 Variac limits
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Sulaiman
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Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
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Since you are drawing 36A from the output and 9A at the input you are probably outputting about 240 x 9/36 = 60 Vac so the suggestion by radiotech is the best;
Connect a 240:60 Vac 2.5 kVA transformer to the variac output.
This has the additional benefit of giving you an isolated/floating ac output so that after the rectifier you can ground/earth the 0V line for 'scoping/metering etc.
Rewinding a variac is non-trivial and you will still be limited by the wiper current rating anyway.
You can use one 2.5 kVA transformer or multiple lower kVa transformers with primaries and secondaries series or parallel depending upon what you can get.
Since transformers are expensive you could use any required number of transformers with 120 + 120 Vac primaries, don't use the secondaries, use the center-tap of the primaries as an auto-transformer.
In this way the kVA rating of each transformer is doubled BUT you will not have an isolated output.
This will allow 20A x 240/120 = 40 Aac output maximum continuous.
Ideal would be a 120:120 transformer wired as 120+120.
The other alternative is to rectify the mains 240 Vac input and use the dc as a supply for a pwm psu. Probably cheaper but much more complex and less reliable.
IF this is the same project as " Limits of an unregulated supply " then why use a voltage doubler?
Direct rectification of the variac output will achieve the desired result!
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IamSmooth
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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
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Sulaiman,
As I change the power into the induction heater input and output currents change; hence, the reason for the variac.
I found that when I used the voltage doubler off of the my rectifier I got more heating with the inductor. I was able to draw more power off of the mains. Maybe I was able to draw more peak current at the right point in the cycle.
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