Winding a conventional AC line frequency transformer

[Eeyore]

It seems to have been my destiny in life to have to learn more about
transformers (and wind a fair few by hand) than I ever suspected I'd need to.

I learnt quite a bit about switchmode type transformers - I suppose actually you
could consider flyback types more like tapped inductors actually - from various
application notes and also here. Epcos's Ferrite Designer application is
additionally an excellent tool for pumping in the numbers for a wide range of
cores in various materials.

It so happens I'd like now to apply some of the techniques I've learnt to
traditional line frequency transformers.

I have several specific things in mind.

A. Reduced flux operation. Notably with toroids this can avoid the typical
'switch on surge' and it also occures to me that stray flux will be reduced both
in proportion to the reduction in magnetisation force and additionally because
operation at lower flux levels keeps core permeability values higher.

B. Improved coupling by using bifilar windings with triple insulated wire
eliminating the need for traditional insulation barriers.

C. Higher than 'typical' VA ratings for a given core size by using more
copper. Again this works well with toroids.

D. I'm sure there was something else but now I've forgotten.

The trouble is that I don't know of any single source of data on cores that's
comparable to the Epcos Magnetic designer. And then again, you can wind almost
any size core you like for toroids which will be my main area of interest.

Suggestions ?

Graham

 

[Phil Allison]

"Eeyore"

It so happens I'd like now to apply some of the techniques I've learnt to
traditional line frequency transformers.

I have several specific things in mind.

A. Reduced flux operation. Notably with toroids this can avoid the
typical
'switch on surge' and it also occures to me that stray flux will be
reduced both
in proportion to the reduction in magnetisation force and additionally
because
operation at lower flux levels keeps core permeability values higher.

** Inrush surge will only be eliminated by doubling the number of turns on
primary and secondary.

Equates to using a 480 volt tranny on 240 volts - results in only half
the normal VA rating.

B. Improved coupling by using bifilar windings with triple insulated
wire
eliminating the need for traditional insulation barriers.

** Completely absurd for a 50/60 Hz supply transformer where high
frequency coupling has no benefit.

C. Higher than 'typical' VA ratings for a given core size by using more
copper. Again this works well with toroids.

** Fan cooling works to add VAs too.

D. I'm sure there was something else but now I've forgotten.

** Exotic core materials ???

Increased VA to size ratio.





.......... Phil

 

[Eeyore]

"Eeyore"

** Inrush surge will only be eliminated by doubling the number of turns on
primary and secondary.

A true 200% doubling is only necessary if the comparison transformer is already
operating on the edge of saturation.

Equates to using a 480 volt tranny on 240 volts - results in only half
the normal VA rating.

Only if you stick to using the same amount of copper. I regularly 'overwind' to
reduce losses and gain VA rating.

It's also possible to use higher temperature insulation if you're pushing for VA
of course.

** Completely absurd for a 50/60 Hz supply transformer where high
frequency coupling has no benefit.

And the harmonics ?

** Fan cooling works to add VAs too.

It certainly would do but I'd like something that's not noisy.

** Exotic core materials ???

Could be interesting.

Increased VA to size ratio.

Always useful.

Graham

 

[Phil Allison]

"Eeysore Fuckhead "

A true 200% doubling is only necessary if the comparison transformer is
already
operating on the edge of saturation.

** How tediously pedantic.

This utter fool does not want not want anyone's input.

Only if you stick to using the same amount of copper.

** Which is the whole point of the comparison.

It's also possible to use higher temperature insulation if you're pushing
for VA
of course.

** At the expense of output voltage regulation.

With a wire temp of say 150C, winding resistance and hence peak IR drop
increases by 50% over the room temp value.

And the harmonics ?

** Yawn ..........

Another Graham Fuckwit Stevenson contextless & meaning free question.

It certainly would do but I'd like something that's not noisy.

** So YOU go invent it and patent it - fuckhead.

This PITA fool REALLY does not want not want ANYONE's input.

Could be interesting.

** Do a Google search on it - fuckwit.



........ Phil

 

[Paul E. Schoen]

It seems to have been my destiny in life to have to learn more about
transformers (and wind a fair few by hand) than I ever suspected I'd need
to.

