Variac Testing

Please describe your "strange things".

I am using two 40 Amp variacs, one to limit the current and one as voltage control. I have wired them up as follows: The current control variac is in series with the input of the voltage variac (this variac has been cut to act as a variable inductor, 0.4 to 27.8mH) and the other end of the voltage variac is connected to the neutral; the brush is the output to my transformer. The problem is that when I turn on the mains, the voltage variac makes a buzzing sound (obviously mains frequency) and the ammeter is reading 10 Amps. I think that the core is saturating. However, if I remove the current control variac and connect the input of the voltage variac to another variac controlled power supply, so that the input to the variac in question is supplied via the brush output of the power supply variac there is only a quiet hum from the variac and the ammeter is reading about 0.5 Amps. What is wrong and how do I fix the problem?

You may have a short between windings where the insulation has been damaged. If this is the case, it might be a tough fix... not impossible, but very difficult without rewinding the entire thing. I have known of people to partially unwind one, re-coat the wire, and carefully re-wind the section that was damaged... but i suspect yours has been damaged by over-amperage, (assumption made with little basis) and the insulation between windings has been compromised, resulting in a short, which cannot easily be repaired. If that is indeed the case I would either (if you had a bunch of time, and needed a project) rewind the entire thing, or buy a new one.

I have only used them as a Current control device to burn in tube amps, never a Voltage control.

Thanks for the suggestion RVZ717 about a possible short between windings, hmm, never thought of that, silly me. So, I am busy stripping the variac and will thoroughly clean it and revarnish and see what happens.

Try swapping the variacs, so the voltage variac is used as the current control, providing this is still possible.

Have you tried the voltage variac direct to the mains to check its operation? If it hums, it's crook, unless you have connected it incorrectly.

A picture is much more descriptive than 1000 words.

please post circuits to understand Internationally as Drawings have the International language.

A variac is an auto-transformer type of a device. The 2 end of the coil winding serve as the (primary side) and are used solely as the input. Where one end of the coil is connected to the neutral wire while the other end to the hot or live wire. The output side (secondary side) should be taken from the wiper or the brush end with the Neutral wire serving as the common terminal between the primary and secondary.

Using the brush or wiper end of the variac as one of the inputs or one of the legs in the primary where 120 volts is fed will cause the humming sound, drawing large current since the transformer winding is being saturated eventually damaging the device.

<< The current control variac is in series with the input of the voltage variac (this variac has been cut to act as a variable inductor, 0.4 to 27.8mH) and the other end of the voltage variac is connected to the neutral; the brush is the output to my transformer. >>

Why you need to cut?

Just use Sliding Point and any one of other 2 points.

Insulate the free end.

Don't put the SOcalled Inductor on lower side but in series with the Input side.

Leave the 'N' to the nuetral. Don't lift it.

Thanks for the info. I'll atempt to rewind it initially. If that fails, then I'll have to get myself a new variac.

Cheers.

If you are going to rewind it, be sure to use the correct size of the magnet wire coated with several layers of insulation then wind them tightly together to achieve the correct inductance. Good luck..

Don't try it if you have not the professional facillities of winding. And Variac is a very special item for winding orr rewinding.

I think that you have already tampered with the one giving abnormal Hum.

"(this variac has been cut to act as a variable inductor, 0.4 to 27.8mH)"
How do you cut a variac to act as a variable inductor?
What you are trying to do is very difficult.
It is possible to connect a variac and a fixed inductor to act as a variable inductor.
Why don't you post a schematic of what you are doing. There may be some helpful comments.

Good Luck

No, you are missing the point here. If you feed the live into one terminal of a variac and use the brush as the output (leaving the other terminal hanging, i.e. not attached to anything), then it will act as a variable inductor. Cutting the core is to prevent saturation. This variac is working perfectly. It is the voltage variac that is the problem. I have connected the same set-up with another variac and it works fine plus the it was working for over three years.

This is the schematic of the variac layout:

Yes !

This is the way I pointed out in post#10.

I'm surprised that the schematic shown would work successfully.

If there is a load on the output, the voltage variac is near maximum output voltage and the inductor variac is set near minimum inductance value, there will be a high volt/turn on the inductor variac and it will hum.

A variac can be checked for shorted turns by applying rated voltage to the variac and using a clamp on AC ammeter with the jaws open. Move the clamp on ammeter around the variac and where the shorted turns are will indicate a high reading.
Yes this sounds stupid, but have used it many times and it does work.

Just for everyone's information: Variac is the trade name for a particular brand of variable voltage transformer. The generic term is variable voltage transformer.

Unless there are shorts in the windings of the voltage variac, the circuit schematic as drawn, will work and is capable of producing a variable output with a range of anywhere from 0 to 240 V maximum without saturating. That is even with the series connected variac being shorted. In this particular case, he will only lose his so called variable series inductance.

Inter-winding shorts will eventually develop even on normal usage if the wiper is frequently moved around or by using substandard brush replacements. Carbon residues, left or that settled in-between or around the windings most often causes this shorts and if not cleaned will eventually create corrosion and arcing between windings caused by the wiper sweeping action.

Hi VSAR,

I have stripped the voltage control variac and there is a lot of debris in between the windings especially where the brush travels. Going to thoroughly clean it all and revarnish it, obviouly not the track area. Then we will see if it work.

Thanks.

You may not need to rewind it if you can clean and remove all carbon deposits! Be sure you replace the brush with the proper type so that you minimize further deterioration. The carbon brush need not be too soft nor too hard that it crumbles as it sweeps the windings, thereby minimizing unnecessary arcing. Brush spring tension is also important..

Hi Carl Pugh,

Thanks for the tip with the clamp meter!!!