Some factors governing the severity of switching surges

The consequences that the ability of a vacuum switching device to interrupt high-frequency currents can have when it is used to interrupt the starting current of a motor are examined. Experimental tests on a system with a 12 Omega surge impedance cable and a 12 Omega surge impedance source showed that a steep-fronted surge with a risetime of 1 PU per 400 ns can be produced at the motor terminals when the switch closes. Measurements also showed that the rate of rise of the voltage is dependent on the surge impedance of the cable. For example, a similar test with a 50 Omega surge impedance cable and a matching source impedance produces a surge with a risetime of 1 PU per 100 ns. If the bus inductance remains unchanged, the rate of rise of the voltage is approximately proportional to the cable surge impedance as theory predicts. To avoid repeated high-frequency-current zeros, either one or the other end of the cable should have a matched termination. However, matching the source end is almost impossible as the number of lines connected to the incoming busbar is a variable. The motor end can be matched using a resistor in series with a surge capacitor of low reactance at high frequencies.<>