[ARC5] Need Help with Variac Diagnosis

[Kenneth G. Gordon]

On 19 Feb 2015 at 14:01, Phillip Carpenter wrote:

> I have a General Radio W20MT3 autotransformer, this one handles 20 amps, or
> about 3,000 watts. It came without meters so I've added a Simpson AC voltmeter
> in parallel on the load side and a matching Simpson Ammeter in series with the
> reset beaker on the load side.

Well done, Phillip. I have done the same thing with a large 20 amp Variac I 
have here.

> I'm having two issues: 1) the unit trips my 20A house breaker at no load. I
> believe this is caused by inrush current so I have ordered a 20A inrush current
> limiter to help solve that problem.

Yes. That problem is EXTREMELY common with large Variacs! I have a 20 
amp, 240 VAC input job here. Whenever I turned it on, it would trip the main 
breaker at least 1/2 the time. I did some reading up on Variacs and found 
that this is a common occurrence with high-current Variacs.

In fact, this particular Variac is one I bought from University surplus after I 
retired, and is the same one I used at work. It is wired as it was from the 
factory, and in my use at work, it would trip the main breaker there too.

The solution suggested was a time-delay relay and a big resistor. I wired the 
resistor in series with the input to the Variac, then used a TD relay to bypass 
the resistor after a few seconds. I mounted both items inside the Variac 
case. It all fit.

I have had no more trouble with the main tripping since I did this.

> 2) originally the voltmeter read 120 volts
> when the Autotransformer was rotated full, but now it only reads 117 volts. If I
> switch the output to 140 volts the voltmeter only reads 134 volts. Something has
> started acting like a resistor and dropping the voltage.

Now THAT sounds as though you either have not wired it quite correctly, or 
there is a shorted turn in the winding somewhere.
 
> Does anyone have an idea what could be causing this? I plan to use this
> Autotransformer to drive my tube radios, such as the ARC-5s, as well as my
> Hallicrafters SR-2000 ( which is why I have the 20A version). The AC voltage
> here runs 120-121 volts, so I want to roll it back to 115-117 volts.

Why don't you simply use a bucking transformer instead of a Variac? A 
bucking transformer is much, much safer in that you cannot accidentally 
crank it to the high voltage output setting. Furthermore, since a bucking 
transformer only has to carry the primary current, it can be much smaller 
than a Variac and can do the same job of lowering your line voltage. Its 
regulation is also better than a Variac.

I know several folks who use a large bucking transformer to lower the voltage 
to their entire station.

See this URL:

http://www.w7ekb.com/glowbugs/rx/receivers.htm

then scroll down to 8) Line Voltage Adjustment

to see two different ways to hook up a bucking transformer. I prefer the first 
way, myself.

> I can't figure why the output voltage dropped below 120 or 140, depending on the
> winding setting. Could there be some metal shaving has fallen into the winding
> that is shorting it?

Yes. If even one turn was shorted, that would cause the voltage to drop. I 
would examine the windings very carefully. You may find a bit of metal 
embedded in the winding somewhere. If you're lucky, all you have to do is 
pry it out...carefully.

If there is a shorted turn, that can cause the Variac to get very hot and even 
burn up.

> Any thoughts from those with experience using a Variac is very much appreciated!

See above. ;-)

Ken W7EKB