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Variac trivia, will you be able to crack this one?

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Mads Barnkob

Thu Jun 19 2008, 04:37PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

So I got offered another variac without any info but a picture, the wire is about 1.5mm(awg15) in diameter. The plastic box in the background is 16cm tall.

NEW INFO: It have been used in a 230vac measurement circuit, in danish "mÃ¥leopstilling", not sure I translated it right.

Anyone that can guess or recognize its current limit?

Finn Hammer

Thu Jun 19 2008, 05:05PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

Mads,

It strikes me that there are awfully few windings on this unit. Probably only good for 120V max.

Current limit in the 25A range is my guess.

Cheers, Finn Hammer

Mads Barnkob

Thu Jun 19 2008, 06:42PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Sounds about right according to what I have found on google so far, for example this variac

Its a dual 0-140V 1500VA unit, as he writes "It can also be used to control both sides of a 240VAC circuit, for 0-280V input to a 3000VA load." I guess thats is either in series or a variac on phase and neutral

I counted windings on the one I got offered and the one in the link, they are both at about 80+ windings.

Would I be able to drive a 120vac variac for shorter periods of 10-15min, or would I fry it way faster at double voltage?

Dr. Dark Current

Thu Jun 19 2008, 06:53PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

MadsKaizer wrote ...

Would I be able to drive a 120vac variac for shorter periods of 10-15min, or would I fry it way faster at double voltage?

If you doubled the voltage it would draw insane currents and blow the fuse. This is because of the core saturating. You might be able to run it at 110% or maybe 120% its rated voltage but no more.

GeordieBoy

Thu Jun 19 2008, 09:33PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

> It strikes me that there are awfully few windings on this unit. Probably only good for 120V max.

It may be a 400Hz military/aviation variac. If so it will still work at 50Hz, but only up to 1/8th of its specified input voltage without core saturation.

-Richie,

Mads Barnkob

Fri Jun 20 2008, 05:36AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Thanks guys, hopefully the seller will find some more information, but for now it seems I cant use it either.

I understand core saturation like this, overvoltage on the primary will destroy my secondary output, as the magnetic flux cant go higher, secondary output will only be peaks.

would the primary current rise at 230vac instead of 120vac?, there must be something im missing

Steve Ward

Fri Jun 20 2008, 06:40AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

would the primary current rise at 230vac instead of 120vac?, there must be something im missing

The magnetizing current would go up, yes. Its important to distinguish magnetizing current from actual current being forwarded to your load. The magnetizing current is generally quite small because the variac winding should present a fair impedance to the line frequency at the input voltage, and not draw considerable idle current (magnetizing current). The magnetizing current should go up pretty linearly with applied voltage, until saturation, where you get lots of current!

Dr. Slack

Fri Jun 20 2008, 08:01AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

I understand core saturation like this, overvoltage on the primary will destroy my secondary output, as the magnetic flux cant go higher, secondary output will only be peaks.

Well, yes, but more to the point, overvoltage on the primary will destroy the *primary* in a sizzle of smoking enamel, if the breakers hold. When Steve said you get lots of current, he meant lots and lots and lots.

I like the 400Hz theory, there's just no way you should be able to count the turns on a mains variac from a photo. Then again, it might just be a low voltage variac. Does the panel side look "aircrafty"?

You can estimate the voltage/frequency limit by finding the cross section of the core, and doing the flux/change sums.

You can estimate the current rating by measuring the cross section of the wire and using your favourite loading factor. 10A/mm2 is a pretty conservative continuous rating, you could go much higher with shorter runs and/or fan cooling.

Mads Barnkob

Fri Jun 20 2008, 08:24AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

So basicly magnetizing current will rise very high at saturation as it no longer can dissipate its energy to the core/secondary load?

Unfortunately thats the only picture I have received so far, asked him for more but haven't got any yet.

To estimate voltage/frequency, you are talking about faraday's law? (V=2*f*N*A*Bs). So a little guesswork on the voltage or frequency and then see if it sounds fair.

Dr. Slack

Fri Jun 20 2008, 09:37AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

So basicly magnetizing current will rise very high at saturation as it no longer can dissipate its energy to the core/secondary load?

No, it's not about dissipating energy. Magnetising current will rise very high at saturation as the core can no longer generate enough back-emf, because it has lost inductance due to the core saturating. The primary wire is very low resistance, the effective applied voltage that this wire sees for Ohm's Law I=V/R is the supply voltage *minus* the voltage that is being generated in the primary inductance due to the rate of change of current. To a first (and generally pretty useful) approximation, consider the wire resistance is zero, and the primary current that flows is entirely controlled by the need for its rate of change to generate a back-voltage equal to the supply voltage across its inductance. As the inductance drops in saturation, the current rises. As the inductance heads towards zero, the current heads towards infinity (OK, now the wire resistance does start to have an effect, as do the breakers).

The point is that even just doubling the applied voltage sends the core core so deep into saturation, you may easily pull 10x rated load current just as magnetisation, which is 100x dissipation at rated power. Whereas doubling the rated load current by overcurrenting the secondary (but at the right voltage) only gives 4x the rated power dissipation, something a bit easier to manage by intermittent working.

Faraday's Law, yes. I haven't checked the formula you've quoted to see if there are any constant factors missing (the odd 2 here or pi there), but that's basically it, I never remember it, always derive it from first principles each time I come to use it. If you put in 60Hz and 400Hz, you will get two different answers for voltage, and they will each be right for their respective frequencies.

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