Eractic Variac

racfan9

racfan9

Super Member
Good Morning, Well I purchased a standard Electric Variac off of that famous auction site. It was sold as is, no testing of the item. I received it yesterday checked it out then plugged it in hooked up to my voltmeter and the dial is off a little to say the least. With the variac dialed to 10 my voltmeter is reading 15 volts at 30 it's reading 45 on the VM, 50 on the variac is 70 on the meter so you get the idea. My questions are is this fixable and if so what's the culprit and second, is it OK to use it this way as long as I know what's actually being put out via the VM.

Thanks,
racfan9
 
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If it is an older one it might be made for 117 VAC. Today in alot of places 125 VAC is nominal.

The only other thing is look see if it lines up with the zero when all the way down. You might have to loosen the set screw and adjust the knob a bit.

I don't count on such verniers much anymore. I would use a voltmeter. You can get a Harbor Freight elcheapo for like five bucks that is well accurate enough.

Same with like a generator, I want to see it on a scope or a frequency counter before trusting the dial.
 
What JURB said, plus many of those dials were calibrated as just approximations. Some manufacturers supplied a meter that could be installed on the output.
 
JURB said:
...I would use a voltmeter...
Click to expand...
This is the best (safest) way to use a Variac -- the 'calibrations' on the autoformer dial were just... shall we say... guidelines. Many of the "120 VAC" Variacs (IME) are good for about 140V 'wide open'.

A Kill-a-Watt is a nice accessory for testing since it can monitor both voltage and current.
 
I'm with JURB, ignore the dial. Use a a Harbor Freight cheapo to measure the voltage. In fact I would ty-wrap the meter to the Variac so it is always available. Or store the meter in the same place as the Variac.
 
Whats the scale marked as, 0-120 or 0-140 ? If you have a 0-120 scale, make sure it hasn't been re-wired to run 0-140. Most variacs could operate either way, but obviously the scale is different depending on the internal wiring. Not sure about yours specifically but often there is a terminal strip or at least a junction box with some wires where the cord comes in. Hopefully it has instructions or markings as to what does what.


I purposely wired mine to run 0-120. I don't have the correct knob on it, and there is no scale so I have to rely on a voltmeter to see what it even does. I just wanted to make sure that if I wound it wide open, it wouldn't blow anything up. Basically I made it dummy-resistant.
 
Thanks everyone for the replies. I took a picture of the plate on the front and I'm kinda confused that the input voltage is 230 and output is 0-270. Shouldn't input voltage be more like the 130-140 range as earlier mentioned. Is it still usable as long as my meter is verifying actual voltage?

racfan9
 

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You have a variac that is designed to run on 230Volts and is a unit like Gadget mentions will produce voltage above the level of the input.

So what you have is a 0-100% scale for 0-270 Volts. So the number on the dial is NOT the voltage.

You can work with that one but it would not be my choice as the 120V input and 0-140 volt output is better for the US 120V power.
 
Sounds like it's working fine. If you assume the dial is a % of the output voltage, and you assume the highest output voltage would be 280 (with modern voltage). Now if you feed it 120 v, then you get 14v per 10% on the dial, which is almost exactly what you are getting. Neat old piece of equipment, we use those in our lab all the time, and I can say I,ve never had one go out, they are essentially a large copper coil, and they either work or they don't. (Engineers forgive my hand waving!).
 
Well I just checked to see what the max voltage output is with the dial turned all the way up to 100 and the voltmeter read 142 volts. How can that be when my voltage at the outlet in my garage is only 119 volts.

racfan9
 
racfan9 said:
Well I just checked to see what the max voltage output is with the dial turned all the way up to 100 and the voltmeter read 142 volts. How can that be when my voltage at the outlet in my garage is only 119 volts.

racfan9
Click to expand...

An autoformer can produce higher than input. It is essentially one side of the transformer tapped off for the output. The turns ratio can go beyond 1:1 -

300px-Tapped_autotransformer.svg.png


http://en.wikipedia.org/wiki/Autotransformer#Variable_autotransformers

Salient part -

"Typically the primary connection connects to only a part of the winding allowing the output voltage to be varied smoothly from zero to above the input voltage"
 
Sort of the same way there are 120V to 240V conversion transformers.

Knowledge of how a transformer works will enlighten you as to the performance on the one you have.

It probably has internal wiring to allow you to change it to provide only full wall voltage output. That would be either 120V or 240V (nominal) depending on what you plug it into. Most of us in the US have 120V max input variacs and can get 120 or 140 out of them. Yours is more versatile.

See if there is a manual for you unit on the www and you can open it up to see if there are taps for the other output voltage, 100% of input.
 
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