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Microwave power transformer

J

James Thompson

Jan 1, 1970
0
I have skipped around Google, now to ask the experts.

To make use of salvaged microwave transformers, how can one stop or lessen
the heating effect of the core. I have cut out the secondary winding and
the magnetron heater windings as well as remove the magnetic shunts. I have
even added about 4 turns to the primary (#14 gauge copper). Still with no
load on secondary (with is #12 copper after rewind), it still gets warm to
the touch. Would I need to also cut out the welds on the core to lessen the
eddy currents?
I wound the secondary till I get 20 volts out with a load of 1 amp, 21.8
with no load.
Anyway, what else might I try to calm the eddies so it runs cooler?
 
J

Joerg

Jan 1, 1970
0
Hello James,

I have skipped around Google, now to ask the experts.

To make use of salvaged microwave transformers, how can one stop or lessen
the heating effect of the core. I have cut out the secondary winding and
the magnetron heater windings as well as remove the magnetic shunts. I have
even added about 4 turns to the primary (#14 gauge copper). Still with no
load on secondary (with is #12 copper after rewind), it still gets warm to
the touch. Would I need to also cut out the welds on the core to lessen the
eddy currents?
I wound the secondary till I get 20 volts out with a load of 1 amp, 21.8
with no load.
Anyway, what else might I try to calm the eddies so it runs cooler?
No idea, really, but with components like that it is often a race to the
rock-bottom cost point. They might have chosen the cheapest core metals
they could get by with, at the longest expected runtime. Minus the
failures allowed to flow into warranty overhead.

Regards, Joerg
 
A

amdx

Jan 1, 1970
0
To make use of salvaged microwave transformers, how can one stop or lessen
the heating effect of the core. I have cut out the secondary winding and
the magnetron heater windings as well as remove the magnetic shunts. I have
even added about 4 turns to the primary (#14 gauge copper). it still gets warm to
the touch.
Snip
Would I need to also cut out the welds on the core to lessen the
eddy currents?
I wound the secondary till I get 20 volts out with a load of 1 amp, 21.8
with no load.
Anyway, what else might I try to calm the eddies so it runs cooler?

I earlier had suggested you add turns to the primary, but I don't think four
extra turns gets you very much, I just counted 142 tuns on the primary of
a microwave transformer I have. It might be a lot of work, but wind 200
tuns on the secondary side and test it. Then start backing off on turns
until to core temp becomes a problem.
Another thought, I think you can just wind 50 turns in the secondary
window
and wire it series adding to the primary, then check to see how it works.
Then
you could back off on turns as allowed.
Mike
 
J

James Thompson

Jan 1, 1970
0
amdx said:
Snip
Would I need to also cut out the welds on the core to lessen the

I earlier had suggested you add turns to the primary, but I don't think
four
extra turns gets you very much, I just counted 142 tuns on the primary of
a microwave transformer I have. It might be a lot of work, but wind 200
tuns on the secondary side and test it. Then start backing off on turns
until to core temp becomes a problem.
Another thought, I think you can just wind 50 turns in the secondary
window
and wire it series adding to the primary, then check to see how it works.
Then
you could back off on turns as allowed.
Mike
It may not be so bad, I am going to put a temp lead on it and see what it
raises to.
What I want this transformer for is for 15 to 20 amp dc regulated supply /
battery charger. Maybe add variable voltage and current limit. The first
transformer I rewound didn't have the mag shunt. I am using right now the
12 guage pvc coated multistrand wire in the secondary. I will redo it with
magnet wire for perm. use.
I saw an article while web hunting on using 2 of these transformers to make
an arc welder, I may give that a try also. Hey thanks for your input on
this, and I will let you know how much the temp rises under no load.
 
