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Variac Amperage?

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Shahn

Fri Apr 28 2006, 02:32AM

Registered Member #210 Joined: Sun Feb 19 2006, 08:25AM
Location:
Posts: 26

I am buying a Variac localy and have the choice between:

VARIAC-2520(USED) - Variac Input 0~240V 50/60Hz, Output 240/280V @ 10A,
Flange fixing with 1/4" shaft and knob - NZ$252 + GST

VARIAC-V50HN(USED) - Variac Input 0~230V 50/60Hz, Output 240/280V @ 15A,
Base fixing with 1/2" shaft and missing dial/wheel - NZ$315 + GST

VARIAC-230V-28A-7K5VA-1256 - Variac Input 0~240V 50/60Hz, OLD open design -
Output 240/280V @ 28A, Flange fixing with 3/4" shaft and control wheel -
NZ$506.25 + GST

I would prefer to get the 10A becaus its cheaper but dont want to have to upgrade any time soon. What sort of size SSTC/DRSSTC will I be able to run from these. It obviously depends on how/what Im making but in general?

Thanks

HV Enthusiast

Fri Apr 28 2006, 02:40AM

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

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.

Colin 99

Fri Apr 28 2006, 03:20AM

Registered Member #192 Joined: Fri Feb 17 2006, 03:08AM
Location: Canada
Posts: 44

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

Desmogod

Fri Apr 28 2006, 06:13AM

Registered Member #139 Joined: Sat Feb 11 2006, 11:01AM
Location: Perth, Western Australia
Posts: 358

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.

hvguy

Fri Apr 28 2006, 06:56AM

Registered Member #289 Joined: Mon Mar 06 2006, 10:45AM
Location: Conroe, TX
Posts: 154

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.

Sulaiman

Fri Apr 28 2006, 07:40AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

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.

Desmogod

Fri Apr 28 2006, 08:00AM

Registered Member #139 Joined: Sat Feb 11 2006, 11:01AM
Location: Perth, Western Australia
Posts: 358

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

Hazmatt_(The Underdog)

Fri Apr 28 2006, 08:43AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

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.

Shahn

Fri Apr 28 2006, 12:43PM

Registered Member #210 Joined: Sun Feb 19 2006, 08:25AM
Location:
Posts: 26

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

hvguy

Sat Apr 29 2006, 09:08AM

Registered Member #289 Joined: Mon Mar 06 2006, 10:45AM
Location: Conroe, TX
Posts: 154

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

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