Variac trips the breaker - need advise for cleaning

[Falkra]

Hello to everyone,

I just bought a Variac from eBay, used, and when I plug it and turn it on, it trips the breaker (only the room was off, not the whole house).
The variac had no load, it's fuse didn't blow.

Could some cleaning of the contacts on the carbon track be a good solution ? With some contact cleaner ? (I have a spray can)
When I measure resistance on the output plug, I get 25 ohms at 0V, and 250 at 240V, but the values jump a little bit, then stabilize. Dirty contacts ?

On the input plug the two pins have 1Meg resistance, it is not shorted, and not shorted to earth either.

[Jeroen3]

Cleaning does not fix the inrush current of large transformers. You need an inrush current limiter.

[ArthurDent]

It sounds like the variac is wired incorrectly so the line is connected to the zero terminal and the wiper. Carefully check the wiring. They rarely need to be cleaned.

Here is representative wiring for one style variac. There may be other taps on your variac. Make sure yours is rated for 240 volts as well. For complete protection fuse both the input and output leads for the current rating of the variac.

[Falkra]

Thank you to both of you for answering. I will check.
It is a 1500VA unit, but it used to run with no special device (no inrush current limiter), directly connected to the mains.

The seller posted a picture where it was working and I called him, I think he is trustworthy (and he said he will accept a refund if there is a problem).


(click to enlarge if needed)

[PKTKS]

 that figures  of 1500VA 2A VARIAC...
should  not trip any break...  it is far below  power line

OR the unit is faulty
OR incorrectly  some how ...

BUT IMHO a VARIAC itself without the isolation transformer
is not very useful - indeed it is dangerous to have live mains.

the isolation transformer should delay the inrush enough
for such  figures (1500VA)

I have some very similar figures myself but always isolated

Paul

[soldar]

You can test it by connecting it to the mains in series with an incandescent light bulb.

If the light bulb shines brightly the transformer is shorted.

If the bulb shines dimly or not at all you can then short the light bulb (without disconnecting anything, just short the light bulb as it is in the circuit). If everything is ok here that means it was the inrush current tripping the breaker.

[Falkra]

Thanks for your answers.

Meanwhile I cleaned it a bit (the wiper track was really dirty) and now I measure 1.8 to 18 ohms on output (power off, of course) instead of previous values.
I will test, but I have no incandescent bulb setup, I'll try to get one or check in my old piles of stuff. 

[HackedFridgeMagnet]

Yes a variac will trip a breaker on no load due to inrush current. Try to time your turn on to the top of the cycle. 

No it isn't an accurate test to use an ohm meter on the brushes, as the carbon contact will read quite high at low currents.

Probably the light bulb test is better.

And best be sure your chassis is earthed.
Your insulation is good if you have a tester.
Your input active goes to one end of the coil, not the brush.
Your neutral is shorted from in to out, not your active.
You have the active fused. 

[Stray Electron]

  I used to have a 35 Amp Variac and it never tripped the breaker either on 115 or on 230 VAC. I use several 6 and 8 amp variacs almost everyday and they have never tripped a breaker on 115 VAC.   IMO someone has most likely connected the input incorrectly. You might also have a breaker that has arc protection or GFI protection in it and that might be making it too sensitive for use with the variac.

 PKTKS said  " 1500VA 2A VARIAC..."  Are you saying that that variac runs on 750 volts??

[james_s]

You can test it by connecting it to the mains in series with an incandescent light bulb.

If the light bulb shines brightly the transformer is shorted.

If the bulb shines dimly or not at all you can then short the light bulb (without disconnecting anything, just short the light bulb as it is in the circuit). If everything is ok here that means it was the inrush current tripping the breaker.

I was going to suggest exactly this.

Also is it a standard circuit breaker? In some parts of the world GFCI (RCD) breakers are common, then over here recent homes have arc-fault breakers, and some combine the two features. With these kinds of breakers there are faults other than excessive current that will cause them to trip.

[Johnny10]

I did a quick test of my Staco 10Amp 1.4KVA Variac and I can get up to 7 AMP Inrush on turn-on no load, 120V.

[Stray Electron]

     A 7 amp inrush on the input side a 10 amp output variac with no load!

     If that lasted longer than a few a microseconds then there's definitely something wrong with your variac!  What did you measure that with?

[Johnny10]

I was wondering the same thing.

Just a bounce on the needle and not every time maybe once in 20 turn-ons.
Most turn-ons no spike,

[richnormand]

My two Variac blow the breaker most times if I turn them on while the adjusted output is close to the mains voltage. If I put them at zero before powering them all is fine.

