Corona discharge occurs when an uneven electric field is generated by a non-smooth conductor. Around the non-uniform electric field and near the electrode with a small radius of curvature, when the voltage rises to a certain level, due to the ionization of air, a discharge will occur, forming a corona.
From the conditions for corona generation, we can find that non-uniform electric fields, non-smooth conductors, and sufficiently high voltages are the necessary conditions for corona generation. Therefore, corona will occur at the end of the windings of high-voltage motors, especially for motors with a rated voltage higher than 6kV, the corona of the stator windings will be more obvious, and the higher the voltage, the more serious the corona problem will be. Therefore, for the windings of high-voltage motors, anti-corona treatment measures are taken by using special electromagnetic wires and wrapping resistance tapes on the outside of the winding coils. Regarding the corona problem of high-voltage motors, people’s understanding is relatively in place. Then, why does corona also occur in variable-frequency motors?
Variable frequency motors are powered by variable frequency drives. The voltage output by the variable frequency drive is not a sine wave like that of the power supply at the industrial frequency, but a square wave with steep rises and falls. This special pulse wave causes the input voltage of the motor to have periodic peak overvoltages that are twice the rated voltage. Due to the extremely fast speed of this pulse overvoltage, it will cause a serious non-uniformity in the electric field distribution of the motor windings. Although most variable frequency motors are low-voltage motors, the special power supply method determines the non-uniformity of the electric field in their windings.
Based on the characteristics of the number of windings and the length of each winding in the motor, the first and last windings of the low-voltage high-power motor winding bear almost all of the voltage amplitude, and they are also the most prone to problems in the motor winding. Moreover, from the analysis of the winding embedding process, the first winding coil is subjected to relatively greater damage, and thus it faces greater danger. This is why many motor manufacturers provide special protection for the first and last windings. For low-voltage high-power variable frequency motors, due to the uneven field strength and the sharp peak voltage of the pulse, the ends of the motor winding have the basic conditions for the generation of corona. To prevent the generation of corona in the variable frequency motor, the winding of the variable frequency motor should adopt special anti-corona electromagnetic wire, and special protective measures should be taken for the first and last windings.
Post time: Jul-31-2025