# A Curious Find

Discussion in 'Electronic Basics' started by Randy, Jul 22, 2003.

1. ### RandyGuest

A friend of mine gave me a transformer that has peaked my curiosity. It is
out of an industrial microwave oven. I took measurements to try and
determine its configuration:

Primary - 500 milliohms, 120 vac, (assuming) 120 turns, there is a shunt
wound around the primary. Its' output is 13 amps.

Secondary - 50 ohms, 4000 volts, (assuming) 4000 turns.

My initial Idea, when I got the transformer, was to modify it and apply it
as a spot welder but, the results from the calculations I made based on the
measurements above, has left me bewildered. Here are my results:

Primary - 80 amps, Power: 9. 6KVA
Secondary - 2.4 amps @ 4000 volts

Modifying the transformer would mean stepping it down but, if these values
are correct, I won't be able to turn enough wire that is able to handle the
current, to get it in my working range.

Opinions?

An Inquiring Mind!
Randy Gross

2. ### John PopelishGuest

How do you come up with a primary current of 80 amps and a 9.6kVA
rating? I am skeptical that any industrial transformer designed to
operate off 120 volts is rated for 9.6kva output.

Does the transformer have a magnetic shunt (bit of core material)
between the primary and secondary windings?

I don't understand by the statement, "there is a shunt wound around
the primary. Its' output is 13 amps." Is this a single turn
secondary? How did you come up with this output current?

3. ### Howard Henry SchlunderGuest

I have a microwave oven transformer removed from a conventional household
model (probably being operated to output between 1100 to 1300 Watts). The
primary is rated by label for 115V 60Hz and it measures 420 milliohms of
resistance. The secondary measures 115.7 Ohms of resistance and while it's
output voltage is unmarked, the oven's capacitor was rated for 2100VAC, so
I'm pretty convinced that the secondary outputs very nearly 2kV.

Comparing the primary resistances, I'm guessing that the "industrial
microwave oven" yours came from is nothing more than a household model with
possibly a shiny stainless steel exterior for the industrial environment,
or, your measurements are in error. Your secondary, stikes me as incredibly
odd being twice the voltage of mine and half the resistance. It is my
understanding that transformers are normally designed such that loss in the
windings are approximately equal to each other. The measurements from my
transformer support that understanding, but yours will have a loss in the
primary which is more than 10 times as much as the secondary. I'm inclined
to believe that at least one of your measurements are erroroneous.
2.4 Amps @ 4000 volts makes for a very good electric chair, but a very light
duty (read: crappy) metal welder. At 80 Amps in the primary alone, you
would have a resistance loss of 3200 Watts, which would melt the insulation
in only a few seconds. My transformer has magnetic shunts in it which, I'm
guessing from things I've read, would limit the short circuit current to
somewhere around 40 Amps or so (in the primary). If yours doesn't have any
such method of limiting the short circuit current, then your transformer is
worthless as a welder without an external ballasting system.

Normally, people who want to weld with microwave over transfomers, will
completely remove the secondary winding and put their own very high current
but low voltage winding in place. Although I've not built my own yet, I
believe a secondary output voltage somewhere between 9 to 60 Volts would
produce a high power welder. I also understand that 12V car batteries make
for some quite decent welding devices as well (save the likely possibility
of the battery releasing hydrogen gas and exploding).

Howard Henry Schlunder

4. ### RandyGuest

<>...
:
: >
: > Primary - 80 amps, Power: 9. 6KVA
: > Secondary - 2.4 amps @ 4000 volts
:
: How do you come up with a primary current of 80 amps and a 9.6kVA
: rating? I am skeptical that any industrial transformer designed to
: operate off 120 volts is rated for 9.6kva output.

My thoughts exactly! The first thing I did was measure the pri. and sec.
resistance. I got .5 ohms and 50 ohms respectively. 120 vac/.5 ohms = 240
amps: impossible. So I figured I did something wrong. So I use the
secondary: 4000 vac/50 ohms = 80 amps but, that can't be right either
because that much current would fry the secondary.

:
: Does the transformer have a magnetic shunt (bit of core material)
: between the primary and secondary windings?:

Yes. The magnetic shunt is there.

: I don't understand by the statement, "there is a shunt wound around
: the primary. Its' output is 13 amps." Is this a single turn
: secondary? How did you come up with this output current?

I apologize John, this is the filament winding and it is a wire making 2
complete turns and a third around the primary coil. The 2 ends of this coil
run up to the top of the transformer to a big terminal block. One end is
soldered to a post marked: 3.2V. The other end is soldered to a post
marked: 13A. It has a resistance of .5 ohms too.

Something else that is peculiar, the primary has a fifth wire coming out of
it. This wire comes out of the primary and goes into the core. Ground
maybe, but why? Not on Schematic!

