What's wrong with the transformer I have. - Page 1
Forum section: electronics / beginners

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 21, 2023, 05:35:47 pm
content: At the company I work for they have a compressed air dryer which has a step down transformer from 480VAC to 2 secondary of 24VAC each. The primary winding actually has 8 tabs which allows for voltage from 220VAC to 575VAC. I don't see the rating for the primary but it's protected by a 2.5A circuit breaker. The secondary rating is 4.2A. When it failed it still work that is if you supply power to the primary you still get power out of the secondary. The problem is that even under no load the primary would drawy about 3A which is enough to trip the breaker. If I would use a larger breaker it would work but it does gets hot and of course it's not something I want to do. The winding resistance reads the same as the good one and there is no current leakage to the core or the metal parts of the transformer. 2 of them failed in the same manner in less than a year. I wonder what really happened here.

author: james_s
author_rank: Super Contributor
timestamp: April 21, 2023, 05:42:22 pm
content: Shorted turn on the primary most likely. A single turn being shunted out of the winding won't affect the DC resistance much, but it will create a short circuit load on the transformer.

author: planet12
author_rank: Regular Contributor
timestamp: April 21, 2023, 11:04:11 pm
content: I second the diagnosis of shorted turns.If you've had multiple fail, it's likely their internal temperature is getting too high during operation and the insulation is breaking down internally (that and/or manufacturing faults).There's a device called a "ring tester" that can be used to detect shorted turns; if you have a known-good transformer to get a baseline reading from you could use one to confirm the diagnosis (a ring tester isn't too hard to build yourself - search the web for "transformer ring tester" and you'll find several options).

author: james_s
author_rank: Super Contributor
timestamp: April 22, 2023, 12:41:24 am
content: IMO it's not really necessary to use a ring tester, if the transformer is drawing excessive current with no load connected there isn't really much else that could be going on.

author: BillyO
author_rank: Super Contributor
timestamp: April 24, 2023, 03:17:23 am
content: 3rd the diagnosis.It sounds like a nice transformer. Given the price of nice transformers these days, it might be worth fixing.Even back in the old days as the earth was cooling just enough to live on I would collect nice transformers that had failed and either re-build them to spec or custom wind them. Just pull them apart, unwind the bobbins and re-wind them as you need. The custom winds were the most lucrative by virtue of numbers, but every now and then I'd get a request for a rebuild of a rare or discontinued unit that would pay my rent for a week ($50 in 1974 $$).Since simple as s#It single output 6.3V 35VA transformers from the likes of Hammond go for near $100 these days, there might indeed be a market for custom and rebuilt units which would go for 5x+. IIRC it would take about 1 hour to wind and finish such a beast. Give a cost of $20 for materials, I'll take $80/hr. all day in my spare time. And that's for an off the shelf item. Custom or special rebuilds .. order up the new Porsche.

author: geggi1
author_rank: Frequent Contributor
timestamp: April 25, 2023, 04:41:28 am
content: If the voltages from the transfomer is rectified you can just replace it with a switchmode powersupply.They are pretty cheep and avalable in many places.If htere are AC loads after the transformer you can start looking at places like digykey,RS and other.480/2x24 is a pretty common type transformer and it would be possible to find a off the shelfe.

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 25, 2023, 10:59:35 am
content: Quote from: geggi1 on April 25, 2023, 04:41:28 amThe outputs are not rectified. The 2 secondaries are used for 2 separate circuits.

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 25, 2023, 11:02:03 am
content: Quote from: BillyO on April 24, 2023, 03:17:23 amI don't know if it's a nice transformer. It's made by mdexx of Germany but made in the Czech Republic. It's rated at 100VA / 330VA. The model number is TAM3896-0FE20-0FA0. I don't think it's worth fixing.

author: geggi1
author_rank: Frequent Contributor
timestamp: April 26, 2023, 08:59:02 pm
content: If there is room in the equipment you could replac the transformer with two transformers.Dual secondary transformers are usually more expencive and less common.

author: james_s
author_rank: Super Contributor
timestamp: April 26, 2023, 11:46:28 pm
content: Is the transformer overheating prior to failing? If you've had two of them fail I wonder if it's getting overloaded by some other fault.