Maker Pro
Maker Pro

Boy, do I feel stupid!

R

Rich Grise

Jan 1, 1970
0
I've been poking around with a salvated microwave oven transformer (MOT),
and doing little diddly simple experiments:
http://www.abiengr.com/~sysop/images/MOT-test.jpg

And the 0.1 ohm resistor in series with the primary seems to be dropping
about .145 volts, +/- .4; that's an amp and a half! At 115 volts, that's
<brain refuses to do arithmetic> over 150 watts!

But nothing got even warm. It was humming merrily at 60 Hz, but it never
even got above room temperature. So, where are these 150 watts going?

Then, finally, after all of these years of dabbling in things electronical,
it hit me - that amp and a half is going through _an inductor_!!! With a
DC resistance of 1.0 ohms +/- 0.1 ohm. That's one and a half watts of
real power.

DUH!!!!!

Next, I'm gonna see what happens when I take the magnetic shunts out;
http://www.abiengr.com/~sysop/images/MOT-primary2.jpg
has anyone ever done this, and is it a good idea?
[the penny is only for scale, but I did have this wild-ass idea that the
induced eddy currents would make magnetic effects that would be physically
observable, but no such luck. )-; ]

And at only 1.4 amps, is it worth the bother to wind more primary
turns?

And are there other newsgroups that might want to participate in a MOT
saga? ;-)

Thanks,
Rich
 
J

John Popelish

Jan 1, 1970
0
Rich said:
I've been poking around with a salvated microwave oven transformer (MOT),
and doing little diddly simple experiments:
http://www.abiengr.com/~sysop/images/MOT-test.jpg

And the 0.1 ohm resistor in series with the primary seems to be dropping
about .145 volts, +/- .4; that's an amp and a half! At 115 volts, that's
<brain refuses to do arithmetic> over 150 watts!

But nothing got even warm. It was humming merrily at 60 Hz, but it never
even got above room temperature. So, where are these 150 watts going?

You have rediscovered power factor current. The 1.5 amps indicates
the storage of energy in the magnetic field of the MOT, and its return
to the power company, twice a cycle. Talk about short term loans.

Then, finally, after all of these years of dabbling in things electronical,
it hit me - that amp and a half is going through _an inductor_!!! With a
DC resistance of 1.0 ohms +/- 0.1 ohm. That's one and a half watts of
real power.

DUH!!!!!

The power company dislikes inductive loads for similar reasons. That
current not only dumps a little power into the MOT winding resistance,
but all the transformers and transmission lines all the way back to
the generator. But the watt hour meter at the service entrance
charges you for only 1.5 watts while this thing sits there and loads
the grid with 150 VA.
Next, I'm gonna see what happens when I take the magnetic shunts out;
http://www.abiengr.com/~sysop/images/MOT-primary2.jpg
has anyone ever done this, and is it a good idea?

It is a good idea if you want to witness what happens.
[the penny is only for scale, but I did have this wild-ass idea that the
induced eddy currents would make magnetic effects that would be physically
observable, but no such luck. )-; ]

And at only 1.4 amps, is it worth the bother to wind more primary
turns?

If you intend to operate the MOT with short bursts of full load
(turning the primary voltage off between those bursts), probably not.
If you want to operate it for hours on end with a wide range of
loads, it probably is. What might you use it for?
 
J

John Popelish

Jan 1, 1970
0
Rich Grise wrote:
(snip)
Next, I'm gonna see what happens when I take the magnetic shunts out;
http://www.abiengr.com/~sysop/images/MOT-primary2.jpg
has anyone ever done this, and is it a good idea?
(snip)

Before you make the measurements, I suggest you draw a picture of the
core and try to imagine the magnetic flux paths, and the magnetic
reluctance (magnetic flux resistance) for the various branches, and
see if you can reason out what to expect from the experiment.
 
T

Tim Williams

Jan 1, 1970
0
Rich Grise said:
And the 0.1 ohm resistor in series with the primary seems to be dropping
about .145 volts, +/- .4; that's an amp and a half! At 115 volts, that's
<brain refuses to do arithmetic> over 150 watts!

*Cough* 150 VA. Sober up, Rich ;-) (Noticed you were posting drunk
earlier...)
But nothing got even warm. It was humming merrily at 60 Hz, but it never
even got above room temperature. So, where are these 150 watts going?

Well, you measured it on a resistor, so if it isn't heat, it's still going
through the wires, and the power company doesn't like that. Oughta toss on
a capacitor (ooh, power-on surge-ified!) or some more turns to reduce the
B-field.
Then, finally, after all of these years of dabbling in things electronical,
it hit me - that amp and a half is going through _an inductor_!!! With a
DC resistance of 1.0 ohms +/- 0.1 ohm. That's one and a half watts of
real power.