I learnt quite a bit about switchmode type transformers - I suppose
actually you
could consider flyback types more like tapped inductors actually - from
various
application notes and also here. Epcos's Ferrite Designer application is
additionally an excellent tool for pumping in the numbers for a wide
range of
cores in various materials.

It so happens I'd like now to apply some of the techniques I've learnt to
traditional line frequency transformers.

I have several specific things in mind.

A. Reduced flux operation. Notably with toroids this can avoid the
typical
'switch on surge' and it also occures to me that stray flux will be
reduced both
in proportion to the reduction in magnetisation force and additionally
because
operation at lower flux levels keeps core permeability values higher.

There is also a switch-on surge caused by remanent magnetism in the core,
which depends on the net DC current caused by odd number of positive and
negative half-cycles. It can be reduced by reducing the applied voltage at
turn-off, or using soft-start turn-on techniques. Another helpful method is
a capacitor or RC snubber on the primary which provides a damped
oscillating current path at turn-off. Toroids also exhibit this, at least
the ones I use in our high current test sets.

B. Improved coupling by using bifilar windings with triple insulated
wire
eliminating the need for traditional insulation barriers.

Bifilar windings are most useful for primaries used in parallel, to
equalize the current. Secondaries are coupled very well because the flux is
concentrated in the small hole in the donut.

C. Higher than 'typical' VA ratings for a given core size by using
more
copper. Again this works well with toroids.

Toroids exhibit the best VA/weight and VA/volume because of the efficient
tape wound steel core, typically made with high quality and very thin
material. The core is very easy and inexpensive to make. Adding the
windings is expensive, because it is labor intensive, requires special
machines, and is difficult with heavy wire. Most of my secondaries are made
of bus bar, so the difficult part is completing the turns outside the hole
of the core.

The size of the hole determines the size of the copper windings, and is
also where the most heat will be concentrated. The outside windings are
well separated and more easily cooled. I am experimenting with a fluid
cooled design that runs thick wall copper tubing through the toroids.

D. I'm sure there was something else but now I've forgotten.

The trouble is that I don't know of any single source of data on cores
that's
comparable to the Epcos Magnetic designer. And then again, you can wind
almost
any size core you like for toroids which will be my main area of
interest.

Suggestions ?

Graham

A good source of information is www.toroid.com. They have toroid kits from
about 50 VA up to 1400 VA. The primaries are already wound on the core, and
you add the secondary according to their design charts for selecting number
of turns and wire size. Volts per turn varies from about 0.1 to 0.7. They
also supply mylar tape for insulation, and steel washers and rubber
bushings for center hole mounting.

You can also use old Variacs or Powerstats as toroidal primaries. They tend
to have larger than usual holes, so you can wind more copper on the
secondary. Some of them are as large as 5 kVA. They have tapped primaries
for 120/140 VAC, or 240/280 VAC. The higher taps can be used for lower
primary current on 50 Hz.

There are also some toroidal transformers that are wound with steel wire
cores, which are supposedly even more efficient. Tape wound toroids have a
rectangular cross section, but a true toroid has a round cross-section,
which can be achieved with wire. I would think there would be more wasted
space, however.

Good luck. Let us know what you are trying to do, and what results you get.

Paul

 

[Robert Baer]

It seems to have been my destiny in life to have to learn more about
transformers (and wind a fair few by hand) than I ever suspected I'd need to.

I learnt quite a bit about switchmode type transformers - I suppose actually you
could consider flyback types more like tapped inductors actually - from various
application notes and also here. Epcos's Ferrite Designer application is
additionally an excellent tool for pumping in the numbers for a wide range of
cores in various materials.

It so happens I'd like now to apply some of the techniques I've learnt to
traditional line frequency transformers.

I have several specific things in mind.

A. Reduced flux operation. Notably with toroids this can avoid the typical
'switch on surge' and it also occures to me that stray flux will be reduced both
in proportion to the reduction in magnetisation force and additionally because
operation at lower flux levels keeps core permeability values higher.

B. Improved coupling by using bifilar windings with triple insulated wire
eliminating the need for traditional insulation barriers.

C. Higher than 'typical' VA ratings for a given core size by using more
copper. Again this works well with toroids.

D. I'm sure there was something else but now I've forgotten.

The trouble is that I don't know of any single source of data on cores that's
comparable to the Epcos Magnetic designer. And then again, you can wind almost
any size core you like for toroids which will be my main area of interest.

