# MOT dissapointment

E

#### ehsjr

Jan 1, 1970
0
In the thread "Need linear supply 3.3V/15A or 5V/25A"
that Joerg started on 12/2 I posted the idea of rewinding
a MOT for ~6V. I've never done it, but decided to try
it. If anything good comes from it, Joerg gets the credit
because his question planted the seed.

Things went so well with the initial experiments that I
decided to actually make a useful transformer for a
13.8V ~25A (peak) supply.

To do that, I need about 100' of #14 enamelled wire.
(Gonna add 24-30 turns on the primary to knock the
5.62 magnetizing current down to 1.4A or lower. 24
turns knocked it down to 1.4. The secondary will be
3 or 4 18 turn windings in parallel.)

The dissapointment: I found another oven yesterday.
Today I ground off the welds and pried off the primary.
Now, get this: it looks like #14 copper, but weighs
only 6 ounces! It has about 132 feet of wire (based
on turns count x average perimeter of the winding)
The damn stuff must be aluminum.

So I'll have to keep on scrounging until I find one
that has real copper. The rewinding will be a bitch,
so I'm going to wait until I can get the real stuff.

If anyone has any pointers to offer - about the winding,
getting the wire, whatever, I'd love to hear them.
By the way, the ~ 25A is not a requirement. I just
want to get the maximum current I can from it, reaonably.
There's no point in going through the effort for
say 10 amps, and my main experimental interest was in
finding out what it would take to get an MOT into
"re-windable condition" . Now it is sort of a challenge
to try to get the most out of it reasonably. I already
know that Joerg could get his 15A at 3.3 or 25A at 5V
from this xformer. It will easily take 50 feet of #14
enamelled wire, with room to spare.

Ed

J

#### James Arthur

Jan 1, 1970
0
ehsjr said:
In the thread "Need linear supply 3.3V/15A or 5V/25A"
that Joerg started on 12/2 I posted the idea of rewinding
a MOT for ~6V. I've never done it, but decided to try
it. If anything good comes from it, Joerg gets the credit
because his question planted the seed.

Things went so well with the initial experiments that I
decided to actually make a useful transformer for a
13.8V ~25A (peak) supply.

To do that, I need about 100' of #14 enamelled wire.
(Gonna add 24-30 turns on the primary to knock the
5.62 magnetizing current down to 1.4A or lower. 24
turns knocked it down to 1.4. The secondary will be
3 or 4 18 turn windings in parallel.)

The dissapointment: I found another oven yesterday.
Today I ground off the welds and pried off the primary.
Now, get this: it looks like #14 copper, but weighs
only 6 ounces! It has about 132 feet of wire (based
on turns count x average perimeter of the winding)
The damn stuff must be aluminum.

So I'll have to keep on scrounging until I find one
that has real copper. The rewinding will be a bitch,
so I'm going to wait until I can get the real stuff.

If anyone has any pointers to offer - about the winding,
getting the wire, whatever, I'd love to hear them.
By the way, the ~ 25A is not a requirement. I just
want to get the maximum current I can from it, reaonably.
There's no point in going through the effort for
say 10 amps, and my main experimental interest was in
finding out what it would take to get an MOT into
"re-windable condition" . Now it is sort of a challenge
to try to get the most out of it reasonably. I already
know that Joerg could get his 15A at 3.3 or 25A at 5V
from this xformer. It will easily take 50 feet of #14
enamelled wire, with room to spare.

Ed

rewinding here in sed:

MOT hacking

Boy, do I feel stupid!

Cheers,
James Arthur

P

#### Paul E. Schoen

Jan 1, 1970
0
James Arthur said:

You might consider using an old Powerstat or Variac (which are really
toroidal transformers), and then add whatever number of turns you need
through the donut hole. I have made transformers that put out 1000 amps
continuous and over 10kA pulses by using one or two or four turns of bus
bar as the secondary. You can also use copper pipe and run water through it
for cooling. Even a burned up Powerstat can be repaired or even totally
rewound, or you can get a complete kit from about 80 VA to 1.4 kVA from
www.toroid.com. Toroids are notoriously quiet, small, and efficient. It's
not too much hassle to wind a low voltage secondary. They are generally 0.2
to 0.8 volts per turn.

Paul

J

#### Joerg

Jan 1, 1970
0
ehsjr said:
In the thread "Need linear supply 3.3V/15A or 5V/25A"
that Joerg started on 12/2 I posted the idea of rewinding
a MOT for ~6V. I've never done it, but decided to try
it. If anything good comes from it, Joerg gets the credit
because his question planted the seed.

Things went so well with the initial experiments that I
decided to actually make a useful transformer for a
13.8V ~25A (peak) supply.

To do that, I need about 100' of #14 enamelled wire.
(Gonna add 24-30 turns on the primary to knock the
5.62 magnetizing current down to 1.4A or lower. 24
turns knocked it down to 1.4. The secondary will be
3 or 4 18 turn windings in parallel.)

The dissapointment: I found another oven yesterday.
Today I ground off the welds and pried off the primary.
Now, get this: it looks like #14 copper, but weighs
only 6 ounces! It has about 132 feet of wire (based
on turns count x average perimeter of the winding)
The damn stuff must be aluminum.