DUH!!!!!
:p

Next, I'm gonna see what happens when I take the magnetic shunts out;
http://www.abiengr.com/~sysop/images/MOT-primary2.jpg
has anyone ever done this, and is it a good idea?

Well, the shunts are bypassing a little magnetic field closer to the
primary, but not much (depending on width), and various parts of the core
are probably running near saturation anyway, so it wouldn't make much
difference.

I have an MOT in regular use, without shunts, but it also has sufficient
turnage that it doesn't saturate much.

You should check the current waveform on that resistor and see what it looks
like... bet it's got some nice nipples (that oughta get you runnin' to the
scope!).
[the penny is only for scale, but I did have this wild-ass idea that the
induced eddy currents would make magnetic effects that would be physically
observable, but no such luck. )-; ]

Nah, you need iron filings for that. Hope the primary is well insulated ;-)
And at only 1.4 amps, is it worth the bother to wind more primary
turns?

I would. 150VA is a lot of current you could be using for ___.
And are there other newsgroups that might want to participate in a MOT
saga? ;-)

No idea... is there a mad scientist or high voltage group?

Tim
 
A

Alan B

Jan 1, 1970
0
I've been poking around with a salvated microwave oven transformer (MOT),
and doing little diddly simple experiments:
http://www.abiengr.com/~sysop/images/MOT-test.jpg

And the 0.1 ohm resistor in series with the primary seems to be dropping
about .145 volts, +/- .4; that's an amp and a half! At 115 volts, that's
<brain refuses to do arithmetic> over 150 watts!

Used your AC meter, didja?
But nothing got even warm. It was humming merrily at 60 Hz, but it never
even got above room temperature. So, where are these 150 watts going?

Not Watts. VAR's - Volt Amps Reactive. The power transmitted that is
not usable (billable) load.
 
C

colin

Jan 1, 1970
0
Rich Grise said:
Next, I'm gonna see what happens when I take the magnetic shunts out;
http://www.abiengr.com/~sysop/images/MOT-primary2.jpg
has anyone ever done this, and is it a good idea?
[the penny is only for scale, but I did have this wild-ass idea that the
induced eddy currents would make magnetic effects that would be physically
observable, but no such luck. )-; ]

AFAIK the shunts only realy come into play when there is a heavy load on the
secondary and act as a current limiter, old type welding transformers have
adjustable shunts to vary the maximum current, when the secondary is short
circuited the current cuases a magnetic field wich oposes that by the
primary wich then finds an easier path through the shunts.

with no secondary at all i suspect they will have an undramatic effect,
altering the effective magnetic path length/cross sectional area only
moderatly.

Colin =^.^=
 
D

Derek Potter

Jan 1, 1970
0
Rich Grise said:
Next, I'm gonna see what happens when I take the magnetic shunts out;
http://www.abiengr.com/~sysop/images/MOT-primary2.jpg
has anyone ever done this, and is it a good idea?
[the penny is only for scale, but I did have this wild-ass idea that the
induced eddy currents would make magnetic effects that would be physically
observable, but no such luck. )-; ]

AFAIK the shunts only realy come into play when there is a heavy load on the
secondary and act as a current limiter, old type welding transformers have
adjustable shunts to vary the maximum current, when the secondary is short
circuited the current cuases a magnetic field wich oposes that by the
primary wich then finds an easier path through the shunts.

with no secondary at all i suspect they will have an undramatic effect,
altering the effective magnetic path length/cross sectional area only
moderatly.

The effect of a shunt magnetic path is simply to introduce a series
inductance in the transformer, which will have the effect of a crude
current limit. It sometimes seems odd that a *shunt* magnetic path
should introduce a *series* impedance, but it's quite logical - think
in terms of the secondary current producing a flux that does not
contribute to transformer action but still creates a back EMF in the
secondary.

Same physical system, different way of describing it :)
 
M

Mark Fergerson

Jan 1, 1970
0
Rich said:
Next, I'm gonna see what happens when I take the magnetic shunts out;
http://www.abiengr.com/~sysop/images/MOT-primary2.jpg
has anyone ever done this, and is it a good idea?
[the penny is only for scale, but I did have this wild-ass idea that the
induced eddy currents would make magnetic effects that would be physically
observable, but no such luck. )-; ]

And at only 1.4 amps, is it worth the bother to wind more primary
turns?

