Mains transformer goodness

[Paul E. Schoen]

Anyone care to speculate on the timeline of failure?
There was no local hotspot and no lacquer damage more than a mm from the
"spot weld", and a bit of very localised smoke staining travelling a cm
or
so along the affected wires in each direction. Nothing to suggest that
the
initial bridge was between the 2 bifilar wires of the primary, so not
running a 120 volt primary in effect on 240V ac for any time. For 240V
use
the 2 primaries seriesed to give about 2.8 ohm originally , after failure
then about 0.4 ohm.
The 2 primaries broke into 5 lengths
9.6,14.6,20.1,20.1 and 25.3m long , measured to about 0.2m accuracy.
So originally probably 2 x 45m. Don't know for sure as did not think to
check but the weld was probably 20.1m from one end, but I would suggest
that
bridge occured after an arc to another layer (higher pd) and then
localised
heating to bridge across to the bifilar fellow wire.

In summary , no evidence that bifilar wiring itself was the reason for
failure but more due to the lack of any interlayer insulation. Because of
the uneven wire spacing between inside and outside faces of the toroid it
is
too easy for the layers to be jumbled.

It would also be helpful to know how the toroid was mounted, and if the
failure occurred where pressure was applied. Toroids are usually mounted
either flat with washers and rubber gaskets with a single screw through the
hole, or vertically in an "Omega" bracket, with some rubber cushioning
material around the periphery. But in either case there may be additional
pressure on a "high spot" where two windings may be crossed, and softening
of the insulation from overheating may also contribute to a short.

It is fairly easy to check for dielectric breakdown between the bifilar
windings, but nearly impossible to detect a weakness between adjacent turns
of a single winding. But it might be possible to use a higher frequency, or
PWM pulses of higher peak voltage, to produce a higher potential between
turns of the same winding, and observe spikes of current due to breakdown.

Paul

 

[N_Cook]

It would also be helpful to know how the toroid was mounted, and if the
failure occurred where pressure was applied. Toroids are usually mounted
either flat with washers and rubber gaskets with a single screw through the
hole, or vertically in an "Omega" bracket, with some rubber cushioning
material around the periphery. But in either case there may be additional
pressure on a "high spot" where two windings may be crossed, and softening
of the insulation from overheating may also contribute to a short.

It is fairly easy to check for dielectric breakdown between the bifilar
windings, but nearly impossible to detect a weakness between adjacent turns
of a single winding. But it might be possible to use a higher frequency, or
PWM pulses of higher peak voltage, to produce a higher potential between
turns of the same winding, and observe spikes of current due to breakdown.

Paul

But the primary is buried under the heavy gauge secondaries windings and
then three layers of 0.05mm polyester film strip, each layer of that
probably overlapped by a factor of 3. So I would have thought immune from
any external mechanical effects, exccept vibration perhaps.

 

[N_Cook]

This one is mounted by central coach bolt and dished disk plus rubber disk.
In use the bolt is vertical and when amp is carried it is horizontal.
I should say I've seen the result of an amp set for USA use used on 240V ,
extensive areas of bare copper due to lacquer melting or burning off and
large amounts of smoke damage.

 

[Phil Allison]

"N_Cook"

This one is mounted by central coach bolt and dished disk plus rubber
disk.
In use the bolt is vertical and when amp is carried it is horizontal.
I should say I've seen the result of an amp set for USA use used on 240V ,
extensive areas of bare copper due to lacquer melting or burning off and
large amounts of smoke damage.

** Now I know for sure you are a posturing BULLSHIT ARTIST.

The only thing that fails when a toroidal of several hundred VA is fed with
double the rated primary voltage is the damn FUSE !!!!




...... Phil

 

[Archon]

"N_Cook"


** Now I know for sure you are a posturing BULLSHIT ARTIST.

The only thing that fails when a toroidal of several hundred VA is fed with
double the rated primary voltage is the damn FUSE !!!!




..... Phil

Assuming the correct value/type of fuse was in place and hadn't been
"repaired" by wrapping aluminum foil (aka ciggy paper) around it.
In my experience not many 120vac transformers survive even momentary
240vac,fused correctly or not. Maybe you are talking about much larger
transformers than I deal with.
JC

 

[Phil Allison]

"Archon"

Assuming the correct value/type of fuse was in place and hadn't been
"repaired" by wrapping aluminum foil (aka ciggy paper) around it.

** 100% WRONG !!

In my experience not many 120vac transformers survive even momentary
240vac, fused correctly or not.

** So YOU only deal with flea powered trannys - right ?

Maybe you are talking about much larger transformers than I deal with.

** Did you bother to read what I posted ????

A dual-primary tranny of 300VA has 5 ohms of resistance if connected for
230/240 volts and 50 Hz operation

If the same tranny is connected for 120 volts - that 5 ohm value falls to
a tiny 1.25 ohms !!!

