the associated overvoltages when the vacuum switch
interrupts the inverter circuit with a load equivalent to
the inverter motor. As a result, the mean
chopping-current level is 3-4 times higher than that in
the normal AC circuit. In particular, high chopping
currents occur when the timing of the current
interruption by the vacuum switch synchronizes with
that of the inverter's switching, which we call 'double
chopping.' The reason that the high chopping current
shows up in the inverter circuit has not been clarified.
These high chopping-current levels due to double
chopping are supposed to be harmful since they cause
the associated high overvoltage to be generated across
the vacuum contacts. However, fortunately, simple
experiments confirmed that a typical surge suppressor
could reduce these overvoltages.