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  #1  
Old Thu 04 December 2008, 02:09
WTI
Just call me: James
 
Detroit (Michigan)
United States of America
First 2 posts copied from elsewhere:

You can get a free, heavy duty transformer for your power supply out of any large microwave oven. These are also free on craigslist every day.

Most broken microwaves fail because the door switches wear out, so the MW blows the safety fuse instantly.

If you take the transformer out and cut off the high voltage windings with a hacksaw, you can quickly wind a few wraps of wire until you get the voltage you need. It is usually about 1v per turn of wire. Don't forget that you want a secondary winding for about 9-12V to power your BOB.

Just Google "microwave transformer power supply" for many easy to follow directions.


EDIT: Don't touch the high voltage capacitor if you take a MW apart. It will still have juice in it unless it has a bleeder resistor.
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  #2  
Old Thu 04 December 2008, 02:18
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
James, that hack makes me nervous! If anyone wants to pursue this for a MM, I'll give you a thread where you can field the lawyers all by yourself!
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  #3  
Old Fri 05 December 2008, 00:09
WTI
Just call me: James
 
Detroit (Michigan)
United States of America
How to rewind a Microwave Transformer

Like always, I must state that I'm telling you NOT to do what I am about to show you. Just buy a complete power supply (or even safer, a complete CNC router). This stuff is dangerous!

On my way home tonight, I see at the curb a Microwave Oven! Not a day after having just discussed that you can rewind the beefy transformers inside into whatever voltage you want. I stopped and picked it up.

Inside was a nice sized, 7 pound transformer. I measured the secondary voltage at 2204 volts. A little too high for what I had in mind! It also had another 3v winding (the thick white wire in the picture).

(It also had a big capacitor, but it was only 1mfd 2500V. Too small of a value to reuse, but zappy enough to treat with respect. )

In this picture you can see we have two main windings, the secondary (fine wire) and the Primary (the thick wire).

I wanted to remove the Secondary and the little 3v winding WITHOUT nicking or breaking the Primary.
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File Type: jpg MW-transformer.jpg (26.3 KB, 983 views)
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  #4  
Old Fri 05 December 2008, 00:14
WTI
Just call me: James
 
Detroit (Michigan)
United States of America
The copper windings are super soft, so a regular wood chisel cuts right through them in about 10 hammer blows. Yeah, I know that's not what quality tools are made for, but I know how to sharpen them again....

I put a piece of 1/4" plywood between the windings in case the chisel got away from me, remember I wanted to protect the Primary windings.
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File Type: jpg copper-chop.jpg (20.1 KB, 988 views)
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  #5  
Old Fri 05 December 2008, 00:19
WTI
Just call me: James
 
Detroit (Michigan)
United States of America
Next I took a piece of oak and knocked out what was left of the Secondary windings. I also knocked out two little laminated sections called Shunts. These limit the current in the transformer. If you leave them in, the transformer will run warmer. I did not need them for a CNC power supply, so out they went.
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  #6  
Old Fri 05 December 2008, 00:24
WTI
Just call me: James
 
Detroit (Michigan)
United States of America
Here you see the cleaned out core. Secondary windings knocked out, 3v winding sniped out, and Shunts tapped out. I left the paper/mica insulator wrapped around the core for additional insulation. Had it come out with the windings, I would have wrapped the core with insulation tape.
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  #7  
Old Fri 05 December 2008, 00:47
WTI
Just call me: James
 
Detroit (Michigan)
United States of America
Nothing really scientific here, I just took some 12 gauge wire (It's what I had, 14g would be fine I'm sure, and much easier to wrap!) and wrapped it around the core. Solid wire is the best, as multi stranded would have more loss. It came out to 17v. It was just what was left on the spool, but If I was going to do it right, I'd figure about 1v per winding. I'll leave a bunch of extra wire so I can wrap or unwrap until I get the voltage I want.

It needs a fuse for sure, as to not burn anything up (like my shop), and I would wind another coil around the core for 12v to power a BOB.

A transformer is rated for about 100 watts per pound of core, so this thing should be good for about 700 watts.

It took 35 minutes a was a fun "make a mess" benchtop project.

Take a look at my post titled "James Webster's 300VA, 71V, $60 power supply" if you want to see how to turn the AC output of this transformer into DC. Making my own transformer would save me $45 bucks out of that $60. A toroidal transformer is more efficient, so keep that in mind too.



Remember I told you NOT to do this. I can't be responsible for your crazy actions.


Last edited by Gerald D; Fri 05 December 2008 at 01:37.. Reason: I spell like a girl
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  #8  
Old Fri 05 December 2008, 00:59
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
James, this exactly the type of thing I would do for myself, but be too nervous to recommend to anyone else.

