Pwr xfmr question

[Larry]

Can one tell if a pwr xfmr is good or not by measuring the (unloaded)
primary current? I have a large surplus unit that I want to use for a
project. It's rated for lots of current and weighs over 11 pounds.
When I measured the pri. current, it was 0.31A at 120VAC with no load on
the transformer.

If this is not a definitive test, is there a relatively simple test that
I could do short of fully loading the secondaries and watching for
excessive temp rise?

My SET amp seems to have a shorted turn in the 6.3v winding, shows 5.6v
unloaded. It would be nice to use the "new" xfmr to rebuild this amp and
make some improvements.

Thanks for any info.

-Larry

[Gerald Stombaugh]

Let us presume that there is approximately 5 lbs.
of steel in the unit. Excitation losses should be about
7 VA/lb = 35VA/120= approximately 291Ma RMS current.

Looks OK to me...................

Gerald





On Thu, 24 Jul 2003 08:11:32 -0700, Larry
> wrote:

>Can one tell if a pwr xfmr is good or not by measuring the (unloaded)
>primary current? I have a large surplus unit that I want to use for a
>project. It's rated for lots of current and weighs over 11 pounds.
>When I measured the pri. current, it was 0.31A at 120VAC with no load on
>the transformer.
>
>If this is not a definitive test, is there a relatively simple test that
>I could do short of fully loading the secondaries and watching for
>excessive temp rise?
>
>My SET amp seems to have a shorted turn in the 6.3v winding, shows 5.6v
>unloaded. It would be nice to use the "new" xfmr to rebuild this amp and
>make some improvements.
>
>Thanks for any info.
>
>-Larry
>



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[Chuck Harris]

Hi Larry,

If you really had a shorted turn in your filament winding, the
transformer would be getting very hot, and growling at you.

If it is not getting getting really hot, and growling,
I would expect that low filament voltage is more likely
he result one of the following:

0) normal
1) primary voltage that is down around 100V, or 120V connected to
a 135V tap on primary
2) excessive filament loading
3) mistaken use of 5V filament winding for 6V filaments.
4) a bad voltmeter
and lastly,
5) an improperly wound transformer.

One thing to note, though, 5.6 V is not even 10% low in voltage. The
tube manufacturers allow that much margin.

-Chuck


>>Can one tell if a pwr xfmr is good or not by measuring the (unloaded)
>>primary current? I have a large surplus unit that I want to use for a
>>project. It's rated for lots of current and weighs over 11 pounds.
>>When I measured the pri. current, it was 0.31A at 120VAC with no load on
>>the transformer.
>>
>>If this is not a definitive test, is there a relatively simple test that
>>I could do short of fully loading the secondaries and watching for
>>excessive temp rise?
>>
>>My SET amp seems to have a shorted turn in the 6.3v winding, shows 5.6v
>>unloaded. It would be nice to use the "new" xfmr to rebuild this amp and
>>make some improvements.
>>
>>Thanks for any info.
>>
>>-Larry

[Robert Casey]

Larry wrote:

> Can one tell if a pwr xfmr is good or not by measuring the (unloaded)
> primary current? I have a large surplus unit that I want to use for a
> project. It's rated for lots of current and weighs over 11 pounds.
> When I measured the pri. current, it was 0.31A at 120VAC with no load
> on the transformer.
>
That could be reactive current. Unloaded, the transformer will look
like an inductance
to the AC powerline. And draw reactive current. This is current 90
degrees phase
shifted from the voltage waveform, and does not consume power.

One way to check this would be to use a "Kill-a-Watt" power meter.
Radio Shack
has (as of a few months ago did) these for $40 to $50.

[Larry]

Chuck Harris wrote:
> Hi Larry,
>
> If you really had a shorted turn in your filament winding, the
> transformer would be getting very hot, and growling at you.
>
> If it is not getting getting really hot, and growling,
> I would expect that low filament voltage is more likely
> he result one of the following:
>
> 0) normal
> 1) primary voltage that is down around 100V, or 120V connected to
> a 135V tap on primary
> 2) excessive filament loading
> 3) mistaken use of 5V filament winding for 6V filaments.
> 4) a bad voltmeter
> and lastly,
> 5) an improperly wound transformer.
>
> One thing to note, though, 5.6 V is not even 10% low in voltage. The
> tube manufacturers allow that much margin.
>
> -Chuck


Mebbe it's not bad, just seems pretty hot after an hour or so. No
growling sounds. Temp of the shell is 47C at that time, and room ambient
23.3C. Uncomfortable to leave hand on it, but not painfully so.