A microwave oven? Considering that they often retail for $49.95 they will cut every corner they can. If non-copper windings save only one penny they'll do it. So I'll have to keep on scrounging until I find one that has real copper. The rewinding will be a bitch, so I'm going to wait until I can get the real stuff. If anyone has any pointers to offer - about the winding, getting the wire, whatever, I'd love to hear them. By the way, the ~ 25A is not a requirement. I just want to get the maximum current I can from it, reaonably. There's no point in going through the effort for say 10 amps, and my main experimental interest was in finding out what it would take to get an MOT into "re-windable condition" . Now it is sort of a challenge to try to get the most out of it reasonably. I already know that Joerg could get his 15A at 3.3 or 25A at 5V from this xformer. It will easily take 50 feet of #14 enamelled wire, with room to spare. What I usually try to get away with is snip some windings out of a transformer to reduce the dissipation in the linear regulator, not wind a new one. Ok, I have wound some transformers but I do not enjoy that. Good transformers will have their laminations in with alternating orientation. So the last 4-5 must be tapped into the pack with a wodden mallet. Often the last one or two just don't want to go in and then it's always knocking wood that it'll be enough and won't hum. By the way 14AWG for 25 amps is, ahm, quite brazen ... watch for any "amperage stench" when drawing lots of current for more than a few minutes. When it ticks like a wood stove better turn it off ;-) E #### ehsjr Jan 1, 1970 0 Joerg said: A microwave oven? Considering that they often retail for$49.95 they
will cut every corner they can. If non-copper windings save only one
penny they'll do it.

What I usually try to get away with is snip some windings out of a
transformer to reduce the dissipation in the linear regulator, not wind
a new one. Ok, I have wound some transformers but I do not enjoy that.
Good transformers will have their laminations in with alternating
orientation. So the last 4-5 must be tapped into the pack with a wodden
mallet. Often the last one or two just don't want to go in and then it's
always knocking wood that it'll be enough and won't hum.

These MOT transformers don't fall into the "good" category.
The core is all E's in one direction with the I's across
the open E's.
By the way 14AWG for 25 amps is, ahm, quite brazen ... watch for any
"amperage stench" when drawing lots of current for more than a few
minutes. When it ticks like a wood stove better turn it off ;-)

Naw - I want to run the #14 windings in parallel to share
the current. I'm hoping for 4 windings, 18 turns each, at
a bit over 6 amps each - about 72 feet of #14 to fit on
the core. Otherwise, I'll have to come to you to ask for
sound effects of a melting transformer.

I'm also looking to wind another ~ 25 turns in series with
the primary, so I need about 100 feet of #14. As I struggle
to get all that wire wound into the existing core openings,
I will _not_ need any sound effects. I'll supply those myself.

Ed

M

#### [email protected]

Jan 1, 1970
0
These MOT transformers don't fall into the "good" category.
The core is all E's in one direction with the I's across
the open E's.

Naw - I want to run the #14 windings in parallel to share
the current.  I'm hoping for 4 windings, 18 turns each, at
a bit over 6 amps each - about 72 feet of #14 to fit on
the core.  Otherwise, I'll have to come to you to ask for
sound effects of a melting transformer.

I'm also looking to wind another ~ 25 turns in series with
the primary, so I need about 100 feet of #14.  As I struggle
to get all that wire wound into the existing core openings,
I will _not_ need any sound effects. I'll supply those myself.

Ed- Hide quoted text -

- Show quoted text -

ok this is tricky...If you are talking about delivering 25 Amps DC at
the output, then the RMS current in the secondary may have to carry
can be as high as 2x that. It depends on the type of input filter on
the rectifier. If you use a big C, the xformer and rectifer will
deliver the current in pulses, and to deliver 25 Amps average current,
the instantaneous value of the pulses is a lot higher than 25 Amps.
When you figure out the RMS value of the pulses (which determines how
hot the wire will get) you will see its can be much higher than 25
Amps. This is the detailed part of designing a high current
supply.

The situation is actually better if you use a smaller filter C so the
current pulses are longer in time and lower in magnitude, but then you
have more ripple on the output. This is not a simple problem.

Mark

Mark

E

#### ehsjr

Jan 1, 1970
0
ok this is tricky...If you are talking about delivering 25 Amps DC at
the output, then the RMS current in the secondary may have to carry
can be as high as 2x that. It depends on the type of input filter on
the rectifier. If you use a big C, the xformer and rectifer will
deliver the current in pulses, and to deliver 25 Amps average current,
the instantaneous value of the pulses is a lot higher than 25 Amps.
When you figure out the RMS value of the pulses (which determines how
hot the wire will get) you will see its can be much higher than 25
Amps. This is the detailed part of designing a high current
supply.

The situation is actually better if you use a smaller filter C so the
current pulses are longer in time and lower in magnitude, but then you
have more ripple on the output. This is not a simple problem.

Mark

Mark

Yes I understand that. It won't be a problem *if* I can get four
secondary windings on there, plus the extra 25 turn primary winding.
The secondary resistance will be low - about 0.0113625 ohms if the
wire tables are right. That's 4 windings at 18 feet of #14 copper
all in parallel. Even at 45 amps, that's only ~ .54 volt drop,
and the peak computes to 20.29, so ~ 17.29 after the rectifiers.
So ripple needs to be less than 3.49 - 80,000 uF should do.

But that's all on paper. I don't know how the damn thing will
perform, and it's really an experiment to get the most I can
from a scrounged MOT. All I've been able to do so far is
reduce the no load, no secondary winding current draw from
5.62 to 1.4 amps by temporarily winding 24 turns and putting
them in series with the primary.

Ed

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