# Boy, do I feel stupid!

Discussion in 'Electronic Basics' started by Rich Grise, Jan 17, 2006.

1. ### Rich GriseGuest

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

2. ### John PopelishGuest

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.

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.
It is a good idea if you want to witness what happens.
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?

3. ### John PopelishGuest

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.

4. ### Tim WilliamsGuest

*Cough* 150 VA. Sober up, Rich ;-) (Noticed you were posting drunk
earlier...)
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.
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!).
Nah, you need iron filings for that. Hope the primary is well insulated ;-)
I would. 150VA is a lot of current you could be using for ___.
No idea... is there a mad scientist or high voltage group?

Tim

5. ### kellGuest

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

6. ### Alan BGuest

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

7. ### colinGuest

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 =^.^=

8. ### Derek PotterGuest

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

9. ### Mark FergersonGuest

Scientist types who actually need to know about it:

http://www.pupman.com/

Specifically:

Don't tell them I sent you.

Mark L. Fergerson

10. ### Derek PotterGuest

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.

11. ### Derek PotterGuest

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

12. ### Rich GriseGuest

[John's excellent comments on power factor snipped]
....
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

13. ### Rich GriseGuest

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

14. ### Rich GriseGuest

Thanks Everybody! This thread has been very enlightening!

Thanks!
Rich

15. ### John PopelishGuest

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.

16. ### Rich GriseGuest

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

17. ### ehsjrGuest

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

Ed

18. ### Derek PotterGuest

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.

19. ### Ron HubbardGuest

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

Ron

20. ### Rich GriseGuest

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

Cheers!
Rich