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Modifying transformer

J

Jon Slaughter

Jan 1, 1970
0
I tore apart the battery transformer I mentioned here a while back. I'm
going to remove a few turns on the primary to increase the voltage across
the secondary(its 24.2VAC now and I'd like to get that to about 30VAC or
so).

I'm wondering if removing turns is a good idea? The core already saturates
at about 110VAC and I would imagine that removing turns on the primary can
only make things worse? Although since it already saturates and wastes
about 1A or 120W I'd imagine that since I'm not loading the secondary to
anything near what its suppose to be that it won't be to bad? (its suppose
to be rated for 15A but I'll probably draw about 4A max on rare occasions).


Another issue was that the top part of the transformer was welded to the
bottom part. Now wouldn't this defeat the purpose of laminates? Surely by
welding them they would be electrically connected and therefore reduce the
reason to use laminates? When I cut the welds I was thinking that maybe it
would be better not to weld it shut to remove any currents between the top
and bottom halfs? The magnetic flux should still concentrated in the core
because the gap would be very small?

I guess I'm going to play around with it and see what happens though. Try
without core first then piece it together. Just want to get a few things
clear before I wrap everything up.

Thanks,
JOn
 
J

Jon Slaughter

Jan 1, 1970
0
lol, now I see why the welded it... anyone know?
 
J

John Larkin

Jan 1, 1970
0
I tore apart the battery transformer I mentioned here a while back. I'm
going to remove a few turns on the primary to increase the voltage across
the secondary(its 24.2VAC now and I'd like to get that to about 30VAC or
so).

I'm wondering if removing turns is a good idea? The core already saturates
at about 110VAC and I would imagine that removing turns on the primary can
only make things worse? Although since it already saturates and wastes
about 1A or 120W I'd imagine that since I'm not loading the secondary to
anything near what its suppose to be that it won't be to bad? (its suppose
to be rated for 15A but I'll probably draw about 4A max on rare occasions).

If it's already close to saturation, removing primary turns could be
dramatic. 30/24 = 1.25, and few transformers are designed 25% away
from saturation.

Saturation isn't an absolute event, but losses climb radically as the
core is driven harder. Smoking varnish is a reasonable expectation.

John
 
J

jasen

Jan 1, 1970
0
I tore apart the battery transformer I mentioned here a while back. I'm
going to remove a few turns on the primary to increase the voltage across
the secondary(its 24.2VAC now and I'd like to get that to about 30VAC or
so).

I'm wondering if removing turns is a good idea?

good! it's not.
The core already saturates at about 110VAC and I would imagine that
removing turns on the primary can only make things worse?

yup.

it might be better to re-wind the secondary with more turns
(maybe use thinner wire)
Another issue was that the top part of the transformer was welded to the
bottom part. Now wouldn't this defeat the purpose of laminates? Surely by
welding them they would be electrically connected and therefore reduce the
reason to use laminates?

if the weld is thin (not right across the core) it doesn't form a shorted turn
for eddy currents.
 
J

Jon Slaughter

Jan 1, 1970
0
I'm not sure if its the laminates that are the main issue or the turns? I
For a given core, it's primary turns that cause saturation; fewer
turns makes higher flux density and heads for saturation. It sounds
like this one was borderline to start with.


yeah, but what I'm saying is that if I would have added turns to the primary
then I could have reduce the saturation? Of course I would have decreased
the secondary voltage too but...
Rewinding sounds like a huge hassle. There are lots of surplus
transformers around.

Well, I will try to look into that and see. In any case its a good learning
experience and things always make more sense when I get my hands dirty.


Thanks,
Jon
 
D

default

Jan 1, 1970
0
I tore apart the battery transformer I mentioned here a while back. I'm
going to remove a few turns on the primary to increase the voltage across
the secondary(its 24.2VAC now and I'd like to get that to about 30VAC or
so).

I'm wondering if removing turns is a good idea? The core already saturates
at about 110VAC and I would imagine that removing turns on the primary can
only make things worse? Although since it already saturates and wastes
about 1A or 120W I'd imagine that since I'm not loading the secondary to
anything near what its suppose to be that it won't be to bad? (its suppose
to be rated for 15A but I'll probably draw about 4A max on rare occasions).
If the core is indeed saturating, you'll only make it worse by
removing turns. It's anyone's guess how it will turn out - mine would
be badly, but if it is only to be used occasionally for short periods
it may work. The transformer is rated at 15 amps at 24 out raising to
30 will lower the output rating as well. 100 watts of waste is a fair
amount of heat and cost if it is to run for protracted periods.

Putting a small 4 amp 6 volt transformer phase wired to boost the
secondary is safer and probably easier.
Another issue was that the top part of the transformer was welded to the
bottom part. Now wouldn't this defeat the purpose of laminates? Surely by
welding them they would be electrically connected and therefore reduce the
reason to use laminates? When I cut the welds I was thinking that maybe it
would be better not to weld it shut to remove any currents between the top
and bottom halfs? The magnetic flux should still concentrated in the core
because the gap would be very small?

Transformer manufacturers do take some short cuts and welds are one of
them - you see it a lot in el-cheapo 12V battery charger transformers.
microwave ovens, and other bulk produced consumer stuff. Not as bad
as it may seem at first because the weld usually has very shallow
penetration and is on a part of the core where the field is weaker -
but it will waste some power and I wouldn't try to re weld it. It is
also hard to do unless you have a TIG setup and are very good and
fast.
I guess I'm going to play around with it and see what happens though. Try
without core first then piece it together. Just want to get a few things
clear before I wrap everything up.

Thanks,
JOn
You can add turns to the secondary if there's room.
 
P

Paul E. Schoen

Jan 1, 1970
0
Jon Slaughter said:
yeah, but what I'm saying is that if I would have added turns to the
primary then I could have reduce the saturation? Of course I would have
decreased the secondary voltage too but...


Well, I will try to look into that and see. In any case its a good
learning experience and things always make more sense when I get my hands
dirty.


Thanks,
Jon

As a learning experience what you are doing is fine, and maybe for a "rough
and dirty" power supply it will be OK. I looked at your previous post in
Mid-March (although the nym was BobJones, so maybe not you), the
transformer was a 12 VCT at 15 A, and another was 25 VCT at 2 A. The
purpose there was for a variable supply 5 V to 12 V.

Anyway, much depends on what you intend to use the transformer for. A
variable DC supply will be more efficient if you run the primary off of a
Variac or Powerstat. At low outputs you will be below saturation and it
will be very happy. You can also boost the 120 input to 140 and push the
output at the expense of saturation.

You could also use a phase modulated triac controller designed for
inductive loads.

If you just want to get higher DC voltage out, you can connect the output
in various ways. A full wave bridge with 12 VCT will give you about 15-18
VDC, while a full wave CT will give you half the voltage and twice the
current. You can make a doubler circuit using two capacitors and two diodes
to get 30-35 VDC. The regulation will be poor but such a circuit is also
somewhat current-limiting, which may be good. You will need about 2000 uF
per ampere.

Have fun with your experiments.

Paul
 
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