On 2019/11/04 8:46 a.m., Bob Engelhardt wrote:
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?Â* Motors, but motors really don't like being run on variacs.
Even universal motors are terrible at less than rated voltage.Â* 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.Â* And a variac has a huge thermal sink to
handle transients (this variac weighs 20lbs).

...

When I was one of the geeks working the theatre lighting at our high
school in Toronto (Northern Secondary School - mid to late 60s) we used
the large lever variac style dimmers to bring the lights up. The school
had (as I recall) 2500 students and a huge auditorium with a great many
row and spot lights for productions.

So the idea was you were supposed to bring the lights up, not in a
switched on rush, but as fast as you would raise the lever - a second or
two - which allowed for the filament to heat up a bit slower and saved
replacement bulbs. There may even have been a minimum setting to act as
a 'keep-alive' to preheat the filaments of the big spots - I just don't
recall...

John :-#)#