Transformer temperature

[Keith Rogers 2Participant @keithrogers2]

I have been aware for many years, and in fact NDIY has just mentioned, that transformer laminations should be electrically isolated from each other.

Can anyone tell me how come modern laminations are welded together, and what effect that has on the efficiency of the transformer?

Keith.

[Tim StevensParticipant @timstevens64731]

The welding is there to hold the laminations together during build. The problem of eddy currents relies on circuits within the iron-work, and the welds should be sited so that they do not provide useful loops. It will reduce the efficiency a bit, but helps to keep the price down – especially if the assembly is put to outworkers rather than in house.

Regards, Tim

Edited By Tim Stevens on 24/06/2017 17:16:02

[Russell EberhardtParticipant @russelleberhardt48058]

The welds are relatively shallow so do not enclose much of the magnetic flux hence the resulting Eddy currents are small and the effect on efficiency is negligible.

Russell

[Keith Rogers 2Participant @keithrogers2]

Thanks for the replies Tim and Russell much appreciated.

Regards, Keith.

[not done it yetParticipant @notdoneityet]

Transformers are designed to run hot.

Have to disagree a tad here. Heat is a wasteful byproduct and so reduces the efficiency of the device. Transformers are built to withstand heat, but are designed for minimum heat production, shirley? If they could, they would run cold and not need high temperature winding insulation. A simple trade off between high efficiency and cheap production cost.

[roy entwistleParticipant @royentwistle24699]

Large transformers are oil cooled

Roy

[Nicholas FarrParticipant @nicholasfarr14254]

Posted by roy entwistle on 25/06/2017 09:06:06:

Large transformers are oil cooled

Roy

Hi Roy, large is a bit ambiguous. I have a large 230 to 110v transformer, which is air cooled. In comparison, I have a 230v welding transformer of about the same physical size which is oil cooled. Neither of them are "Bill massive" though.

Regards Nick.

Edited By Nicholas Farr on 25/06/2017 09:55:31

[Russell EberhardtParticipant @russelleberhardt48058]

Posted by not done it yet on 25/06/2017 07:39:53:Transformers are built to withstand heat, but are designed for minimum heat production, shirley?

No, they are designed for best cost/performance ratio. It all depends on the intended application. Designing for minimum heating would result in transformers that are too big and too expensive.

Russell

[John HaineParticipant @johnhaine32865]

Posted by mick H on 23/06/2017 21:48:45:

………………….

I had a look at the charger unit for my Bosch drill and the power output is much the same as the pond transformer 14.4V at 1.5 amps and this unit is barely warm if left on all night. Strange. Something just does not feel right although as one can deduce, my knowledge of these things is scant and I may be comparing apples with pears. I have a feeling that I will probably end up with a transformer with a greater output but I find this problem intriguing.

 

Mick

Your Bosch charger almost certainly uses a switched-mode supply (certainly the ones I have for my drill and garden trimmer do). This rectifies the mains to a high voltage, then uses a high-frequency inverter with a small transformer for isolation and to reduce the voltage to the required level. As it's running at high frequency, it doesn't need large inductance so the core and windings are much smaller and efficiency can be significantly better than a cheap 50 Hz transformer. If your transformer is outside so won't set the house on fire if it blows I suggest you don't worry (but worth making sure the pump etc are clear to minimise the work the pump has to do and so the load on the transformer).  Oh, and if you leave the Bosch charger on all night, the charging circuit will probably be turning off the charge well before morning so the load will reduce anyway.

Edited By John Haine on 25/06/2017 10:24:18

[not done it yetParticipant @notdoneityet]

they are designed for best cost/performance ratio

You think so? This is referring to these cheap chinese wall warts and similar (the subject of this thread)? They don't care about the whole life cost (buying and energy usage). Cheap, cheerful and hopefully it will outlast the guarantee.

This application is likely a continuously running one, where the energy losses are relatively horrendous as a percentage of work done. I saved nearly ten quid a year simply by buying a phone/ansafone with a better dc power supply

.

A couple of pounds extra, for a decent transformer, would likely have been saved on energy use over the period of ownership and it a) would last a lot longer and b) the cost/performance ratio would be favourable for the purchaser rather than the vendor. Routers from ISPs are likely in a similar state – the transformer likely loses nearly as much energy as the router uses.

Transformers are still not designed to run hot, per se. They are designed to cope with the heat generated. Yes, cold running is an ideal situation, but some of these chinese wall warts are only bought by people that either don't know or don't care.

[SillyOldDufferModerator @sillyoldduffer]

Posted by not done it yet on 25/06/2017 12:17:20:

…

A couple of pounds extra, for a decent transformer, would likely have been saved on energy use over the period of ownership and it a) would last a lot longer and b) the cost/performance ratio would be favourable for the purchaser rather than the vendor. Routers from ISPs are likely in a similar state – the transformer likely loses nearly as much energy as the router uses.

