Teardown and repair of year 1961 Soviet autotransformer

[jtu]

I wanted to add an autotransformer to my lab equipment. I could buy new one reasonably priced, but I'm going for used equipment first as I want to learn something in the process.

I stumbled on one on the Facebook Marketplace. Absolutely by chance while selling some stuff myself. There's a picture in the gallery, where you can see it between other old equipment. Lady selling seemed absolutely unaware of what it is, no confirmation if it is working condition. Whatever, offered 10 Eur, got it.

Dave's classic - teardown before turning it on - paid back. It had few burned windings. First I thought: ok, wasted time and money, but it turned out a minor thing upon closer inspection. And so teardown transformed into quick repair.

I don't know what did actually happen, what caused the burn. My guess is the design flaw was the reason: both ends of the transformer's winding where too close, just couple millimetres, and the moving contact or some debris made the short there somehow.

I removed the burned parts. Just two working windings from the "upper" end of the transformer. Fixed the ends with heat resistant tape so nothing flaps around and reassembled. First tested with 10Vpp from my signal generator, then with 200W incandescent bulb from the mains. Not for a long time, longer test will come later. Seem working ok. Hums a bit.

That's it, enjoy the test video and the gallery.

[Electro Detective]

Nice score   

A bit of hum is normal, as is a freaky surge on turn on sometimes    

As long as the top windings are clean, and the carbon brush is making smooth firm contact in a narrow region, you are good to go

Make sure the wiring is absolutely 100% correct, get some fuses happening in case of a blunder,   
and don't assume this aurotransformer has never been messed with since 1961, even if it 'appears' untouched and stock    

Does the sticker mean 250va ?
and is the unit wired/designed for 0 to 127v or 0 to 220v
or 127v to 220v ...very confusing   
it may explain any loud hum = magic smoke may soon be coming   

Edit: it appears the carbon brush arm frame shorted on those windings at one point, perhaps the carbon brushes got worn unevenly,
perhaps someone pushed a bit too hard and it skipped at the stop block,
or perhaps the arm has a secondary insulating stop block that unglued/failed ? 

What are the grey carbon brushes like, do they still have some healthy 'meat' left on them ?
 
Hang on to those removed wire bits btw, you may need them one day,
and they don't take up a lot of storage space   

 

[jtu]

Nice score   

Make sure the wiring is absolutely 100% correct, get some fuses happening in case of a blunder,   
and don't assume this aurotransformer has never been messed with since 1961, even if it 'appears' untouched and stock    

There are no fuses in the device. By design

For practical purposes I use external fuse for any experiments anyway.

I could add fuse. I have no intent to keep this as an authentic / museum item. It seem I could, but I don't see why currently. I don't believe there is historical value and I repaired it without keeping some historic properties already.

Regarding if anybody has messed with it previously. It is really hard to tell for some Soviet products Production quality of this unit is as if somebody made it partly manually. I'd say rather yes, somebody has. Seem that on/off switch on the top has been removed. From the other hand, there are photos of versions without the switch. Plus there was insulation tape used on some connections instead of tube as for others. Using tape is unusual for manufactured items, as far as I know.

Does the sticker mean 250va ?
and is the unit wired/designed for 0 to 127v or 0 to 220v
or 127v to 220v ...very confusing   
it may explain any loud hum = magic smoke may soon be coming   

What are the grey carbon brushes like, do they still have some healthy 'meat' left on them ?
 
Hang on to those removed wire bits btw, you may need them one day,
and they don't take up a lot of storage space   

 

Yes, 250VA.

0 to 127v or 0 to 220v I'm sure. There is a jumper switch on the back. But switch is not connected to anything and by the insides (windings, wiring) it seem this has never been populated. This is 220V unit. From the other hand, there is mechanical 127V limiter (little black knob to be screwed in) and voltmeter has both voltage markings in red.

BTW, it is possible to calculate, I know wire diameter and total resistance.

Finally, yes, I kept those couple windings. I am thinking of adding a few windings because nowadays in my country it is 230V instead of Soviets' 220V.

[jtu]

I think you are right saying that moving the arm with it too far might gave caused the problem. Btw, it’s not carbon brush, it’s carbon wheel.

[Gyro]

Remember that, regardless of any input fuse, you also need to fuse the Output. It's very easy to overcurrent the wiper and destroy the windings at low output voltage settings without the input seeing any overload. This has caused the death of many autotransformers (Variacs).

[Electro Detective]

Gyro is on the  money, input and output fuses = cheap insurance

Suggest a T fuse on the input for startup surge, and any flavor you like on the output

Both 500ma or less IIRC
you may have to sizzle a couple to get the right value if less than 500ma


 

[Doctorandus_P]

Btw, it’s not carbon brush, it’s carbon wheel.

One thing I never quite understood about variac's is that the carbon brush (almost) always connects to 2 windings at the same time, which creates a short circuit.
I have a very small (700mA) Variac with thin wire and on some of the photographs I saw of bigger Variacs the Carbon brush has a big piece of Aluminimum connected to it, probably for cooling.
Could it be that this brush has some extra resistance build in to reduce that shor circuit current?
Can anybody do a thermal image of the temperature rise of the shorted winding?

A carbon wheel should not only have less wear, but it's point contact to the windings will also mean there is less of an opportunity for a short circuit in a winding.

[jtu]

Both 500ma or less IIRC

Device is 250VA, should take more than 1A continuously, you sure you mean 500mA = 0.5A?

[Decapitator]

...I saw of bigger Variacs the Carbon brush has a big piece of Aluminimum connected to it, probably for cooling.
Could it be that this brush has some extra resistance build in to reduce that short circuit current?

Exactly. The carbon brush has a resistance that is designed to bridge 2 adjacent turns while dissipating as little power as possible, both from the "shorted" turn and from the I2R losses due to the output current. The brush carrier and the copper winding each dissipate a portion of the heat generated. I have an open frame 5A 120V Variac on my bench that appears to have about 500 turns. The voltage between adjacent turns would be about 0.25 volts. The 1935 US Patent (US2009013A) goes into specific details about optimizing brush resistance. The 1951 patent (US2949592A) has additional details.

[FlyingHacker]

An NTC works well to limit the inrush current on autotransformers. Like any heat based device, though, it will do little if there is a momentary power outage after it is already warm... neverthelsss for most case it works quite well.