This produces a reflected voltage at the end of the cable approximately equal in magnitude, and with the same sign, but twice the magnitude of the incident voltage at the motor terminals. The over-voltage at the motor terminals causes winding dielectric insulation stresses that are most pronounced at the turn insulation adjacent to the coil terminals. Thus, in the coil-insertion winding process, the first and last turn within a coil have a chance to cross over or lie adjacent to each other in the stator slot. This is the point at which the insulation is stressed the most. 1000V can exceed the dielectric withstand capability of standard motors.
Failures are inevitable.
Due to the dominating winding inductance, the characteristic impedance of motors smaller than 25hp can be ten to one hundred times that of the cable connecting the drive to the motor. Therefore, the incident wave voltage will be reflected back towards the inverter, and the voltage amplitude at the terminals of the motor will approximately double. Significant over-voltages at the terminals of the motor can be prevented, however, if the cable is terminated with the cable surge impedance so that the incident voltage will not be reflected. In many applications, such as submersible pumps, the motor terminals may not be accessible. Increasing the rise time of the PWM inverter output voltage applied to the cable above a critical value will significantly reduce over- voltages due to reflections.