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Old 19th Jul 2021, 10:07 pm   #1
Julesomega
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Default What is a reasonable magnetising current for a mains transformer?

I have been testing some mains tfmrs to offer on the forum, and because I hate bangs and showers of sparks in my face I use a lamp limiter. The larger ones give a dim glow from a 60W bulb, but when I tried this one the lamp lit brightly. It is from some Japanese consumer equipment, I guess from the 80s or 90s. However the off-load output voltages seemed quite respectable, though after correction for the reduced input they seem a little high, at 31V, 27V, and 27.5V

These in turn seem unusual for consumer equipment, where one usually finds something like 30V C-T, 15V C-T and maybe 9V for a bit of glue logic. I suppose it could have a shorted turn on what should be a second 27.5V winding

I added another 100W to the limiter, whereupon both lit at the same brightness and the transformer buzzed loudly. I discontinued the test as violence alarms me

I see we have a 'sticky' thread in this section, Testing a transformer for shorted turns which also recommends the use of a lamp limiter but doesn't give expected values according to transformer size. My transformer measures 605035mm and weighs 0.6kg, which should give a power rating of at least 25W so I am guessing a nominal 30W. I wonder what is going on here
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Old 19th Jul 2021, 10:28 pm   #2
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Question Re: What is a reasonable magnetising current for a mains transformer?

Those primary winding wires - black and white - could imply a 115 v.a.c. primary. Do you know what the primary voltage is? Before you can do anything with any mains transformer, that data must be known.

Al.
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Old 19th Jul 2021, 10:45 pm   #3
Chris55000
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Default Re: What is a reasonable magnetising current for a mains transformer?

Hi!

Often is the case that black and white primary wires of a mains transformer do imply a 100/115V primary, but it's not set in stone by any means!

All Japanese mains transformers must be assumed to be 100 V 50 Hz unless the transformer, a label on the gear or the relevant s.m. clearly specified otherwise!

Chris Williams
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Old 19th Jul 2021, 11:10 pm   #4
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Default Re: What is a reasonable magnetising current for a mains transformer?

115V? What an outrageous idea
I'll see if it's any happier at 115V, but I'm stuck with 50Hz
Won't matter for a short while
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Old 19th Jul 2021, 11:20 pm   #5
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Default Re: What is a reasonable magnetising current for a mains transformer?

Also, Magnetising current can depend on many other things within the transformer.
To mention a few:
1. The physical size and type of core material.
2. The efficiency of the design.
3. The winding material. SOME modern stuff uses aluminium wire NOT copper.

To enlarge a little bit, The size and shape of the core has a huge amount to do with magnetising current. We all know that a modern toroid is more efficient than an older C core or double C core design. It will almost always be more efficient than an EI style transformer.
BUT !!! this may not always be the case, as some very fancy iron used in very fancy transformers can be very efficient. Frequency of operation also has a HUGE influence on the current. A 400 Hz transformer used in aircraft can be one tenth the size of one designed for 50 Hz. Even America with its 60 Hz mains means its slightly more efficient than our 50 Hz.

The flux density that the design is based on will also affect the current. If low flux densities are used, it generally means a very much lower current. If a design is made for a transformer calculated around a maximum flux density of 2 Teslas, thats a fairly large amount of magnetic current roaring around the core. If, the same sized core is operated
at 1 Tesla it will have much lower magnetising current, but may suffer from higher operating temperatures because there is so many more turns in a low flux density design, caused by the longer wire length and consequent higher resistance.

The resistance of the windings will also add to the current. Pure copper ( NOT audiophool grade ) has a conductivity of 1. Aluminium has a conductivity of 5.

https://www.thoughtco.com/table-of-e...ctivity-608499.

The above is very short of a thorough explanation, but will do for now. My technical writing ability is not good enough, nor my aging brain being smart enough to go over all the elements required. I do make quite respectable transformers however.

Colours of wires varies by country and the designer so I will stay out that argument.
BUT, if you run into my transformers, EI will be brown and blue for primary with green/yellow for a screen. It will be designed for a nominal 240 volts.

For dual primaries, 120/240 volt operation they will be:
pri one, brown beginning, orange end.
pri two black beginning, white end.

These are operated in series for 240 volts, and parallel for 120 volts.
Screen will again be green/yellow, if fitted.

Why dont I provide multi taps for the primary?.
I make my transformers to be a generous design which are more than capable of operating on slightly higher or lower primary voltages. These voltages will of course change the magnetising current.

Hope that helps a little.

Joe
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Old 20th Jul 2021, 12:38 am   #6
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Default Re: What is a reasonable magnetising current for a mains transformer?

A more informative test is available when you have a variac, and can measure/plot the primary magnetising current as a function of primary voltage, and identify when the onset of saturation starts.
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Old 20th Jul 2021, 4:11 am   #7
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Default Re: What is a reasonable magnetising current for a mains transformer?

Japan has both 50Hz and 60Hz areas. 100V supplies are common.

It is not surprising that a 120v transformer for the US goes into saturation by 240v and the magnetising current shoots up due to a drastic fall in inductance. This effect was used by serious designers to pull mains fuses when someone plugged an instrument in in the UK with its mains settings in the USA condition.

The saturation current of 115v mains fans hit with 240v helped with the fuse pulling, too.