Next test, as suggested above, is to put a 100W incandescent light bulb (if you can still find them) in series with the Variac without any load applied. If it glows brightly you have a short (you might hear the hum or see it getting hot) or it is mis-wired.

Also keep in mind that Variac are autotransformers. The output is not isolated from the input so there are similar risks as playing with the mains.

[HackedFridgeMagnet]

My two Variac blow the breaker most times if I turn them on while the adjusted output is close to the mains voltage. If I put them at zero before powering them all is fine.

If you disconnect the load it wont matter what voltage the variac is set too.

[Psi]

I have that exact variac and it never blows breakers when i turn it on, or makes lights dim.

I have blown it's internal 2A fuse a couple of times over-driving it though, but that's all my fault

One thing though, the volt meter on mine doesn't always read correctly. And the wire connections to the mains output socket came loose.
Maybe the design is susceptible to wiring breaks/faults and yours got one from the vibration in shipping.
If it was me that would be the first thing to check.

[PKTKS]

 PKTKS said  " 1500VA 2A VARIAC..."  Are you saying that that variac runs on 750 volts??

no the VARIAC itself is rated (NOMINAL  rated) with such figures.

It actually runs standalone from an isolation transformer which transforms
a nominal 110 AC  to a nominal 240 AC totally isolated secondary.

So the VARIAC itself runs at proper rated input voltage which is 240V AC.

Maximum nominal amperage is around 2A on the VARIAC itself.
But it should support 3A just fine for short periods.

The transformer is far more capable running the secondary safely
with a 5A fuse - the VARIAC line contains a SAFE only 3A fuse in case of short

The primary contains a  ZING EAR ZE-800 safety circuit brake

BTW none of them never tripped with the loads I regular use them
which are several mixed types of SMPS rated up to 1000W
- mostly with very  high inrush amps

Paul

[Calambres]

I have a variac (230V 50Hz 8A) that consistently tripped the breaker on cold start. It was not because it was incorrectly plugged (damn SCHUKOs!  ) I installed a neon lamp in its chassis between Line and Earth to know when I was pluggin it the other way 'round (Lit -> OK). It kept tripping the breaker until I put an Ametherm "MS35 1R040" NTC in it to tame the inrush current. Never failed from then on.

[PKTKS]

I installed a neon lamp in its chassis between Line and Earth to know when I was pluggin it the other way 'round (Lit -> OK).

I have made similar thing but placed a 90V NEON on the primary of
the isolation transformer. That position allows me to know even if
the primary circuit break tripped

Just hacked the NEON resistor to 110 ACV

Paul

[Falkra]

Hi, thank you all for posting here !

I cleaned the contacts, verified there was no loose connection or unsoldered wire, which wasn't the case.
It started twice, unloaded, and seemd to work. Then I connected a multimeter to read the output voltage and started it again : breaker tripped. Again.

Here is what I found inside and picture of my breakers, for science. 


(click for full size)


(click for full size)

I think I will return the unit to the seller (I paid using Paypal on ebay). The problem is still there and it doesn't look like a quick and easy fix (at least to me).

[edpalmer42]

I have a Variac that's rated for 250V, 1.6KVA.  I wanted to measure the no-load current so I carefully wired my ammeter in series.  When I powered it up, LIGHT came out of my meter!!    There was no place for the light to come out, but it did anyway!    When I opened my meter, I couldn't see anything wrong.    I finally noticed that the 2A glass fuse now had a perfect mirror coating on the inside.  I replaced the fuse and all was well.    So yes, I can confirm that a Variac can have a huge starting surge.  The surge occurs because until the magnetic field is set up in the core, the only thing to limit the current is the resistance of the winding.  Depending on where in the AC cycle you happen to hit the switch, you may or may not get a huge surge.

It just occurred to me that if you put a zero-crossing detector and AC switch on the front end, you might be able to avoid a surge completely.  Has anyone thought about that?

Like Calambres and PKTKS, I wanted some lights for safety, so I added two neons.  One from hot to neutral, and one from hot to ground.  The neon doesn't draw enough current to upset a GFCI.  So if both lights are on, everything is as safe as possible.  If either light is off, something has gone horribly wrong!

Ed

[Falkra]

I think my setup is too sensitive, and I don't really have time to mod it (the NTC and neons are great ideas).
In addition, I was not convinced by the internal quality of the wiring and soldering. 

[Jeroen3]

Interesting, you have C curve breakers, usually t's B curve. Inrush must be quite bad then.

[Falkra]

That may also be part of the issue.

The seller accepted the return and I sent it back, I am waiting for the refund.

Thank you (to everybody).