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Old 27th Nov 2018, 5:32 pm   #1
Skywave
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Question Hot mains transformer

This transformer has a nominal 230-v. primary and a tapped secondary: 0v. - 12v. - 24v. So a total N/L voltage of 36v. It's of a conventional construction: E and I laminations, pri. & sec. wound on it. Physically, it is about 3½ x 2" x 2½". I think it might have come from a Fidelity UA6 music centre.

With a resistive load on the sec'y., a 3-amp. draw caused the sec. voltage to drop from 36v. to 34v. I ran that test for 1 minute.

However, prior to serious use, I put it on a 'soak test' with no load. After about 30 minutes, I touched to top of the laminations clamp: Yow! That is really hot: two seconds of finger contact was enough! Intrigued, I made a few measurements:
Pri. resistance: 15 Ω; pri. inductance: 800 mH; no-load current about 1 amp.
From that, it seems to me that it is the iron loss that is the main contributory factor to that heat. (So N/L power factor approaching unity ).
So, as far as I'm concerned, this transformer is not suitable for use. However, is there a fault in the pri. winding? (To me, seems unlikely) or is this simply a case of poor design? (To me, seems most likely).

Al.
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Old 27th Nov 2018, 5:43 pm   #2
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Default Re: Hot mains transformer

Shorted turns?
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Old 27th Nov 2018, 5:45 pm   #3
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Default Re: Hot mains transformer

How do you get 36 volts from a 24 volt secondary?

Lawrence.
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Old 27th Nov 2018, 7:04 pm   #4
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Default Re: Hot mains transformer

Quote:
Originally Posted by ms660 View Post
How do you get 36 volts from a 24 volt secondary?

Lawrence.
maybe its a typo and he means its 12-0-24 V?
Which strikes me as a bit odd.
rather than 0-12-24 I wonder if it should be a 12-0-12 CT job? which means it would be capable of 24V end to end on the primary (You can;t wire it in series it's already in one piece.) or 12V if FW rectified (at full load).
I am sure Al will elucidate.

I'm suspecting a shorted turn on one side of the centre tap and maybe its really 24-0-24V, the short being around halfway down one side?

Andy.
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Old 27th Nov 2018, 7:14 pm   #5
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Arrow Re: Hot mains transformer

ms660: The secondary is not 24-v.

The secondary winding has a tap, so there are 3 connections from that secondary. Calling one end of the whole winding 0v., relative to that, the tap is 12v. Relative to that same 0v. end, the other end of the winding is 36v. So the voltage between the tap ('12v.') and at the end of the winding ('36v.') is 36v. minus 12v. = 24v.

Yes, I should have explained things clearer in my O.P. (It was written amongst doing 3 other jobs - all urgent! )
My apologies to all those here who were confused.

Al.

Last edited by Skywave; 27th Nov 2018 at 7:25 pm.
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Old 27th Nov 2018, 7:24 pm   #6
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Default Re: Hot mains transformer

Quote:
Originally Posted by bikerhifinut View Post
I'm suspecting a shorted turn on one side of the centre tap and maybe its really 24-0-24V, the short being around halfway down one side?
Ah! I like that - since the N/L voltages are asymmetrical w.r.t. the tap. Although if we call the tap 0v., we get 24v. and 12v. - and having just the right amount of s/c turns on that '12v. winding' seems a remarkable co-incidence to produce 12v.

To me, at the moment, s/c turns on the pri. seems the most likely cause, especially since the wire on the pri. will be much thinner than that on the sec.

Al.
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Old 27th Nov 2018, 7:28 pm   #7
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Default Re: Hot mains transformer

So 0v-12v-36v, not 0v-12v-24v as was originally stated, it might be the 3 amp current draw that's damaged it?

Lawrence.
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Old 27th Nov 2018, 8:07 pm   #8
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Arrow Re: Hot mains transformer

Yes, 0v-12v-36v. As I wrote, sorry about the confusion and my poorly written OP.
Possible cause: excessive current draw on the sec. which has damaged it . . . well, yes, maybe. I can see that the wire on the sec. is quite thick.
Perhaps I'll run some voltage regulation tests of the secondary; might be informative.

Al.
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Old 27th Nov 2018, 8:35 pm   #9
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Default Re: Hot mains transformer

It does sound rather hot, there's a lot of iron losses there!

