Which is the best way to estemate the max power / current of a transformer? - Page 1
Forum section: electronics / projects

author: Barny
author_rank: Frequent Contributor
timestamp: April 21, 2013, 06:47:47 pm
content: HiMy name is Bernhard.Sorry, my English is really bad.Some time ago, I got some little transformer at a scap yard.They was originally used to power up one and two 12V/25W light bulbs in a railway signal.Known technical Data:Weight:9 KgPrimary:220V / 230V AC50HzWire size prim.:0.35mm^2 to 0.4mm^2 cross section (0.7mm diameter)One primary winding with 4 taps (logarithmic turn count)Secondary:9V to 110V AC (depending on the choosen tap)Wire size sec.:1.0mm^2 to 1.1mm^2 cross section (1.15mm to 1.2mm diameter)One secondary winding with 7 taps (logarithmic turn count)Size of the iron stack:150mm x 100mm x 68mmSize of the iron core:45mm x 68mmAge of the transformer:Guessed 20 to 40 years.By the way:The hand written word "weiß" on the picture means white in German and means that this trafo was used to power up the white signal.Which is the best way to estemate the max power / current?I've heard and readed many theories:*)The cross section of the iron core in mm / 10 = max. power in VA*)The cross section of the secondary wire in mm * 5 = max. current.*)Increase the load til the voltage drops 20% = max. loadIs someone able to help a poor machine fitter?

author: SeanB
author_rank: Super Contributor
timestamp: April 21, 2013, 06:55:09 pm
content: Being so old that it is conservatively designed load it till the secondary drops 5%, then take that value as being the peak current, constant current being half of that. Driving 50VA of light means the 12V side can handle 50VA, and thus it will not be likely to be designed to supply more than 100VA maximum. Older units often only have a very low temperature rise allowed on the insulation as well, as it is paper and varnish.

author: BravoV
author_rank: Super Contributor
timestamp: April 21, 2013, 07:03:11 pm
content: There is a way to get the ballpark or "estimation" on the VA of an iron core transformer, check this ->POST.

author: IanB
author_rank: Super Contributor
timestamp: April 21, 2013, 07:10:11 pm
content: That transformer seems quite large and heavy. I don't think I would call it "little"I would guess it can supply 5 A comfortably on the 12 V output, but the temporary short circuit current will go much higher (and the transformer will get hot if you try it). When I was a kid I used to overload old transformers salvaged from valve/tube equipment and the windings would start to hiss when they got too hot.But loading up the secondary is a sure way to determine the capacity. Just feel the outside of the winding bobbin and if it is not getting more than a little warm the transformer is OK. It should not be hot to the touch and it certainly should not start hissing and bubbling.

author: 4to20Milliamps
author_rank: Regular Contributor
timestamp: April 21, 2013, 07:11:53 pm
content: Find a marked one that is similar in size.or you can do the math:http://www.smoke.com.au/~ic/ham/transformers/used_transformers.htmlhttp://ebookbrowse.com/transformers-figuring-out-unmarked-ones-pdf-d241775604

author: Kevin.D
author_rank: Frequent Contributor
timestamp: April 21, 2013, 07:37:45 pm
content: 9 Kg is that, thats quite a large one ,you can estimate power by weight and size of iron core ,that would put it (conservatively) around the 500VA mark if I was estimating it for my own use . You can also estimate current rating of a winding by the wire gauge used .that would put your secondary at ~ 5A going by the chart I am looking at .That would make it a 5X110 =550VA secondary which matches close to the weight/power estimation.Dont take my estimates as being spot on ,because I am using data I have here for estimating 25VA-100VA tran's ,and extrapolating upto 500VA .It might be wrong because of that large difference in size..

author: Barny
author_rank: Frequent Contributor
timestamp: April 21, 2013, 09:49:31 pm
content: Quote from: SeanB on April 21, 2013, 06:55:09 pmYou are right.It looks like they used paper to isulate the layer.The wire itself are isulated with plastic coating.What is the maximum themperature rise of this old transformer?(Thumb multiplied by pi)Quote from: SeanB on April 21, 2013, 06:55:09 pmThis are railway transformers.The railway (OBB) is known for use excessive safety coefficient.Quote from: BravoV on April 21, 2013, 07:03:11 pmThank you for the link.Quote from: IanB on April 21, 2013, 07:10:11 pm1999 to 2002 I had to replace some little bit larger transformer.(15KV 16 2/3Hz primary and filled with 2000L oil.)Compared to this, my transformer are small.Quote from: IanB on April 21, 2013, 07:10:11 pmI dont knew why, but I have a natural aversion to touch something extreme close to live wire.(Even if it's isolated)Quote from: 4to20Milliamps on April 21, 2013, 07:11:53 pmI have a 1.1KVA rated transformer which is only a few cm larger.I dont think that this is a reliable methode to get the maximum rating.Quote from: kevotronic on April 21, 2013, 07:37:45 pmThat's promising.But I think that this little copper lumps are not that powerful.I think, i'll going to search enough power resistors (light bulbs) to get more detailed informations.