Suggestions ?

Graham

I have info only on "E/I" and "C" core shapes (silicon steel
laminations), nothing on toroids (sorry).

 

[Robert Baer]

There is also a switch-on surge caused by remanent magnetism in the core,
which depends on the net DC current caused by odd number of positive and
negative half-cycles. It can be reduced by reducing the applied voltage at
turn-off, or using soft-start turn-on techniques. Another helpful method is
a capacitor or RC snubber on the primary which provides a damped
oscillating current path at turn-off. Toroids also exhibit this, at least
the ones I use in our high current test sets.




Bifilar windings are most useful for primaries used in parallel, to
equalize the current. Secondaries are coupled very well because the flux is
concentrated in the small hole in the donut.

*Incorrect!* The flux is concentrated by the magnetic material - in
this case the body of the toroid.
Go back to a magnetics class...

 

[Paul E. Schoen]

*Incorrect!* The flux is concentrated by the magnetic material - in
this case the body of the toroid.
Go back to a magnetics class...

If you are picking nits, you are correct. Perhaps it is better stated that
the location where a conductor will be most influenced by the magnetic flux
in the core is concentrated in the hole. Or perhaps I should have said
magnetic field, rather than flux. In any case, the point I was trying to
make is that the conductors within this relatively small space have current
induced in them only there, and it is a much smaller space than that of an
equivalent size EI core.

There is no need to be condescending.

Paul

 

[Phil Allison]

"Paul E. Schoen"

"Robert Baer"


If you are picking nits, you are correct.

** He ain't nit picking.

Perhaps it is better stated that the location where a conductor will be
most influenced by the magnetic flux in the core is concentrated in the
hole.

** Utter bollocks.

Or perhaps I should have said magnetic field, rather than flux. In any
case, the point I was trying to make is that the conductors within this
relatively small space have current induced in them only there,

** Your proof of this absurd claim is where ?

and it is a much smaller space than that of an equivalent size EI core.

** The usable copper window area is LARGER with GOSS strip toroidals than
with similar E-I cores.

My god - windings surround the ENTIRE core instead of just the middle
stump or part of it !!

( Cricket allusion will mystify all Yanks ..... )

There is no need to be condescending.

** Gotta bend down a long way to even SEE a Lilliputian like you.



........ Phil

 

[John Larkin]

My god - windings surround the ENTIRE core instead of just the middle
stump or part of it !!

( Cricket allusion will mystify all Yanks ..... )

Cricket has been explained to me many times. It still makes no sense.

John

 

[Tony Williams]

If you are picking nits, you are correct. Perhaps it is better
stated that the location where a conductor will be most
influenced by the magnetic flux in the core is concentrated in
the hole. Or perhaps I should have said magnetic field, rather
than flux.

[snip]

The imagery I have is that a straight piece of wire
carrying a current has the magnetic field arranged
as concentric rings around it. If a toroidal magnetic
core is slipped over that wire then the 'rings' get
naturally concentrated in the higher permeability core.

I've done experiments with bar-turns and toroids.
The interesting thing is that the current ratio is
not particularly affected (<0.1% anyway) by the bar
not being central, tilted, or even not round.

 

[Paul E. Schoen]

If you are picking nits, you are correct. Perhaps it is better
stated that the location where a conductor will be most
influenced by the magnetic flux in the core is concentrated in
the hole. Or perhaps I should have said magnetic field, rather
than flux.

[snip]

The imagery I have is that a straight piece of wire
carrying a current has the magnetic field arranged
as concentric rings around it. If a toroidal magnetic
core is slipped over that wire then the 'rings' get
naturally concentrated in the higher permeability core.

I've done experiments with bar-turns and toroids.
The interesting thing is that the current ratio is
not particularly affected (<0.1% anyway) by the bar
not being central, tilted, or even not round.

That is a good description of a toroid being used as a current transformer,
where a high current flows through a large conductor (or several turns of a
smaller conductor) and the smaller secondary current is used for
measurement. I recently tested a PCB mounted toroid with 1000:1 ratio, with
a load of 10 ohms, and it was quite linear from about 1 ampere to several
hundred.

