The vast majority of motor damage is the damage of the motor coil winding, and the damage of the coil is mainly overheating damage, and the mechanical loss, iron loss and other constant loss generated by the motor during operation and the copper consumption proportional to the square of the load current become the heat source of the motor. The heat output of the motor increases exponentially with the increase of the motor current. If the temperature rise exceeds the limit allowable temperature of the insulating material, it will cause damage to the motor or reduce the service life of the motor.

The thermal relay can only protect the positive sequence current of the motor under limited environmental conditions (temperature), and has no protective effect on negative sequence current, zero sequence current, and high harmonics. In important occasions, when the thermal relay protection trip stops, the thermal relay can not indicate the cause of the trip and the current of the motor at the time of the trip, so it can not accurately analyze and judge the cause of the stop. The electronic motor protection relay overcomes the major defects of the thermal relay, it can not only protect the positive sequence current of the motor, but also effectively protect the negative sequence current, zero sequence current, etc. When the digital electronic relay trip stops, it can indicate the cause of the trip and the current of the motor at the time of the trip, which is an important basis for judging the cause of the stop.

The causes of overheating damage to the motor coil winding mainly come from the power supply overvoltage, negative sequence current (three-phase imbalance, lack of phase), undervoltage, high harmonics and the motor's positive sequence current (gridlock, overload), zero sequence current (single-phase grounding leakage) and so on.