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Rewinding a transformer

J

JSF

Jan 1, 1970
0
We have a toroidal transformer stepping 5 Vac 400 Hz to 25 Vac. The input
could be up to 5-7 amps and the output 1.5 amp or about a 40 watt load.
Running at full load it is getting somewhat hot.
There is allot of room left on the coil so to lower the losses also heat,
would it be possible to rewind it with a larger gauge wire , both pri and
sec windings and keep the ratio the same. If so the inductance will be lower
so do I need more turns on the pri and sec or Not.
Any ideas. I think the core is way out of saturation yet. There is about 28
turns on the pri, and 140 on the sec.
 
T

Tim Wescott

Jan 1, 1970
0
JSF said:
We have a toroidal transformer stepping 5 Vac 400 Hz to 25 Vac. The input
could be up to 5-7 amps and the output 1.5 amp or about a 40 watt load.
Running at full load it is getting somewhat hot.
There is allot of room left on the coil so to lower the losses also heat,
would it be possible to rewind it with a larger gauge wire , both pri and
sec windings and keep the ratio the same. If so the inductance will be lower
so do I need more turns on the pri and sec or Not.
Any ideas. I think the core is way out of saturation yet. There is about 28
turns on the pri, and 140 on the sec.
I'm not a transformer expert -- but that sounds like a workable
solution. You'll have more inter-winding capacitance, which may or may
not be a problem.

Make sure that a high winding resistance wasn't an intentional part of
the design -- many wall-wart type transformers are intentionally
inefficient to limit short-circuit current without having to pay for
extra components.

You can test for core loss issues by running the thing with the
secondary open -- generally with transformers the core loss is a
function of the primary voltage, with copper losses being a function of
the currents. If it only gets hot when power is flowing then chances
are the windings are the culprit.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/
 
P

Paul Mathews

Jan 1, 1970
0
JSF said:
We have a toroidal transformer stepping 5 Vac 400 Hz to 25 Vac. The input
could be up to 5-7 amps and the output 1.5 amp or about a 40 watt load.
Running at full load it is getting somewhat hot.
There is allot of room left on the coil so to lower the losses also heat,
would it be possible to rewind it with a larger gauge wire , both pri and
sec windings and keep the ratio the same. If so the inductance will be lower
so do I need more turns on the pri and sec or Not.
Any ideas. I think the core is way out of saturation yet. There is about 28
turns on the pri, and 140 on the sec.

If you're careful to get the polarities correct and to match the turns
counts, you can add windings in parallel with those existing. Only
connect them in parallel after verifying that voltage and polarity are
almost identical.

You can determine saturation margin by increasing primary voltage with
secondary unloaded. As you approach saturation, primary current will
rise rapidly. This is best observed with a current probe, but you can
observe the voltage across a small series resistor. For a toroid,
abrupt current spikes will occur near voltage zero crossings.

You can also drive either winding with a signal generator, lowering
drive frequency until you observe saturation characteristics as
described above. Your operating margin in frequency is approx. the same
as the operating margin in voltage. For example, if you can go down to
300 Hz, driving a winding with its normal operating voltage, you could
also raise primary voltage by 33% at 400 Hz.
Paul Mathews
 
M

Mark

Jan 1, 1970
0
The inductance will not significantly change due to larger wire, you
can use the same number of turns as you have now with heavier wire and
it will reduce the copper loss and no change to the core loss.

Does the xformer get hot when the primary is powered but the sec is not
loaded? If so, then you have core losses.

The optimum design fills up the core opening with wire and usually has
about equal copper and core looses. If you have mostly copper losses
and no core losses then you can use heaverier wire and FEWER turns.
This will reduce the copper loss and increase the core loss.

Fewer turns = more core loss and less copper loss
More turns = less core loss more copper loss
heavier wire = less copper loss no change to core loss

Also core loss is constant vs load and is a function of input voltage
and frequency
Copper loss is a squared function of load current

Again the optimum design fills up the space and has about equal core
and copper loss at the desired load.

(This discussion assumes 60 Hz i.e. we are not talking about high
frequency effects)


Mark
 
J

JSF

Jan 1, 1970
0
Mark said:
The inductance will not significantly change due to larger wire, you
can use the same number of turns as you have now with heavier wire and
it will reduce the copper loss and no change to the core loss.

