but the rotation time will not be too long. If the motor phase loss occurs during shutdown, the motor cannot be restarted .

(3) All three-phase windings are burned out, and it looks like the entire winding is burned out and discolored . In less serious cases, the insulation of the end windings can be seen to be cracked. In serious cases, the windings and insulation are burned very black, and the end windings are loose. This type of problem may be caused by the motor running under low voltage , overload , or wiring errors , rotor jamming, and unsmooth starting .

Long-term motor overload is related to the matching of motor and load, that is, there is a problem of a small horse pulling a big cart; low-voltage heavy-load operation generally occurs when the power grid capacity is insufficient, or the lines are temporarily connected in the wild, which will lead to insufficient rated voltage of the motor; wiring problems generally occur in the case of triangle and star connection. For example, if the motor that should be connected in star is mistakenly connected in triangle, the motor winding will quickly burn out due to heat; the jamming during the operation of the motor is equivalent to blocking the motor rotor. At this time, the current will be particularly large, which will also cause the motor winding to burn out in a short time. Of course, the inherent temperature rise of the motor and the overheating of the winding during operation cannot be ignored.

(4) Phase-to-phase and ground faults. This type of fault only occurs locally and will not spread to a larger area. We have described this issue in detail in our original article.

In order to prevent fatal problems in the winding, the motor temperature should be monitored in an appropriate manner. If the motor is overheated, the relevant parameters should be tested, and the measured parameter values have certain characteristics: Detect the three-phase current of the motor. When a turn-to-turn short circuit occurs in one phase of a three-phase motor, the current of two phases is often larger than the normal value, and the current of the other phase is smaller, or even smaller than the normal value. When measuring the resistance of the three-phase winding, the one with a small resistance value may have been short-circuited. Measure the insulation resistance between the winding phases. The two phases with low resistance values may be short-circuited. Detect the insulation to the ground. The resistance of the broken winding phase is extremely small.

In addition to the above-mentioned turn-to-turn faults to the ground and between phases, if one phase of the three-phase winding is reversely connected, the starting torque will drop significantly, the rated speed cannot be reached with a slight load, the three-phase no-load current will increase significantly and be unbalanced, the noise and vibration will be abnormal, and the no-load will also generate serious heat. When the motor is running continuously at full load, the power supply voltage is too high or too low, the three-phase voltage is unbalanced, and the star connection is mistakenly connected to a triangle, which will cause a sharp increase in load current and overheating of the winding.