When 240 is applied in error, the AC current draw is limited only by that
1.25 ohms of resistance - since the core is instantly driven into * total
magnetic saturation* and effectively disappears.

The resulting AC current flow is in the order of **150 ** amps - so any
tubular glass or ceramic fuse blows INSTANTLY.

Any domestic power circuit (ie thermal magnetic, 15 or 20 amp rated )
breaker will open in a few milliseconds under such a gross overload.

Get real pal.



....... Phil

 

[N_Cook]

"N_Cook"


** Now I know for sure you are a posturing BULLSHIT ARTIST.

The only thing that fails when a toroidal of several hundred VA is fed with
double the rated primary voltage is the damn FUSE !!!!




..... Phil

It was someone who brought their 160VA Alchemist APD22 Nemesis Amplifier
over from the USA, transformer survived long enough for the electrolytics
to short , then they overloaded the primary enough to blow the , still set
for USA use, mains fuse.

 

[Paul E. Schoen]

It was someone who brought their 160VA Alchemist APD22 Nemesis Amplifier
over from the USA, transformer survived long enough for the
electrolytics
to short , then they overloaded the primary enough to blow the , still
set
for USA use, mains fuse.

So the fuse should be about 2 amp slow blow type, which should blow within
0.1 sec at 20 amps and within 1 or 2 cycles at higher current. For 240 VAC
use, it would be about half that. But the parallel primary connection would
also be able to handle twice the current, so the fuse should have been
adequate.

I have tested some toroidal trannies that seem to saturate quite slowly
with increased primary voltage. In a 2 kVA unit rated 240 VAC I had about
50 mA at 240V, 100 mA at 280V, and 600 mA at 320V. Of course it would have
drawn a lot more current at 2x rating, but perhaps not enough to blow a
properly rated fuse instantly. I have seen a rather severe failure in a
toroidal 480 to 240 VAC autotransformer that was connected backward to a
high capacity mains source, thus placing the protective fuse on the load
side.

Paul

 

[N_Cook]

So the fuse should be about 2 amp slow blow type, which should blow within
0.1 sec at 20 amps and within 1 or 2 cycles at higher current. For 240 VAC
use, it would be about half that. But the parallel primary connection would
also be able to handle twice the current, so the fuse should have been
adequate.

I have tested some toroidal trannies that seem to saturate quite slowly
with increased primary voltage. In a 2 kVA unit rated 240 VAC I had about
50 mA at 240V, 100 mA at 280V, and 600 mA at 320V. Of course it would have
drawn a lot more current at 2x rating, but perhaps not enough to blow a
properly rated fuse instantly. I have seen a rather severe failure in a
toroidal 480 to 240 VAC autotransformer that was connected backward to a
high capacity mains source, thus placing the protective fuse on the load
side.

Paul

I was trying to find a table or formula for toroidal inrush current versus
power rating, but nothing found. A toroidal Tx , power for power comparison
to conventional Tx, has to be larger fuse rating, to avoid blowing at switch
on or after a few switch-ons.
If a toroidal Tx has nothing wrong with it at switch one then there is more
headroom for some later failure inducing problem not to blow the fuse. But
does this headroom increase with higher wattage ? does the ratio of
practical fuse rating/ normal maximum power consumption rise with higher
wattage toroidals ?

 

[Phil Allison]

"N_Cook"

Phil Allison
with double the rated primary voltage is the damn FUSE !!!!

It was someone who brought their 160VA Alchemist APD22 Nemesis Amplifier
over from the USA,

** Has a 3 ohm primary.

Current draw on a 240 volt supply is circa 70 amps !!!

transformer survived long enough for the electrolytics
to short,

** Pure fantasy.

then they overloaded the primary enough to blow the , still set
for USA use, mains fuse.

** Pack of lies.

The supply side fuse would have blown in milli-seconds.

The electros would not have be over voltaged.


...... Phil

 

[Phil Allison]

"Paul E. Schoen"

I have tested some toroidal trannies that seem to saturate quite slowly
with increased primary voltage.

** Not true of any normal commercial toroidal sold for use with AC supply,

In a 2 kVA unit rated 240 VAC I had about 50 mA at 240V, 100 mA at 280V,
and 600 mA at 320V.

** Memory plays tricks and your test results are probably wrong.



...... Phil

 

[Eeyore]

I was trying to find a table or formula for toroidal inrush current versus
power rating, but nothing found.

Well of course you won't you mental retard. It depends on the exact construction
of the transformer, the load and the instant at which it is switched on in the
mains cycle.

Why do you have to clog up these groups with your infantile questions ?

If in doubt fit a Surge-Gard in series.

Graham

 

[Paul E. Schoen]

Well of course you won't you mental retard. It depends on the exact
construction
of the transformer, the load and the instant at which it is switched on
in the
mains cycle.

