Measurement of 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 pulse width modulation (PWM) drives, utilizing fast switching IGBTs, 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. Work has been progressing on overvoltages at the machine terminals as a result of VSDs operating with long cables connecting 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. To this end, this paper presents a measurement technique which can be used to study PWM generated surges and their propagation within the coil, considering each turn at a time. This requires the inclusion of the differences in the surge impedances in each section of the coil. Detailed parameter measurements are made of each turn section (slot vs. overhang), which are then used to determine the surge impedance of each section. Reflection and refraction coefficients are calculated, then used to map, via lattice diagrams, the propagation of surges within the turns of one coil.