Thanks for the info.

-Larry

[Larry]

Gerald Stombaugh wrote:
> Let us presume that there is approximately 5 lbs.
> of steel in the unit. Excitation losses should be about
> 7 VA/lb = 35VA/120= approximately 291Ma RMS current.
>
> Looks OK to me...................
>
> Gerald
>

Aha! Thanks, Gerald. This one goes into to my rules of thumb collection...

-Larry

[Chuck Harris]

Hi Larry,

It is normal for most commercially wound power transformers
to be uncomfortable to touch under normal loading. The insulation
is generally rated for continuous operation at 105C (above boiling
point for water) your transformer is less than half that.

-Chuck

Larry wrote:

> Mebbe it's not bad, just seems pretty hot after an hour or so. No
> growling sounds. Temp of the shell is 47C at that time, and room ambient
> 23.3C. Uncomfortable to leave hand on it, but not painfully so.
>
> Thanks for the info.
>
> -Larry
>
>

[Patrick Turner]

Chuck Harris wrote:

> Hi Larry,
>
> It is normal for most commercially wound power transformers
> to be uncomfortable to touch under normal loading. The insulation
> is generally rated for continuous operation at 105C (above boiling
> point for water) your transformer is less than half that.
>
> -Chuck

Many commercially well made transformers don't get hot,
eg, Quad 405 power trannies, and lots more I could name.
Only the cheap and nasty ones run at 105C, imho.
If only the bloody bean counters said "yes"
to the engineers who like cool designs, we'd have little to complain about.
But the BCs are allergic to "yes", and can only say "No!",
or "Smaller!", or "Lighter!" and above all they like the word "cheaper!"

Don't buy hot running transformers.

I have wound a couple of transformers using second hand iron
of high loss E&I material,
and if you have B = 0.8T, it is still difficult to get
low copper losses, and low temperature running.
I rewound an ST70 tranny which I thought ran too hot,
and I added 50% to the existing stack height with material I had laying
around,
of similar poor quality to the Dynaco's.
It still runs quite warm, even though I'd lowered the Bm by increasing th
stack height.
A 25C rise above ambient isn't unusual with such crap iron.
So on a day of 30C, expect 55C for the iron.
The wire could be slightly cooler.
55C is a bit too hot to me, and the whole amp then gets pretty warm
on a hot day, and to minimise the effects, a row of 10mm holes
around the tranny, and a well perforated bottom cover and 20mm high
amp feet allow a healthy air flow through the amp, and it all helps.
Potting makes the tranny run hotter, its like a banket.
But Grain oriented silicon steel cores are the best, and the losses at 0.85T
can be extrememely low.
In a couple of power trannies with cores rated at 1.9 kW, (125 mm stack of 50
mm tongue,)
I was able to get iron losses to less than 5 watts, so negligible iron
heating occurs, even on a hot day, and all the majority of the heat
is in the copper. Overall efficiency of the GOSS cored transformer
can be 94%, at the rated load for the copper.
Many 25 watt trannies one might buy from Radio Shack etc, will
have 7 watt iron losses. Notice how 6the suppliers seldom state the no-load
idle current?
The mains voltage multiplied by the no-load idle current, ( magnetising
current flowing through
the primary indictance ), is the idle VA, ( or watts ),
and always this should be less than around 8% of the total input VA
with the rated load on the tranny.
Even better, is to design for a 50% continuous overload.
Just use plenty GOSS, and forget lightweight philosophy with tube gear.

Toroidal power trannies can be marvels, if one gets one wound with low Bmax,
and get them done with impregnation, and no crossed over turns in any layer,
etc,
but most I see are crummy. They often hum as well.
Because the toroid iron is a continuous strip of spiral wound GOSS material,
the iron U is high, often over 20,000, so the Lp is high,
so Iron losses are very low, IF THE BM IS NO HIGHER THAN 0.85T.
This usually results in a cool and quiet toroidal, and an 800 VA toriodal
can be only little more than 1/2 the weight of a well done E&I
tranny running with the same load, and same temperature.

Patrick Turner.

>
>
> Larry wrote:
>
> > Mebbe it's not bad, just seems pretty hot after an hour or so. No
> > growling sounds. Temp of the shell is 47C at that time, and room ambient
> > 23.3C. Uncomfortable to leave hand on it, but not painfully so.
> >
> > Thanks for the info.
> >
> > -Larry
> >
> >