Why do you have to clog up these groups with your infantile questions ?

If in doubt fit a Surge-Gard in series.

Toroidal trannies do seem to have higher inrush currents due to remanent
magnetism. If there are an unequal number of positive and negative
half-cycles applied, the effect is a net DC component that magnetizes the
core. Depending on the phase of the next application of voltage, this could
add to the normal surge and cause very high inrush for a couple of mSec,
which can trip some sennsitive breakers or pop fuses.

I have tried adding series thermistors, but they only work when they are
cold, and in our application we applied multiple surges that heated them
and reduced their effectiveness. In normal applications where power is
switched infrequently, they work well. Another method is to apply soft
turn-on or gradual turn-off using a triac with phase modulation to
demagnetize the core or reduce the turn-on surge.

Paul

 

[Phil Allison]

"Paul E. Schoen"

Toroidal trannies do seem to have higher inrush currents due to remanent
magnetism.

** Nonsense.

If there are an unequal number of positive and negative half-cycles
applied, the effect is a net DC component that magnetizes the core.

** Utter bollocks.




...... Phil

 

[N_Cook]

Toroidal trannies do seem to have higher inrush currents due to remanent
magnetism. If there are an unequal number of positive and negative
half-cycles applied, the effect is a net DC component that magnetizes the
core. Depending on the phase of the next application of voltage, this could
add to the normal surge and cause very high inrush for a couple of mSec,
which can trip some sennsitive breakers or pop fuses.

I have tried adding series thermistors, but they only work when they are
cold, and in our application we applied multiple surges that heated them
and reduced their effectiveness. In normal applications where power is
switched infrequently, they work well. Another method is to apply soft
turn-on or gradual turn-off using a triac with phase modulation to
demagnetize the core or reduce the turn-on surge.

Paul

I thought the disjunctures between the laminations of conventional Tx set up
miniature magnetic circuits in opposition to the magnetising field and so
reduced the current inrush. Toroidal formers don't have these.

 

[Eeyore]

Toroidal trannies do seem to have higher inrush currents due to remanent
magnetism. If there are an unequal number of positive and negative
half-cycles applied, the effect is a net DC component that magnetizes the
core. Depending on the phase of the next application of voltage, this could
add to the normal surge and cause very high inrush for a couple of mSec,
which can trip some sennsitive breakers or pop fuses.

I have tried adding series thermistors, but they only work when they are
cold, and in our application we applied multiple surges that heated them
and reduced their effectiveness.

True, but effective in 'most' instances according to application. Another
solution is to place a time-delayed relay across the surge-gard so it cools
quickly again.

In normal applications where power is
switched infrequently, they work well. Another method is to apply soft
turn-on or gradual turn-off using a triac with phase modulation to
demagnetize the core or reduce the turn-on surge.

Never tried the triac idea. With the relay shunt mentioned above, some people
also used fixed value power resistors. Fine as long as no appreciable current
is drawn before the relay closes.

Graham

 

[bz]

It ain't necessarily so.

Start with this for current US regulation:

http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&tpl=/ecfrbrowse/Titl
e40/40cfr761_main_02.tpl
.

More useful for answering questions:
http://en.wikipedia.org/wiki/Polychlorinated_biphenyl





--
bz

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

[email protected] remove ch100-5 to avoid spam trap

 

[Eeyore]

I thought the disjunctures between the laminations of conventional Tx set up
miniature magnetic circuits

They're called air gaps between the E and I laminations and cannot be completely
avoided.

in opposition to the magnetising field

They alter the permeability of the core.

and so reduced the current inrush. Toroidal formers don't have these.

Indeed, toroids don't have air gaps.

Graham

 

[Eeyore]

Yes yes yes. If you want something special you can get it.

It isn't what N Cook was talking about though. Stop obfuscating. It makes you look
silly.

Graham

 

[Paul E. Schoen]

"Paul E. Schoen"



** Nonsense.

The Wiki says otherwise. And I have had a lot of experience with high power
toroids and have seen the effects of high inrush current.
http://en.wikipedia.org/wiki/Inrush_current

** Utter bollocks.

I have personally observed this in our circuit breaker test sets. We take
care to apply voltage at about 70 degrees phase angle to obtain an initial
current peak equal to those that follow, and adjust the timing so that
there are equal numbers of positive and negative half-cycles. Inrush
current on successive pulses is minimal. But if there is an unequal number,
and a net DC component, there is always a much higher peak inrush current.
This is easily seen with short duration pulses of several cycles. In
breaker testing, of course, the breaker may trip at any time, sometimes
causing net DC, and the next operation usually exhibits high inrush.

Some other references:
http://relays.tycoelectronics.com/appnotes/app_pdfs/13c3206.pdf
http://powerelectronics.com/mag/power_magnetic_solutions_solving/
http://www.melcontransformers.info/images/transformer.pdf

Paul