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rewinding MOT not working out.....

 
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renegade1957



Joined: 03 Feb 2008
Posts: 5

PostPosted: Sun Feb 03, 2008 3:08 pm    Post subject: rewinding MOT not working out..... Reply with quote

hello,
i'm wondering why my voltages are not up to par on a welder project i am building. i removed the secondary winding from a working MOT and as per the instuctions, wound about 5 turns of 12 ga. wire in the secondary place as a temporary winding just to check volts per turn of wire. should be about 3 or 4 volts ac but i get .1 volts. increased to about 10 turns and got .2 volts. this is not right. any ideas? pri. is 110 volts, 60 c. there is no buzz or hum like i might expect. when the power is turned off to the transformer, i get a voltage spike between 1.5 and 10 volts on the secondary temporary winding......thanks for any input!!
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torbjorn



Joined: 07 Jun 2007
Posts: 370
Location: Sweden

PostPosted: Mon Feb 04, 2008 10:15 am    Post subject: Reply with quote

What kind of multimeter do you use to measure the secondary voltage? Remember that many cheap digital multimeters, in particular those that only have 200 V and 1000 V AC ranges, do not give correct readings below about 20-30 V in the 200 V range.

Also, make sure that you have continuity through the primary winding. Many transformers have an integral thermal fuse that might get open circuit by itself when messing around with the transformer.
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PostPosted: Mon Feb 04, 2008 12:54 pm    Post subject: Reply with quote

hello torbjorn,
yes i am using a cheap (sears craftsman)digital multimeter with a 200/600vac setting. never knew that..... i do have a sweep needle type analog meter i will try and see what happens, thanks for the reply, i will post back later..........renegade.
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renegade1957



Joined: 03 Feb 2008
Posts: 5

PostPosted: Mon Feb 04, 2008 6:56 pm    Post subject: rewinding MOT not working out..... Reply with quote

hi again,

well it is the same with my analog meter, about .5 volts with 10 turns. the resistance on the primary reads about 110 ohms. looks to be several hundred turns of very fine copper wire, not sure of the gauge/size of the wire. i am reasonably certain there is no thermal fuse or relay in the primary, as i can see no place where it could hide...........hmmm
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vtech



Joined: 08 May 2006
Posts: 1264
Location: USA

PostPosted: Mon Feb 04, 2008 9:19 pm    Post subject: Reply with quote

Is your project something like this?
http://www.5bears.com/welder.htm
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renegade1957



Joined: 03 Feb 2008
Posts: 5

PostPosted: Tue Feb 05, 2008 12:24 am    Post subject: rewinding MOT not working out..... Reply with quote

hello,
no, it is more like this with the side by side windings on the MOT

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



Joined: 08 May 2006
Posts: 1264
Location: USA

PostPosted: Tue Feb 05, 2008 6:24 am    Post subject: Reply with quote

hmmm...It is basically the same concept. You are using insulated wire right?

As far as the actuall reading goes, You will not get the same results(he is using a 240VAC txformer(UK ) & you mentioned 110VAC txformer?

In txformer design, the output is directly related to a ratio of number of turns between primary/secondary (240vac txformer has a lot more turns than 110Vac). So you may have to get a different txformer.

some info
http://www.tpub.com/neets/book2/5i.htm
http://en.wikipedia.org/wiki/Transformer

(Reason for the spike is because of an EMF which is normally generated as the magnetic field collapses)
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renegade1957



Joined: 03 Feb 2008
Posts: 5

PostPosted: Tue Feb 05, 2008 2:48 pm    Post subject: rewinding MOT not working out..... Reply with quote

well boys and girls,
i am thinking now that i messed up and took out the primary and am doing this backward, trying to power up the secondary winding Confused . the one winding i have left has one end of the winding for output and the other end is grounded to the frame. the one i took out had two tabs one on each end of the heavier winding which was a much larger wire with fewer turns and was not grounded.........does this sound like i blew it?
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vtech



Joined: 08 May 2006
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Location: USA

PostPosted: Tue Feb 05, 2008 5:44 pm    Post subject: Re: rewinding MOT not working out..... Reply with quote

renegade1957 wrote:
well boys and girls,
i am thinking now that i messed up and took out the primary and am doing this backward, trying to power up the secondary winding Confused . the one winding i have left has one end of the winding for output and the other end is grounded to the frame. the one i took out had two tabs one on each end of the heavier winding which was a much larger wire with fewer turns and was not grounded.........does this sound like i blew it?


Sounds like you are not familiar with how a transformer constructed to start with. ---Do NOT go any further as you are running a chance of getting a shock or blow something up Shocked

No-- you could not have taken out the primary without destroying it. Plus, there is no way to power the secondary without blowing out a fuse or circuit braker. Secondary IS the outside wire (white wire in the picture)
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renegade1957



Joined: 03 Feb 2008
Posts: 5

PostPosted: Wed Feb 06, 2008 11:29 pm    Post subject: rewinding MOT not working out..... Reply with quote

thanks vtech,

i am careful when i do these things. self preservation rules my actions. i was using the side by side type as shown in the link i provided above, not the inner/outer like you keep refering to. others have used 110v mots to build these welders as voltage is not that important. anyway, it is possible to power up the secondary and all i did was make a step down transformer by doing what i did, just that it stepped down way too much. i will look for another transformer and do it right next time. Very Happy .......thanks again for the input!
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