Calculation of neutral voltage shift in LCI driven induction motors

Load commutated inverter (LCI) drives are being used for adjustable speed control of induction motors and synchronous motors. It has been found that the motors and transformers connected to these drives are subjected to higher than normal voltage stresses to ground and premature failures of motors have been reported. This has been shown to be due to a voltage difference between the source side neutral voltage and the motor neutral voltage. This voltage is known as the neutral shift. This neutral shift voltage is added to the normal line to neutral or phase voltage causing a much higher than normal line or terminal to ground voltage stress in the ungrounded side of any DC linked AC converter. Voltages as high as 3.3 pu normal phase voltage may be seen for certain types of inverters. A method of determining the neutral shift voltage is presented. Results of a computer simulation of an LCI circuit and test results are presented to verify this analysis