How to test the primary coil on a microwave oven transformer

T

trg-s338

Guest
I'm building a spot welder using a modified microwave oven
transformer. The project calls for removing the secondary coil
windings. In doing so, I accidentally "bumped" the primary coil with
a hammer resulting in 2 or 3 wires in the primary coil becoming
slightly displaced by about 1/8 of an inch. I hope someone can tell
me how to test the primary coil for a short circuit as a result of
this slight damage. I currently have a 1/2 inch diameter copper cable
wound 1.5 times around the secondary core. I just want to make
certain the primary is not shorted out. I have a multimeter I can
use. Help, anyone? Thanks in advance.

Manny
 
On Sun, 31 Jan 2010 23:20:05 -0800 (PST), trg-s338
<americansun@gmail.com> wrote:

I'm building a spot welder using a modified microwave oven
transformer. The project calls for removing the secondary coil
windings. In doing so, I accidentally "bumped" the primary coil with
a hammer resulting in 2 or 3 wires in the primary coil becoming
slightly displaced by about 1/8 of an inch. I hope someone can tell
me how to test the primary coil for a short circuit as a result of
this slight damage. I currently have a 1/2 inch diameter copper cable
wound 1.5 times around the secondary core. I just want to make
certain the primary is not shorted out. I have a multimeter I can
use. Help, anyone? Thanks in advance.

Manny
Click to expand...
Just apply line voltage, maybe through one of those power strips that
has a circuit breaker. If there are shorted turns, the breaker will
trip or the transformer will smoke.

John
 
"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message
news:ebrdm5919ioiabrs04pt03jfufrndcpa8u@4ax.com...
On Sun, 31 Jan 2010 23:20:05 -0800 (PST), trg-s338
americansun@gmail.com> wrote:

I'm building a spot welder using a modified microwave oven
transformer. The project calls for removing the secondary coil
windings. In doing so, I accidentally "bumped" the primary coil with
a hammer resulting in 2 or 3 wires in the primary coil becoming
slightly displaced by about 1/8 of an inch. I hope someone can tell
me how to test the primary coil for a short circuit as a result of
this slight damage. I currently have a 1/2 inch diameter copper cable
wound 1.5 times around the secondary core. I just want to make
certain the primary is not shorted out. I have a multimeter I can
use. Help, anyone? Thanks in advance.

Manny

Just apply line voltage, maybe through one of those power strips that
has a circuit breaker. If there are shorted turns, the breaker will
trip or the transformer will smoke.
Click to expand...
It is unlikely that the short will be solid enough to cause either
scenario, and the breaker in most outlet strips is 10A with a fairly long
time delay which could allow some dangerous overheating and arcing. There
are probably 1000 turns on the primary, with perhaps #22AWG wire. Each turn
is about 0.12 volts. A solid short between two windings could create a
current of perhaps 100 amps, but at 1000:1 the primary current due to the
short would be only 100 mA. But that is still high enough to indicate a
shorted turn.

I'm not sure what the normal unloaded current draw might be on a MOT, but a
shorted turn would cause an increase, although it might vary as the area
around the short is disturbed. If the damaged windings are accessible, and
if the wires are fairly loose, and not soaked in varnish, it might be
possible to separate them using a plastic toothpick, and then apply some
insulating varnish to stabilize it.

Then it is important to make sure the primary stays well insulated from the
secondary, since that winding will be applied to the workpiece and any
leakage could result in hazardous voltages. I'd recommend heavy Mylar or
"fish paper". But if the secondary wire is well insulated, it might not be
needed.

I'm not sure if 1.5 turns will generate enough voltage to be useful as a
spot welder. It can certainly provide several hundred amps, but only 0.2
volts or so, which may not be enough to overcome the inductance of the
output wiring and the resistance of the material to be welded. But that is
another discussion. The main interest in this project should be safety.