Transformers are still not designed to run hot, per se. They are designed to cope with the heat generated. Yes, cold running is an ideal situation, but some of these chinese wall warts are only bought by people that either don't know or don't care.

I don't think you can assume that a cool transformer is more efficient than a hot one.

For the same power input, a heavy transformer takes more heat to reach a given temperature than a lighter equivalent. The heavy transformer runs cooler because it has more mass and a greater surface area, not because it's electrically more efficient. The light one runs hotter because it has less mass and surface area, not because it's less efficient.. And the light one will not become significantly more efficient if it's fitted with a cooling fan. It is true that physically big transformers are more efficient than tiny ones, but that's not due to their operating temperatures.

A hot transformer isn't a problem provided the insulation's up to it. Over the years insulation has improved considerably. In the 1920's power transformer temperatures were limited to 65C; by WW2 the limit had been raised to 85C; today, the lowest rating you can get is 100C or 110C (depends on where you are).

So, measuring temperature alone doesn't tell you much. A hot transformer may be operating as per design, or it may be accidentally overloaded, or it might faulty, or, as NDIY points out, perhaps it was under specified for profit. To work out which is the case requires more testing than pointing a thermometer at it.

In a world where consumer electronics are replaced regularly it makes sense not to over specify. Think of all those beautiful high-end transformers ending up as landfill.

Dave

[roy entwistleParticipant @royentwistle24699]

Nick I was thinking substation sizes

Roy ( A bit big for OP )

[duncan webster 1Participant @duncanwebster1]

Posted by SillyOldDuffer on 25/06/2017 14:06:49:

 

I don't think you can assume that a cool transformer is more efficient than a hot one.

For the same power input, a heavy transformer takes more heat to reach a given temperature than a lighter equivalent. The heavy transformer runs cooler because it has more mass and a greater surface area, not because it's electrically more efficient. The light one runs hotter because it has less mass and surface area, not because it's less efficient.. And the light one will not become significantly more efficient if it's fitted with a cooling fan. It is true that physically big transformers are more efficient than tiny ones, but that's not due to their operating temperatures.

A hot transformer isn't a problem provided the insulation's up to it. Over the years insulation has improved considerably. In the 1920's power transformer temperatures were limited to 65C; by WW2 the limit had been raised to 85C; today, the lowest rating you can get is 100C or 110C (depends on where you are).

So, measuring temperature alone doesn't tell you much. A hot transformer may be operating as per design, or it may be accidentally overloaded, or it might faulty, or, as NDIY points out, perhaps it was under specified for profit. To work out which is the case requires more testing than pointing a thermometer at it.

In a world where consumer electronics are replaced regularly it makes sense not to over specify. Think of all those beautiful high-end transformers ending up as landfill.

Dave

 

Just to be pedantic (who me?), mass doesn't affect steady state temperature, only surface area. More mass will mean it takes longer to get to that temperature.

If you want to make the existing set up run cooler, solder a copper pipe to a copper plate, glue the transformer to this plate and make the water outlet flow through the pipe, bit is it really worth the work?. If the Transformer hasn't got a flat face, use metal loaded epoxy (can you still get Devcon?)

Edited By duncan webster on 25/06/2017 16:22:35

[SillyOldDufferModerator @sillyoldduffer]

Posted by duncan webster on 25/06/2017 15:49:49:

Posted by SillyOldDuffer on 25/06/2017 14:06:49:

…

Just to be pedantic (who me?), mass doesn't affect steady state temperature, only surface area. More mass will mean it takes longer to get to that temperature.

I 'm sure you're right but thinking about it has made my head hurt. I've had to delete 3 perfectly convincing counter-arguments simply because they're wrong and now I've run out of steam.

I heard about Dunning Kruger Effect this week; it's the observation that the poorest performers are least aware of their own incompetence. I thought Dunning Kruger Effect was great until I realised that I've got it.

Very educational this forum!

Dave

[duncan webster 1Participant @duncanwebster1]

They must have discovered that after research on politicians, who think they and only they have the answer to the world's problems, whereas in reality they case most of them!

[Russell EberhardtParticipant @russelleberhardt48058]

Posted by not done it yet on 25/06/2017 12:17:20:

they are designed for best cost/performance ratio

You think so?

Yes, I do but as I said the design parameters will vary according to the application. If you don't consider cost as well as meeting your customers' requirements you will soon go out of business. If you try to sell someone a Rolls Royce when what they need, and can afford, is a Mini you won't get very far.