Does the xformer get hot when the primary is powered but the sec is not
loaded? If so, then you have core losses.

The optimum design fills up the core opening with wire and usually has
about equal copper and core looses. If you have mostly copper losses
and no core losses then you can use heaverier wire and FEWER turns.
This will reduce the copper loss and increase the core loss.

Fewer turns = more core loss and less copper loss
More turns = less core loss more copper loss
heavier wire = less copper loss no change to core loss

Also core loss is constant vs load and is a function of input voltage
and frequency
Copper loss is a squared function of load current

Again the optimum design fills up the space and has about equal core
and copper loss at the desired load.

(This discussion assumes 60 Hz i.e. we are not talking about high
frequency effects)


Mark

Thanks all, I will work on it now.
Test showed that there was very little loss in the core.
 
P

Pooh Bear

Jan 1, 1970
0
Mark said:
The inductance will not significantly change due to larger wire, you
can use the same number of turns as you have now with heavier wire and
it will reduce the copper loss and no change to the core loss.

Does the xformer get hot when the primary is powered but the sec is not
loaded? If so, then you have core losses.

The optimum design fills up the core opening with wire and usually has
about equal copper and core looses. If you have mostly copper losses
and no core losses then you can use heaverier wire and FEWER turns.
This will reduce the copper loss and increase the core loss.

Fewer turns = more core loss and less copper loss
More turns = less core loss more copper loss
heavier wire = less copper loss no change to core loss

Also core loss is constant vs load and is a function of input voltage
and frequency
Copper loss is a squared function of load current

Again the optimum design fills up the space and has about equal core
and copper loss at the desired load.

(This discussion assumes 60 Hz i.e. we are not talking about high
frequency effects)

Optimum design for a toroid is a somewhat limited concept.

Losses in a toroid are almost always determined by copper losses IME.

Graham
 
P

Pooh Bear

Jan 1, 1970
0
JSF said:
Thanks all, I will work on it now.
Test showed that there was very little loss in the core.

Correct.

Using more copper works brilliantly. I've used the technique for ages.

Graham
 
P

Phil Allison

Jan 1, 1970
0
"Mark"@y43g2000cwc.googlegroups.com...
The optimum design fills up the core opening with wire and usually has
about equal copper and core looses.


** Not for a GOSS strip toroidal core.

Core losses are much lower than the copper loss.

If you have mostly copper losses
and no core losses then you can use heaverier wire and FEWER turns.


** Insane.

The core may suddenly saturate, then you *are* in trouble.

This will reduce the copper loss and increase the core loss.


** Crapology when the core is a GOSS strip toroidal.

Again the optimum design fills up the space and has about equal core
and copper loss at the desired load.


** See above.

This is an old hat theory based on large transformers using high loss core
designs.



........ Phil
 
J

JSF

Jan 1, 1970
0
Phil Allison said:
"Mark"@y43g2000cwc.googlegroups.com...


** Not for a GOSS strip toroidal core.

Core losses are much lower than the copper loss.




** Insane.

The core may suddenly saturate, then you *are* in trouble.



OK if I have less turns, why does the core saturate easier? Is it that more
field from the same voltage applied?
If the turn ratio is keep the same then where does one determine on how
many turns to start with on the primary?
At this point the orginal transformer has 30 turns of #16 on th pri and 150
turns of #22 on the sec. The loss is a little high not bad but runs hot at
40 watts out. Will # 14 30 T and 15T of # 18 be better?
 
P

Phil Allison

Jan 1, 1970
0
"JSF"
OK if I have less turns, why does the core saturate easier? Is it that
more field from the same voltage applied?


** Basically yes, less turns = less inductance = closer to saturation for
a given applied voltage and frequency.

If the turn ratio is keep the same then where does one determine on how
many turns to start with on the primary?


** There are complicated formulae and then you need to see the core maker's
data.

Or, just do a simple test and all is revealed.

At this point the orginal transformer has 30 turns of #16 on th pri and
150 turns of #22 on the sec. The loss is a little high not bad but runs
hot at 40 watts out. Will # 14 30 T and 15T of # 18 be better?