Paul
 
On Mon, 1 Feb 2010 21:00:04 -0500, "Paul E. Schoen"
<paul@peschoen.com> wrote:

"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message
news:ebrdm5919ioiabrs04pt03jfufrndcpa8u@4ax.com...
On Sun, 31 Jan 2010 23:20:05 -0800 (PST), trg-s338
americansun@gmail.com> wrote:

I'm building a spot welder using a modified microwave oven
transformer. The project calls for removing the secondary coil
windings. In doing so, I accidentally "bumped" the primary coil with
a hammer resulting in 2 or 3 wires in the primary coil becoming
slightly displaced by about 1/8 of an inch. I hope someone can tell
me how to test the primary coil for a short circuit as a result of
this slight damage. I currently have a 1/2 inch diameter copper cable
wound 1.5 times around the secondary core. I just want to make
certain the primary is not shorted out. I have a multimeter I can
use. Help, anyone? Thanks in advance.

Manny

Just apply line voltage, maybe through one of those power strips that
has a circuit breaker. If there are shorted turns, the breaker will
trip or the transformer will smoke.

It is unlikely that the short will be solid enough to cause either
scenario, and the breaker in most outlet strips is 10A with a fairly long
time delay which could allow some dangerous overheating and arcing. There
are probably 1000 turns on the primary, with perhaps #22AWG wire. Each turn
is about 0.12 volts. A solid short between two windings could create a
current of perhaps 100 amps, but at 1000:1 the primary current due to the
short would be only 100 mA. But that is still high enough to indicate a
shorted turn.
Click to expand...

0.12 sounds low. Most refs for microwave transformers cite around 1
volt per turn.

John
 
"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message
news:q13fm5ppaonm24cl341egmeai2q9g9f2p9@4ax.com...
On Mon, 1 Feb 2010 21:00:04 -0500, "Paul E. Schoen"
paul@peschoen.com> wrote:


"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in
message
news:ebrdm5919ioiabrs04pt03jfufrndcpa8u@4ax.com...
On Sun, 31 Jan 2010 23:20:05 -0800 (PST), trg-s338
americansun@gmail.com> wrote:

I'm building a spot welder using a modified microwave oven
transformer. The project calls for removing the secondary coil
windings. In doing so, I accidentally "bumped" the primary coil with
a hammer resulting in 2 or 3 wires in the primary coil becoming
slightly displaced by about 1/8 of an inch. I hope someone can tell
me how to test the primary coil for a short circuit as a result of
this slight damage. I currently have a 1/2 inch diameter copper cable
wound 1.5 times around the secondary core. I just want to make
certain the primary is not shorted out. I have a multimeter I can
use. Help, anyone? Thanks in advance.

Manny

Just apply line voltage, maybe through one of those power strips that
has a circuit breaker. If there are shorted turns, the breaker will
trip or the transformer will smoke.

It is unlikely that the short will be solid enough to cause either
scenario, and the breaker in most outlet strips is 10A with a fairly long
time delay which could allow some dangerous overheating and arcing. There
are probably 1000 turns on the primary, with perhaps #22AWG wire. Each
turn
is about 0.12 volts. A solid short between two windings could create a
current of perhaps 100 amps, but at 1000:1 the primary current due to the
short would be only 100 mA. But that is still high enough to indicate a
shorted turn.


0.12 sounds low. Most refs for microwave transformers cite around 1
volt per turn.
Click to expand...
You are correct. MOTs are driven hard into saturation as compared to high
quality isolation transformers of similar size. So at 120:1 ratio it would
still take something like a 2000 amp short to trip a 10A breaker even with
a fast curve.

I found this page describing a spot welder. It recommends about 4 VRMS
which would probably be at least 4 or 5 turns. I would estimate that you
could get at least 500 amps for the weld if you run the tranny at a 25%
duty cycle, which is about right for a welder.

http://www.teralab.co.uk/Electronics/Spot_Welder/Spot_Welder_Page1.htm

The Wiki says the open circuit voltage is 5-10 volts and the current is
4000-24,000 amps, for 100 mSec or so. The voltage at the weld itself is
only about 1-1.5 volts.

http://en.wikipedia.org/wiki/Spot_welding

Here are other DIY references:
http://hackaday.com/2009/06/23/how-to-build-your-own-spot-welder/
http://www.instructables.com/id/Build-a-Microwave-Transformer-Homemade-Welder/

Many high power industrial spot welders use high frequency (400-4000 Hz),
and rectifiers so that the weld is made with DC. Or they can use capacitive
discharge:
http://www.powerstream.com/spot-welder.htm

Learning a bit as I go...
Paul
 
On Feb 2, 5:16 am, "Paul E. Schoen" <p...@peschoen.com> wrote:
"John Larkin" <jjlar...@highNOTlandTHIStechnologyPART.com> wrote in message

news:q13fm5ppaonm24cl341egmeai2q9g9f2p9@4ax.com...





