Effect of the overhang capacitance on fast-rising surge distribution in winding of a large AC motor

A special aspect of the voltage distribution in the coils of electrical machines when exposed to a fast-rising surge, namely the influence of the winding parameters, especially the capacitance matrix for the slot and overhang parts of the winding, is considered. To model the capacitance, three methods have been applied. For voltage calculation, the multiconductor transmission line theory has been combined with the scatter matrix in accordance with previous authors. It is shown that, whichever method is to be used for the computation of the capacitance, care must be taken to evaluate the capacitance to the ground in overhang region. Experimental and computational results are presented. Although no account has been taken of the effect of the high frequency magnetic flux penetrating into the laminated core at high frequency, yet peak voltages across turns that might be of immediate importance to machine designers are well predicted. In a motor with a multi-turn coil winding, the effect of the high frequency magnetic flux penetrating into the laminated core may not be the key factor that determines the amplitude of the inter-turn voltage in the multi-turn winding when confronted by a fast-rising surge. The amplitude of the inter-turn voltage to a certain extent depends on the dielectric permittivity used when computing the capacitance to the ground in the overhang part, and the dielectric permittivity used when calculating the velocity of surge propagation in the winding