Transformer - strange current waveform and "overheating" ?? - Page 1
Forum section: electronics / projects

author: max_torque
author_rank: Super Contributor
timestamp: August 01, 2019, 09:28:44 am
content: I installed an COTS 3kVA 240VAC single phase isolation transformer on a project:http://www.solahevidutysales.com/pdf/transformers/AutomationXfrmrs.pdfI wired the Taps to suit (240VAC primary, 240VAC secondary, ie 1:1 setup)The unit powered up, but sometimes trips the (12A rated) circuit breaker on it's primary feed when you first turn it on. Without any secondary load, the unit gets extremely hot after around 10 min of operation (i'd guess an external case temperature of around 80 degC). I measured the primary voltage and current, shown below:Yellow - Channel A = primary current via current probe at 100mV/ampBlue - Channel B = primary voltageit seems the current is extremely non-sinusoidal with 20A "Pulses" lasting 5ms lagging the voltage waveform, almost like something is saturating and the current is suddenly ramping up.The unit does provide a 240VAC output voltage, as measured on a multimeter - sorry, didn't get time to scope the secondary yet.So i guess questions to any transformer experts:1) how hot should this sort of transformer run under no load? - rating is "80 degC rise at peak load2) It's a 60Hz rated unit, we are at 50Hz, so i guess i'd expect a higher current ripple, but that should still be more sinusoidal surely?3) Measuring the primary and secondary taps with an LCR meter shows inductances as expected (around 70mH Prim and Sec when wired 1:1). If taps were miss-labelled and hence mis-wired, i'd expect to see odd inductances across the taps?4) Could a shorted turn cause this sort of behavoir?

author: The Soulman
author_rank: Super Contributor
timestamp: August 01, 2019, 09:36:35 am
content: Most likely it is saturating because it's running on 50Hz instead of the intended 60Hz.Transformers these days are designed and build to such a precision that they function to specbut not beyond that, just to save few pennies on resources.

author: richard.cs
author_rank: Super Contributor
timestamp: August 01, 2019, 09:58:23 am
content: Agreed, it looks like it's saturating.You don't say which part number you have but if it has an option to wire the taps for a higher voltage you could try that. A transformer designed for 240 V 60 Hz is only usable at 5/6 the voltage, or 200 V on 50 Hz unless it was over-specced in the first place. 80 C case temperature suggests a much higher winding temperature, if you leave it on long enough it will probably overheat and fail.You should probably just replace it with a transformer designed for 50 Hz use, it's not the right component for you.

author: max_torque
author_rank: Super Contributor
timestamp: August 01, 2019, 10:14:16 am
content: I had considered saturation from 50hz operation, but it just doesn't feel quite right to me for several reasons:1) The transformer is sold in the UK by a major distributor - (RS components) surely they would have noticed if everyone returned the units they bought?2) There is no "min frequency" spec for the unit in any documentation for it. Surely, if it saturated at 50Hz, it would tell you that?3) Looking at the wave form, a minor reduction in the time for each cycle (5/6ths) won't be enough to prevent the current rising suddenly, because the rise time only takes 1 ms?I can certainly try connecting to a high voltage tap on the primary and seeing what happens!

author: max_torque
author_rank: Super Contributor
timestamp: August 01, 2019, 10:16:20 am
content: Unit bought from here:https://uk.rs-online.com/web/p/wall-mount-transformers/8908801no mention of not being suitable for 50Hz.

author: Zero999
author_rank: Super Contributor
timestamp: August 01, 2019, 10:26:53 am
content: Quote from: max_torque on August 01, 2019, 10:16:20 amThere's no mention of it being suitable for 50Hz either and the data sheet only specifies the HS5F3AS at 60Hz.https://docs-emea.rs-online.com/webdocs/14a6/0900766b814a6acd.pdfIt appears you have the wrong part for the job. You need a 50Hz transformer.

author: max_torque
author_rank: Super Contributor
timestamp: August 01, 2019, 10:34:47 am
content: I've actually got a frequency controlled AC source, so i'll stick 240VAC at 60Hz into it and see what the current waveform looks like!

author: Benta
author_rank: Super Contributor
timestamp: August 01, 2019, 10:46:36 am
content: The datasheet is quite clear on this point: it's a60 Hztransformer.

