VARIAC

[EEVaditya]

What are the specifications to be considered while buying Variac?

[nowlan]

I believe they have
1. voltage
2. kva
3. ampere

Depending on your load, you may need a larger one (kva)

[helius]

Additionally, some are designed to sweep 0-100% of input voltage, and others 0-110%.
Often this can be changed by the wiring, if the autotransformer is multitapped.
Some are designed to be mounted on the rear of a panel and rotate counter-clockwise.

[jbryan2]

The voltage and current that your load requires.

It took several tries a couple of years ago for me to get an unbroken one.  The sellers didn't appreciate how much the copper and laminations weigh and underpacked them.  They were reputable sellers and I got refunds, but it was really heartbreaking to get beautiful, never-used variacs all busted up in shipping.

[Shock]

Some other differences:
Max voltage 120v, 240v, 280v, 480v, 560v (the larger ones are normally ganged)
50Hz or 60Hz or both
Additional output taps on the winding (e.g. 240v gives you set outputs at 110/120/220)
Sustained verses peak current rating
Output socket
If it has a voltage or current meter or both (some cheap models have these so beware)
Replaceable brushes
Enclosure
Panel mountable or reversible
Cheap vs quality models

[Electro Fan]

Not sure where you are located but if you can call David Riddle.  His company makes a large array of variacs including some beautiful units.

http://www.ebay.com/itm/NEW-GENERAL-RADIO-240-VOLT-MAINS-METERED-VARIAC-TUBE-AUDIO-AMP-MCINTOSH-C22-/111183396920?pt=LH_DefaultDomain_0&hash=item19e30c0038

If you call him he will walk you and talk you through whatever you want to know about variacs.

[rickselectricalprojects]

the main things are:
safety
is it from a reliable seller
voltage (input and max output (like on some variacs the overwind might be 30 volts so the max output would be 270 volts)
max current
max KVA
i hope i helped and i am only 14 so please dont be to harsh if i got something wrong.

[LaurenceW]

No-one has spelled it out, yet (perhaps it's obvious to all reading, here?) but remember that the output of a variac is REFERRENCED TO THE MAINS NEUTRAL SIDE and NOT ISOLATED! Worse still; wire the variac primary into the main power supply incorrectly, and the entire output will be referrenced to Mains Live! ZZZT....

Also many units - even quality ones - may be supplied as "open frame" - that is, not contained in a safe end-user case, but designed to be built into an equipment cabinet of some kind.

[ManateeMafia]

Some units are isolated like my EJ901V. However, I think a GFCI at the wall or bench is a must have option.

It has a rectified dc output that may be of benefit.

I was leery of some of the new units that are being sold with high current specs. 20A models with 15A outlets and plugs do not seem to be a good combination.

[avvidclif]

Some units are isolated like my EJ901V. However, I think a GFCI at the wall or bench is a must have option.

It has a rectified dc output that may be of benefit.

I was leery of some of the new units that are being sold with high current specs. 20A models with 15A outlets and plugs do not seem to be a good combination.

Nothing wrong with that, having excess capability is good. Think of the opposite, a 15A Variac with a 20A socket!!!!!

[ManateeMafia]

Agreed. The units I have seen have 20A fuses with 15A receptacles!

[saturation]

As variacs are electrical devices, its specs depend heavily on the quality of its materials and assembly.

If you buy a no-name I'd derate it 50% at least and test it thoroughly before deploying it.