Vintage Variac Restoration!

[bobhaha]

Hey guys!

I've acquired a vintage (probably around 25 year old) variac from a good friend of mine. He worked as an electrical engineer for many years and has an enormous assortment of different test gear and equipment he has acquired over the years. Anyway... One day I was over at his house and told him that I was looking for a variac and low and behold... he said he had a spare one that needed work. I hesitantly took it off him knowing these are pretty expensive; so I promised that I would make it all pretty and give it a good home.

So this is what I got from him...

Notice the extreme angle of the top plate... Carbon brushes are not touching and there is no end stop.

It's an Australian made variac, made by Warburton Frank

It's an 8 Amp rated variac built for 240V 50Hz input.


So the first thing I did was take the top plate off and unscrewed the middle plastic insert. It was apparent that this variac has seen some use, but the windings looked great! No signs of over heating or arcing! I was really happy :drool:

The copper contacts are quite oxidized and full of muck... So I got to work cleaning it with a tiny bit of isopropyl alcohol and a rag.

I then used some 1000 Grit wet and dry sand paper and carefully went around sanding down to the clean copper surface. I then polished it up with 1200 Grit sandpaper just to make it look pretty

You can see the state of the contacts before on the right and after on the left.

Next thing to fix was the base... It was pitted and oxidized and didn't particularly look good.

I gave it a quick sanding with 1000 Grit sandpaper...

Then a little wash with isopropyl alcohol and a rag.

The next thing to do was fix this badly aligned top plate...

I placed the assembly into the vice and decompressed it inorder to remove the circlip.

I replaced it with a stronger clip then removed it from the vice...

It helped, but didn't fix the problem... Still no endstop and it still sat kinda wonky..

So I placed a washer to the outside edge, removed all spacers on the inside edge and replaced the circlip...

And that worked great!!

In this picture the plate looks crook because there is no shaft running through the middle and the brushes springs are pushing one side up. But it's atleast touching the contacts now

I placed a test shaft to see what it looks like...

Looks great! Now there is pressure on the contact and it glides soooo smoothly! I'm really happy!

Here is a short video showing it all working together....



Next step is to measure up and get a custom lasercut case for it. I'm hoping to do it all in acrylic so you can see inside. I also need to find a sturdy fibreglass rod to replace the aluminium one I have in there right now. Then lastly I need a voltage scale and knob made up... Those will probably be laser cut/etched as well.

So stay tuned for more!

-Adrian

 

[Meatball]

Hey nice work on that! It should have many years of service left on it.

Just curious though, did you actually test it yet? Its a smooth voltage adjustment, no obscene "hops" between taps?

 

[Things]

Wish I could get a free variac

Laser cut case should be fine with that solid base, although yeah, handles will take a bit of consideration.

 

[Presentteck]

NICE
That thing looks awesome and I hope you get it up and running soon.

I'm not quite as lucky getting one for free, but I did manage to get a variac for $7 USD at a thrift shop with all the bells and whistles. No really, It has bells and whistles...

LOL :crackup: (for those who dont know, it's for model trains. It has a bell, whistle, and reverse button)

It is an 80W model, 0-18V. I think it goes up to something like 4 amps max.
I'll probably take it outta that case, but I haven't needed it just yet. Just another place to look for those who are looking though.

 

[lasersbee]

Great job restoring that Variac...:gj:
They are great when you want to test for low power
or brownout conditions of electronic designs.


Jerry

 

[bobhaha]

Hey nice work on that! It should have many years of service left on it.

Just curious though, did you actually test it yet? Its a smooth voltage adjustment, no obscene "hops" between taps?

There is a video in the OP showing me using it. I just read the resistance of the coil as I moved it and it looks pretty nice!

Wish I could get a free variac

Laser cut case should be fine with that solid base, although yeah, handles will take a bit of consideration.

I think 8mm or 10mm acrylic should be strong enough to handle it (get it hahahahahhahahaha no :scowl

 

[Speedy78]

I had no idea what a variac was untill I saw this thread. I then proceeded to google it. These are pretty neat. Putting the whole thing in an acrylic case would look good.

 

[millirad]

Nice toy! About the only thing that could go wrong would be a windings insulation could crack and short out, due to it's age. But it looks good to go. Have fun!

 

[bobhaha]

I have a quick question for someone with a bit more knowledge....

If I want to over-volt this thing, i.e. place the hot side on the center tap and pass it with the wiper getting double the input voltage... what kind of ballast do I need?

I read somewhere that it's recommended to use a 700-1000W ballast. Where can I get one cheaply?

Would I be able to use the primary of a MOT, with the secondary shorted, as a ballast? Those things are rated to ~2500W... that should allow ~10.5A max right?

 

[Sigurthr]

Needing a ballast at all depends on the load you're going to put the variac under. Going up to the overwinding (1:>1) section does not intrinsically require the use of a ballast.