And are there other newsgroups that might want to participate in a MOT
saga? ;-)

You're not the first to wonder about this. There's a whole buncha Mad
Scientist types who actually need to know about it:

http://www.pupman.com/

Specifically:

http://www.google.com/custom?q=mot+...ains=www.pupman.com&sitesearch=www.pupman.com

Don't tell them I sent you.


Mark L. Fergerson
 
D

Derek Potter

Jan 1, 1970
0
Rich Grise wrote: [...]
You're not the first to wonder about this. There's a whole buncha Mad
Scientist types who actually need to know about it:

http://www.pupman.com/

Specifically:

http://www.google.com/custom?q=mot+...ains=www.pupman.com&sitesearch=www.pupman.com

Don't tell them I sent you.

And there are others who like to connect up several MOTs and try to
get a few KW out of a magnetron in order to create plasma balls. Or
to kill burglars and stray cats, I forget which.
 
D

Derek Potter

Jan 1, 1970
0
Next, I'm gonna see what happens when I take the magnetic shunts out;
http://www.abiengr.com/~sysop/images/MOT-primary2.jpg
has anyone ever done this, and is it a good idea?
[the penny is only for scale, but I did have this wild-ass idea that the
induced eddy currents would make magnetic effects that would be physically
observable, but no such luck. )-; ]

And at only 1.4 amps, is it worth the bother to wind more primary
turns?

Not unless it's running far too hot. By using more turns, you just
waste the magnetic capacity of the core by reducing the flux. You're
also adding series copper which will increase the losses.

You want minimum turns for the highest acceptable flux density and
then use as much copper as the core will take. That gives you amximum
power throughput.
And are there other newsgroups that might want to participate in a MOT
saga? ;-)

Not yet..... newgroup alt.mot.flash.flash.flaaaaaaaaa
 
R

Rich Grise

Jan 1, 1970
0
Rich Grise wrote:
[John's excellent comments on power factor snipped]
....
If you intend to operate the MOT with short bursts of full load
(turning the primary voltage off between those bursts), probably not.
If you want to operate it for hours on end with a wide range of
loads, it probably is. What might you use it for?

I haven't really decided yet. I'm entertaining thoughts of a spot
welder, or just a general purpose bench supply; in any case I'm
basically going to just play with it for awhile. :)

Thanks!
Rich
 
R

Rich Grise

Jan 1, 1970
0
Rich Grise wrote:
(snip)
(snip)

Before you make the measurements, I suggest you draw a picture of the
core and try to imagine the magnetic flux paths, and the magnetic
reluctance (magnetic flux resistance) for the various branches, and
see if you can reason out what to expect from the experiment.

Uh, yeah, right after I finish that course in transformer physics. ;-)

I don't even know how the length of the flux path relates to inductance
yet! )-;

I would think that there'd be a better coupling factor, since more of
the flux goes through the secondary, but when I start to think about
what this does to the primary current, my brain starts to hurt. 8-|

I once worked for a guy who had many years' experience with transformers,
and he designed ferroresonant units for battery chargers, and basically
did it by the seat of his pants with liberal doses of Black Magick. ;-)

Thanks!
Rich
 
R

Rich Grise

Jan 1, 1970
0
Next, I'm gonna see what happens when I take the magnetic shunts out;
http://www.abiengr.com/~sysop/images/MOT-primary2.jpg
has anyone ever done this, and is it a good idea?
[the penny is only for scale, but I did have this wild-ass idea that the
induced eddy currents would make magnetic effects that would be physically
observable, but no such luck. )-; ]

And at only 1.4 amps, is it worth the bother to wind more primary
turns?

Not unless it's running far too hot. By using more turns, you just
waste the magnetic capacity of the core by reducing the flux. You're
also adding series copper which will increase the losses.

You want minimum turns for the highest acceptable flux density and
then use as much copper as the core will take. That gives you amximum
power throughput.
And are there other newsgroups that might want to participate in a MOT
saga? ;-)

Not yet..... newgroup alt.mot.flash.flash.flaaaaaaaaa

Thanks Everybody! This thread has been very enlightening! :) :)

Thanks!
Rich
 
J

John Popelish

Jan 1, 1970
0
Rich said:
Uh, yeah, right after I finish that course in transformer physics. ;-)

I don't even know how the length of the flux path relates to inductance
yet! )-;

Inductance is proportional to the total flux created by a given
current. The easy flux path through the primary coil is the solid
metal path through the center of the core and back around through both
outside legs. The shunts (with their series air gaps) add just a tiny
bit of additional flux path in parallel to the solid metal loop out
and back past them. This is like putting a 10k resistor in parallel
with a 10 ohm resistor. Very little additional (flux) conductivity
results.

