Corona, because the non-smooth conductor produces an uneven electric field, around the uneven electric field, near the electrode with a small radius of curvature, when the voltage rises to a certain degree, due to air ionization will occur discharge, the formation of corona.
From the conditions of corona generation, we can find that non-uniform electric field, non-smooth conductor, and high enough voltage are necessary conditions for corona generation, so there will be corona generation at the end of the high-voltage motor winding, especially for the motor with a rated voltage 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 will be carried out by using special electromagnetic wires and adding resistance belts outside the winding coil. For the corona problem of 3 phase induction motors, everyone’s understanding is relatively in place, so why do variable frequency motors also have corona generation?
Variable frequency motor through the inverter power supply, the inverter output voltage is different from the sine wave of the power frequency power supply, but the steep rise and steep drop of the square wave, this special pulse wave leads to the input voltage of the motor will have a periodic, higher than the rated voltage 2 times the sharp overvoltage, due to the pulse overvoltage corresponding to the speed is extremely fast, will cause the motor winding electric field distribution is seriously uneven. Although most variable frequency motors are low-voltage motors, the special power supply mode is doomed to the uneven characteristics of its winding electric field.
From the analysis of the characteristics of the number and length of turns of three phase motor, the first and last loops of the winding of the low-voltage high-power motor bear almost all the voltage amplitude, which is also the most prone to problems in the motor winding. Moreover, from the analysis of the winding’s laying process, the damage to the first turn coil is relatively greater, so the risk is greater. This is also the reason why many motor manufacturers have special protection for the first and last coils. For low-voltage and high-power variable frequency motor, because of the 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 of variable frequency motor, the winding of variable frequency motor should adopt special electromagnetic wire to prevent corona, and special protective measures should be taken for the first and last coils.
Post time: Sep-13-2024
