Re: Variac Amperage?
HV Enthusiast, Fri Apr 28 2006, 02:40AM

For Tesla coil use (not continous use), you can generally run the variacs at 200% of their rated current value. So, if you need a 20A variac for a Tesla coil, you can use a 10A variac with no problem. Just keep run times to a minimum to keep heating down.

Re: Variac Amperage?
Colin 99, Fri Apr 28 2006, 03:20AM

I found out the hard way on that 200% rating thing. I had bought two Genrad 5 amp variacs to use one on my tesla coil thinking that it would be ok for a 30mA NST. While I was running it, I couldn't get any more power from around 50% to 120% on the variac dial. I finally connected an ammeter to it and found out the variac would not deliver more than 5 amps, as it was rated. I then tried a 10 amp variac from work and it worked fine, even up to a full (canada here) 15 amps using a 60mA NST. So the bottom line is that it depends on the manufacturer.

Shaun

Re: Variac Amperage?
Desmogod, Fri Apr 28 2006, 06:13AM

If in doubt, buy the biggest one you can afford.
You'll use it in this hobby, and it still probably won't be big enough.

Re: Variac Amperage?
hvguy, Fri Apr 28 2006, 06:56AM

Colin 99, It sounds to me like your variac may have had an isolated secondary. A "normal" or non-isolated variac can not behave like you mention as full scale (100%) effectively bypasses the variac, connecting the load to the line.

I second Dans comment, you can usually run 2x the current rating with out any problems provided the variac is not an unusual (isolated or multiple secondary) type and the duty cycle is short. For example, I have run 110A through a 28A variac for three minutes with out any problems. I would, however, recommend buying the biggest one practical as they are tools you will use often in this hobby.

Re: Variac Amperage?
Sulaiman, Fri Apr 28 2006, 07:40AM

My choice would be influenced by the availability of spare carbon brushes.
Variac mechanicals don't seem to wear out,
The winding contact areas need occasional cleaning,
(absolute worst case - rewinding)
Carbon brushes need replacing.

Since nowadays we can't rely on continuity of supply
I advise you to buy spare brushes when you buy the variac
If you can't get brushes, change choice of variac.

The higher the current rating the better!
- until you try to move your variac.
If you have a convenient permanent location for the variac
then get the largest you can justify to yourself.
If only for occasional use then I'd consider the weight/storage.

Re: Variac Amperage?
Desmogod, Fri Apr 28 2006, 08:00AM

Don't worry about availabilty of brushes. The type used in car alternators are more than up to the job.
May need a reshaping with a dremel though. But they are cheap as

Re: Variac Amperage?
Hazmatt_(The Underdog), Fri Apr 28 2006, 08:43AM

mmm....I contacted Newark about Staco brushes for my 1256B and they said that I could get replacement brushes for 1256D, which is basically the same.

Point is, depending on make, you can get parts.

Re: Variac Amperage?
Shahn, Fri Apr 28 2006, 12:43PM

Ok well I will try getthe biggest. What can a 15A run as compared to 10A?

Re: Variac Amperage?
hvguy, Sat Apr 29 2006, 09:08AM

It should pass at least 30A intermittently. At 240V that’s 7.2KW, plenty of power for most TC work.

Re: Variac Amperage?
Shahn, Sat Apr 29 2006, 01:18PM

So you mean the 28A should be heaps or the 15A at 200%?

Re: Variac Amperage?
Part Scavenger, Mon May 01 2006, 02:20AM

I hope I'm allowed to do this. Here's an excerpt from pupman, sounds like a deal. Especially if you're planning on spending $300 anyway. I was going to PM this to Shahn, but I thought, "heck, why not help everybody out?"

Original poster: **link**

I have a truck load of variacs to sell you can see 2 pictures at the
link listed below.

The Variac in the picture is a 7.8 KW, 28 amps, input voltage
240/120, Outout voltage 0-280. This is a 2 gang unit, Double unit, 2
variacs connected together. You can use them seperate or wire them
in parallel to get 15.6 KW.

I have a lot of these to sell and I am not sure how many I actually
have probably 40 or 50 total.

I have maybe 4 or 5 double units. I have maybe 10 tripple units. I
have 1 maybe 2 units that are a 5 variac stack. I might even have
a unit that is a 4 variac stack not sure until I move some other
stuff out of the way to take a look.

The values are unknown at the moment. Most are 240/120 input voltage
and 0-280 output voltage, rated about 5KW, 6KW up to about 7.8 KW per
variac. There might even be a 10KW unit somewhere in the lot.

I have a single 5KW variac rated at 120 volts input, 140 volts
output. There might be 2 or 3 of these.

These things are heavy. The double variac unit weight is about 100
pounds maybe more I have not actually weighed it. It took 2 people
to move it so I could take pictures.

These things sell for about $650.00 to $850.00 per variac NEW. I
will take $300.00 per variac or any "REASONABLE" offer. Absolutely
NO trading I have too much stuff as it is already.

I perfer the units be picked up at my house in Murfreesboro, TN. 37129.

Buyer is responsible for all shipping costs including package and
handling fees.

If anyone is interested in the entire LOT of variacs I will take
$5000.00 for them all. I can provide pictures of them all. You will
need a 3/4 ton or maybe a 1 ton truck to haul them all. Also bring
several very strong guys with you to load them in your truck.

