# Boy, do I feel stupid!

6 views

### Rich Grise

Jan 16, 2006, 7:08:38 PM1/16/06
to
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

### John Popelish

Jan 16, 2006, 7:34:21 PM1/16/06
to
Rich Grise wrote:
> 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?

### John Popelish

Jan 16, 2006, 7:36:40 PM1/16/06
to
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.

### Tim Williams

Jan 16, 2006, 7:37:52 PM1/16/06
to
"Rich Grise" <rich...@example.net> wrote in message
news:pan.2006.01.17....@example.net...

> 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

--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

### kell

Jan 16, 2006, 7:52:27 PM1/16/06
to

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

http://forum.4hv.org
Don't tell them I sent you.

### Alan B

Jan 16, 2006, 8:26:26 PM1/16/06
to
On Tue, 17 Jan 2006 00:08:38 GMT, via
<pan.2006.01.17....@example.net>, Rich Grise
<rich...@example.net> spake thusly:

>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?

Not Watts. VAR's - Volt Amps Reactive. The power transmitted that is

### colin

Jan 17, 2006, 7:09:46 AM1/17/06
to
"Rich Grise" <rich...@example.net> wrote in message
news:pan.2006.01.17....@example.net...
> 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 =^.^=

### Derek Potter

Jan 17, 2006, 8:10:59 AM1/17/06
to

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 :)

### Mark Fergerson

Jan 17, 2006, 9:42:16 AM1/17/06
to
Rich Grise wrote:

> 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? ;-)

Scientist types who actually need to know about it:

http://www.pupman.com/

Specifically:

Don't tell them I sent you.

Mark L. Fergerson

### Derek Potter

Jan 17, 2006, 10:23:13 AM1/17/06
to
On Tue, 17 Jan 2006 07:42:16 -0700, Mark Fergerson <nu...@biz.ness>
wrote:

>Rich Grise wrote:
[...]

>Scientist types who actually need to know about it:
>
>http://www.pupman.com/
>
> Specifically:
>
>
> 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.

### Derek Potter

Jan 17, 2006, 10:28:16 AM1/17/06
to
On Tue, 17 Jan 2006 00:08:38 GMT, Rich Grise <rich...@example.net>
wrote:

>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

### Rich Grise

Jan 17, 2006, 12:03:02 PM1/17/06
to
On Mon, 16 Jan 2006 19:34:21 -0500, John Popelish wrote:
> Rich Grise wrote:
[John's excellent comments on power factor snipped]
...

>> 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?

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

### Rich Grise

Jan 17, 2006, 12:09:45 PM1/17/06
to

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

### Rich Grise

Jan 17, 2006, 12:14:23 PM1/17/06
to

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

Thanks!
Rich

### John Popelish

Jan 17, 2006, 1:13:45 PM1/17/06
to
Rich Grise wrote:
> On Mon, 16 Jan 2006 19:36:40 -0500, John Popelish wrote:
>
>
>>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.
>
>
> 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.

### Rich Grise

Jan 17, 2006, 1:53:03 PM1/17/06
to

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

### ehsjr

Jan 17, 2006, 3:45:37 PM1/17/06
to

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

Ed

### Derek Potter

Jan 17, 2006, 8:55:28 PM1/17/06
to
On Tue, 17 Jan 2006 20:45:37 GMT, ehsjr <eh...@bellatlantic.net>
wrote:

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.

### Ron Hubbard

Jan 18, 2006, 2:30:54 AM1/18/06
to
Try some of the Tesla coil groups; they just love to blow things up or
do tricks with high voltages (and sometimes currents).

Ron

### Rich Grise

Jan 19, 2006, 8:09:08 PM1/19/06
to
On Tue, 17 Jan 2006 20:45:37 +0000, ehsjr wrote:
> Rich Grise wrote:

>> 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

### ehsjr

Jan 19, 2006, 11:58:32 PM1/19/06
to
Ohhh - my bad. I meant what voltage & current do you
expect? I dunno if you're going for an isolation
transformer, a 12 v at 20 amps or ???

Ed

### Rich Grise

Jan 20, 2006, 11:55:54 AM1/20/06
to
On Fri, 20 Jan 2006 04:58:32 +0000, ehsjr wrote:
> Rich Grise wrote:
>> On Tue, 17 Jan 2006 20:45:37 +0000, ehsjr wrote:
>>>Rich Grise wrote:
>>
>>
>>>>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. ;-)
>>
> Ohhh - my bad. I meant what voltage & current do you
> expect? I dunno if you're going for an isolation
> transformer, a 12 v at 20 amps or ???

Oh, that. I haven't decided anything yet - heck, it could sit there
on the bench for months just the way it is befor I feel like dicking
around with it some more; it's more a diversion than anything else.

But the idea of a 12V, 20A supply does sound attractive; or maybe
3 windings, and make a +12, -12, +5 supply, or maybe even those new
low voltages that chips seem to be using these days. IOW, I can
do whatever I want to do with it, I just don't know what I want yet.
:-)

Thanks!
Rich