Using a 120V Variac on 230V - Page 1
Forum section: electronics / beginners

author: Swake
author_rank: Frequent Contributor
timestamp: December 06, 2018, 04:33:06 pm
content: Found an original 120V Variac in very good condition. It is a 60ties or 70ties model. Is there anything that would refrain me from using it on 230V?The amperage rating should be halved to maintain same power rating of course, but anything else?

author: Andy Watson
author_rank: Super Contributor
timestamp: December 06, 2018, 04:36:30 pm
content: Because it will likely saturate the core and release the magic smoke.

author: perieanuo
author_rank: Frequent Contributor
timestamp: December 06, 2018, 04:43:13 pm
content: You can use it, but only once...Envoyé de mon iPad en utilisant Tapatalk

author: Swake
author_rank: Frequent Contributor
timestamp: December 06, 2018, 04:54:54 pm
content: Duh... forgot about that one.Thanks for the reminder. There is also something with the frequency if I recall correctly. And I didn't check but this is probably a 60Hz model while we're at 50Hz.Subsidiary question: can core saturation be measured/detected (using DIY level equipment that is)?

author: Totalsolutions
author_rank: Contributor
timestamp: December 06, 2018, 04:58:33 pm
content: Watch Dave release the magic smoke!A transformer rated at 120 v ac on 230 v ac.https://youtu.be/8itTKH5tj3s

author: Zero999
author_rank: Super Contributor
timestamp: December 06, 2018, 05:04:14 pm
content: Quote from: Swake on December 06, 2018, 04:33:06 pmYes it's frequency dependant. You could theoretically use it at 230V, if the frequency were increased to 120Hz. Of course there's a limit to how far this can be pushed as hysteresis losses in the core increase with frequency and the insulation can only take a certain voltage before failing.Measuring the current would do.

author: Gyro
author_rank: Super Contributor
timestamp: December 06, 2018, 07:01:51 pm
content: I suspect that most variacs operate pretty close to core saturation - just on the basis of practical physical size, weight and cost. Trying to squeeze another 10-20% - maybe. An additional 100% - No chance.Just a thought, depending on quality and cost of your variac (probably wouldn't be worth it with a cheap chinese one). You could put a 240 to 120V transformer on the input, and another, as 120 to 240V on the output. You could use autotransformers, or, more usefully, make one of them an isolation transformer. It depends how the economics work out foryou.

author: Wolfram
author_rank: Frequent Contributor
timestamp: December 06, 2018, 08:01:54 pm
content: Quote from: Hero999 on December 06, 2018, 05:04:14 pmFlux density is inversely proportional to applied frequency, everything else being the same. Since the flux density exponent larger than the frequency exponent in the Steinmetz core loss equation (at least for GOSS used in mains transformers), hysteresis losses should go down with increasing frequency. They only increase with frequency if the flux density is kept constant.

author: Zero999
author_rank: Super Contributor
timestamp: December 06, 2018, 09:55:31 pm
content: Quote from: Wolfram on December 06, 2018, 08:01:54 pmBut everything else isn't the same. The voltage is being doubled as well as the frequency, so the flux density is kept constant, hence higher hysteresis losses.Eddy losses will also increase with double the voltage and frequency.

author: Wolfram
author_rank: Frequent Contributor
timestamp: December 06, 2018, 11:13:06 pm
content: Quote from: Hero999 on December 06, 2018, 09:55:31 pmYou're right. I completely missed the fact that the voltage is being increased proportionally as well.

author: perieanuo
author_rank: Frequent Contributor
timestamp: December 07, 2018, 08:30:45 am
content: Quote from: Hero999 on December 06, 2018, 05:04:14 pmcan't stop laughing omgold variac=old transformer with voltage taken from intermediary transormator position.FREQUENCY DOESN'T CHANGE.come back after learning electronics basics.https://en.wikipedia.org/wiki/Autotransformerhttps://www.ebay.fr/itm/Autotransformateur-VARIAC-500VA-tension-variable-0-250-VAC-3-Amp-230V-NEUF-/272778656159pierre

author: Zero999
author_rank: Super Contributor
timestamp: December 07, 2018, 08:53:42 am
content: Quote from: perieanuo on December 07, 2018, 08:30:45 amWhat are you on about? Of course transformers don't change the frequency. I never said they did. I was talking about how the voltage can be doubled, without cooking the transformer. Go back and re-read my post!

author: perieanuo
author_rank: Frequent Contributor
timestamp: December 07, 2018, 09:02:41 am
content: Quote from: Hero999 on December 07, 2018, 08:53:42 amin fact you're right, I misread.but don't forget, wire gauge on transformer has current in equation, you can't just double the voltage amplitude AND the freq and put this as possible scenario, tha's just not engineering.putting aside core saturation, everyone knows every respected manufacturer put just the material to do the job with maybe 20% margin but not 100%.not speaking of isolation matters added by doubling voltage.Scenarios like this are just sci-fy.he can't do what he ask, point.you can make drawings all day long, I say no, bad idea.I noticed lot of people on this forum put on paper idea

author: perieanuo
author_rank: Frequent Contributor
timestamp: December 07, 2018, 09:05:23 am
content: Quote from: Hero999 on December 07, 2018, 08:53:42 amin fact you're right, I misread.but don't forget, wire gauge on transformer has current in equation, you can't just double the voltage amplitude AND the freq and put this as possible scenario, tha's just not engineering.putting aside core saturation, everyone knows every respected manufacturer put just the material to do the job with maybe 20% margin but not 100%.not speaking of isolation matters added by doubling voltage.Scenarios like this are just sci-fy.he can't do what he ask, point.you can make drawings all day long, I say no, bad idea.I noticed lot of people on this forum put on paper idealistic things.Reality is sadly different.I never want to argue for nothing, but you cannot advice a beginner to try such thing, the guy can loose an eye for example, burn his thumb or whatever cause somebody gives 'maybe will work like this' ideas.pierre

author: Zero999
author_rank: Super Contributor
timestamp: December 07, 2018, 09:19:49 am
content: Quote from: perieanuo on December 07, 2018, 09:05:23 amThe magnetising current wouldn't change and the extra heating could easily be avoided by not running it at its full current rating.In practise, I wasn't seriously suggesting the original poster should actually use double the voltage at double the frequency. It just isn't practical as they're highly unlikely to have a 230VAC 120Hz power supply available. They could theoretically use a variable frequency drive, but that has other issues, such as high frequency harmonics and voltage spikes, so would be a bad idea.