Isolation Transfomer gets hot without load - Page 1
Forum section: electronics / beginners

author: belzrebuth
author_rank: Frequent Contributor
timestamp: June 13, 2020, 12:58:05 pm
content: Hi all,I've collected an isolation transformer from the side of the road just besides a trash can.I saw it while driving so I stopped and picked it up.Anyways I got home and after checking for any dead shorts I powered it up and it seems to work fine.I got about 4.5Ω resistance on the primary and 3.6Ω on the secondary and this is a 500VA E transfomer.Only problem is that it gets a bit hot after leaving it on wihout load for about 20-30minutes.Ambient temperature is 27 celsius and transformer temperature is about 43 degrees celsius.It's not rising to the point it's too hot to touch or anything but it's not like other transformers I have.I think most stay at about room temperature when energised.I've then proceed to test the current draw on the primary with a clamp meter and it's about 165mA which seems a bit high (should be zero AFAIK theoretically).I've tested a toroid I have lying around and indeed after a short peak (inrush current) current reading is about 0 when the transformer is plugged with no load.Is the isolation transformer bad/dangerous?Or this is somehow normal?It seems in a good shape and has a protective case around it so I don't think it's mechanically abused but I don't know what it's been through (electrically).I should remove the metal case and check for any signs of overheating I guess.EDIT : Ι've tested another known good 500VA 220V~12V E transformer and current draw on the primary side 0.3A (with the same clamp meter) so I don't think the current draw of 0.165A is a bad indication.The step down transformer sits at about 32 degress celsius.I now wonder why the toroids have about 0 current draw, while the iron core ones have so much higher current.Is this due to losses?Are the toroids that much more efficient than their E core counterparts?!

author: sahko123
author_rank: Frequent Contributor
timestamp: June 13, 2020, 01:27:47 pm
content: E I transformers tend to be really inefficient compared to toroidal transformers. This inefficiency means that they heat up quite a lot but i don't think you should be worried about it E I transformers have always had quiescent current draw and the higher the capability 500va in this case the higher the passive current draw

author: belzrebuth
author_rank: Frequent Contributor
timestamp: June 13, 2020, 01:41:50 pm
content: Nice! I was getting a bit worried since it's about 20 degrees more (measured about 47 degrees on its core now) than ambient temperature without load which I considered a bit high.Need to load test at some point though to see what I can get out of it.I'll leave it on for the rest of the day to check if the temperature stays relatively constant.

author: sahko123
author_rank: Frequent Contributor
timestamp: June 13, 2020, 01:45:51 pm
content: Be careful though on the off chance it is a fucky transformer do check up on it from time to time but otherwise even without load the transformer should heat up

author: belzrebuth
author_rank: Frequent Contributor
timestamp: June 13, 2020, 01:48:17 pm
content: I could put a sensor with a relay to switch it off of it rises a above 60 degrees.Or just leave it outside

author: belzrebuth
author_rank: Frequent Contributor
timestamp: June 13, 2020, 01:53:04 pm
content: Btw for about 500VA should I be able to draw about 1.7A@235V, right?

author: bdunham7
author_rank: Super Contributor
timestamp: June 13, 2020, 02:04:19 pm
content: Just the quiescent current number without the phase angle doesn't tell you the whole story. There will always be some current but how much of is is dissipated and thus drawn as active power will vary. I don't know why the DCR of the primary and secondary would vary on a 1:1 isolation transformer--perhaps it has some shorted windings? That would cause some current to flow and make some heat. Are you sure it is 1:1? What is the measured voltage on the secondary compared to the primary?

author: belzrebuth
author_rank: Frequent Contributor
timestamp: June 13, 2020, 02:27:05 pm
content: Input voltage is 237Vac and output voltage is 235Vac.Measured with a Brymen Bm869s.It's not exactly a match but I'd say they're pretty close.I would expect if the transformer was compensated to have a slightly upped output voltage compared to the input.It's not as it seems pretty basic so it's probably a 1:1 configuration but I'm out 2 volts which may or may not matter in this case.

author: sahko123
author_rank: Frequent Contributor
timestamp: June 13, 2020, 02:31:22 pm
content: is it possible that the transformer is actually backwards and does actually slightly increase the voltage?

author: belzrebuth
author_rank: Frequent Contributor
timestamp: June 13, 2020, 02:35:26 pm
content: Hmm hadn't thought of thatI'll check though..Since it's 1:1 should this affect it somehow? if it's unloaded of course..I'd guess that it's wired correctly though since I get a higher resistance on my input winding so that must be the thinner magnet wire?

author: bdunham7
author_rank: Super Contributor
timestamp: June 13, 2020, 02:37:45 pm
content: How is it marked?

author: ArthurDent
author_rank: Super Contributor
timestamp: June 13, 2020, 03:13:49 pm
content: As others have said, the primary should have the lower resistance and the larger wire. The output under no load should be slightly higher than the primary input voltage so that when it is loaded to its rated capacity the output voltage will drop to be about equal to the input voltage. This makes sense because the transformer will have less than 100% efficiency because of winding resistance and other factors so there is what is called 'watts loss' that doesn't contribute to the output but is given off as heat. The turns ratio difference tries to compensate for the voltage drop.A lot of cheaper E-I type transformers are cost reduced to the point that the Es and Is aren't interleaved but all are put in the same direction and then welded together (see photo) which is less than ideal but makes assembly faster and cheaper. I think you'll find all microwave transformers made this way and they are rated for a reduced duty cycle to keep them from overheating.

