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Author Topic: Using a 120 volt rated variac on 220-230 volt system  (Read 1692 times)
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Sideshow Bob
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« on: February 02, 2014, 08:17:52 20:17 »

I might get my hands on a variac rated as follows
120VAC 50/60 3.8A
0-140VAC out
Is it posible to use this safe on a 220 volt system. As long as I work with the same Volt times current product?

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zab
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« Reply #1 on: February 03, 2014, 05:54:02 05:54 »

 
It is not safe as you are applying double voltage on its input. The variac core will saturate on this voltage and ultimately it is burnt or atleast got heat up
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hate
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« Reply #2 on: February 03, 2014, 07:41:28 07:41 »

Damn me! I misunderstood your post thinking you were asking about a varistor. My eyes must be in need of glasses, my brain may have some permanent damage too. Wink Pickit's comment made me realize that you were asking about something that resembles a variac. Wink Sorry about that, please ignore my comment below which is of no relevance to a variac but a varistor.

Basically variacs are used to supress unwanted voltage peaks by being inserted parallel with the input voltage. Using a 120VAC rated variac with 220VAC means it will always be exposed to peak voltages (very high compared to its rating) and get burnt eventually.
« Last Edit: February 04, 2014, 09:29:00 09:29 by hate » Logged

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pickit2
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« Reply #3 on: February 03, 2014, 03:17:36 15:17 »

Some are rated at both 120VAC &/ 220VAC at one voltage the Rated Amps is half that of the other voltage.
If model label says only one voltage and that is 110VAC you could use two variacs of the same type.

We had such set up in workshop of my last employer, On another setup we had two transfomers 220:50VAC at 6AMP
So mains to 50Vac then 50V to mains, this was to isolate mains from unit under test.

To use one you can add a load to drop half the mains at rated output.
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zac
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« Reply #4 on: February 04, 2014, 05:31:37 05:31 »

I think you may be able to use a 120V variac (autotransformer) on 220, but I wouldn't recommend it unless you had no available alternatives.  I think core saturation will limit the output power and there is a very slight risk of breakdown of the insulating varnish.  To test it, I would add an inline fuse and ammeter on the input winding.  Determine the fuse rating by the maximum output wattage divided by input line voltage.  If the fuse blows or the ammeter reading is high with no output load, it's not going to work as it will overheat.  
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Sideshow Bob
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« Reply #5 on: February 06, 2014, 12:26:04 12:26 »

Thank you all for the help. I have by my self also come to the fact. That using this on 220 volt. Will most probably cause some sort of core saturation problems. Giving rise to  potential problems like over heating and output waveform distortion to mention a few. It would probably end up shelfed and then thrown out 5 years later. Been there and done that way to often
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solutions
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« Reply #6 on: February 07, 2014, 12:07:26 00:07 »

I thought core saturation was current dependent?
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optikon
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« Reply #7 on: February 14, 2014, 03:09:02 03:09 »

I thought core saturation was current dependent?

Like all things electronic, there is a relationship between core saturation and current of course. Concerning transformers (and close cousins), usually the working equations are for Bsat and "volt-time" products in order to relate your applied waveforms to the core material datasheets.

Good read covering the fundamentals:

http://en.wikibooks.org/wiki/Electronics/Transformer_Design#Equations
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« Reply #8 on: February 15, 2014, 10:58:52 22:58 »

I thought core saturation was current dependent?

Like optikon said it's primarily a volts/seconds issue.  Whenever designing transformers one of the earliest steps is to set max flux density (which impacts power loss in the core) and then looking at the applied volt/seconds (usually appears in the equations as a V/f term, to calculate the required minimum number of primary turns.
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