author: eevblogfan
author_rank: Frequent Contributor
timestamp: April 22, 2013, 08:13:47 am
content: Hey , i'ts easy , just tell us the core deminetions ...

author: Barny
author_rank: Frequent Contributor
timestamp: April 22, 2013, 09:48:37 am
content: Sorry, my english is realy bad.I don't knew the meaning of the worddeminetions.If you want to knew the dimension of the iron core:45mm x 68mm

author: eevblogfan
author_rank: Frequent Contributor
timestamp: April 22, 2013, 10:12:05 am
content: I hope you know what deminetions I've asked for , because based on what you told me it's 935W transformer ( very unlikely )umm this video will show you what you shall measure and please tel me either calcultage by urself , thank youlink :Cheers

author: eevblogfan
author_rank: Frequent Contributor
timestamp: April 22, 2013, 10:13:45 am
content: heyther's link for my thread on that subject ( which I cant understand why he is not sticky !)link :https://www.eevblog.com/forum/projects/transformer-calculations-and-equations-in-here/Cheers and sorry for double posting

author: IanB
author_rank: Super Contributor
timestamp: April 22, 2013, 02:57:09 pm
content: Quote from: Barny on April 22, 2013, 09:48:37 amIt seems that eevblogfan's English is not even EnglishSince earlier you posted photos with a scale I've no idea what information eevblogfan thinks is missing.

author: Balaur
author_rank: Unknown
timestamp: April 22, 2013, 03:16:56 pm
content: Quote from: eevblogfan on April 22, 2013, 10:12:05 amYou used the formula Power = (Section [sq. cm])^2 which is somewhat optimistic.I likethis pagethat gives an estimated power of 495VA

author: eevblogfan
author_rank: Frequent Contributor
timestamp: April 22, 2013, 03:26:16 pm
content: simply the core's area section , say I've got 3.5 C'm by 4 C'm so 3.5*4= 14 so 14^2= 196less the transormer's efficiency so it varies . I got 80% on the AC side and around 70+ on the DC side after all , so the transformer's efficiency is about 80% but the DC is only 70% so you should have at least 137.2W to use . now get 10% less ( for mains voltage variation ) and you'll get 123W continues at 70C or so within the windings of the transformerthe data I was in need is as described in the video I've linked ,BTW : I am student and english is not my language hence that awful

author: eevblogfan
author_rank: Frequent Contributor
timestamp: April 22, 2013, 03:30:16 pm
content: based on the pic's the transformer's rating is 786W ( the core's capcity )

author: BravoV
author_rank: Super Contributor
timestamp: April 22, 2013, 03:35:36 pm
content: Quote from: eevblogfan on April 22, 2013, 03:26:16 pmYes, agree, but you're also demonstrated at tons of your posts that you are lazy to "RE-CHECK" your spelling either manually or using your built-in spelling checker at your browser, that is not an excuse.C'mon, "deminetions" ?Since you are "absolutely" not aware, here, a lesson to improve your skill, when typing your post, there is a button called "PREVIEW" beside the post button, try it, I bet you will love it and I'm pretty darn sure that you've never try to use it.

author: eevblogfan
author_rank: Frequent Contributor
timestamp: April 22, 2013, 03:54:34 pm
content: Bravov I am using that button , but when typing from the phone you have no ability , there used to be " spell check " on the forum but he's gonebesides me being student etc . I have got "Ignoring spelling errors in English" within my list of many other thing such as 25% more time when being tested etc ...shall I add "sorry my english" down on every post ?

author: BravoV
author_rank: Super Contributor
timestamp: April 22, 2013, 04:22:28 pm
content: Quote from: eevblogfan on April 22, 2013, 03:54:34 pmThen all you need is just re-read again, again and again, yes, three times before you press the post button, how hard is that ? Especially you're aware of your weakness and assuming you respect people that will read your post, aren't you ?If after intensive 3 times re-read and still the words like "deminetions" managed to pass your brain & eyes filter, then its clear, no need to bother with "sorry my English" statement anymore, just do NOT post anything at all again, since you don't even understand any words you've made, let alone other people.Just send a message any moderators requesting to put a permanent ban on yourself.