If there's a shorted turn, I'd expect there to be an obvious hot-spot, and 30 minutes would make it smoke. You don't have that.

Sometimes, transformers for intermittent use may be operated at a very high flux density, so core losses are high. This allows fewer turns, of thicker wire, to be used, so resistances are less and regulation is better - and you can suck more power from a smaller sized unit.

However, if it really came from a music centre, that won't be the case. So knowing the original use would be really helpful!
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Old 27th Nov 2018, 9:34 pm   #10
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Default Re: Hot mains transformer

Could it be that the lack of a secondary load means the transformer is saturating, so losing inductance and consuming lots of magnetising current - which then heats up the primary copper. It is quite common for cheaper transformers to be run quite near saturation, especially when off load. Also, the transformer will have been designed to power a device; surviving not powering the device may not have been in the spec.
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Old 27th Nov 2018, 11:25 pm   #11
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Default Re: Hot mains transformer

Quote:
Originally Posted by Skywave View Post
I put it on a 'soak test' with no load. After about 30 minutes, I touched to top of the laminations clamp: Yow! That is really hot: two seconds of finger contact was enough! Intrigued, I made a few measurements:
Pri. resistance: 15 Ω; pri. inductance: 800 mH; no-load current about 1 amp.
(To me, seems most likely).

Al.
It is both iron losses and copper losses, there are at least 15W copper losses on no load. I have encountered this quite a bit, especially on old power transformers designed for 60Hz rather than 50. I'm also convinced the core changes over time. The reason is, in the application of a vintage electrostatic TV set, the CRT beam was grossly deflected by the magnetic field and grossly high magnetization current, and it could not of been like this from new. Also, replacing it with a modern core the magnetization (off load primary current) dropped from 1.5A to 50mA !

I think the core has in some way deteriorated. The core's losses are Eddy current and Hysteresis losses. If we postulate that the magnetic properties of the core have not changed from new, then perhaps the Eddy current losses have skyrocketed due to the laminations connecting themselves together over time. Clean some of the lamination edges about 1cm apart and check for continuity with the meter.
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Old 28th Nov 2018, 12:27 am   #12
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Arrow Re: Hot mains transformer

Perhaps I should have stated this earlier. This xfmr. has the following printed on the winding protective covering. T3573 DE Ltd.

A Google search yielded nothing.

Al.
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Old 28th Nov 2018, 9:30 am   #13
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Default Re: Hot mains transformer

Being too hot to touch is not necessarily a fault, some are intended to do that, simply because there is less iron, so they are cheaper and lighter. A cheap music centre will be engineered for minimum cost of manufacture, with little concern for cost of operating it, so the transformer may well be cheap and lossy
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Old 28th Nov 2018, 10:00 am   #14
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Default Re: Hot mains transformer

Quote:
Originally Posted by G8HQP Dave View Post
Could it be that the lack of a secondary load means the transformer is saturating...
This is possible. I made a transformer using a rather undersized core - winding resistance losses are high (lose 10% of primary voltage overcoming winding resistance!). So, off-load, when primary current is much lower, the core runs at around 10% higher flux, so closer to saturation; and about 20% higher core losses, as iron loss is approx. proportional to square of flux density.

Quote:
Originally Posted by G8HQP Dave View Post
...so losing inductance and consuming lots of magnetising current - which then heats up the primary copper.
I can't envisage of a set of circumstances where the off-load current would be MORE than the on-load, though.

Quote:
Originally Posted by G8HQP Dave View Post
Also, the transformer will have been designed to power a device; surviving not powering the device may not have been in the spec.
As above, total losses unloaded can never be more than total losses when loaded (stated without proof - feel free to challenge me on that till I prove it!). All that happens as you ramp up the load from zero is that core losses may drop (as above), while copper losses rise. But rise of copper losses is always greater than fall in core losses.
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Old 28th Nov 2018, 10:46 am   #15
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Default Re: Hot mains transformer

It's the 3 amp current draw through a resistor load that concerned me, that's a lotta watts for the resistor and the transformer, 102 watts so far as I can make out.

Lawrence.
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Old 28th Nov 2018, 11:24 am   #16
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Default Re: Hot mains transformer

Dimensions of a modern 100VA transformer available from RS: 89 x 68 x 75mm

Approx dimensions of this transformer, converted to metric: 89 x 51 x 63mm

So it's probably not 100VA, and 3A would have been asking a bit much. Although it shouldn't have damaged it in the space of a minute.