J

James Thompson

Jan 1, 1970
0
amdx said:
Snip
Would I need to also cut out the welds on the core to lessen the

I earlier had suggested you add turns to the primary, but I don't think
four
extra turns gets you very much, I just counted 142 tuns on the primary of
a microwave transformer I have. It might be a lot of work, but wind 200
tuns on the secondary side and test it. Then start backing off on turns
until to core temp becomes a problem.
Another thought, I think you can just wind 50 turns in the secondary
window
and wire it series adding to the primary, then check to see how it works.
Then
you could back off on turns as allowed.
Mike
After 1 hour of no load this transformer temp went from 80 degrees (room
temp) to 131 degrees. Maybe it would be best to use these for maybe a car
battery charger so as not to run continuous.
 
J

James Thompson

Jan 1, 1970
0
amdx said:
Snip
Would I need to also cut out the welds on the core to lessen the

I earlier had suggested you add turns to the primary, but I don't think
four
extra turns gets you very much, I just counted 142 tuns on the primary of
a microwave transformer I have. It might be a lot of work, but wind 200
tuns on the secondary side and test it. Then start backing off on turns
until to core temp becomes a problem.
Another thought, I think you can just wind 50 turns in the secondary
window
and wire it series adding to the primary, then check to see how it works.
Then
you could back off on turns as allowed.
Mike
I think I understand what you are saying. More turns = more impedance and
less current = lower magnetic flux = less core saturation and less eddy
heating.
Correct ?
 
R

Rich Grise

Jan 1, 1970
0
amdx said:
To make use of salvaged microwave transformers, how can one stop or
lessen
the heating effect of the core. I have cut out the secondary winding and
the magnetron heater windings as well as remove the magnetic shunts. I have
even added about 4 turns to the primary (#14 gauge copper). it still gets warm to
the touch.
Snip [More Snip]
Another thought, I think you can just wind 50 turns in the secondary
window
and wire it series adding to the primary, then check to see how it works.
Then
you could back off on turns as allowed.
It may not be so bad, I am going to put a temp lead on it and see what it
raises to.
What I want this transformer for is for 15 to 20 amp dc regulated supply /
battery charger. Maybe add variable voltage and current limit. The first
transformer I rewound didn't have the mag shunt. I am using right now the
12 guage pvc coated multistrand wire in the secondary. I will redo it with
magnet wire for perm. use.
I saw an article while web hunting on using 2 of these transformers to make
an arc welder, I may give that a try also. Hey thanks for your input on
this, and I will let you know how much the temp rises under no load.

I wouldn't add any more turns to the secondary until I get that primary
figured out. I have an MOT:
http://www.abiengr.com/~sysop/images/MOT-primary2.jpg
That, if I guesstimate accurately, has 10 turns on the outside layer, and
15 layers, of which 7 are only 9 turns - they're in the valleys between
layers, so there's 10 * 8 + 7 * 9 = 143 turns. It draws a horrendous
amount of current - like, 2 or 3 amps, when just idling. I put the penny
there just to see if it got eddy currents, but nothing. The wire is .040"
diameter, which is real close to #18, so I've decided, whenever I can
scrounge about 25 feet of #18 formvar magnet wire, I'm going to add 15
turns, in a pie, right up against the face of the winding loop, going
through where the shunts are now. Oh, yeah, I'm going to knock out the
shunts and measure the current again one of these days, but haven't got
a round tuit yet. I'll probably do that when 25' or so of #18 magnet wire
mysteriously shows up at my doorstep. ;-)

Then, I intend to use it to dick around with, winding various secondaries
and such; maybe even adding and subtracting windings to/from the primary. ;-)

Cheers!
Rich
 
R

Rich Grise

Jan 1, 1970
0
After 1 hour of no load this transformer temp went from 80 degrees (room
temp) to 131 degrees. Maybe it would be best to use these for maybe a car
battery charger so as not to run continuous.

I don't think 131F is dangerous for a transformer - it's too hot to touch,
but I don't think it'll start any fires. You are talking F, aren't you?
I wouldn't want to be in a room whose temperature is 80C!