author: capt bullshot
author_rank: Super Contributor
timestamp: August 01, 2019, 10:52:03 am
content: It's been already said, your current waveform is the typical "saturated core" waveform.The RS page says max. 60Hz operation frequency, but that's just a sign on non-understanding what they're selling. Pretty normal today.

author: ogden
author_rank: Super Contributor
timestamp: August 01, 2019, 10:53:21 am
content: Quote from: Benta on August 01, 2019, 10:46:36 amRight. 50Hz generates 20% more flux lines than at 60Hz. That thing is expensive, do not have any resale value in the UK even new. I would try to negotiate about resolution of this mistake.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 01, 2019, 10:58:19 am
content: Quote from: max_torque on August 01, 2019, 10:14:16 amYou're putting an awful lot more confidence in RS than they deserve, or so it sounds based on others' impressions of them!Tim

author: max_torque
author_rank: Super Contributor
timestamp: August 01, 2019, 11:03:55 am
content: Swapping it for a different unit shouldn't be an issue, i just want to confirm the root cause. In my pics, if you reverse the current or voltage wave forms (ie the same as swapping L & N around, or changing the direction of current through the current probe) then the current "spike" is leading the voltage by 90deg, which is what you would expect for the position of peak magnetic field strength, so that does tally up.

author: BMK
author_rank: Regular Contributor
timestamp: August 01, 2019, 11:06:59 am
content: Quote from: capt bullshot on August 01, 2019, 10:52:03 amI've used RS for 20 years. Even back in the paper catalogue days you could bet your life on the accuracy of specifications. In the past 3-4 years I have seen a lot of nonsense in specifications. It seems that they don't use technically literate people anymore to parse datasheets into their systems. You cannot trust those search filters. In my opinion they have become the worst offenders of all the vendors I use.You should be able to return the traffo, I've done this many times even after their supposed 14 day cut-off. One thing for sure is they will answer the phone and solve problems like this with little argument.

author: ogden
author_rank: Super Contributor
timestamp: August 01, 2019, 11:12:50 am
content: Quote from: T3sl4co1l on August 01, 2019, 10:58:19 amThey do not know and do not care - you will use component locally or you will export it as part of product you manufacture. Nobody complains that UK RS components sell various kinds of power plugs that would not fit local mains sockets. The same about transformers. RS components is not your local grocery store and even there buyer is responsible to know what he is buying.Quote from: BMK on August 01, 2019, 11:06:59 amSure. By customer protection rules/laws they are obligated to handle returns even if mistake is yours.

author: richard.cs
author_rank: Super Contributor
timestamp: August 01, 2019, 11:14:00 am
content: I would expect it to be marginally better on the 252 V tapping and to not saturate at all on the next one up which is 456 V, though it probably won't be useful for your application either as the secondary voltage will be about 126 V on the 240 V output.I agree the datasheet is poor not putting upper and lower frequencies on it, but as they describe it as a 60 Hz device without giving limits it could legitimately fail at 59 Hz and still meet its specification. It's a transformer, the physics dictates that it has limits to its operating frequency and them not specifying those limits should be a warning sign. I have no idea why RS stocks it in the UK, it's clearly made for American applications with its 60 Hz rating and the options for voltages like 208, 277 and 480 which aren't really used in the rest of the world. I suspect 60 Hz systems probably exist in a few UK factories for imported specialist machinery but it's quite possibly less common even than 400 Hz.If RS are willing to exchange or refund that would probably be your best approach. Otherwise stick it on ebay or something (to paraphrase CQ magazine it might make a fine anchor for a small boat). Take a look at Airlink Transformers for a UK manufacturer of similar transformers in a wall-mount case.I generally find that the data RS pull out of datasheets and onto the product pages is very poor, worse than Farnell and much much worse than Digikey.

author: max_torque
author_rank: Super Contributor
timestamp: August 01, 2019, 11:18:58 am
content: The company that bought the unit spend some thing like £200k () with RS every year, so i don't see any issue getting it swapped (or a refund, as i can't find any 3kVA 1:1 enclosed iso transformers listed that specifically say "50Hz" on RS ......... )

author: ogden
author_rank: Super Contributor
timestamp: August 01, 2019, 11:22:43 am
content: Quote from: max_torque on August 01, 2019, 11:18:58 amWith such purchase amount you can just call them and let them hop while they offer what you needHard to believe that you can't find 50Hz isolation trafo in the UK. It would be truly laughable.