On Mon, 1 Feb 2010 21:00:04 -0500, "Paul E. Schoen"
p...@peschoen.com> wrote:

"John Larkin" <jjlar...@highNOTlandTHIStechnologyPART.com> wrote in
message
news:ebrdm5919ioiabrs04pt03jfufrndcpa8u@4ax.com...
On Sun, 31 Jan 2010 23:20:05 -0800 (PST), trg-s338
american...@gmail.com> wrote:

I'm building a spot welder using a modified microwave oven
transformer.  The project calls for removing the secondary coil
windings.  In doing so, I accidentally "bumped" the primary coil with
a hammer resulting in 2 or 3 wires in the primary coil becoming
slightly displaced by about 1/8 of an inch.  I hope someone can tell
me how to test the primary coil for a short circuit as a result of
this slight damage.  I currently have a 1/2 inch diameter copper cable
wound 1.5 times around the secondary core.  I just want to make
certain the primary is not shorted out.  I have a multimeter I can
use.  Help, anyone?  Thanks in advance.

Manny

Just apply line voltage, maybe through one of those power strips that
has a circuit breaker. If there are shorted turns, the breaker will
trip or the transformer will smoke.

It is unlikely that the short will be solid enough to cause either
scenario, and the breaker in most outlet strips is 10A with a fairly long
time delay which could allow some dangerous overheating and arcing. There
are probably 1000 turns on the primary, with perhaps #22AWG wire. Each
turn
is about 0.12 volts. A solid short between two windings could create a
current of perhaps 100 amps, but at 1000:1 the primary current due to the
short would be only 100 mA. But that is still high enough to indicate a
shorted turn.

0.12 sounds low. Most refs for microwave transformers cite around 1
volt per turn.

You are correct. MOTs are driven hard into saturation as compared to high
quality isolation transformers of similar size. So at 120:1 ratio it would
still take something like a 2000 amp short to trip a 10A breaker even with
a fast curve.

I found this page describing a spot welder. It recommends about 4 VRMS
which would probably be at least 4 or 5 turns. I would estimate that you
could get at least 500 amps for the weld if you run the tranny at a 25%
duty cycle, which is about right for a welder.

http://www.teralab.co.uk/Electronics/Spot_Welder/Spot_Welder_Page1.htm

The Wiki says the open circuit voltage is 5-10 volts and the current is
4000-24,000 amps, for 100 mSec or so. The voltage at the weld itself is
only about 1-1.5 volts.

http://en.wikipedia.org/wiki/Spot_welding

Here are other DIY references:http://hackaday.com/2009/06/23/how-to-build-your-own-spot-welder/http://www.instructables.com/id/Build-a-Microwave-Transformer-Homemad...

Many high power industrial spot welders use high frequency (400-4000 Hz),
and rectifiers so that the weld is made with DC. Or they can use capacitive
discharge:http://www.powerstream.com/spot-welder.htm

Learning a bit as I go...
Paul- Hide quoted text -

- Show quoted text -
Click to expand...
Hey that's interesting!
Took a welding course couple of years ago. Mainly 'stick' welding,
little bit of wire welding.
No spot welding.
Gathered that voltage before striking arc could be anywhere up to 75
volts and while welding at up to 150 amps etc.somewhere around 25
volts.
So those 4 to 5 volts sounds very low! But I guess we are talking a
lot of amps in a very small contact (spot weld) area?
Maybe the outcome of the welding project using the ex-microwave
transformer could be published here?
TIA.
 