But, don't forget, when under continuous duty in an oven, it's got
forced-air cooling going all of the time that it's on.

Cheers!
Rich
 
R

Rich Grise

Jan 1, 1970
0
I think I understand what you are saying. More turns = more impedance and
less current = lower magnetic flux = less core saturation and less eddy
heating.
Correct ?

Pretty much, yeah. :)

Cheers!
Rich
 
J

John Popelish

Jan 1, 1970
0
James said:
To make use of salvaged microwave transformers, how can one stop or lessen
the heating effect of the core.

Lower the volts per turn applied to it.
I have cut out the secondary winding and
the magnetron heater windings as well as remove the magnetic shunts. I have
even added about 4 turns to the primary (#14 gauge copper). Still with no
load on secondary (with is #12 copper after rewind), it still gets warm to
the touch.

4 turns isn't enough to lower the flux swing enough to make it idle
efficiently.
Would I need to also cut out the welds on the core to lessen the
eddy currents?

They are placed where they do very little harm.
 
A

amdx

Jan 1, 1970
0
James Thompson said:
I think I understand what you are saying. More turns = more impedance and
less current = lower magnetic flux = less core saturation and less eddy
heating.
Correct ?
Yes, that is my thinking. And to reiterate what I said above, you have
removed the secondary, so you could add 20 to 50 turns in that window
and put them in series with the primary and see if that is enough to get
your core out of saturation.
 
M

Mark

Jan 1, 1970
0
Yes, that is my thinking. And to reiterate what I said above, you have
removed the secondary, so you could add 20 to 50 turns in that window
and put them in series with the primary and see if that is enough to get
your core out of saturation.


Or you could see how much you need to lower the primary voltage until
it runs cool....
then you can figure out how many more turns you need... but remember
when you put it under load, the wire will then heat up too...

Mark
 
J

James Thompson

Jan 1, 1970
0
amdx said:
Yes, that is my thinking. And to reiterate what I said above, you have
removed the secondary, so you could add 20 to 50 turns in that window
and put them in series with the primary and see if that is enough to get
your core out of saturation.
To add that many windings, I would also need to use a smaller wire and
rewind the whole primary. The wire I have on the secondary fills all the
space in the core now. I believe I understand enough now to go from bare
core to custom transformer so I will experiment and see if it all works out.
I will like you said, add windings to the core ( about 200 or so ) and
measure the idle current is steps as I remove some windings. When the
current begins to rise, then I will know the lowest number of turns to make
the primary winding. That way it will not get the core into saturation and
give me some headroom so when it draws secondary current.
Thank you for your help. I have allways wanted to figure out these
transformers.
 
J

John Popelish

Jan 1, 1970
0
James said:
To add that many windings, I would also need to use a smaller wire and
rewind the whole primary. The wire I have on the secondary fills all the
space in the core now. I believe I understand enough now to go from bare
core to custom transformer so I will experiment and see if it all works out.
I will like you said, add windings to the core ( about 200 or so ) and
measure the idle current is steps as I remove some windings. When the
current begins to rise, then I will know the lowest number of turns to make
the primary winding. That way it will not get the core into saturation and
give me some headroom so when it draws secondary current.
Thank you for your help. I have allways wanted to figure out these
transformers.

A simple way to use MOTs for other uses is to take two similar units
and wire the primaries in series. Then you can wire the secondaries
in parallel for low voltage, high current outputs. This, of course,
lowers the volts per turn by half.
 
C

Clifford Heath

Jan 1, 1970
0
John said:
They are placed where they do very little harm.

Not to mention that if you cut them out, the transformer will fall
apart. Cutting them is actually a good way to get new windings on,
provided you weld them back after ensuring *tight* contact between
the E and the I parts. Bolt through the holes especially on the I
parts *before* cutting the welds, and leave the bolts in place
until after it's re-welded, or you'll wind up with a gapped xformer.
 
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