Transformer question - Page 1
Forum section: electronics / beginners

author: metrologist
author_rank: Super Contributor
timestamp: March 11, 2025, 11:22:54 pm
content: I came across this transformer that has a label. It's rated 250VA.One side labeled X1-X2 is 120V.The other side labeled H1, H3, H2, H4, where H1-H3 is 240V, and H2-H4 is 480V. These two coils are strapped in parallelDoes this mean you can put either 240 or 480 on the input and get 120 out? Do transformers work in reverse with same power rating?What I mean is,what would I get out by supplying 120V to H1H3-H2H4 and how much current could I draw on the output?what would I get out by supplying 120V to X1-X2 and how much current could I draw on the output?What if I unstrapped the coils and ran them independent or in series?What can I use this for in terms of a lab supply with a DC-DC converter or automotive type applications?

author: jmelson
author_rank: Super Contributor
timestamp: March 11, 2025, 11:32:49 pm
content: Yes, This is called a step-down transformer, and is commonly found in machine tools and industrial equipment. With the correct straps, it can be set to convert either 480V or 240V to 120 V. If you strap for 480 V input and feed it 240 V, you would get 60 V on the output. However, you cannot get 250VA at half voltage, as the wire inside has a current limit. So, it would only provide 125 VA when set up that way.Jon

author: metrologist
author_rank: Super Contributor
timestamp: March 12, 2025, 12:31:52 am
content: Still not sure why the inputs are strapped in parallel. What's the difference if I remove the straps and just feed 240 to the 240 coil?So the output is the current limit, roughly 2A at any voltage?

author: CaptDon
author_rank: Super Contributor
timestamp: March 12, 2025, 01:53:24 pm
content: If you remove the straps and run only one 240 volt primary you can only get 125VA from the secondary side. Drawing more than that amount from the 120 side will overcurrent the primary and burn it up. Can you see that each primary winding is only capable of supplying 125VA to the core without overheating?

author: microwhat
author_rank: Contributor
timestamp: March 12, 2025, 02:52:35 pm
content: That sounds unusual for a control transformer. I'd expect something as shown in the attached document page 4https://cdn.automationdirect.com/static/specs/hpsxformers480240.pdfwhere each primary winding is rated for 240VAC and can be connected either in series for 480V service or parallel for 240V.I'd run a test: remove the high side links, apply 240V to H1-H3, and measure the voltage between H2-H4 and X1-X2.Once you know what each winding is actually doing, then you can decide which is going to be your primary and secondary.Regardless of the operating voltage, you can't draw more current through a winding than its rating. So, if you apply 120VAC to X1-X2 and connect the high side windings in parallel, you will get ~240VAC and somewhat less than 1.04A, since you cannot allow more than 2.08A to flow in the X1-X2 winding.If your transformer is intended for 50/60Hz use, then I would not expect it to be useful at any higher frequency. It might transfer nameplate power at 400Hz, but I wouldn't try to use it in a DC-DC converter.

author: BeBuLamar
author_rank: Super Contributor
timestamp: March 12, 2025, 03:27:40 pm
content: It wouldn't work in reverse. If you supply 120AC to the 120V side you would get the high voltage on the other side but the transformer draws a lot of current even if you have nothing connected to the other side.

author: jmelson
author_rank: Super Contributor
timestamp: March 12, 2025, 03:42:31 pm
content: Quote from: metrologist on March 12, 2025, 12:31:52 amThe primary winding is sized so that when the paralleled windings share the current, they stay within ratings. If you only power one winding, it may (likely) overheat.Jon

author: metrologist
author_rank: Super Contributor
timestamp: March 12, 2025, 04:00:40 pm
content: OK, I looked at the label again and cleaned it off to get a better view. The diagram wasn't quite what I thought it was.So it's like what microwhat said and is currently strapped for 2:1 step down.So if I supply H1H3-H2H4 with 120V, I'd get to use 60V 4A. Then I could attach a FBR circuit and use it with my RuiDeng DC-DC converter, but I'd have to lose another 10V. Or I could put the H windings in series and get to use 30V 8A, and that might be more desirable.I suppose if my input voltage was under spec the transformer wont operate very efficiently.Oh well, I already have a nice TDK Lambda CFE-400M 48V 8.3A switching supply for that mess.Thanks

author: Ian.M
author_rank: Super Contributor
timestamp: March 12, 2025, 04:11:44 pm
content: No. it doesn't work that way. The VA rating is at nominal voltage. If you reduce the voltage, the current rating does NOT increase. The X1-X2 winding is only good for fractionally over 2A no matter the input voltage and strapping.

author: metrologist
author_rank: Super Contributor
timestamp: March 12, 2025, 06:48:17 pm
content: In that case I'm so glad I did not drop this thing on my toe, because then I'd be a bit more than disappointed...But it's served a great purpose already here.

