Abstract- When small currents are interrupted by vacuum
switch, it is well known that chopping current and the
associated overvoltage are observed. Various
investigations on the current-chopping phenomena in AC
circuits have been reported, but there are no data
available on inverter circuits in spite of their recent wide
application to synchronous motor systems. In this paper,
chopping-current levels and the overvoltages in an inverter
circuit under a simple three level pulse width modulated
(PWM) control are investigated. As a result, the mean
chopping-current level is 3-4 times higher than that in an
AC circuit. In particular, high chopping currents occur
only when the timing of the current interruption by the
vacuum switch synchronizes with that of the inverter's
switching, which we call 'double chopping'. High chopping
current due to double chopping might be harmful since the
high overvoltage proportional to the chopping-current
level could damage the insulation of electric equipment.
However, we experimentally confirmed that a typical
surge suppressor could reduce overvoltage.