Practical calculation of switching surges at motor terminals

Switching of motors results in steep-fronted surges which cause a relatively large turn-to-turn stress in windings. Simplified and detailed methods are presented for calculating the surge level and risetime at the motor terminals for particular configurations. The levels are shown to be strongly affected by shield or conduit grounding practices and are less dependent on motor size or the number of other loads supplied by the metal-clad bus. The risetimes depend mainly on the motor cable impedance, the shield grounding practices, and an equivalent motor capacitance. Less sensitive parameters include metal-clad bus length, the breaker position, and cable losses associated with the dielectric, skin effect, and semiconductive layers. Comparisons are made with surges monitored during normal switching