Different instantaneous undervoltages of residual voltage attenuation of three-phase asynchronous motor can be divided into three types: power failure, short-distance short-circuit and long-distance short-circuit

1. Power failure refers to the failure caused by the disconnection of motor group and power supply and distribution system. The main reason is misoperation.

For example, the power supply of the operating substation is disconnected by mistake. In case of power failure, the residual voltage of the motor decays slowly because the electromagnetic energy in the motor rotor is not consumed. The probability of power failure in instantaneous undervoltage is the lowest.

2. Short circuit fault in short distance refers to the short circuit fault that occurs near the electrical distance of the motor.

In the case of short-circuit fault in short distance, the electromagnetic energy in the rotor of the motor decays rapidly due to the short-circuit current provided to the short-circuit point, so the residual voltage decays rapidly. Short circuit fault in short distance has a high incidence in instantaneous undervoltage.

3. Long distance short circuit fault refers to the short circuit fault occurring at a distance from the motor.

In the case of long-distance short-circuit fault, the electromagnetic energy in the motor rotor also attenuates due to the short-circuit current provided to the short-circuit point, and the residual voltage attenuates faster, but slower than the short-circuit fault. The occurrence rate of long-distance short circuit fault is the highest in instantaneous undervoltage.

The residual voltage of three-phase asynchronous motor is not only related to the electrical distance of short-circuit fault, but also to the form of fault. If there is compensation capacitor in the power supply and distribution system, the decay speed of residual voltage will be reduced.