On Feb 2, 10:45 am, terryS <tsanf...@nf.sympatico.ca> wrote:
On Feb 2, 5:16 am, "Paul E. Schoen" <p...@peschoen.com> wrote:





"John Larkin" <jjlar...@highNOTlandTHIStechnologyPART.com> wrote in message

news:q13fm5ppaonm24cl341egmeai2q9g9f2p9@4ax.com...

On Mon, 1 Feb 2010 21:00:04 -0500, "Paul E. Schoen"
p...@peschoen.com> wrote:

"John Larkin" <jjlar...@highNOTlandTHIStechnologyPART.com> wrote in
message
news:ebrdm5919ioiabrs04pt03jfufrndcpa8u@4ax.com...
On Sun, 31 Jan 2010 23:20:05 -0800 (PST), trg-s338
american...@gmail.com> wrote:

I'm building a spot welder using a modified microwave oven
transformer.  The project calls for removing the secondary coil
windings.  In doing so, I accidentally "bumped" the primary coil with
a hammer resulting in 2 or 3 wires in the primary coil becoming
slightly displaced by about 1/8 of an inch.  I hope someone can tell
me how to test the primary coil for a short circuit as a result of
this slight damage.  I currently have a 1/2 inch diameter copper cable
wound 1.5 times around the secondary core.  I just want to make
certain the primary is not shorted out.  I have a multimeter I can
use.  Help, anyone?  Thanks in advance.

Manny

Just apply line voltage, maybe through one of those power strips that
has a circuit breaker. If there are shorted turns, the breaker will
trip or the transformer will smoke.

It is unlikely that the short will be solid enough to cause either
scenario, and the breaker in most outlet strips is 10A with a fairly long
time delay which could allow some dangerous overheating and arcing. There
are probably 1000 turns on the primary, with perhaps #22AWG wire. Each
turn
is about 0.12 volts. A solid short between two windings could create a
current of perhaps 100 amps, but at 1000:1 the primary current due to the
short would be only 100 mA. But that is still high enough to indicate a
shorted turn.

0.12 sounds low. Most refs for microwave transformers cite around 1
volt per turn.

You are correct. MOTs are driven hard into saturation as compared to high
quality isolation transformers of similar size. So at 120:1 ratio it would
still take something like a 2000 amp short to trip a 10A breaker even with
a fast curve.

I found this page describing a spot welder. It recommends about 4 VRMS
which would probably be at least 4 or 5 turns. I would estimate that you
could get at least 500 amps for the weld if you run the tranny at a 25%
duty cycle, which is about right for a welder.

http://www.teralab.co.uk/Electronics/Spot_Welder/Spot_Welder_Page1.htm

The Wiki says the open circuit voltage is 5-10 volts and the current is
4000-24,000 amps, for 100 mSec or so. The voltage at the weld itself is
only about 1-1.5 volts.

http://en.wikipedia.org/wiki/Spot_welding

Here are other DIY references:http://hackaday.com/2009/06/23/how-to-build-your-own-spot-welder/http......

Many high power industrial spot welders use high frequency (400-4000 Hz),
and rectifiers so that the weld is made with DC. Or they can use capacitive
discharge:http://www.powerstream.com/spot-welder.htm

Learning a bit as I go...
Paul- Hide quoted text -

- Show quoted text -

Hey that's interesting!
Took a welding course couple of years ago. Mainly 'stick' welding,
little bit of wire welding.
No spot welding.
Gathered that voltage before striking arc could be anywhere up to 75
volts and while welding at up to 150 amps etc.somewhere around 25
volts.
So those 4 to 5 volts sounds very low! But I guess we are talking a
lot of amps in a very small contact (spot weld) area?
Maybe the outcome of the welding project using the ex-microwave
transformer could be published here?
TIA.- Hide quoted text -

- Show quoted text -
Click to expand...
Thanks for the answers, gentlemen. If the project is successful, I
will post a summary writeup here, otherwise, I'll continue modifying
it. Perhaps add more wind to the secondary.
 
