Variac capacity

"Otherwise" would be non-resistive loads - which would be reactive (capacit ance) loads, or inductive (Motors and such) loads.

From another website, put simply:

When electricity flows through a circuit, there are points on the circuit, called loads, where energy is drawn away. Loads, in essence, are objects th at use electricity--such as light bulbs. There are a variety of classificat ion systems, but one way you can divide loads is into resistive, capacative , inductive or a combination of these types.

So:

A Variac "sees" a resistive load as a constant, whereas reactive and induct ive loads will vary, so the amount of current the variac "sees" may be grea ter than the constant load at onset, shutdown, or when the load varies. Hen ce the derating.

Peter Wieck Melrose Park, PA

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".

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.

cs.

suming

nk to

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 :-#)#

Keep in mind that the Kleig Light (Carbon-arc) and mercury-vapor lamps, som e of which are dimmable to a point, are reactive loads. And, the Kleig ligh t as stage lighting persisted into the 1970s on movie sets (with proper fil tering against dangerous actinic wavelengths) and mercury-vapor devices are still used in some lighting designs. So, if a lighting device required a s trike function, and repetitive in some cases, that would certainly de-rate the Variac as compared to a simple incandescent/resistive load.

Note also that brush-type AC motors may have variable speed controls - such as on your corded hand-drill. I can think of any number of devices that wo uld use such a motor and also allow for the use of a large Variac. A theatr ical variable wind machine comes immediately to mind.

Peter Wieck Melrose Park, PA

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** The variac itself has no concern about the load other than the RMS current value. Cos that is what heats the windings.

Is there a switch on the unit ?

That part may be rated at 14A inductive due to potential arcing at switch off.

.... Phil

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** What bullshit.

AC/DC brush motors respond beautifully to Variac control.

So do the motors in regular fans.

Go try it yourself sometime.

.... Phil

On 11/4/2019 12:20 PM, John Robertson wrote: ...

That makes sense, thanks.

I never would have thought of them as reactive, but it's interesting.

My limited experience with single-speed universal (brushed) motors is that reducing the voltage reduced the speed, but also the torque. To an extent that it was not useful.

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** No so.

Torque is proportional to current draw, which depends in turn on the load applied. Brush motors have a max current limit beyond which the com begins to burn.

So the applied voltage can be well below the rating and the torque available remain the same, but at a lower rpm.

A triac controller does a good job too and SCR ones have back emf ( ie speed ) feedback that is very useful.

Induction fans can be varied from full to just ticking over with Variac.

The usual speed controls use series switched inductors or capacitors to drop voltage.

...... Phil

Fluorescent lighting was also pf not 1. We had a lot of fl lighting on a bi g variac. It had standard electromagnetic ballasts, and contrary to oft sta ted opinion it did dim well enough - we knew to not keep it running dimmed, and it blacked out at a low light level, it wasn't perfect but certainly g ood enough.

Discharge light sources were always pf not 1 in the pre-electronic days, wh ether fluorescent, carbon arc or mercury vapour. i was never proportional t o v with them, and in most cases the ballasts were inductive not resistive. And of course projector carbon arcs ran through mercury rectifiers to give asymmetric electrode heating to minimise the light source size.

NT

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** Irrelevant to the RMS current draw.

** Irrelevant to the RMS current draw.

The lying f****it dose not even understand " I squared R" heating.

... Phil

It's not, and I didn't claim it was, but hey.

that makes you the lying fool.

I see you've not learnt to spell moron yet. And at the risk of stating the obvious, we don't have any morons (or 'molrons') on this group. Maybe you should look the word up some time.

NT

** Three deliberate lies in one short sentence.

Might be a bullshit record.

..... Phil

you've outstupided yourself.