Question on Isolation Transformer - Page 1
Forum section: electronics / beginners

author: StubbornGreek
author_rank: Frequent Contributor
timestamp: October 30, 2013, 04:02:34 pm
content: OK, the answer seems obvious to me but I wanted to ensure that I wasn't making an error here as I'm connecting this to mains:I have a KGS Electronics Isolation Transformer (model number 1275t) that was scavenged out of a medical equipment (I think it was a laser if memory serves). Regardless, its rated at 115VAC, 60Hz input at a max of 1500 watts and outputs 115VAC (obviously). My question is since the transformer is wound with a 1:1 ratio (I'm assuming here but since output is the same as input...) what issue could arise using it for 220VAC @ 50Hz input? I'm sure it would run a little warmer at the frequency but would this work OK?Getting ready for an international move ; thanks in advance for your input.

author: jahonen
author_rank: Super Contributor
timestamp: October 30, 2013, 04:07:56 pm
content: Probably it doesn't work, number of primary turns is designed for 115 VAC. Feeding it with 220 VAC will cause saturation of the transformer core.Regards,Janne

author: edpalmer42
author_rank: Super Contributor
timestamp: October 30, 2013, 04:16:44 pm
content: I agree with jahonen. Plugging a 115V transformer into 220V would cause saturation which would cause a big current surge, possible blown fuses, loud noises from the transformer, maybe smoke. Could be an impressive YouTube video.Ed

author: ConKbot
author_rank: Super Contributor
timestamp: October 30, 2013, 04:24:40 pm
content: Little warmer is probably a bit of an understatementOnce the core saturates (the volt-second product is exceeded) It stops acting like and inductor and acts more like a piece of wire. An AC half cycle in the us works out to 1 volt-second (1/120 second * 120 volts RMS), in 220V 50Hz land its 2.2 V-sOnce the transformer hits saturation the back-emf of the primary stops increasing, and your current is limited by the resistance of your primary/house wiring.Going from 60Hz to 50 Hz at the same voltage would be plausible but unlikely (20% increase in volt-second product) So if its a well built transformer with a bit of margin on the amount of iron, it could possibly take the change from 60 to 50 Hz

author: richard.cs
author_rank: Super Contributor
timestamp: October 30, 2013, 04:33:43 pm
content: I'll agree with the others here, 220V into the primary at any frequency much less than 120Hz will kill it, possibly with smoke and flames.Or are you proposing using it as an autotransformer to run 115V equpiment with both windings in series? That would work, it will run a bit warm on 50 Hz but only the most cheap and nasty transformer would actually fail. If you actually want a 220V:220V 50Hz isolation transformer it'll never work.

author: IanB
author_rank: Super Contributor
timestamp: October 30, 2013, 04:52:20 pm
content: I wondered about the auto-transformer idea too. It seems like a good way to re-purpose the transformer.

author: alxnik
author_rank: Regular Contributor
timestamp: October 30, 2013, 04:59:37 pm
content: Although the question has been thoroughly answered from a theoretical point of view, I'll just add the results of an experiment I did:I have a transformer with a primary with 2 taps, one for 110 and one for 220. I connected it to mains in series with an incadecent lamp. When I powered the thing on with no load and on the 220 tap, the lamp was totally off. When connected the 110 tap to mains, the lamp light up like a christmas tree, and more interestingly the transformer started a rather loud buzz at 50hz - and that is from an ~150VA transformer.I think a fire is rather unlikely since mains trasformers have fuses but nevertheless...

author: StubbornGreek
author_rank: Frequent Contributor
timestamp: October 30, 2013, 05:26:41 pm
content: OK, got it and thanks. I didn't even consider the saturation level... I must say, transformers are such a gray area for me. I've gathered tons of literature on the subject over the last couple of months but haven't really had a chance to sit down and study it (so much going on right now).Thanks again for the many (quite immediate) responses.

author: SeanB
author_rank: Super Contributor
timestamp: October 30, 2013, 06:06:51 pm
content: Quote from: alxnik on October 30, 2013, 04:59:37 pmI have seen a lot of transformers with no fusing other than the mains fuse. Often the wiring inside will burn out hopefully fast, but I have seen many catch fire and burn, even those supposedly run at the correct voltage with the correct load. Often you find the winding open circuit with the winding being burnt out internally deep inside where it was poorly cooled.

author: alxnik
author_rank: Regular Contributor
timestamp: October 30, 2013, 06:50:23 pm
content: Quote from: SeanB on October 30, 2013, 06:06:51 pmInteresting. I probably haven't burned enough transformers, but all of the ones I had to disembowel had protection thermal fuses. Funny thing, I always thought of them as kinda useless because you have to dig deep in order to replace (or remove...) and a normal fuse is supposed to be connected before the transformer