Russell

[not done it yetParticipant @notdoneityet]

If you try to sell someone a Rolls Royce when what they need, and can afford, is a Mini

Capital cost or running cost? Clearly a Roller costs more to buy and more to run. That is not what a sensible company would do – they would look at the whole life cost. Most joe public buyers don't have a clue. That is why some Chinese manufacturers can get away with such shoddy goods. They can supply good quality – but at a slightly higher price. Not really the difference between a Roller and a mini, mind.

How often is it stated on the forum "buy cheap, buy twice"?

[John OlsenParticipant @johnolsen79199]

The resistance of the copper wire goes up as the temperature rises, by around 0.39% per degree C. So if it gets to 125 degrees C the resistance will have increased by about 40%. So the losses will increase as the temperature rises, tending to make things worse. This effect can be noticeable on small welding transformers, as they get hot the current you can get falls off because the winding resistance has increased.

But it is quite likely that the primary simply doesn't have enough turns. If this is the case, the core will go into saturation and the transformer will take too much magnetising current, even if there is no load on the secondary.

The only real solution is a bigger transformer with a bit more margin built in.

John

[michael howarth 1Participant @michaelhowarth1]

Just an update on the subject of this transformer. You will recall that it has a nominal output of 20 watts and that the pump requires 10Watts. The transformer still heats up even when the pump is disconnected, reaching a temperature of 38 deg C. It reaches about 50 deg C when the pump is connected. Unfortunately I do not have a meter that measures AC current. Anyway, given these rough and ready figures my grandson has calculated that the transformer is dissipating about 8+ Watts of heat. Not a great deal in the grand scheme of things but still adding £10+ to the electricity bill. I suspect that if £10 extra was spent on making a more efficient transformer everyone would be a winner. Typically, Interpet/Blagdon have not replied to my query. Finis.

Mick

[John HaineParticipant @johnhaine32865]

Assuming that the pump wants a dc supply then you could spend slightly more than the cost of a year's electricity on one of these:

**LINK**

and save probably a fiver a year – though you would need to package it up. I have something similar running the LED spotlights in my workshop and it gets hardly warm.

[Andrew TinsleyParticipant @andrewtinsley63637]

I was always taught one motto when it came to transformers. "You can either have a lot of iron and little copper or vice versa!"

The ones with lots of iron are inefficient compared to the ones having a lot of copper. You can imagine which way most cheap transformers go. Yes lots of iron because it is cheaper than copper!

My apologies to those electrical engineers that can work out the equations for the above two extremes of transformer design. It is in fact a very complex subject. I well remember a lecturer on my first degree course, writing down the equation for an audio transformer V out = and then 4 pages to take the RHS of the equation. Even then he said he had made some approximations. A bit simpler for a 50 Hz power Tx!

Andrew.

[Neil WyattModerator @neilwyatt]

I found a document on the standards for external PSUs

It looks like decent efficiency for external PSUs will become mandatory in the EU witha target date of 2017 or 2018… err…

Neil

[Russell EberhardtParticipant @russelleberhardt48058]

Posted by mick H on 28/06/2017 09:33:40:

The transformer still heats up even when the pump is disconnected, reaching a temperature of 38 deg C. It reaches about 50 deg C when the pump is connected.

I assume that by "transformer" you are referring to a plastic cased d.c. power supply containing a transformer and other components. Those temperatures are within the limits specified by the safety standard BS EN 60065, unless it has been changed since my old copy.

If you want lower temperatures you should change to a switched mode power supply as has already been suggested. They have typical efficiencies of over 80%.

Russell

[SillyOldDufferModerator @sillyoldduffer]

I've been musing on Mick's transformer warming up on no load. That could be caused by the core saturating due to over-voltage on the primary.

Could the symptoms be down to the transformer being a 220V 60Hz type designed for the US but considered 'close enough' for use in the UK? Slightly too few turns coupled with not quite enough iron might cause it to run hot (but not dangerously so.)

Dave

[john fletcher 1Participant @johnfletcher1]

I have followed this discussion with great interest. Years ago when rewinding motor and not have sophisticated test gear available we would as some one earlier has mentioned, measure the winding resistance cold ( workshop temperature) Run the motor and after some time, measure the resistance a second time. Subtract the first (T1) from the second T2 ( =the difference) divide the difference by the first, then divide the result by .004 temp co eff of copper. We then added the resulting temperature calculation to first original temperature which gave us the new the winding temperature, I think this could be easily carried out on a transformer .This appears to be a bit drawn out, but easy when you put in a bit of paper. I remember when the firm bought some ex Naval 60Hz motors, it was said we could expect them to run hotter on 50Hz supply as the flux density would 6/5 of what it would be on 60Hz, because the iron losses would be much greater. So what Dave above says could be correct.John

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