On Feb 3, 4:55 am, trg-s338 <american...@gmail.com> wrote:
On Feb 2, 10:45 am, terryS <tsanf...@nf.sympatico.ca> wrote:



On Feb 2, 5:16 am, "Paul E. Schoen" <p...@peschoen.com> wrote:

"John Larkin" <jjlar...@highNOTlandTHIStechnologyPART.com> wrote in message

news:q13fm5ppaonm24cl341egmeai2q9g9f2p9@4ax.com...

On Mon, 1 Feb 2010 21:00:04 -0500, "Paul E. Schoen"
p...@peschoen.com> wrote:

"John Larkin" <jjlar...@highNOTlandTHIStechnologyPART.com> wrote in
message
news:ebrdm5919ioiabrs04pt03jfufrndcpa8u@4ax.com...
On Sun, 31 Jan 2010 23:20:05 -0800 (PST), trg-s338
american...@gmail.com> wrote:

I'm building a spot welder using a modified microwave oven
transformer.  The project calls for removing the secondary coil
windings.  In doing so, I accidentally "bumped" the primary coil with
a hammer resulting in 2 or 3 wires in the primary coil becoming
slightly displaced by about 1/8 of an inch.  I hope someone can tell
me how to test the primary coil for a short circuit as a result of
this slight damage.  I currently have a 1/2 inch diameter copper cable
wound 1.5 times around the secondary core.  I just want to make
certain the primary is not shorted out.  I have a multimeter I can
use.  Help, anyone?  Thanks in advance.

Manny

Just apply line voltage, maybe through one of those power strips that
has a circuit breaker. If there are shorted turns, the breaker will
trip or the transformer will smoke.

It is unlikely that the short will be solid enough to cause either
scenario, and the breaker in most outlet strips is 10A with a fairly long
time delay which could allow some dangerous overheating and arcing. There
are probably 1000 turns on the primary, with perhaps #22AWG wire. Each
turn
is about 0.12 volts. A solid short between two windings could create a
current of perhaps 100 amps, but at 1000:1 the primary current due to the
short would be only 100 mA. But that is still high enough to indicate a
shorted turn.

0.12 sounds low. Most refs for microwave transformers cite around 1
volt per turn.

You are correct. MOTs are driven hard into saturation as compared to high
quality isolation transformers of similar size. So at 120:1 ratio it would
still take something like a 2000 amp short to trip a 10A breaker even with
a fast curve.

I found this page describing a spot welder. It recommends about 4 VRMS
which would probably be at least 4 or 5 turns. I would estimate that you
could get at least 500 amps for the weld if you run the tranny at a 25%
duty cycle, which is about right for a welder.

http://www.teralab.co.uk/Electronics/Spot_Welder/Spot_Welder_Page1.htm

The Wiki says the open circuit voltage is 5-10 volts and the current is
4000-24,000 amps, for 100 mSec or so. The voltage at the weld itself is
only about 1-1.5 volts.

http://en.wikipedia.org/wiki/Spot_welding

Here are other DIY references:http://hackaday.com/2009/06/23/how-to-build-your-own-spot-welder/http......

Many high power industrial spot welders use high frequency (400-4000 Hz),
and rectifiers so that the weld is made with DC. Or they can use capacitive
discharge:http://www.powerstream.com/spot-welder.htm

Learning a bit as I go...
Paul- Hide quoted text -

- Show quoted text -

Hey that's interesting!
Took a welding course couple of years ago. Mainly 'stick' welding,
little bit of wire welding.
No spot welding.
Gathered that voltage before striking arc could be anywhere up to 75
volts and while welding at up to 150 amps etc.somewhere around 25
volts.
So those 4 to 5 volts sounds very low! But I guess we are talking a
lot of amps in a very small contact (spot weld) area?
Maybe the outcome of the welding project using the ex-microwave
transformer could be published here?
TIA.- Hide quoted text -

- Show quoted text -

Thanks for the answers, gentlemen.  If the project is successful, I
will post a summary writeup here, otherwise, I'll continue modifying
it.  Perhaps add more wind to the secondary.
Click to expand...
In the interest of transparency, I am building this MOT Spot Welder
along the lines of that built by Bob Engelhardt at this link:

http://www.metalworking.com/dropbox/MOTSpotwelderPrintable.pdf
 
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