author: eevblogfan
author_rank: Frequent Contributor
timestamp: April 22, 2013, 04:49:41 pm
content: Bravov you are depressing - please let me do my side , I think I am contributing and you can understand my stupide spelling anyhow , no . after 10^78 times I *wont* find and erors since I know how to say the word but not how to spell correctly - got the problem ?instead of depressing me and not letting me post and contribute , you lose one more voice in here.* not good* if I am allowed to speak

author: c4757p
author_rank: Super Contributor
timestamp: April 22, 2013, 04:53:50 pm
content: Who pissed in your cornflakes, BravoV? I refuse to believe a little broken English really bothers you that much.

author: Barny
author_rank: Frequent Contributor
timestamp: April 22, 2013, 08:34:06 pm
content: Pleace, everybody calm down.The toppic is my old transformerDo someone knew which themperature raise is in the save area at this type of transformer?Is a rise of 20°C or 30°C at the surface all right, or is this to much?

author: SeanB
author_rank: Super Contributor
timestamp: April 22, 2013, 08:40:13 pm
content: For one that old the hottest point on the surface definitely should be no more than 30C above ambient, inside it will be possibly 60c or more. Max temp is around 90C for the paper and varnish.

author: Barny
author_rank: Frequent Contributor
timestamp: April 22, 2013, 08:42:56 pm
content: Thank you verry much.

author: ftransform
author_rank: Frequent Contributor
timestamp: April 23, 2013, 05:13:48 am
content: Can you dip that transformer in some sort of thinned glue to increase its temperature rating?

author: Barny
author_rank: Frequent Contributor
timestamp: April 23, 2013, 10:19:37 am
content: I don't have a vacuum pump.But to get proper sealed windings the coating should aplied within vacuum.Only to seal one old transformer a vacuum pump would be a little to expensive although its for my hobby.(The definition of hobby ist to get minimum results with maximum effort and costs.)

author: SeanB
author_rank: Super Contributor
timestamp: April 23, 2013, 03:47:33 pm
content: One old fridge from the roadside = one vacuum pump for free.

author: IanB
author_rank: Super Contributor
timestamp: April 23, 2013, 04:05:28 pm
content: Quote from: SeanB on April 23, 2013, 03:47:33 pmIt's hard to find a fridge by the roadside in Europe these days because of recycling requirements and CFCs. (It doesn't mean people don't throw them out, but mostly there is no need as old fridges get collected and taken away when a new fridge is delivered.)

author: SeanB
author_rank: Super Contributor
timestamp: April 23, 2013, 06:22:42 pm
content: Here if you leave it out it vanishes in about 5 minutes, even if it is actually being delivered.

author: Barny
author_rank: Frequent Contributor
timestamp: April 26, 2013, 11:34:54 am
content: QuoteYou're right.It's nearly inpossible to get parts from old fridges.At all recycling centre dumpster diveing is prohibited.By the way:I have tested the transformer.It looks like they are able to deliver continous 4A at maximum.I've thought about the reason why the iron core is that big compared to the windings.The reason could be that they wantet to run the transformer at 16 2/3Hz.I think this transformer will be used as anti theft system in some of my battery charging device.

author: nukie
author_rank: Frequent Contributor
timestamp: April 26, 2013, 02:19:51 pm
content: Barny your English is good you don't have to apologise even if its true, most people here is kind enough to 'try to make sense' because we are all engineers here in our heart and not to shame or make fun of the weak. Anyway I am wondering if its the language barrier that discourage some users from participating actively on these forums. If you are out there, please don't be shy, we are here to learn and share.As for typo and spelling errors, its very confusing and quite comical at first but if it happens repeatedly it really annoying. Imagine you pick up a book and it has typo all over. If everyone ignores spelling then how would the search engine work? Wildcard? For the courtesy of forum users and the net. Using a tablet is not an excuse, I find swipe keyboard spells quite well!

author: G7PSK
author_rank: Super Contributor
timestamp: April 26, 2013, 03:16:11 pm
content: I don,t know if this is of any help but I have a very similar transformer size wise, it measures 15 X 15 X 13 cm. it is made in Belgium I purchased it as new surplus in the early 1980's and it is rated at 700 VA which goes very close to an observation made in a previous post.

author: Barny
author_rank: Frequent Contributor
timestamp: June 10, 2015, 08:11:31 pm
content: HiToday I got the technical data for my transformer from an service technican.This iron weigt is designed to deliver25VAat 12VAC secondary.Yes.This trafo is designed to run ONE 12V 25W light bulb (the same type of light bulb like the brake light in older cars) which every imagiable input voltage at 16 2/3Hz or 50Hz.The little transformer on my workbench was running 24/7 for more than 40 yearsNow this little transformer is working as crude multi purpose Voltage source up to 50VA.