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Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
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Posts: 1546
If you increase the size of the line fuses I imagine the core winding of the variac rated at 140 volts running on 240 may output a square wave of 35 amps for the brief time the insulation is smoking just before it catches fire and shorts out completely.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
No.
If a variac is meant for 140v, then you can only use it to 140. Well, OK, maybe 150 if you don't mind it starting to draw huge currents and get hot as it saturates like a MOT. But double is absolutely out of the question.
Now if you had asked 'it's rated for 20A, can I use for 40A for short duration?', the answer is Yes
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
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...the maximum inrush current.These relations are all based upon the fact that after the iron in a transformer is sufficiently saturated, any increase in flux must be supported by the air core inductance of the windings....and the crest value of inrush current is ~~
Icrest = sqr(2E)/6,28FL) *2(2- (S/Bo)) E = RMS lnput voltage L = inductance of winding without steel core S= Saturation of steel i.e. 2T Bo Flux density of normal operation i.e 1.2 T edit oops left F out (frequency) (Peter's formula)
the above lifted from Westinhouse 3rd ed 1944 Electrical Transmission and Distribution Reference Book. probably elsewhere, just happened to know it was there
Registered Member #2463
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I agree with just R. This Westinghouse stuff is for gigantic trransformers, and the other formula they give (Fahnoes) includes resistance,and a bunch of other magnetic stuff where units are operated far closer to saturation.
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variac used for higher voltage
