Corona is caused by the uneven electric field generated by uneven conductors. Around the uneven electric field and near the electrode with a small radius of curvature, when the voltage rises to a certain level, a discharge will occur due to free air, forming a corona.
From the conditions for corona generation, we can find that uneven electric fields, uneven conductors, and sufficiently high voltages are necessary conditions for the generation of corona. Therefore, corona will be generated at the ends of high-voltage motor windings, especially for rated voltages. For motors higher than 6kV, the corona of the stator winding will be more obvious, and the higher the voltage, the more serious the corona problem will be. Therefore, for high-voltage motor windings, anti-corona treatment measures are taken by using special electromagnetic wires and adding resistive tapes to the outside of the winding coils.
The variable frequency motor is powered by a frequency converter. The voltage output by the frequency converter is different from the sine wave of the industrial frequency power supply, but a square wave with steep rise and fall. This special pulse wave causes the input voltage of the motor to have periodic and high voltage. A sharp overvoltage that is twice the rated voltage, due to the extremely fast speed of this pulse overvoltage, will cause serious uneven distribution of the electric field in the motor windings. Although most variable frequency motors are low-voltage motors, the special power supply method is destined to have uneven electric fields in their windings.
From the analysis of the characteristics of the number of turns and length of the motor, the first and last turns of the low-voltage high-power motor winding bear almost all the voltage amplitude, and are also the most prone to problems in the motor winding. Moreover, from the analysis of the embedding process of the winding, The damage to the first turn coil is relatively greater, and therefore the risk is greater. This is why many motor manufacturers provide special protection for the first and last coils. For low-voltage high-power variable-frequency motors, due to uneven field strength and pulse spike voltage, the end of the motor winding has the basic conditions for corona generation. In order to prevent the occurrence of corona in the variable frequency motor, special anti-corona electromagnetic wires should be used in the windings of the variable frequency motor, and special protective measures should be taken for the first and last coils.
Post time: Jan-06-2025