[bad question!] Can one recover a variac (autotransformer) from bad rust damage?

[technogeeky]

Are variacs like the ones pictured below recoverable when rust has set in to this extent? Does anyone have an estimate of the probability of success of recovery (or perhaps the rust does not actually mean they are broken?)

Or perhaps that isn't rust?

Any advice would be appreciated. These units would be a nice purchase, but I don't want to just buy some metal weights.

[helius]

Well, the windings are copper, and copper doesn't turn into iron oxide. If the windings aren't broken, and the track the brush moves on isn't cracked or worn out, they are serviceable. It may not be worth your time to clean the crud off if the price isn't much lower than others in good condition.

[edpalmer42]

What rust?  Are you talking about the brown stuff around the cylindrical part?  That's not rust, that's enamel insulation on the wire.  They're a bit dirty, but they don't look too bad, as far as I can see.

Ed

[Electro Detective]

Well, the windings are copper, and copper doesn't turn into iron oxide. If the windings aren't broken, and the track the brush moves on isn't cracked or worn out, they are serviceable. It may not be worth your time to clean the crud off if the price isn't much lower than others in good condition.

Spot on comment from Helius 

If you're feeling brave, looks like you'll need a sunny afternoon and some elbow grease (and WD40) to sort those out,

and if the carbon brushes and top surface are in good shape, you'll have a winner, assuming no shorts/damage on the windings etc.
If you haven't got the dials that go on the rotating shaft, it means extra work and time sourcing parts and stuffing around.

Posting a picture of the terminal block section is recommended, just in case these were ripped out of an installation by wire cutting knuckle draggers   

If you're not familiar with variac wiring and testing, and have no REAL need for one, give these a miss

or if you've bought them, safer to re-sell them 'as is, condition unknown, for parts' etc
grab some cash and let someone else deal with it that's been there before.   

[helius]

I would estimate the value of a working variac of 900 VA including knob and voltage scales to be around $50. Without a nameplate the rating of the units in the picture could be hard to figure, but about 50 VA per pound of weight is a starting point.

[technogeeky]

What rust?  Are you talking about the brown stuff around the cylindrical part?  That's not rust, that's enamel insulation on the wire.  They're a bit dirty, but they don't look too bad, as far as I can see.

Ed

Yes.

After reading the responses it was clearly a stupid question. Other pictures of this variac I had seen had the same black coating (which appears to be like potting material or something) all the way up and down the coil. Thinking about it now, clearly you would only need as much of that substance as necessary to provide rigidity at the section where the mechanical wiping takes place.

The price on offer is $60 for the pair, and they are 1.4 kVA, so I think it's worth pursuing even though I only want one variac, and don't need that kind of power handling.

I was already prepared (perhaps as evidenced by my earlier stupidity, not so much prepared as aware of the potential need ) to complete a restoration of the wiper section.

[technogeeky]

Here are the rest of the pictures I have if anyone has any more comments. I appreciate the advice!

[Electro Detective]

From those pics they look pretty much complete and ready for a poor mans refurbish to me 

Worst case you may get one good working one from parts of both (FrankenVariac)   

[edpalmer42]

The brush is the only part that wears on a typical variac.  If it's worn or broken it could be difficult to replace.  But you'll have a second unit so no problem.  Other than that, it's nice that they have an output of 0 - 140 Volts instead of just going from 0 to the input voltage.

Just remember, it's an autotransformer, not an isolation transformer.

Ed

[technogeeky]

The brush is the only part that wears on a typical variac.  If it's worn or broken it could be difficult to replace.  But you'll have a second unit so no problem.  Other than that, it's nice that they have an output of 0 - 140 Volts instead of just going from 0 to the input voltage.

Just remember, it's an autotransformer, not an isolation transformer.

Ed

Yep, I have some of those also. None of them can handle this sort of power, however... but that is another problem for another day.

[bsalai]

I haven't bought one for years, but see them advertised occasionally. I think that's a good price for that sized variac. They just look a little dirty. If they turn easily, they will probably clean up nicely. Can you check the winding resistance? It will be low, but if it increases smoothly as you rotate it, that would be a good sign.


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[Electro Detective]

Just curious...   

These units are labelled to work on 50hz and 60 hz

from 0 to 140 volts

max current output 1.4 KVA

120 volt input.

-------------------

would they be work with a 240 volt input

at 50 hz

and produce variable output from 0 to 280 volts

at a reduced total output of 0.7 KVA

-------------------

It's a variable autotransformer and should work  , taking into account the reduced current output limits, right?


That said, would a labelled 240 volt unit work in the U.S. the same way, from 0 to 120 volts at 60hz with twice the KVA output?

[helius]

Yes, that will work. The autotransformer is just a primary, no secondary. The brush contact is a tap on the primary that is a percentage of the input voltage, usually 5 to 115 %. It should be obvious why the output is unlikely capable of 0 V.
As to the respective rating, it could be determined by both the saturation of the core (and so lower under 240 V, 50 Hz input) and the wire's ampacity (and so higher under 240 V, 50 Hz input!). The insulation between adjacent loops may also place a limit on the input voltage.

[Electro Detective]

Thanks, whenever I see gear like this with 50hz and 60hz specs, I assume some manufacturers are making devices that can be re-labelled and shipped out to different countries, to work with their respective voltages.
 
May not always be the case, but a nice twofer when it is 

[cdev]

variac windings are that brown color.

Were it not for the rust on the screw yours would look brand new.

Don't use a variac thats meant for 120 volts on 220!

[edpalmer42]

Yes, that will work. The autotransformer is just a primary, no secondary. The brush contact is a tap on the primary that is a percentage of the input voltage, usually 5 to 115 %. It should be obvious why the output is unlikely capable of 0 V.
As to the respective rating, it could be determined by both the saturation of the core (and so lower under 240 V, 50 Hz input) and the wire's ampacity (and so higher under 240 V, 50 Hz input!). The insulation between adjacent loops may also place a limit on the input voltage.

Are you sure about that?  Wouldn't the core saturate due to doubling the voltage, even with no output current?

Ed

[Electro Detective]

ok, here we go... 

[1audio]

You don't what 240V on az 120V variac. The core will saturate and lots of loud hum and smoke will ensue. The 50Hz /60Hz rating indicates that the core has enough iron to handle 50 Hz at 120V (probably +20%) without issue. This can be important. I have never seen a variac with two primarys to support 120-240. Its possible to drive the center tap to one end with 120 and get 240 out but with reduced current capability. 50Hz / 60Hz seems to be common. You can find 50-400 Hz examples and (be careful) 400Hz only examples. The 400Hz units will be really unhappy on 60 Hz. Not enough iron.

Japan for example is 50 Hz on one side of the island and 60 Hz on the other, all at 100V. Not a place to sell traditional electric clocks easily.

[eKretz]

How about an "it definitely won't work." Back when I first started working with electronics I thought this should work also. Then I tried it. There was no smoke, but there was a vaporized carbon brush.

[Electro Detective]

The scores so far:   

Yes, in theory it might/should work     >  1

No way, unless you like magic smoke  >  2

It definitely won't work                      >  1

Try it... with the window open            >   0

[PartialDischarge]

Even if it was a 220V variac it may not work depending on the power of your home line. Why? because of the initial current when the core is not magnetized may trip the overcurrent protections. This is an inductive load not a resistive one so you cant compare with for example a heater.