Re: Modifying transformer


"Jon Slaughter" <Jon_Slaughter@xxxxxxxxxxx> wrote in message
news:iOt%h.7195$2v1.3168@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx

"John Larkin" <jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in
message news:edos33p70ts989a3blt3ue5iil915e2og2@xxxxxxxxxx
On Sun, 06 May 2007 22:46:24 GMT, "Jon Slaughter"
<Jon_Slaughter@xxxxxxxxxxx> wrote:

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.



If I remember correctly it was only pulling 1A before without any
secondary load. Now its about 2.5A and I've removed about 5 turns. That
doesn't seem right but thats my readings. I think the extra current is
because the core isn't welded so there is a great more deal of leakage.
The voltage only went from 24.2 to 25.7VAC so I'd have to remove a lot
more.

I'll remove some more turn to get me to about 27VAC and then weld it shut
again and then try and run it through some stress tests. Unfortunately I
can't easily add to the primary because of the huge wire it uses(I don't
have any even close to it. I guess its about AWG 16-18 or something.

I suppose if I could add to the secondary that I wouldn't have any of
these issues? Would it even reduce the saturation if I did? (I could
always rewind the bobbins if I got the wire. I don't need the 15A the
transformer is rated for but just about 8 or would be perfectly fine.

Thanks,
Jon

It seems like a whole lot of bother to make a transformer do what it was
not originally designed to do, and taking turns from the primary is the
worst way possible. Once saturation occurs, the output voltage actually
becomes somewhat regulated, so a large change in primary voltage produces
much less change in the secondary. If you are not using a true-RMS meter,
your readings will be false because of the increased distortion (clipping,
or a flattened peak). Your current readings will also be erroneous because
you will see even more distortion, but for current this will be sharp
peaks corresponding to the flattened output voltage waveform.


Well, maybe a bother to you but was fun for me. Learned a little bit about
the practical side of transformers.

Oh, the reason why I didn't want to mess with the secondary was that it is
center tapped. I figured it would be much more complicated to mess with?
I'd also have to worry about keep the taps even. (Although I suppose it
wouldn't be hard to add turns but since I didn't have any wire handy I just
decided to mess with the primary.

#16 or #18 wire is not huge, but of course I am a high current guy and I'm
used to #4-0 welding cable and 6" x 1/2" bus bar. If you are willing to go
to the trouble of unwelding a transformer and pulling out primary turns,
you should be able to find an old automotive alternator or generator with
wire heavy enough to use for a proper secondary. A motor winding shop
might give you some old wire, or maybe an entire motor, for the price of
its recycle value.


Well, I probably could have found something to use but wasn't interested in
it so much. I kinda ruined the transformer as now its pulling 4A but I am
geting my 30V on the secondary. Makes more noise than before but I didn't
weld it all that well. Only took a min to cut it open and another min to
weld it closed. The hardest part was getting the bobbin out.

Better yet, you should be able to find a more suitable transformer surplus
or at a hamfest. The heavy items are often left behind. I've picked up
(with some difficulty!) some big transformers and iron core chokes that
were in the freebie piles and trash bins. Sometimes you have to pull them
out of old tube equipment. If you can find a Variac or Powerstat, you
basically have a toroidal core primary, and you can wind your own
secondary for any voltage you want. I have a couple dozen of these with
power rating of 500 VA to 2500 VA or more, and I've even repaired some
that were burned, because all the windings are accessible.



Yeah, I'm going to get me a better quality transformer. I was kinda
experimenting with this one. I'm sure I could always rewind it if I wanted.
I assume that I could reduce the saturation point by using more windings on
the primary?

The transformer died in a good cause though as atleast I have learned a
great deal about them. Wasn't a good transformer to start with so no big
loss.

Thanks,
Jon


.

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