Now, if there were a secondary on the other side of that shunt, and it
was circulation big ampere turns that produced a field that bucked the
flux that passed through it, then the alternate flux path through the
shunts and their air gap would provide the primary with a reduced but
not zero amount of flux per ampere turn, regardless, so the primary
inductance could not be forced effectively toward zero, but would
retain a minimum inductance, and thus, a minimum impedance across the
line. That is the current limiting effect of the shunts.
 
R

Rich Grise

Jan 1, 1970
0
Rich Grise wrote:

Inductance is proportional to the total flux created by a given current.
The easy flux path through the primary coil is the solid metal path
through the center of the core and back around through both outside
legs. The shunts (with their series air gaps) add just a tiny bit of
additional flux path in parallel to the solid metal loop out and back
past them. This is like putting a 10k resistor in parallel with a 10
ohm resistor. Very little additional (flux) conductivity results.

Now, if there were a secondary on the other side of that shunt, and it
was circulation big ampere turns that produced a field that bucked the
flux that passed through it, then the alternate flux path through the
shunts and their air gap would provide the primary with a reduced but
not zero amount of flux per ampere turn, regardless, so the primary
inductance could not be forced effectively toward zero, but would retain
a minimum inductance, and thus, a minimum impedance across the line.
That is the current limiting effect of the shunts.

Thank you! I must admit, John (may I call you John?) when you describe
something, you really do it good! When are you going to write your
book? You could probably get a lot of material right off google! ;-)

Thanks again!
Rich
 
E

ehsjr

Jan 1, 1970
0
Rich said:
Next, I'm gonna see what happens when I take the magnetic shunts out;
http://www.abiengr.com/~sysop/images/MOT-primary2.jpg
has anyone ever done this, and is it a good idea?
[the penny is only for scale, but I did have this wild-ass idea that the
induced eddy currents would make magnetic effects that would be physically
observable, but no such luck. )-; ]

And at only 1.4 amps, is it worth the bother to wind more primary
turns?

Not unless it's running far too hot. By using more turns, you just
waste the magnetic capacity of the core by reducing the flux. You're
also adding series copper which will increase the losses.

You want minimum turns for the highest acceptable flux density and
then use as much copper as the core will take. That gives you amximum
power throughput.

And are there other newsgroups that might want to participate in a MOT
saga? ;-)

Not yet..... newgroup alt.mot.flash.flash.flaaaaaaaaa


Thanks Everybody! This thread has been very enlightening! :) :)

Thanks!
Rich

Keep on posting as you play ... er ... experiment.
What do you expect to get out of the secondary, once
you wind it?

Ed
 
D

Derek Potter

Jan 1, 1970
0
Rich said:
Next, I'm gonna see what happens when I take the magnetic shunts out;
http://www.abiengr.com/~sysop/images/MOT-primary2.jpg
has anyone ever done this, and is it a good idea?
[the penny is only for scale, but I did have this wild-ass idea that the
induced eddy currents would make magnetic effects that would be physically
observable, but no such luck. )-; ]

And at only 1.4 amps, is it worth the bother to wind more primary
turns?

Not unless it's running far too hot. By using more turns, you just
waste the magnetic capacity of the core by reducing the flux. You're
also adding series copper which will increase the losses.

You want minimum turns for the highest acceptable flux density and
then use as much copper as the core will take. That gives you amximum
power throughput.


And are there other newsgroups that might want to participate in a MOT
saga? ;-)

Not yet..... newgroup alt.mot.flash.flash.flaaaaaaaaa


Thanks Everybody! This thread has been very enlightening! :) :)

Thanks!
Rich

Keep on posting as you play ... er ... experiment.
What do you expect to get out of the secondary, once
you wind it?

2 oz of boiling resin, 8 oz of molten copper, 1/2 litre dioxin vapour,
3 joules of green light and a dead cockroach travelling at 180 mph.
 
R

Ron Hubbard

Jan 1, 1970
0
Try some of the Tesla coil groups; they just love to blow things up or
do tricks with high voltages (and sometimes currents).

Ron
 
R

Rich Grise

Jan 1, 1970
0
Rich Grise wrote: [about MOT]
Thanks Everybody! This thread has been very enlightening! :) :)

Keep on posting as you play ... er ... experiment.
:)

What do you expect to get out of the secondary, once
you wind it?

Well, I _expect_ it to act like an ordinary transformer, but that's
what the play^H^H^H^Hexperimenting is for. ;-)

Cheers!
Rich
 
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