For the test sets we build, we use a Rogowski coil for current measurement.
It is built using 8 air-core inductors positioned across the top and bottom
of a 6" x 1/2" bus bar. The axis of each inductor is perpendicular to
current flow. The output is a voltage which is proportional to dV/dT of
current, so an RC integrator is used. The coils read the magnetic field,
and can be affected by external fields and the presence of magnetic
material nearby (like steel brackets and bolts), but once solidly mounted,
the output is very linear and accurate over a range of about 50 amps to
50,000 amps or more.

The point I was trying to make is that only the very small portion of the
conductor through the hole of the toroid is affected by its magnetic field.
The conductors outside this small space serve merely to complete the
circuit. There is no appreciable difference if the conductors are wrapped
tightly around the toroid, or are routed far away from the outer surface.
Of course, tighter wraps use less wire and there is less resistive loss.

Paul

 

[Fred Bartoli]

John Larkin a écrit :

Cricket has been explained to me many times. It still makes no sense.

What's that?

 

[Fred Bartoli]

Eeyore a écrit :

It seems to have been my destiny in life to have to learn more about
transformers (and wind a fair few by hand) than I ever suspected I'd need to.

I learnt quite a bit about switchmode type transformers - I suppose actually you
could consider flyback types more like tapped inductors actually - from various
application notes and also here. Epcos's Ferrite Designer application is
additionally an excellent tool for pumping in the numbers for a wide range of
cores in various materials.

It so happens I'd like now to apply some of the techniques I've learnt to
traditional line frequency transformers.

I have several specific things in mind.

A. Reduced flux operation. Notably with toroids this can avoid the typical
'switch on surge' and it also occures to me that stray flux will be reduced both
in proportion to the reduction in magnetisation force and additionally because
operation at lower flux levels keeps core permeability values higher.

B. Improved coupling by using bifilar windings with triple insulated wire
eliminating the need for traditional insulation barriers.

C. Higher than 'typical' VA ratings for a given core size by using more
copper. Again this works well with toroids.

D. I'm sure there was something else but now I've forgotten.

The trouble is that I don't know of any single source of data on cores that's
comparable to the Epcos Magnetic designer. And then again, you can wind almost
any size core you like for toroids which will be my main area of interest.

Suggestions ?

Bifilar winding isn't possible, unless you want a 1:1 xformer, or a
total mess of series/parallel windings.

 

[Eeyore]

Bifilar winding isn't possible, unless you want a 1:1 xformer, or a
total mess of series/parallel windings.

Why need it be a mess ?

Graham

 

[John Larkin]

John Larkin a écrit :

What's that?

Really, you are better off not knowing.

John

 

[Spehro Pefhany]

Really, you are better off not knowing.

John

I'm always suspicious of endeavors that require learning a new lexicon
or require new clothes.



Best regards,
Spehro Pefhany

 

[Fred Bartoli]

Eeyore a écrit :

Why need it be a mess ?

Suppose you want a 24V xformer (a 1:10 ratio). You'll need ten winding
sections, each primary part in series, each secondary part in parallel.

I qualify this as a mess, since this level of complication isn't needed
at all for the purpose.

 

[Robert Baer]

If you are picking nits, you are correct. Perhaps it is better
stated that the location where a conductor will be most
influenced by the magnetic flux in the core is concentrated in
the hole. Or perhaps I should have said magnetic field, rather
than flux.

[snip]

The imagery I have is that a straight piece of wire
carrying a current has the magnetic field arranged
as concentric rings around it. If a toroidal magnetic
core is slipped over that wire then the 'rings' get
naturally concentrated in the higher permeability core.

I've done experiments with bar-turns and toroids.
The interesting thing is that the current ratio is
not particularly affected (<0.1% anyway) by the bar
not being central, tilted, or even not round.

As long as the cross-sectional area of the wire used (not the shape
as you correctly mentin) is the same, this is correct.
Where one can see slight modifictions of this, is when a substantial
percentage of the wire cross-section is rather close to the higher
permeability core.

 

[Robert Baer]

Eeyore a écrit :


Bifilar winding isn't possible, unless you want a 1:1 xformer, or a
total mess of series/parallel windings.

Bifilar windings for N:M ratios have beendone for at least 30 years;
even using coax cable!