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Come-on, is my 10A variac REALLY only good for 10A?

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Josh Johnson

Mon Sept 21 2009, 02:52AM

Registered Member #793 Joined: Sun May 20 2007, 06:50PM
Location: Minneapolis, MN
Posts: 35

Hi All,

After getting my spiky IGBT voltage under control I have been running my DRSSTC at higher power levels. I increased my OCD to ~400A and am now getting consistent 1 meter streamers. The only problem is with >90% on the variac, almost anytime there is a ground strike I blow the 10A fuse in the control box. The fuse is there to protect my 10A variac.

So my question is how solid is that 10A rating. I am assuming it is a continuous duty rating so for tesla coil use of say 2 min runs, maybe I can exceed this by a couple amps? Possibly fuse at 12A or so? I don't plan on increasing the power draw of my DRSSTC much beyond this current level since I have TO-247 IGBTs and I am pushing my tank cap's ratings as it is.

None of my other projects TC or otherwise have been at this power level so I don't have any practical experience with beating up on variacs.

Thanks,
Josh

Arcstarter

Mon Sept 21 2009, 04:16AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Well first of all, 1 meter sparks from TO-247's is quite an accomplishment. Well done!

Yes, you can push the variac further, for awhile. I would say 12 amps would even maybe be conservative :P.

Proud Mary

Mon Sept 21 2009, 05:43AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Well Josh, you can buy very cheaply domestic wall socket plug in AC watt/hour amp/hour meters aimed at the power conscious consumer, and small business user who wants to know how much it really costs to run this and that around the home. One of them would be more than good enough for your purposes.

Stick it on the mains line before the Variac, lad, so you can read I(in) directly, and deduce I(out) from the Variac voltage setting (if yours is of the calibrated dial type) and your question will be solved.

You could use an AC ammeter which are easy enough to buy very cheaply from Russia, but the domestic wall meter will allow you to make an energy inventory of all the devices in your house, which is why they've been introduced as a consumer item.

Be sure to tell us how you get on.

Harry

Dr. Slack

Mon Sept 21 2009, 07:32AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

A transformer, including a variac, can overcurrent happily until temperature kills some part of it.

In a fixed autotransformer, that's a lot, teh mass of the windings will allow it to soak up excess heat, uniformly, precisely where it is generated, and you can use huge overcurrents, especially if you calibrate the I2t, which is easy by measuring the resistance of the windings. Stick it until the windings get to 100C, then stop before the insulation burns.

In a variac however, the sliding contact is the weak point. The contact point between the windings and the brush will be a poorly-cooled hot-spot. If the slider is carbon, there will be heating in that as well. It's not possible to calibrate as there is no easy way to measure a relevant temperature, ie at the interface, under pulse conditions.

One strategy is to say to yourself that 10A is probably conservative, and you can push pulses to 20A, maybe. If you have two, another strategy would be to turn the wick up on one until it dies, then back off a bit on the other one.

An important factor is your duty cycle. Does it ground strike continuously, or are there only a few? In other words, do you want to overcurrent it continuously, in which general heating is a problem, or just for very short and infrequent pulses, where the I2t of the brush is most important.

If you increase the value of a fuse to handle pulses, don't forget that it will also happily pass larger continuous current, leaving your variac open to smoking under other conditions. It may be better, if you can find or build it, to use a time delay fuse. This will maintain a low current limit for contionuous duty portecting the mass of the windings, but small mass bits like the wiper still protected at a higher pulse current.

It may be possible to repair minor damage to the brush/winding interface with very fine abrasive paper with it dismantled. Maybe you can try the smoke test on your only unit? Do you feel lucky, punk?

While I'm all for Harry's suggestion of measuring what your present application is taking (what is an engineer without measurement), it's not going to tell you what will kill your variac. But it might tell you what did kill it! Or might not, those plug-in monitors are not so hot for pulses.

I rather foolishly connected a 13A kettle to my 2A variac as a load, forgot what I was doing, and just wound it straight up. At about 2/3rds full scale, I was alerted by a sparking sound from the wiper, so wound it straight back down again. It lived to tell the tale, however YMMV!

If the current is reasonably steady over a long 2 minute run, and you are talking about sneaking it up a little bit, like 12A, then fan-cooling the variac will give you an improvement in power handling. However, as before, the winding resistance is easy to measure, the brush temperature is next to impossible, so you won't know by how much it has improved.

Steve Conner

Mon Sept 21 2009, 07:52AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

A 10 amp variac will take more than 10 amps for short periods. However, so will a 10 amp fuse. Usually a fuse can pass twice its rated current for a few minutes before it pops.

The fuse should be in the output from the variac, so it's in series with the sliding contact and sees the same current as it. As Dr. Slack said, it's the weak point of the whole thing.

I've seen big DC motors fitted with temperature probes in their brushes. The application was EV rock crawling, and the guy had a "Brush temperature" gauge on the dashboard. This would almost certainly be overkill for your variac, though.

I think the fundamental problem is with your OCD. If it's limiting streamer length already, then the current should not increase much on ground strikes, since the OCD should limit it there too. I'd check to make sure that EMI from the ground strikes isn't getting into your control electronics.

HV Enthusiast

Mon Sept 21 2009, 10:59AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

For Tesla Coil use, you can typically use a variac for 2x its rated power. So if you have a 10A variac, you can run it up to 20A for short durations such as in Tesla coils.

Mads Barnkob

Mon Sept 21 2009, 03:07PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Look at figure 1 in this general datasheet, should do for most variacs

Herr Zapp

Mon Sept 21 2009, 04:38PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Josh -

Here is a "generic" graph showing the overload characteristics of a typical variable autotransformer. Since there is so much thermal mass in the windings and core, variacs can withstand pretty significant overloads, with the usual thermal "hot-spot" being the contact between the brush and the windings.

For fusing a 10A variac, you can use a 15A, slow-blow type, but bear in mind that you have disabled the thermal protection for the variac. To help keep the variac as cool as possible, make sure that the commutator (portion of the winding where the brush rides) is clean and burnished, and that the contact surfaces of the "wiper" (the second sliding electrical-mechanical contact carrying current to the rotating brusholder) are also clean and bright.

If your variac is an open-frame model, or you remove the sheet-metal enclosure, you can easily measure the brush temperature using a cheap non-contact IR optical thermometer.

Regards,
Herr Zapp

1253550912 480 FT76353 Staco Variac Overload Graph

HV Enthusiast

Mon Sept 21 2009, 04:44PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Yes, that is a good point. The weak point *is* the contact between the brush and windings, and yes, if you have an overload condition and a fuse rated for higher power than the variac, you'll find some fusing or damage at this point in some cases. I actually have a variac that has lots of these "damage spots" on it due to overcurrent conditions.

Dr. Slack

Mon Sept 21 2009, 09:01PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

That's handy, a couple of sources suggesting 2x and 3x nominal for a 2 minute run. If those figures are for "big" variacs, you might want to back off a bit for smaller ones with a lower thermal time constant.

Remember that moving the wiper under load will dramatically reduce the current carrying capacity compared to a static wiper.

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