I just found a schematic under the terminal block. All of the ohms values I
gave you are correct. No other values are given except the line voltage:
120V. 60hz and, 13 amp supply to the magnetron. Let me know what you come
up with.
My Ohms Law isn't working.

Randy Gross

: --
: John Popelish
:

5. ### RandyGuest

You are quite right and, yes, I am a novice. The transformer will have to
be rewound but, I have to know what I'm dealing with before I can proceed
and, with guys such as yourself here on the group, I'm sure I will. I'm
taking a course now but I'm still crawling. Thanks for responding.

Randy Gross

<3f1d9acc\$>...
: "Randy " wrote in message : > A friend of mine gave me a transformer that has peaked my curiosity. It
is
: > out of an industrial microwave oven. I took measurements to try and
: > determine its configuration:
: >
: > Primary - 500 milliohms, 120 vac, (assuming) 120 turns, there is a
shunt
: > wound around the primary. Its' output is 13 amps.
: >
: > Secondary - 50 ohms, 4000 volts, (assuming) 4000 turns.
:
: I have a microwave oven transformer removed from a conventional household
: model (probably being operated to output between 1100 to 1300 Watts).
The
: primary is rated by label for 115V 60Hz and it measures 420 milliohms of
: resistance. The secondary measures 115.7 Ohms of resistance and while
it's
: output voltage is unmarked, the oven's capacitor was rated for 2100VAC,
so
: I'm pretty convinced that the secondary outputs very nearly 2kV.
:
: Comparing the primary resistances, I'm guessing that the "industrial
: microwave oven" yours came from is nothing more than a household model
with
: possibly a shiny stainless steel exterior for the industrial environment,
: or, your measurements are in error. Your secondary, stikes me as
incredibly
: odd being twice the voltage of mine and half the resistance. It is my
: understanding that transformers are normally designed such that loss in
the
: windings are approximately equal to each other. The measurements from my
: transformer support that understanding, but yours will have a loss in the
: primary which is more than 10 times as much as the secondary. I'm
inclined
: to believe that at least one of your measurements are erroroneous.
:
: > My initial Idea, when I got the transformer, was to modify it and apply
it
: > as a spot welder but, the results from the calculations I made based on
: the
: > measurements above, has left me bewildered. Here are my results:
: >
: > Primary - 80 amps, Power: 9. 6KVA
: > Secondary - 2.4 amps @ 4000 volts
:
: 2.4 Amps @ 4000 volts makes for a very good electric chair, but a very
light
: duty (read: crappy) metal welder. At 80 Amps in the primary alone, you
: would have a resistance loss of 3200 Watts, which would melt the
insulation
: in only a few seconds. My transformer has magnetic shunts in it which,
I'm
: guessing from things I've read, would limit the short circuit current to
: somewhere around 40 Amps or so (in the primary). If yours doesn't have
any
: such method of limiting the short circuit current, then your transformer
is
: worthless as a welder without an external ballasting system.
:
: Normally, people who want to weld with microwave over transfomers, will
: completely remove the secondary winding and put their own very high
current
: but low voltage winding in place. Although I've not built my own yet, I
: believe a secondary output voltage somewhere between 9 to 60 Volts would
: produce a high power welder. I also understand that 12V car batteries
make
: for some quite decent welding devices as well (save the likely
possibility
: of the battery releasing hydrogen gas and exploding).
:
: Howard Henry Schlunder
:
:
:
:
:
:
:
:

6. ### John LarkinGuest

It won't work this way for transformers. It's primary inductance, not
DC resistance, that determines no-load primary current, and even that
is unrelated to actual transformer power capacity, which is mainly
limited by ohmic heating of the wire. Figure roughly 15-20 watts per
pound.

John

7. ### FleetieGuest

"piqued"! It's "piqued"! You're about the fourth poster I've seen in a
a WEEK who doesn't know this word, so why use it if you don't know
it? It just makes you look stupid.

8. ### RandyGuest

article <>...
: On Tue, 22 Jul 2003 20:38:02 +0000 (UTC), "Randy "
:
:
: >My Ohms Law isn't working.
:
: It won't work this way for transformers. It's primary inductance, not
: DC resistance, that determines no-load primary current, and even that
: is unrelated to actual transformer power capacity, which is mainly
: limited by ohmic heating of the wire. Figure roughly 15-20 watts per
: pound.
:
: John
:
:
This gets more interesting by the minute.

Thanks,
Randy Gross

9. ### Howard Henry SchlunderGuest

Yo buss dis. whuz up dere. I'z sho we all has our personal gripes ovuh how
othuh folk use de English language, but in my eye, a peakin' curiosity or a
curiosity at its peak be a perfectly legitimate thin to have. Sheeit!

in message

10. ### Fred AbseGuest

Maybe that's why it was in the trash