How to test the primary coil on a microwave oven transformer for a short.

Started by trg-s338 February 1, 2010
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
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.
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.
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.
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=A0am, "Paul E. Schoen" <p...@peschoen.com> wrote:
> "John Larkin" <jjlar...@highNOTlandTHIStechnologyPART.com> wrote in messa=
ge
> > 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. =A0The project calls for removing the secondary coil > >>>>windings. =A0In doing so, I accidentally "bumped" the primary coil wi=
th
> >>>>a hammer resulting in 2 or 3 wires in the primary coil becoming > >>>>slightly displaced by about 1/8 of an inch. =A0I hope someone can tel=
l
> >>>>me how to test the primary coil for a short circuit as a result of > >>>>this slight damage. =A0I currently have a 1/2 inch diameter copper ca=
ble
> >>>>wound 1.5 times around the secondary core. =A0I just want to make > >>>>certain the primary is not shorted out. =A0I have a multimeter I can > >>>>use. =A0Help, anyone? =A0Thanks 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 lo=
ng
> >>time delay which could allow some dangerous overheating and arcing. The=
re
> >>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 t=
he
> >>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 woul=
d
> still take something like a 2000 amp short to trip a 10A breaker even wit=
h
> 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-Tra= nsformer-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 capaciti=
ve
> 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.
On Feb 2, 10:45=A0am, terryS <tsanf...@nf.sympatico.ca> wrote:
> On Feb 2, 5:16=A0am, "Paul E. Schoen" <p...@peschoen.com> wrote: > > > > > > > "John Larkin" <jjlar...@highNOTlandTHIStechnologyPART.com> wrote in mes=
sage
> > >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. =A0The project calls for removing the secondary coil > > >>>>windings. =A0In 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. =A0I hope someone can t=
ell
> > >>>>me how to test the primary coil for a short circuit as a result of > > >>>>this slight damage. =A0I currently have a 1/2 inch diameter copper =
cable
> > >>>>wound 1.5 times around the secondary core. =A0I just want to make > > >>>>certain the primary is not shorted out. =A0I have a multimeter I ca=
n
> > >>>>use. =A0Help, anyone? =A0Thanks in advance. > > > >>>>Manny > > > >>> Just apply line voltage, maybe through one of those power strips th=
at
> > >>> 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. T=
here
> > >>are probably 1000 turns on the primary, with perhaps #22AWG wire. Eac=
h
> > >>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 hi=
gh
> > quality isolation transformers of similar size. So at 120:1 ratio it wo=
uld
> > still take something like a 2000 amp short to trip a 10A breaker even w=
ith
> > 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 yo=
u
> > 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-bui=
ld-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 capaci=
tive
> > 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.
On Feb 3, 4:55=A0am, trg-s338 <american...@gmail.com> wrote:
> On Feb 2, 10:45=A0am, terryS <tsanf...@nf.sympatico.ca> wrote: > > > > > On Feb 2, 5:16=A0am, "Paul E. Schoen" <p...@peschoen.com> wrote: > > > > "John Larkin" <jjlar...@highNOTlandTHIStechnologyPART.com> wrote in m=
essage
> > > >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. =A0The project calls for removing the secondary coil > > > >>>>windings. =A0In doing so, I accidentally "bumped" the primary coi=
l with
> > > >>>>a hammer resulting in 2 or 3 wires in the primary coil becoming > > > >>>>slightly displaced by about 1/8 of an inch. =A0I hope someone can=
tell
> > > >>>>me how to test the primary coil for a short circuit as a result o=
f
> > > >>>>this slight damage. =A0I currently have a 1/2 inch diameter coppe=
r cable
> > > >>>>wound 1.5 times around the secondary core. =A0I just want to make > > > >>>>certain the primary is not shorted out. =A0I have a multimeter I =
can
> > > >>>>use. =A0Help, anyone? =A0Thanks 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 wi=
ll
> > > >>> 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 fairl=
y long
> > > >>time delay which could allow some dangerous overheating and arcing.=
There
> > > >>are probably 1000 turns on the primary, with perhaps #22AWG wire. E=
ach
> > > >>turn > > > >>is about 0.12 volts. A solid short between two windings could creat=
e 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 indica=
te 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 VRM=
S
> > > 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 2=
5%
> > > 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-b=
uild-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 capa=
citive
> > > 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. =A0If the project is successful, I > will post a summary writeup here, otherwise, I'll continue modifying > it. =A0Perhaps add more wind to the secondary.
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