David
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Old 20th Jul 2021, 8:49 am   #8
Julesomega
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Default Re: What is a reasonable magnetising current for a mains transformer?

Seems much happier at 120V: gives 16.75, 14.5 and 14.75V, magnetising current now 70mA (was 200mA at 160V)

Another mystery solved, only remaining mystery is what did it come out of? Might be from a Sony XDR-F1HD FM tuner which I modified for external power input
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Old 20th Jul 2021, 10:03 am   #9
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Default Re: What is a reasonable magnetising current for a mains transformer?

A reasonable rule of thumb is that for transformers up to 50W or so, current taken off-load could be 20% of the full-load primary current. (Because they're in quadrature, this does not mean that on-load current is 20% higher than what you'd expect from simple watts calculations.)

Transformers for higher powers generally have magnetising currents of a smaller percentage.

Just a couple of point's I'd query about Joe's comprehensive post:

Quote:
Originally Posted by joebog1 View Post
Also, Magnetising current can depend on many other things within the transformer.
...

3. The winding material. SOME modern stuff uses aluminium wire NOT copper.

...

The resistance of the windings will also add to the current. Pure copper ( NOT audiophool grade ) has a conductivity of 1. Aluminium has a conductivity of 5.
First, conductor material should make hardly any difference to magnetising current. Conductor resistance is low, otherwise there'd be a significant voltage drop on load; and as such the low winding resistance contributes hardly anything to the off-load impedance, which is dominated by primary inductance.

Secondly, if copper has a conductivity of 1, aluminium has conductivity 0.6, not 5!! It's actually a worse conductor, not better. Ratio of resistivities is about 1 : 1.6, the table in the link agrees

Copper 1.68E−8
Aluminum 2.82E−8


The advantage of aluminium is it is considerably less dense, so even after using beefier wire gauges in a necessarily larger transformer, relative to copper, you can still end up with something not as heavy which is a plus for aircraft transformers (been there, done it...). But it's a pain to terminate, and minimum order quantities for aluminium wire is huge...
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Old 21st Jul 2021, 3:29 pm   #10
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Default Re: What is a reasonable magnetising current for a mains transformer?

Yes a lot of Japanese equipment was designed for 100/110V: there was a bit of a 'thing' a couple of decades back when some grey-import Japanese ham-radios started to arrive here and to 'make them work' on UK mains the two 100V primaries [which were usually wired in parallel for 100V supplies] were rewired-in-series.

They got rather hot but produced plenty of RF output.

For a while.

No point crying to the legitimate UK importers/distributors when the thing inevitably went pop!
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Old 21st Jul 2021, 4:12 pm   #11
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Default Re: What is a reasonable magnetising current for a mains transformer?

70mA still seems q bit high for a 30VA transformer I think what you have there is one meant for 60 cycles supply I'm pretty sure it should at bit less than this now I've said that I'm going to have to check one of my junk box TXs now
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Old 21st Jul 2021, 8:20 pm   #12
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Default Re: What is a reasonable magnetising current for a mains transformer?

If the TX were run from 60Hz, the mag current would drop to less than 60mA, even further if there is still some saturation occurring on peaks at 50Hz.

Looking at the primary current waveform with a 'scope can be very revealing. There should be very little if any trace of "pips" on the top of the sinewave at the nominal supply condition.
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Old 22nd Jul 2021, 5:10 am   #13
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Default Re: What is a reasonable magnetising current for a mains transformer?

kalee20.


This table has an easier to understand table of conductivity of metals. It is listed in order from best to worst.

Silver is the best conductor, followed by copper. Aluminium ( pure ) is listed well down the table. Aluminium alloys are worse than pure aluminium.

With respect, Joe.
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Old 22nd Jul 2021, 7:31 pm   #14
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Default Re: What is a reasonable magnetising current for a mains transformer?

JoeB, the numbers Kalee gives are resistivities. You are (NOW) both saying the same things but from opposite directions. Your first go was wrong however.
At room temperature, my copy of Kaye and Laby (THE bible of such stuff) gives Aluminium with resistivity of 2.50 with copper given as 1.55 (units ignored to save my head).
Les.
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Old 22nd Jul 2021, 7:43 pm   #15
kalee20
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Default Re: What is a reasonable magnetising current for a mains transformer?

Yes - the table Joe included in his post (#5) gives resistivities and conductivities. I picked on the resistivities because that's what I use routinely - but one is the reciprocal of the other, so if ratio of resistivities is 1.6 : 1, the ratio of conductivities is 1 : 1.6.

I'm struggling to see how to derive Joe's 5 : 1 though, even the resistivity-density product doesn't give a ratio of 5... between Cu and Al.
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Old 22nd Jul 2021, 10:32 pm   #16
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Default Re: What is a reasonable magnetising current for a mains transformer?

So at high school we were taught that the following table was normally used for such work. Dad also taught me the same thing. He sort of knew his stuff as he held engineering degrees from

Poland/France
Great Britain
Australia

https://en.wikipedia.org/wiki/Electr...d_conductivity

The table is a few pages down, but is identical to the one I still use today. Yes, I agree that conductivity and resistivity are equal and opposites of each other.
I have also stated that my writing skills are very 'strayan, so my meaning may have been misunderstood.

My apologies for any confusion.

With respect

Joe
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