Inrush current of variac

Thread Starter

tom66

Joined May 9, 2009
2,591
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?
 

jimkeith

Joined Oct 26, 2011
539
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:

russ_hensel

Joined Jan 11, 2009
825
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.
 

crutschow

Joined Mar 14, 2008
23,157
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.
 

strantor

Joined Oct 3, 2010
5,025
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.
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
When you turn on the breaker/switch the voltage across the transformer primary can be quite high on the sine curve,which can cause a transient condition that can saturate the transformers primary. If you have a zero crossing switch, this will never happen.
@jaclement: That's not correct, because current is lagging

Please have a look at this link:
http://www.opamp-electronics.com/tutorials/inrush_current_2_09_12.htm
Especially the paragraph below the first picture and below the 3rd picture.
of course I didn't mean there was a bigger inrush current at the exact moment (zero crossing) where the transformer was connected.

Just to make it clear: the inrush current that appears AFTER you apply AC voltage at zero crossing is bigger than the inrush current you'd have if you applied AC voltage at its peak value. :D:D
Strantor, I may not make any difference, the relay on the secondary, because it's not only the caps of your load that contribute to your inrush current. Please have look at the following link:

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

praondevou

Joined Jul 9, 2011
2,935
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.
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. :)
 

K7GUH

Joined Jan 28, 2011
191
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.
 

t_n_k

Joined Mar 6, 2009
5,448
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.
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.
 

jimkeith

Joined Oct 26, 2011
539
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.
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.
 

Thread Starter

tom66

Joined May 9, 2009
2,591
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.
 

jimkeith

Joined Oct 26, 2011
539
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.
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.
 

Thread Starter

tom66

Joined May 9, 2009
2,591
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.
I see this in a lot of plasma and some LCD TVs which often have ~600µF of bulk capacitance.
 
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