Parameter measurements to study surge propagation in induction machines

Winding failures in induction machines have been a major concern in the past several years, and more so recently with the addition of variable-speed drives (VSDs). Both the introduction of the vacuum breaker, and the use of pulsewidth-modulation (PWM) drives, utilizing fast switching insulated gate bipolar transistors, have resulted in an increase in winding failures in induction machines. Two mechanisms that cause winding failures are steep-fronted surges, like those caused during the opening and closing of vacuum breakers, and transient overvoltages caused by impedance mismatch between the cable and load during VSD operation. There has been a fair amount of work done to date on the propagation of vacuum-breaker-induced steep-fronted surges in the windings of the induction machine. More recently, work has been progressing on overvoltages at the machine terminals as a result of VSDs operating with long cables connecting the drive and machine. However, the propagation of these surges down the coils and the interference of these PWM surges with each other in the coils as well as the coupling between turns and coils have not been thoroughly investigated. Such an investigation would allow more benign PWM strategies to be developed, which do not build up in the machine to unacceptably high levels. The relevant parameters to be used are unknown and difficult to calculate and measure. To this end, this paper presents a measurement technique to determine the machine parameters for surge studies in machines. Detailed parameter measurements are made of each turn section (slot versus overhang), which are then used to determine the surge impedance of each section. Also included are the mutual capacitive and inductive couplings. Reflection and refraction coefficients are calculated which may then be used for further analysis of surges in machines.