author: belzrebuth
author_rank: Frequent Contributor
timestamp: June 13, 2020, 03:40:39 pm
content: It's unmarked.It just the number 220 on the front and at the back.On the front the two cables are different color and the back the same color.So I assumed that the different colors should indicate primary.That and the higher resistance.But ArthurDent says the lower resistance should be the primary.I really don't know.Conductor gauge seem pretty much identical..So only clue is the resistance.On one end it's 3.7ohm and on the other 4.5ohm

author: sahko123
author_rank: Frequent Contributor
timestamp: June 13, 2020, 03:44:40 pm
content: since in this case its "1":"1" instead of a step down transformer it needs to compensate for voltage drop on the other end so the lower resistance would be the primary

author: belzrebuth
author_rank: Frequent Contributor
timestamp: June 13, 2020, 03:49:27 pm
content: Makes sense..When I wire it like this I get just 1 volt higher on the "secondary".I really don't get the colour coding on the secondary but that may be just the manufacturer's stupid decision.So lower resistance is primary.Got it!I'm going to leave it wired like this and see if this changes anything heat wise, I really doubt it though.Will load test it as well to see how it behaves.Maybe put on an old TV or something..

author: belzrebuth
author_rank: Frequent Contributor
timestamp: June 13, 2020, 03:57:44 pm
content: Just another thing.If I were to put a variac along with this should I wire the variac before or after the isolation transformer?The variac is also 500VA.My no1 guess would be the variac first as to be sure that the current is mostly output from the isolation transformer but since toroids are more efficient and the variac is essentially a toroid could it be after as well?Or it doesn't matter at all?

author: bdunham7
author_rank: Super Contributor
timestamp: June 13, 2020, 04:04:07 pm
content: Variac first. The less stuff you have after the output of the isolation transformer the less chance of something going wrong and referencing your output to ground--which would defeat the purpose of isolation. Make sure you fuse both the input and output of the variac.

author: sahko123
author_rank: Frequent Contributor
timestamp: June 13, 2020, 04:05:14 pm
content: I think a variac should go after the isolation transformer so the isolation transformer is always operating in the correct voltage range but i think it would be isolated with variac after anyway?

author: richnormand
author_rank: Unknown
timestamp: June 13, 2020, 04:10:22 pm
content: Do you have one of these meters that measure voltage and load for mains? They are quite cheap on ebay and such.The idea would be to hook up the meter at the transformer output and a load like a 60 to 100 watt filament light bulb (or other resistive load) and measure the power used.Then hook the load directly to the transformer output and measure the power used at the primary to the mains. The difference should be the transformer loss (assuming a reasonable 1:1 for the voltage).Then reverse the transformer and redo to see if there are any difference.Using two and then three and then four,,, bulbs in parallel to increase the load should allow you to plot the transformer losses vs load and extrapolate the line down to zero load to see if it is reasonable.

author: bdunham7
author_rank: Super Contributor
timestamp: June 13, 2020, 04:14:14 pm
content: Quote from: sahko123 on June 13, 2020, 04:05:14 pmTransformers are generally perfectly happy at lower-than-rated voltages. Isolation doesn't do you any good if your isolated source becomes ground referenced--you just have a new shock hazard that happens to be totally isolated from your mains supply. The variac is just one more opportunity for that grounding to happen.

author: Zero999
author_rank: Super Contributor
timestamp: June 13, 2020, 04:25:17 pm
content: Yes, as mentioned above, the primary will have a slightly lower resistance, than the secondary and the transformer will be slightly more than 1:1 to make up for the voltage loss, so the transformer was most likely wired backwards, although a temperature rise of 15oC isn't that bad.Regarding which is better: isolation transformer then variac or vice versa: I'd put the isolation transformer first, simply because you'll get better load regulation. If the transformer's secondary voltage drops about 14V, at full load, it won't make much difference to the output, if the variac is on the secondary, but it will if the variac is on the primary and set to a comparatively low voltage.

author: bdunham7
author_rank: Super Contributor
timestamp: June 13, 2020, 04:32:18 pm
content: Quote from: Zero999 on June 13, 2020, 04:25:17 pmI suppose it depends on whether your goal is isolation or load regulation. Your observation is correct and iso-variac setups have that issue at high currents and low voltages. In most use cases that is only a transitory phase--you only use it in that mode as you are bringing up the power. I'm assuming that somone that goes to all the trouble of setting up an apparatus like this is primarily interested in the isolation.

author: AVGresponding
author_rank: Super Contributor
timestamp: June 14, 2020, 09:38:02 am
content: One thing that doesn't seem to have been mentioned yet is that this type of transformer is often given dual VA ratings; a continuous and intermittent.Be careful you don't try to load a 500VA intermittent at 500VA continuous!Peak power duty cycle varies by manufacturer and purpose, but as a rule of thumb, if it starts to smell a bit iffy after 10 or 15 minutes at a given load, it's probably at its maximum intermittent load...

author: belzrebuth
author_rank: Frequent Contributor
timestamp: June 14, 2020, 09:45:55 am
content: Thanks for mentioning it.I wouldn't probably even reach the rated VA though but good to know.

author: Vovk_Z
author_rank: Super Contributor
timestamp: June 14, 2020, 11:34:00 am
content: Quote from: belzrebuth on January 17, 1975, 03:58:03 pmThe windings are winded one onto another so one winding has larger diameter and longer wire, of cause.