variac trouble (not really)

[JC@]

Too many times, when I turn on a big variac I use (10 - 12A), the magnetothermic breaker that belongs to that part of the house jumps off and I have to go there and reset it.

What may be causing this? I have two smaller variacs that don't do this.
I'm thinking about soldering a .02 high voltage cap from each lug at the input side of the variac's ON/OFF switch to ground.
Would this help?

Thank you

[Ray Ivers]

JC,

Here's a link to a Mouser catalog page - check out the 'Inrush Current Limiters'.

http://www.mouser.com/catalog/627/468.pdf

FWIW, I picked out the 527-CL30 for my amp; haven't tried them yet, though.

Ray

[Just Bob]

Circuit berakers are supposed to open instantly on a short circuit, stay closed at the rated current, and open at progressively shorter times with overloads. Many motors for household appliances have substantial inrush current and breakers are not supposed to trip under that condition. They should stay closed for several seconds with a 100% overload. Your variac shouldn't have that much inrush current. Two things could be a problem. You may already have a substantial load on that circuit. What else is it running? Lights, etc? If you already have 10 amps load, no surprise that the breaker would trip. Second, the breaker could be bad. I've had some that trip lower than they're supposed to. Breakers are cheap. If you don't have other stuff running on that circuit, get a new breaker or swap it with another of the same rating in the box to see if that's the problem.

[Tom Phillips]

I'm thinking about soldering a .02 high voltage cap from each lug at the input side of the variac's ON/OFF switch to ground.
Would this help?
Adding the caps as you describe will not help your problem.
Tom

[JC@]

JC,

Here's a link to a Mouser catalog page - check out the 'Inrush Current Limiters'.

http://www.mouser.com/catalog/627/468.pdf

FWIW, I picked out the 527-CL30 for my amp; haven't tried them yet, though.

Ray

Thank you Ray, I'll give a try to one of those.

[JC@]

Circuit berakers are supposed to open instantly on a short circuit, stay closed at the rated current, and open at progressively shorter times with overloads. Many motors for household appliances have substantial inrush current and breakers are not supposed to trip under that condition. They should stay closed for several seconds with a 100% overload. Your variac shouldn't have that much inrush current. Two things could be a problem. You may already have a substantial load on that circuit. What else is it running? Lights, etc? If you already have 10 amps load, no surprise that the breaker would trip. Second, the breaker could be bad. I've had some that trip lower than they're supposed to. Breakers are cheap. If you don't have other stuff running on that circuit, get a new breaker or swap it with another of the same rating in the box to see if that's the problem.



Thank you vey much Bob, for your clear explanation. Very helpful.
I guess I'll have to do a little bit more of investigation. There's a lot of equipment sharing that part of the line, but most of the time when I switch this variac on it is set to 0, that is no load on it... I don't know.

[Just Bob]

With no load on the variac, there is still a good bit of current flowing. It's a reactive load. Current and voltage are out of phase so there's very little actual power being consumed, but the breaker will still register the reactive current. It's better to use a smaller variac if you don't need the 10 amp capacity. You could try using an extension cord to power it from another circuit that is lightly loaded. I have dozens of extension cords running everywhere. Not a great idea, but that's how it goes. It is possible to use a motor-starting oil-filled capacitor to resonate the variac at low loads, but that's a BIG capacitor and not something I'd do casually. You'd also need to know the inductance of the variac to calculate a capacitor with the opposite reactance. Better to add a circuit or find a long cord.

[JC@]

With no load on the variac, there is still a good bit of current flowing. It's a reactive load. Current and voltage are out of phase so there's very little actual power being consumed, but the breaker will still register the reactive current. It's better to use a smaller variac if you don't need the 10 amp capacity. You could try using an extension cord to power it from another circuit that is lightly loaded. I have dozens of extension cords running everywhere. Not a great idea, but that's how it goes. It is possible to use a motor-starting oil-filled capacitor to resonate the variac at low loads, but that's a BIG capacitor and not something I'd do casually. You'd also need to know the inductance of the variac to calculate a capacitor with the opposite reactance. Better to add a circuit or find a long cord.

Wow. Let me digest all this information. It is more than I asked for, but exactly what I needed to know.
So thank you Just Bob.. Thank you.

[Steve Conner]

Power factor correction (what the previous poster mentioned) and inrush current limiting are two totally different things. JC'A's problem is inrush current, and I imagine PFC would make no difference.

[JC@]

With the help of a proper inrush current limiter the problem is definetely solved.
Great! No more trips upstairs to reset the magnetothermic thing.