Inrush current of variac

Discussion in 'General Electronics Chat' started by tom66, Dec 19, 2011.

  1. tom66

    Thread Starter Senior Member

    May 9, 2009
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    Whenever I turn on my variac (which is a 15A rated device I got for cheap) it trips the 32A MCB for the sockets. I fear it's the inrush current, as it makes the lights dim briefly. Eventually with enough flicks of the breaker it will work properly. How can I limit the inrush, or is there a way to make the breaker less sensitive to this inrush?
     
  2. strantor

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    Oct 3, 2010
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  3. jimkeith

    Active Member

    Oct 26, 2011
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    I assume that you are powering it from your 50hZ 240VAC mains.
    This is big variac and in my opinion it is behaving normally.
    Inrush will be worse on 50hZ as it is closer to saturation than if powered by our 60hZ (North American) system.

    Do you have it set up to deliver overvoltage? If overvoltage is not necessary, you may rewire the input to the top of the winding--this will run the flux lower.

    Other solutions include:
    1. slower trip breaker--they are available with other response curves
    2. NTC (negative temperature coefficient) thermistor http://www.ametherm.com/
    3. Apply power via a series power resistor--then short the power resistor with a switch
     
    Last edited: Dec 19, 2011
  4. russ_hensel

    Well-Known Member

    Jan 11, 2009
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    In rush current into a inductor seems a bit odd to me -- assuming the output is not connected to a load. I would be interested in what others think.

    That said the solutions above seem reasonable.
     
  5. crutschow

    Expert

    Mar 14, 2008
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    You could turn it on and off through a zero-crossing type solid-state relay. The high inrush current results when the variac is arbitrarily turned on at or near the peak input waveform voltage. Turning it on at the zero-crossing should reduce the inrush close to it's normal operating current.
     
  6. strantor

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    This was covered in the post that I linked to in the first reply. I think it was decided that the zero crossing idea doesn't hold up, although I never tested it. have a read:
    http://forum.allaboutcircuits.com/showthread.php?t=57011&page=2
     
  7. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Correct. I tested it with small 60Hz transformers.

    www.opamp-electronics.com/tutorials/inrush_current_2_09_12.htm

    edit:eek:ops you already the link posted. :)
     
  8. K7GUH

    Member

    Jan 28, 2011
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    I like solution #3 posted above. My father was an electrician, and would use a 60 or 100 watt bulb in series with an unknown transformer before applying voltage. Never saw him melt anything, except on purpose.
     
  9. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    It's probably a similar situation to the 'well-known' phenomenon of power transformer magnetizing current inrush. It usually depends at what point in the supply AC cycle the magnetic circuit is energized. Presumably the OP's variac has a laminated steel magnetic core.
     
  10. jimkeith

    Active Member

    Oct 26, 2011
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    A zero-crossing solid state relay will do, provided that it is also zero-state turn off as well. In other words, it must always start on the positive half cycle and always turn off at the negative half. That way, the set is always the same.

    I devised one like this years ago for a chain welding transformer control--wish I still had the circuit--really worked neat for how simple it was.

    Another way to do it would be to turn it on via a solid-state soft-start control. Any set remaining in the transformer control would be 'ironed' out via phasing the SCRs on gradually. For something this big, I would use anti-parallel SCRs rather than a triac. That way, the turn-off state would be a non-issue.
     
  11. tom66

    Thread Starter Senior Member

    May 9, 2009
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    I was thinking of having a 10 ohm resistor (or 500W lamp) wired in series with the variac, and a 15A relay to short this out after ~100ms has passed (just use a cheap transformer from an alarm clock/radio to get 9VAC and put this into a cap/BJT switch for the relay.) Would overcomplicate it a lot, though... and I'm not even sure if that would work that well.
     
  12. jimkeith

    Active Member

    Oct 26, 2011
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    This will work and is not unduly complicated.

    http://en.wikipedia.org/wiki/Pre-charge

    Typically, AC Inverter drive controls have a delay circuit, limiting resistor and contactor to get the DC link capacitor charged without blowing a line fuse or tripping a breaker--otherwise know as capacitor pre-charge.
     
  13. tom66

    Thread Starter Senior Member

    May 9, 2009
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    I see this in a lot of plasma and some LCD TVs which often have ~600µF of bulk capacitance.
     
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