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Bob Engelhardt

On 11/4/2019 10:32 AM, wrote:
"Otherwise" would be non-resistive loads - which would be reactive (capacitance) loads, or inductive (Motors and such) loads. ...

Right ... I should have been more specific: what DEVICES would be
reactive? Motors, but motors really don't like being run on variacs.
Even universal motors are terrible at less than rated voltage. Assuming
theatrical stage use makes it more mysterious as to what devices might
be "otherwise".

A Variac "sees" a resistive load as a constant, whereas reactive and inductive loads will vary, so the amount of current the variac "sees" may be greater than the constant load at onset, shutdown, or when the load varies. Hence the derating.

But even resistive loads, especially theatrical lights, have in-rush
much higher that steady state. And a variac has a huge thermal sink to
handle transients (this variac weighs 20lbs).

Maybe it's the high voltage kick on inductive turn-off that's the
problem. The kick would be proportional to current, wouldn't it (more
energy to dissipate)? Do motors show inductive kick on turn-off?

Another thought - Ward-Leonard's main business was motor controllers,
So the theatrical use assumption may be wrong.