author: CaptDon
author_rank: Super Contributor
timestamp: March 12, 2025, 10:30:04 pm
content: Quality control transformers can be used 'in either direction' without harm if you remain within the VA rating!! A step down indeed can be used as a step up. We have not only done it but also did it in three transformer three phase configurations!!

author: metrologist
author_rank: Super Contributor
timestamp: March 13, 2025, 05:54:05 am
content: speaking of three phase, usually 440V is so. I'd gathered this is commonly used to tap the machine tool supply line to power typical 110 devices. I guess you just tap two phases and it's good enough?

author: CaptDon
author_rank: Super Contributor
timestamp: March 13, 2025, 02:02:27 pm
content: Yes, we often used 1KVA up to 3KVA transformers as step down for 'domestic power' at a desk or workstation. 480 or 277 on the high side and 120 on the low side. Proper grounding must be observed on the low side in the event the transformer would fail and also for every imaginable safety reason. Many industrial 480VAC machines actually have domestic power stepdown transformers built in to run computers, lights and accessories. They often have 120VAC duplex outlets on the machine for operating test and diagnostic equipment.

author: MrAl
author_rank: Super Contributor
timestamp: March 14, 2025, 06:12:19 am
content: Quote from: metrologist on March 12, 2025, 04:00:40 pmHi,That diagram is nice, but it does not match up with your description of how the transformer was wired when you first found it.From your text, it sounds like when you first found this transformer some of the windings were "strapped" as you called it. Unfortunately, that's not a very good description of maybe how it was wired, and if two of the windings were connected in parallel, then that diagram does not match up with that functionality.Instead, due to the way two of the windings were found to be wired in parallel (presumably), the winding labeled with "440" should actually be labeled "220" just like the other winding. The reason for this is because you could not place a 440v winding in parallel with a 220v winding or the transformer would draw so much current it would quickly blow out any fuse or circuit breaker providing the power to it.If the windings were 220v and 220v, then you could place them in parallel and then you would have your 220v to 110v transformer. If you put the two 220v windings in series, then you would have a transformer that works as a 440v to 110v (or 480v to 120v).That's the way transformers work.Why they labeled them the way you are showing is a little puzzling, and it may be a good idea to test the voltage of each winding before any are connected together.Also, when you place the two 220v windings in series, you have to make sure you have them connected in "series aiding" so that the voltage add. If you connect either of them in reverse, you get zero volts output because then the two voltages subtract, and 220-220 equals zero.So it would be a good idea to test these windings before you do anything. You can apply a smaller voltage like 12vac to test the winding ratios.Once you figure out what configuration you want to use and with what voltages, you should test the transformer to see if it overheats. You have to let it run for some time, like 12 hours, and the standard life test is overnight (like 12 hours).

author: BeBuLamar
author_rank: Super Contributor
timestamp: March 14, 2025, 11:02:05 am
content: I agree with MrAI. If the 2 HV winding are wired in parallel for 240V input then the 2 windings should have closely similar resistance. If one winding is for 240 and one is for 480 like your drawing then the resistance would be different. That's the quick test. Then you can do the voltage test with low voltage like MrAI suggested.

author: metrologist
author_rank: Super Contributor
timestamp: March 14, 2025, 11:15:05 am
content: Quote from: MrAl on March 14, 2025, 06:12:19 amYes, precisely why I was confused. I was going from memory as the transformer was at work, connected to a large metal frame that had been mostly stripped and was on a scrap heap, and not very easy to access nor read due to residue and dirt.As one does, imagine if it could be salvaged and put to use, but I could not figure out what I thought I saw nor how I could use it. I mean I had a pretty good idea of it's intended purpose, but that's not an application I could use.Anyway, after taking a second look, the diagram is like this, and not being able to see the middle part very clearly is what caused all this discussion.I mentioned straps because there is what appears to be a sheet of Bakelite sandwiched in the laminated core where the winding wires are riveted to terminal mounting points, and there are metal straps screwed to the terminals - in this case in parallel (i.e., left configuration).Now I remember when wiring up my CNC I was using a large toroid of a similar makeup and also as you describe, I messed up the primary and had them in reverse - there were just loose wires coming out of the winding and I couldn't figure out the direction of the windings.. Of course I powered it up to discover that. Just a shudder as I quickly cut the power before anything got hot.

author: MrAl
author_rank: Super Contributor
timestamp: March 14, 2025, 08:38:19 pm
content: Quote from: metrologist on March 14, 2025, 11:15:05 amHello again,I am afraid that drawing makes it even more confusing to figure out what this transformer can doI have to strongly suggest you take snap shots of the transformer so we can see the actual labels. That could help a lot.Of course there are a few tests you can do to figure this out too.