SEE PICTURES AT,

Boy, I wish I had 300 bucks right now!

Re: Variac Amperage?
Shahn, Mon May 01 2006, 06:01AM

Hey thanks heaps Part Scavenger. Not to sure how well it will work out with our shitty dollar. But thanks.

Re: Variac Amperage?
Part Scavenger, Mon May 01 2006, 12:04PM

Oh, that's right. You live in NZ don't you? Sorry, I forgot.

Re: Variac Amperage?
hvguy, Fri May 05 2006, 07:42AM

So you mean the 28A should be heaps or the 15A at 200%?

The 15A at 200% should be fine.

Re: Variac Amperage?
Shahn, Sat May 06 2006, 05:49AM

ok thanks!!!

Re: Variac Amperage?
Marcus Wagner, Tue Sept 12 2006, 03:23AM

Hi!

Nice to have this variac-thread.... so I dont need to open a new one.. (I need some help...)

some questions...

1) why does my variac heat excessively as soon as I plug it into the wall and no load is connected? It is drawing 0.2 Amp at 240 Volts without anything and is rated up to 2A / 0-250V continously. May it be defective? If yes, what can be the reason... I mean because a variac is a very simple device where you see damages almost immediately...


2) How is it possible to draw 110A with a 28A variac for even 3 minutes??? What is the 110A - Peak Pulse, RMS? Most lines in Europe are only good to 16A RMS and fuses and protection-circuits will trigger if you pull to much to long.... I ever thought 240Vx16A=3840Watt was the upper maximum for what I can draw from my wallplug. So how do you guys manage to get 20... 50.. 100 Amperes from your mains? Where do you connect to, to your main-house-connection without any fuses/circuits? Well what is the maximum I can draw from my mains?


Greets,

Marcus

Re: Variac Amperage?
Steve Conner, Tue Sept 12 2006, 10:08AM

Hi Marcus

1) Pretty much all magnetic devices get hot when plugged in with no load, due to core losses. I would say 0.2A no-load current seems reasonable.

2) I live in the UK and I've managed to get over 30A RMS @ 240V from a wall plug for a few minutes. Our wall plugs are wired on ring circuits protected by 32A breakers. Each plug has a 13A fuse inside, but you can pull twice the rated current through a fuse for several minutes before it pops. (most circuit breakers aren't so generous though)

If you want more, you need to wire your experiment to a separate circuit in your house. The Americans have high current 240V outlets for things like air conditioners and electric cookers, and some American coilers seem to plug their mega coils into these.

All of this varies a lot between countries. For instance here your house gets a single phase 240V supply with an 80A fuse. Other European countries seem to give you less current to play with. In Denmark, I heard the houses get 3 phase 230/400V supply with each leg fused at 16A, and any high powered appliances are designed to run off 400V.

A variac should be able to deliver twice its rated current for a few minutes without any bother.

Re: Variac Amperage?
Marcus Wagner, Tue Sept 12 2006, 08:28PM

Hi, thanks for the answers!

My questions No.2 is answered to my full happiness... ;c)
Now I know that there are big differences in different countries. In Germany each leg is usually fused at 16A, too.

But what me still wories is my questio No.1 my variac.... it heats VERY STRONG in a few seconds with out a load to the point that you can NOT touch it anymore and the insulation also begins to stink. Any ideas? While closely examining the device I found a tiny crack... may it be a bigger crack inside and this causing some asymetric heating? I mean only a small portion of the device gets hot at first and then the temperature spreads onto the full mass...


Greets,

Marcus

Re: Variac Amperage?
Steve Conner, Tue Sept 12 2006, 08:49PM

:-O It sounds like your variac has a shorted turn, assuming you didn't wire it up wrong.

Re: Variac Amperage?
Nik, Tue Sept 12 2006, 08:51PM

Only 80A max on your 240v line! Wow, where I am the older homes have their (single phase) 240v line fused (or breakers) at 100A, all the new homes (20 years or news it seems) have their 240v line fused (or breakers) at 200A! It must be the area.

Re: Variac Amperage?
Marcus Wagner, Tue Sept 12 2006, 08:53PM

well, I got it from ebay for about 5 Euros...... it was working properly at first but on some day it felt down to the floor... I never saw the crack until now...

but there is one more thing... since the terminals broke off, I simply soldered hot& neutral to the endpoints of the variac, effectively shorting 5 turns each together to make a spot great enough to solder the wires to.... was it my failure by shortcircuiting a few wires on each end? I though this would only effect the voltage availabe but produce no shortcircuit current... ouch!

Re: Variac Amperage?
Reaching, Tue Sept 12 2006, 08:56PM

since i build a complete 5A variac by myself i know whats going on with your variac. in my homemade variac i had the problem that the brush shorted some windings of the variac which got very hot in a few seconds,. maybe its the same with your variac, . look at the brush and at the surface where the windings are blank, search for black sprinkles or shorts, maybe you have a short somewhere.

Re: Variac Amperage?
Steve Conner, Wed Sept 13 2006, 10:55AM

I'm pretty sure that shorting those two lots of 5 turns together is what is making the thing overheat.