Transformer temperature

Thank you for your interest and comments gents. The outfit that I have is a " Blagdon 3000 Inpond 5 in 1". So quite a reputable make but I would guess having origins in China. The outfit performs well and does what it says it will do, keeping the pond crystal clear and apparently well oxygenated. It is about 10 months old and the transformer has always run very warm from day 1. As previously mentioned, the original transformer failed after about 7 months. I will be giving it a routine clean out over the weekend and will obviously check for obstructions etc.

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.

I emailed the manufacturer this morning but their replies are within 7 days.

Mick

Quite likely the transformer is a bit mean on copper and iron and so is not up to the job. After all, it only has to last until the guarantee runs out. One thing to try is to measure the magnetising current, eg the input current from the mains with no load on the output. This should be quite low, less than 10 % of the current for full load.

Many years ago where I was working we were having trouble with the mains transformers in a PYE VHF receiver. The transformer was rated at 23 VA. With no load on the secondary, they were typically taking 18 VA just for magnetising current. So they ran quite hot just on no load. An interesting check was to gradually reduce the mains input voltage with a Variac transformer. A 10 % reduction in input voltage dropped the magnetising current right back to about what is should have been, eg a couple of VA or so. So we had a transformer rewound with about 10 % more turns, which worked perfectly. Of course, we were not allowed to do this to the rest of the sets, since that was an unauthorised mod, so we had to put up with just changing them all the time.

Actually, if you don't happen to have suitable gear to measure the input current, just running it no load and seeing how warm it gets would be worthwhile.

John

John is quite right about modern day transformers, particularly from china. I believe all new units are required to have a rating shown. This ranges from one to five, or six, in roman numerals, the best being the higher numbers which waste far less power than the poor ones.

Basically the magnetic flux is lost from the unit, instead of being concentrated and used where it is effective or the windings are substandard in the poor units.

Think about things a little and you will realise the turns ratio, to reduce the voltage from 240 to 12V is 20:1. This can theoretically be achieved with 20 turns on the primary and just 1 on the secondary. Or it might be achieved wit 200 and 10,

2000 and 100 or any other combination of that ratio. The lower numbers will not work as well as higher numbers.

Also, the transformer core needs to be designed so as not to retain its magnetism (it needs to change at the mains frequency) and the induced currents (eddy currents) in the core material need to be minimised (that is why cores are laminated and electrically isolated between laminations).

Further design parameters, like isolating the primary (at mains voltage) from the safe secondary voltage, to avoid mains voltage being fed to the output in the case of failed windings, can be arranged with windings separated to each end of the core or one winding enclosing the other. Both work, but the efficiency is not the same!

Good transformer design is not a simple matter as all the above aspects are inter-related!

Transformers are designed to run hot. Modern winding insulation materials can withstand quite high temperatures. A 27 C temperature rise is by no means excessive. When we used to get transformers made we would specify a 125 C thermal fuse to be embedded in the windings.

Are you measuring the temperature of the windings, the laminations, or a plastic case? You can measure the winding temperature by measuring the primary resistance when cold ( at a known temperature) and then again after giving it a long run at full load. It is then a simple matter to calculate the temperature from the known temperature coefficient of the resistance of copper.

Even the lowest spec wire insulation is rated for 100 C continuous and the more usual class F is rated for 140 C.  Maximum surface temperature of an enclosure that can be touched is, if I remember right, 65 C.

Russell.

Edited By Russell Eberhardt on 24/06/2017 09:31:13

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.

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

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

Thanks for the replies Tim and Russell much appreciated.

Regards, Keith.

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.

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

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

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

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.

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

Nick I was thinking substation sizes

Roy ( A bit big for OP )

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

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

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!

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