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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

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Old 26th Nov 2004, 7:51 pm   #1
Radio_Dave
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Default Variac voltage problems

Hi,

I recently brought a bare variac. I have fitted it into a case and when I first wired it up the knob worked in reverse and the maximum voltage was only 264V ( not 276V ) I rectified my wiring mistake and tried again, this time the knobs is working in the right direction but the voltage is well over 350V and the series lamp limiter (which I was using for precautions ) is now glowing quite brightly.

I can't figure out what is wrong any ideas? I am sure the connections are right now

Many Thanks

David

Last edited by Station X; 6th Jan 2005 at 9:49 pm. Reason: Import
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Old 26th Nov 2004, 10:17 pm   #2
jim_beacon
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Default Re: Variac voltage problems

David,

it sounds as if you have one of the variacs with a multi tapped winding (often 4 or 5 connections to the winding, plus the wiper). Generaly, the neutral for the feed and the load go to the tapping at the anti-clockwise end of the winding, the live load goes to the wiper, and the live feed goes to either the most clockwise tap (max output = feed volts), or to the most clockwise but one (max ouput = feed + 10%). The other two taps are noemally intende for 110V, and give either 250 out (nearest the ccw end, or 276 for the next along).

I would surmise that you have connected your live feed to one of the 110V taps, hence the high ouput.

If you use your multi-meter on a very low ohms range, place one probe on the anti-clockwise end of the winding and measure the resistance to the connectors on the panel. In increasing resistance order you should get:

neutral (0 ohms)
110V in, 250V+10% tap
110V in, 250V out
250V in, 250V +10% out
250V in, 250V out

Assuming the winding has 5 connections.

Hope that helps

Jim.
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Old 26th Nov 2004, 11:12 pm   #3
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Default Re: Variac voltage problems

The common neutral can be placed at either end of the winding depending on which way the variac is mounted, the knob can be removed and placed on the other end of the spindle.

The full winding measures 16.4 ohms
The tap for the anticlockwise 115% output measures 10.4 ohms
The tap for the clockwise 115% output measures 14.8 ohms

Why aren't the two 115% taps the same? The instructions also say that the spindle is earthed, and it isn't! would this cause a problem

Thanks

David

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Old 26th Nov 2004, 11:47 pm   #4
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Default Re: Variac voltage problems

David,

considering the small number of turns on these machines, the difference in readings for the 115% taps may indicate shorted turns - does it get hot if run with no load?

Jim.
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Old 27th Nov 2004, 12:02 am   #5
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Default Re: Variac voltage problems

Hi Jim,

The variac is brand new, I have dismantled it to look at the windings and they are fine. The two ( supposed 115% tappings are soldered in the wrong place. I wonder if this was made as a special order? ( I did buy it off eBay )

Would I be right in thinking that if I multiplied the total winding of 16.6 ohms by 115% and then subtracted it I would find the position for the 115% over voltage tap? If I'm right the same maths means the tapping I'm using is giving me 137% over voltage

Many Thanks

David
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Old 27th Nov 2004, 12:42 am   #6
jim_beacon
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Default Re: Variac voltage problems

David,

the tap should appear at 14.8 Ohms from the neutral end. I did just type the proof in, but the computer lost it

You use the normal transformer equation:

Np/Ns=Vp/Vs

where N is number of turns, V is volts, p is primary and s is secondary. Also, as we are measuring a DC resistance, number of turns is proportional to resistance (note: this will not work for most auto-transformers found in sets, as the neutral end of the winding is wound with heavier gauge wire - it carries both the primary and secondary current, where as the top end of the winding only carries one or the other, depending on whether it is a step up or a step down transformer).

Therefore we can say

Vp/Vs=Rp/Rs,

and work out the tap point from this.

*MODIFYED 30/11/04 TO REMOVE INCORRECT STATEMENT* JB.

Jim.
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Old 27th Nov 2004, 10:43 am   #7
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Default Re: Variac voltage problems

Thanks Jim,

I used your equation and it does prove that I have a tapping for a 136% over voltage with a clockwise rotation of the spindle and a 110% over voltage tapping with a anti clockwise rotation. The variac is supposed to be a Regavolt 402 with two 115% taps for clockwise or anticlockwise rotation and it is rated at 2.5 amps

My plan is to count the number of turns from the live end to the 110% tap and then replicate that tap on the other side so I sould have a 110% overvoltage with a clockwise rotation

I need to know if the spindle should be earthed. the instructions say it is earthed at one end but it is running in nylon bushes and is completely isolated. The top bush does have a slot in and it wouldn't take alot of imagination to make something from brass which would carry an earth it. Can anybody confirm this?

Many Thanks

David
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Old 27th Nov 2004, 8:12 pm   #8
Colin
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Default Re: Variac voltage problems

Hi Dave,

My Regavolt autotransformer has a brass insert in the rear bush that connects the spindle to earth.

rgds
Colin
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Old 28th Nov 2004, 5:38 pm   #9
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Default Re: Variac voltage problems

Thanks Colin for the confirmation about the earth contact It was easy to make a contact and now the spindle is earthed

Thanks Jim for you help with finding the places for correct tappings, Unfortunately it proved too difficult to attach the tapping to the winding in a satisfactory manner so I have connected the variac as ordinary a 1:1 240v output. I decided that I really had no need for the overvoltage settings anyhow

Many Thanks

David
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Old 30th Nov 2004, 9:32 pm   #10
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Default Re: Variac voltage problems

I HAVE EDITTED ONE OF MY EARLIER POSTS TO REMOVE INCORRECT INFORMATION, PLEASE RE-READ THE THREAD

Jim.
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