The no-load primary current draw of 1 amp is the real indicator here - most likely a shorted turn, as suggested earlier. But a quick test on a variac might help to establish if the problem is with the core.

I can't find any data on-line about the Fidelity UA6, but I really doubt it would have been fitted with a 100VA transformer. Any idea what the power output would have been? Even if it was 50 watts per channel - which would have been a lot of power for even an upmarket Fidelity - the peak-to-mean ratio (or crest factor) of music means that you can seriously cut corners with transformer ratings, and I'm sure Fidelity would have taken full advantage of that.

Either way, given that class B amplifiers don't draw much current when idle (and don't draw all that much more on average when playing music), I seriously doubt this transformer was designed to require a significant load. Having rescued way more transformers from audio gear than I have room for, I'd expect the no-load draw from the mains to be a few 10s of milliamps at most.
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Old 28th Nov 2018, 12:20 pm   #17
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Default Re: Hot mains transformer

Do you have a variac so that you could repeat your no-load temp expt - but with the voltage to the transformer reduced to 220 volts say? This is just in case the transformer was designed for what was the typical European mains voltage.
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Old 28th Nov 2018, 12:42 pm   #18
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Default Re: Hot mains transformer

Quote:
Originally Posted by SteveCG View Post
Do you have a variac so that you could repeat your no-load temp expt - but with the voltage to the transformer reduced to 220 volts say? This is just in case the transformer was designed for what was the typical European mains voltage.
The maximum flux density Tm, that the mains transformer core experiences is proportional to the applied voltage V and inversely proportional to the frequency f, the equation is:

Tm = V/(4.44fNm^2)

where m^2 is the core cross sectional area and N the number of turns.

For power transformers like this, that are struggling with high flux densities that push the core into the saturation region, lowering the applied voltage (or increasing the frequency say from 50 to 60Hz) will help. So if the transformer was intended for 220V the problem will obviously be worse on 230 to 240V.

As I mentioned the ones I have had trouble with in this respect were intended for 60Hz and running them on 50Hz is quite problematic.

One way to examine if the core is being pushed into heavy saturation in the cycle, is to place a 1 Ohm current sense resistor in series with the primary. I have a scope with totally isolated inputs to examine the voltage developed across the resistor , a Tek 222PS, but in this absence of that isolated scope, feed the transformer primary by an isolating transformer and earth the primary connection that has the 1 Ohm resistor, so as to measure the voltage across the resistor with a standard scope. The current is not a sine wave, it has a characteristic shape relating to the core properties.

Last edited by Argus25; 28th Nov 2018 at 12:50 pm. Reason: typo
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Old 28th Nov 2018, 12:44 pm   #19
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Default Re: Hot mains transformer

Quote:
Originally Posted by SteveCG View Post
Do you have a variac so that you could repeat your no-load temp expt - but with the voltage to the transformer reduced to 220 volts say? This is just in case the transformer was designed for what was the typical European mains voltage.
If you do not have a Variac you could use a lamp limiter to drop the voltage. Bulbs of different wattages can be used to obtain different voltages.

Scoping the current will tell if the core is saturating or if there is resistance loading.
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Old 28th Nov 2018, 1:28 pm   #20
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Default Re: Hot mains transformer

Quote:
Originally Posted by Argus25 View Post
One way to examine if the core is being pushed into heavy saturation in the cycle, is to place a 1 Ohm current sense resistor in series with the primary. I have a scope with totally isolated inputs to examine the voltage developed across the resistor , a Tek 222PS, but in this absence of that isolated scope, feed the transformer primary by an isolating transformer and earth the primary connection that has the 1 Ohm resistor, so as to measure the voltage across the resistor with a standard scope. The current is not a sine wave, it has a characteristic shape relating to the core properties.

Good advice from Hugo!

Another way, which might be simpler still - use a Variac to power the transformer and measure the current at 50V, 100V, 150V, 200V, 250V. Plot a graph.

If there's a shorted turn, the current will be proportional to voltage, more or less (assuming there's no arcing going on in there). If it's just a saturation issue, current will be low at lower voltages and then ramp up disproportionally at higher
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