** You have failed to describe the core - at all.

Now is not too late.

Do you have a Variac available and an AC amp meter?




........ Phil
 
P

Pooh Bear

Jan 1, 1970
0
JSF said:
OK if I have less turns, why does the core saturate easier? Is it that more
field from the same voltage applied?

Yes - in one. The magnetising force increases.
If the turn ratio is keep the same then where does one determine on how
many turns to start with on the primary?

Use the same as you have now.
At this point the orginal transformer has 30 turns of #16 on th pri and 150
turns of #22 on the sec. The loss is a little high not bad but runs hot at
40 watts out. Will # 14 30 T and 15T of # 18 be better?

Just increase the copper gauge. Messing about with the working flux is likely to
lead you into trouble.

Graham
 
J

JSF

Jan 1, 1970
0
Pooh Bear said:
Yes - in one. The magnetising force increases.


Use the same as you have now.

The above should have been 150T of #18 not 15T. Its a 1:5 winding ratio.
 
J

JSF

Jan 1, 1970
0
JSF said:
The above should have been 150T of #18 not 15T. Its a 1:5 winding
ratio.

Yes the applied ac voltage can be vaired from 0 to 7 v .
 
P

Pooh Bear

Jan 1, 1970
0
JSF said:
The above should have been 150T of #18 not 15T. Its a 1:5 winding ratio.

That should reduce your losses quite a bit.

Graham
 
J

John Popelish

Jan 1, 1970
0
JSF said:
OK if I have less turns, why does the core saturate easier? Is it that more
field from the same voltage applied?

The peak core flux is proportional to the voltage per turn times the
time it is applied for each half cycle (actually the time integral of
the per turn voltage over the half cycle). Adding turns lowers the
volts per turn.
If the turn ratio is keep the same then where does one determine on how
many turns to start with on the primary?

Use the number it has now, to keep the core flux swing the same as it
is now. Add a proportional bit to both windings to lower the flux
swing a little.
At this point the orginal transformer has 30 turns of #16 on th pri and 150
turns of #22 on the sec. The loss is a little high not bad but runs hot at
40 watts out. Will # 14 30 T and 15T of # 18 be better?

The original wire gauges have cross sections of 2583 and 642 circular
mils, with a ratio of about 4 to 1. So the primary is producing more
heat per volume of copper than the secondary is (the current density
is higher). I would probably go up two gauges on the primary and 1 on
the secondary, to 14 and 21, with cross sectional areas of 4107 and
810 circular mils for a ratio of about 5 to 1, to better match the
current ratios. If I had room, I might add 2 turns to the primary and
10 to the secondary (32 turns and 160 turns), to drop the flux by 6%, too.

http://www.pupman.com/listarchives/1998/April/msg00222.html
 
R

Roy L. Fuchs

Jan 1, 1970
0
We have a toroidal transformer stepping 5 Vac 400 Hz to 25 Vac. The input
could be up to 5-7 amps and the output 1.5 amp or about a 40 watt load.
Running at full load it is getting somewhat hot.
There is allot of room left on the coil so to lower the losses also heat,
would it be possible to rewind it with a larger gauge wire , both pri and
sec windings and keep the ratio the same. If so the inductance will be lower
so do I need more turns on the pri and sec or Not.
Any ideas. I think the core is way out of saturation yet. There is about 28
turns on the pri, and 140 on the sec.
Insulated wires or magnet wire? Also what gauge are they now?
Are they on opposite sides of the toroid or stacked?

If insulated, you could switch to SPC (silver plated copper), and
increase gauge.

Also, could you have some DC riding in there? That will cause
saturation or heating effects.

If magnet wire, you can double the wire up (bifurcated) to
effectively double the area of the conductors while keeping the same
turns count and ratio. You can do this on both the primary and
secondary.

How big is the toroidal core? Also, it could be cracked if it is
ferrite construction. That will cause problems like you describe.
 
R

Roy L. Fuchs

Jan 1, 1970
0
OK if I have less turns, why does the core saturate easier? Is it that more
field from the same voltage applied?

The term is "volts per turn". Another to look up is "ampere/turn"
 
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