Help identify variac

[chrish]

Hello everyone,

I managed to get a good deal on eBay (or so I thought) on a pair of variacs. The reason they were cheap, I believe, is because they had no specification.

In the photos below it is difficult to see, but the output is a US style 2 hole socket. The adjustment knob has 'YAMABISHI' printed on it. I am assuming therefore that it is intended for the US or Japanese market. The dimensions of the case are 13cm diameter, 11cm tall (not including the knob). The unit weighs about 3.8kg.

I live in Australia where the voltage is 230-240 50Hz. I am guessing that even if this unit was made for the US or Japanese market, it would not be that critical using it on 240 volts as long as I remained within the current limit? This leads to next question, what would be a sensible current limit for this device given the dimensions and weight?

I intend to house it in an insulated box with fuse protection, a voltage and current meter, and a proper line cord and socket for the device under test if the variac can be used.

Thanks in advance,

Chris



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[zigzagflux]

If the unit was designed for 60 Hz, and you intend to use it on 50 Hz, you may have a problem, as the core could saturate. Might want to place a current meter in series with the excitation voltage and slowly increase voltage to the final desired value (yes, this would require an additional variac to perform the test). The idea is that current excitation will quickly increase as the core nears saturation. Unfortunately, this variac doesn't have the 260V end tap that you could use.

As far as current capability, you have two choices: compare the size and weight to new units in a Newark or Mouser catalog, and assume the capability to be the same. Or, get an approximation of the wire size and reverse engineer. I prefer the former.

Edit: I see you actually bought two of them. Good, now it will be easier to test.

[UncleArt]

Strictly based on size, using a similar ballpoint pen for scale, your unit is the same size as my Calrad 45-740, which as a nameplate rating of 5 amps.

But, that's at an AC input of 115 volts, with an output range of 0-130v


-regards

art

[sawreyrw]

Chrish,

How do you know you can use this unit at 240 volts, 50 Hz? If it is designed for 120 volts and you apply 240 to it, you will have big trouble.

[chrish]

I don't know I can use it, and have not yet...

My assumption is that it is a transformer, and that if it was designed for 110 volts and 50 Hz, it might be able to be used for 240 volts 60 Hz, but with limitations. I am guessing that the issues will be:

1. Breakdown voltage for the insulation. I am hoping that if it is mains rated, 240 will still be low enough not be an issue.

2. Current rating of the windings.

3. Saturation of the core for lower frequency.

I think I read in Building Valve Amplifiers that variacs generally have a amperage limitation due to winding gauge rather than a VA rating. Any idea if that would translate to having to halve any 120 volt rating for 240 volts input?

My guestimate was 5 amps (even if it is at 120 volts), thanks for the confirmation of that at least UncleArt.

I am hoping that with these limitations, the units might still be usable for my purpose, testing amplifiers etc. As I am not an engineer, I am hoping that if I am about to do something stupid by trying, one of you knowledgeable gentlemen might warn me

Thanks for the input so far!

Regards,

Chris

[rmyauck]

I've also got an old (problably 40+ yrs)10A one off of eBay last year, same ht. (11cm) and 16cm wide. 5.4 kg. The way its hooked up it doesn't give more than the line voltage 121-123V at max setting. Had pay almost as much for shipping as the unit as it came from the U.S..

Why not try using both for your 240V 50HZ. One each in parallel so you are spreading the load on the two units. Maybe only would work with the two in series. Hook up a cheap voltmeter to see what your getting. Maybe 50HZ won't matter if you don't put a big load on them either.

Maybe totally wrong! Just a thought and probably out to lunch!

Are there any converter devices available that you could borrow to try them on 120V first to make sure it is designed for that voltage?

Randy

[zigzagflux]

If you don't know it's 240V rated, assume it's only 120.

In order to know if it is in fact 240V rated, you must run the excitation current test.

[kevinkr]

The Yamabishi I owned was not confidence inspiring, in fact mine appeared to have been built for the 100V Japanese market and it was real marginal thermally on 120V .. I believe these were sold here as cheap competition to the rugged and expensive Genrad Variacs that were common lab equipment here back in the day.. (I have one of those Genrad it is now about 50yrs old, I've owned for about half that time.)

I'll keep my fingers crossed but I suspect it may mis-behave at 240V/50Hz. I would just stick a 100W or larger lamp in series with the AC to the variac, if it cannot handle the voltage and frequency the lamp will just light up quite brightly signifying that the core has saturated and you'll know - no need even for a second variac to do this test. (This is a non destructive test.) If this passes then I would proceed to the excitation current test mentioned, anything over a hundred mA or so would be very suspect IMHO.

Good luck!

[chrish]

Thanks Zigzag,

How should I conduct the test with the two variacs? First variac - ammeter - second variac, have first variac set for minimum voltage, second for max and then slowly wind up voltage of first variac watching current? Should I place a load on the second variac? I have several DMMs, so can watch amps at input of first variac too if required.

Thanks for the advice so far,

Chris

Thanks, you posted while I was writing...

These were pretty cheap, about $25 for the pair, so not such a problem if they do not work out.

[gingertube]

Chris,
If these were intended for 115V Ac 60Hz then they are not safe to connect to a 250V AC 50Hz supply - not safe from a overheating failure point of view that is.

Variacs of any description, even correctly rated units, have no input to output isolation, they are a single variable tapped winding, an inductive voltage divider if you like - so from an electrical safety point of view, no variac can be called safe anyway. I won't have one in my workshop even though it is properly wired with Residual Current Breakers (or Earth Leakage breaker or whatever you like to call them).

I must admit to a certain trepidation each time I see posts about using Variacs. I wonder how many people who use them understand that the variac output has exactly the same inherent safety risks as the power connections on their input. I have used variacs in the past, powered from an isolation transformer.

Cheers,
Ian