Variacs are usally rated in terms of VA (volt-amps) for a certain voltage (the 1:1 voltage). So if you have a 1200VA 120V variac, it is rated for 10A draw at 120V, regardless of where the 120V mark takes place (center of winding or end). It should be noted that the current carrying capacity of the winding does not care what voltage is being applied or sourced. It can handle 10A at 10V or 100V or 240V. Copper losses are determined by I^2 * R, where R is the resistance of the winding.

So, if your load only draws 8A or less at whatever voltage you're pumping in to it then you do not need a ballast. Typically 240V variacs go up to 336V when overwound, so for full output voltage the load would have to be at least 42-Ohms to draw 8A or less. If it will draw more than 8A at whatever your chosen voltage is then you will need a ballast to increase the impedance to at least the required amount to limit current to 8A or less.

Xl = 2pi f L
where Xl = inductive reactance of ballast needed, L = inductance of ballast needed

Xl = 6.28 x 50 (in aussieland) x L
Xl = 314 x L
Xl = Vout / 8
Vout / 8 = 314 x L
L =

example: 336V out you need a 42-Ohm ballast:
42 = 314 x L
42 / 314 = L
L = 0.1337H or 133.7mH


That all being said variacs can survive being overloaded for a short period of time. I only have a 500VA variac rated for 5A @ 120V but I regularly run the thing at 18A @ 120V (~2200VA) for short run times. It did overheat once, melt the winding it was on, arc from the brush to the next winding, and destroy several turns, but I repaired it afterwards. Just watch the temperature of the winding and the brush and you can safely run it overloaded for a short period of time. If you're planning on running it for more than a minute or so at more than the rated current, you will want a ballast.

 

[Cyparagon]

What do you mean overvolting? If you're talking about 0-270V instead of 0-240V just read the label:
For overvoltage connection: input: 2&5; output: 2&3. That sets you up for 0-270V instead of 0-240.
If you intend on getting anything over 270V out of this thing, it's gonna be more tricky than just sticking an inductor on it.

 

[Pman]

Had a friend give me an old one that I cleaned up and is now smooth. Man these things weigh a ton for such a small package (about 15lbs). Hampden BPS-16-F rated for 10 amps at 120V. Don't know how old it is but the new ones have a keyed on/off switch while mine has a different type of key switch.

 

[Cyparagon]

the new ones have a keyed on/off switch while mine has a different type of key switch.

Why? Do you have key switches on your outlets too?

 

[LaserCo]

I have a few variacs here I can use. I have never heard of a keyswitch on them, weird!

What is really handy with them is to be able to read an ammeter while you are cranking up the juice, I suggest that you plan one into your case design.
The old style meter, not a digital readout also.

 

[bobhaha]

Needing a ballast at all depends on the load you're going to put the variac under. Going up to the overwinding (1:>1) section does not intrinsically require the use of a ballast.

Variacs are usally rated in terms of VA (volt-amps) for a certain voltage (the 1:1 voltage). So if you have a 1200VA 120V variac, it is rated for 10A draw at 120V, regardless of where the 120V mark takes place (center of winding or end). It should be noted that the current carrying capacity of the winding does not care what voltage is being applied or sourced. It can handle 10A at 10V or 100V or 240V. Copper losses are determined by I^2 * R, where R is the resistance of the winding.

So, if your load only draws 8A or less at whatever voltage you're pumping in to it then you do not need a ballast. Typically 240V variacs go up to 336V when overwound, so for full output voltage the load would have to be at least 42-Ohms to draw 8A or less. If it will draw more than 8A at whatever your chosen voltage is then you will need a ballast to increase the impedance to at least the required amount to limit current to 8A or less.

Xl = 2pi f L
where Xl = inductive reactance of ballast needed, L = inductance of ballast needed

Xl = 6.28 x 50 (in aussieland) x L
Xl = 314 x L
Xl = Vout / 8
Vout / 8 = 314 x L
L =

example: 336V out you need a 42-Ohm ballast:
42 = 314 x L
42 / 314 = L
L = 0.1337H or 133.7mH


That all being said variacs can survive being overloaded for a short period of time. I only have a 500VA variac rated for 5A @ 120V but I regularly run the thing at 18A @ 120V (~2200VA) for short run times. It did overheat once, melt the winding it was on, arc from the brush to the next winding, and destroy several turns, but I repaired it afterwards. Just watch the temperature of the winding and the brush and you can safely run it overloaded for a short period of time. If you're planning on running it for more than a minute or so at more than the rated current, you will want a ballast.

Ok this doesn't make sense to me... sorry if this sounds stupid, but a volt-amp is literally the output voltage multiplied by the amperage... right?

So if the windings in my case can handle 8A at 240V giving a VA rating of 1920VA... Doesn't this mean at 336V the current handling capability now diminishes to ~5.7A? Otherwise you are violating the 1920VA rating.... :thinking:

I have a few variacs here I can use. I have never heard of a keyswitch on them, weird!

What is really handy with them is to be able to read an ammeter while you are cranking up the juice, I suggest that you plan one into your case design.
The old style meter, not a digital readout also.

I plan on doing just that... and possibly a circuit breaker built in too for the output (and possibly a sneaky by pass switch for the breaker )

 

[Cyparagon]

We can't help you until you explain to us what you're trying to do.