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Registered Member #2288
Joined: Wed Aug 12 2009, 10:42PM
Location: Cambridge, MA
Posts: 179
I tried to make a variable voltage doubler using two variacs, one of which being used "backwards".
It seems like a pretty straightforward design, but it doesn't work at all. I would expect the variac settings in the schematic to give 110V out with 110V in, and it does, but the second "doubling" variac is just starting to saturate at this point. If I turn up the left mains varrying variac any more, the doubler variac starts violently buzzing and saturating. If I run 110V in, the doubling variac better be in the no-doubling position, if it's even one winding into "multiplication range", it starts vigorously shaking/fully saturated.
The doubling variac says right on it that it's rated for 250V, so why is this not working?
Registered Member #816
Joined: Sun Jun 03 2007, 07:29PM
Location:
Posts: 156
Sounds like it could be the brush on the 2nd variac not making contact with the windings properly and interrupting the magnetizing current, another reason why variacs should not be used backwards in this way, also the brush has now to handle twice the output current if you use the full doubling range.
A better way is to use an isolation transformer and use the variac to alter the pri volts and put the sec in series with the hot and the load. Make sure there are no earthed taps on the transformer. If the load volts decrease when you wind up the variac you have to reverse the connections on one side of the isolation transformer.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
dude 500 first, it should work so let's find the fault.....
1) Just using the left variac, apply 110 Vac and check that you get 0% to 100% output (or more)
2) Just using the right variac, apply 110 Vac to the 250 Vac input and check that you get 0% to 100% output (or more)
3) Re-connect as per your diagram turn the wiper on the right variac to the top end / maximum turn the wiper on the left variac to minimum apply 110 Vac input to the left variac slowly turn up the left variac - should go to 100% with no problems slowly turn the wiper of the right variac down (increasing the output voltage) untill you hear humm/buzz ... that's the limit.
If you put a mains filament lamp (about 100W) in series with the mains you can watch the current being drawn, if you draw too much the lamp will limit current/damage and be very bright.
P.S. I too would rather use a transformer (any transformer with dual 110 Vac primaries can be used as an auto-transformer) Preferably an isolating transformer for safety. I don't like dials that can be set to autodestruct !
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Sulaiman wrote ...
dude 500 first, it should work so let's find the fault.....
If the rotor on T1 is set at maximum volts and the rotor on T2 is set to zero, we will have what was called until extremely recently a short circuit.
If we now reduce T1 rotor by, say, a single turn to 109V and increase T2 rotor by a notional single turn to 1V, will a mysterious transformer action have replaced the near short condition so that the fuse no longer blows? And what about 108V and 2V? And so on - until the inductance of T2 rotor to Neutral has sufficient reactance to reduce current to a level which the circuit can sustain without failure.
I might add that this explanation is completely consistent with the T2 range marked 'off limits range' in Dude 500s diagram above.
Registered Member #2288
Joined: Wed Aug 12 2009, 10:42PM
Location: Cambridge, MA
Posts: 179
Sulaiman wrote ...
dude 500 first, it should work so let's find the fault.....
1) Just using the left variac, apply 110 Vac and check that you get 0% to 100% output (or more)
2) Just using the right variac, apply 110 Vac to the 250 Vac input and check that you get 0% to 100% output (or more)
3) Re-connect as per your diagram turn the wiper on the right variac to the top end / maximum turn the wiper on the left variac to minimum apply 110 Vac input to the left variac slowly turn up the left variac - should go to 100% with no problems slowly turn the wiper of the right variac down (increasing the output voltage) untill you hear humm/buzz ... that's the limit.
If you put a mains filament lamp (about 100W) in series with the mains you can watch the current being drawn, if you draw too much the lamp will limit current/damage and be very bright.
P.S. I too would rather use a transformer (any transformer with dual 110 Vac primaries can be used as an auto-transformer) Preferably an isolating transformer for safety. I don't like dials that can be set to autodestruct !
This is basically the exact procedure I followed. As long as the right transformer is at the maximum, no multiplication state, I can put the full 110V in, but then even a single winding turn (barely turning the knob at all), the transformer is buzzing away and in saturation, if I turned it any more, bad stuff would start to happen.
My current thought is maybe it has to do with the fact that variacs short two adjacent loops frequently with the brush, which seems to be fine in normal operation, but maybe in this mode of operation there isn't enough leakage there to handle that?
Also, it is note worthy that this is a 3-phase setup, both variacs being triple, but as far as I know it's all the same.
I have a bunch of big iron transformers so if I need to I'll go with this, but I liked the elegance of this setup, kind of combining the "double enable switch" and the doubler transformer into one item.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I think Proud Mary said it right. If you have it reversed and you bring the wiper on the right down to the bottom you are going to short the hot and neutral on the left.
Registered Member #1617
Joined: Fri Aug 01 2008, 07:31AM
Location: Adelaide, South Australia
Posts: 139
As well as shorting the first variac if you turn the second right down, the second will saturate if it is turned too low while the first is up high....
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Variac Voltage Doubler