While chiselling/cutting the secondary coil, I think one should be careful to avoid damaging the primary coil. As you said, you don't want to nick that coil with the sharp end of the chisel, but I would avoid directing the blows in the direction of the primary coil. If the primary coil takes too much strain, it could damage the varnish layers inside the coil. I have seen motor re-winders plying their trade, and they do hammer the varnished coils into shape, but we aren't those professionals.

Also, I would prefer the chisel to a hacksaw or grinder. I wouldn't want filings or copper chips to get into the primary coil and make a little hard spot between the varnish layers. The coils do vibrate slightly when powered up (that's what maks the hummm) and a little hard lump could wear through the varnish.

So, be gentle and clean on that primary coil.

Thanks for the thread, James
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  #9  
Old Fri 05 December 2008, 03:36
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Quote:
Originally Posted by WTI View Post
Nothing really scientific here, I just took some 12 gauge wire (It's what I had, 14g would be fine I'm sure, and much easier to wrap!) and wrapped it around the core. Solid wire is the best, as multi stranded would have more loss. It came out to 17v. It was just what was left on the spool, but If I was going to do it right, I'd figure about 1v per winding. I'll leave a bunch of extra wire so I can wrap or unwrap until I get the voltage I want.
I would wrap 10 turns, measure the voltage, divide by ten to get the "Volts/turn", then divide that number into the desired Volts to get the number of turns desired.

If the selected wire is too thin, its resistance makes it cook inside that coil and melt the insulation in there. Magnet wire might look like it has only varnish on it, but it is rated for the high temps inside coils.

Quote:
Originally Posted by WTI View Post
. . . . . I would wind another coil around the core for 12v to power a BOB.
The PMDX-122 only wants 9V AC.


I think you are being nice and conservative when you judge 700VA for that transformer. That thing has probably been powering a 2000VA (plus) microwave
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  #10  
Old Fri 05 December 2008, 03:41
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Build your own welder
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  #11  
Old Fri 05 December 2008, 08:11
domino11
Just call me: Heath
 
Cornwall, Ontario
Canada
Gerald,
In North America, dont forget, Microwaves are limited to about 1200 watts. (115V Line). Most are in the 700 to 1000 watt range. Sometimes I think it would be nicer if we went to 230V instead. Would save a lot on copper.
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  #12  
Old Fri 05 December 2008, 08:14
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
. . . . and cook faster too!
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  #13  
Old Fri 05 December 2008, 08:15
domino11
Just call me: Heath
 
Cornwall, Ontario
Canada
Or if it was my wife, burn faster!
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  #14  
Old Fri 05 December 2008, 08:25
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Have checked now . . . . our microwave ovens are also less than 1200W on the magnetron side. Some of those ovens also have grill/browning elements and that's where they get the >2000Watt figures they advertise.
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  #15  
Old Mon 16 February 2009, 16:53
Mr.Chips
Just call me: Mr.Chips
 
Tucson Arizona
United States of America
Quote:
Originally Posted by Gerald D View Post
I would wrap 10 turns, measure the voltage, divide by ten to get the "Volts/turn", then divide that number into the desired Volts to get the number of turns desired.

If the selected wire is too thin, its resistance makes it cook inside that coil and melt the insulation in there. Magnet wire might look like it has only varnish on it, but it is rated for the high temps inside coils.
I am taking a shot am making a 40V transformer out of a Microwave transformer. It looks just like WTI's.

I hackedsawed the secondary off and am ready for the next step, but I want to understand the process prior to starting.

I will use solid 14 ga wire and wrap 10 turns per your post. Now that will not yield the desired voltage, so after I determine the number of turns and if it comes out to be more than 10 turns, can I splice more wire on to the 10 turns, or start over with a continious length of wire?

What size Cap and rectifier should I use for 40V

Thanks

Last edited by Mr.Chips; Mon 16 February 2009 at 16:54.. Reason: text change
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  #16  
Old Tue 17 February 2009, 04:14
jbmclain
Just call me: Joe #42
 
Birmingham, AL
United States of America
I ran across this web site one night and thought it very interesting. http://unique3phase.com/
It talks about building your own transformer and even a welder.

Its right in the line of what we are working with, I had to order the DVDs.

Joe
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  #17  
Old Tue 17 February 2009, 10:33
isladelobos
Just call me: Ros
 
Canary Islands
Spain
Send a message via MSN to isladelobos Send a message via Yahoo to isladelobos
Thanks !!!
exactly what I wanted

this is a good thing for my garage
http://www.convertidorestrifasicos.c...licaciones.htm
one phase to tree phase converter... for the welder and the compressor...
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  #18  
Old Tue 17 February 2009, 12:14
jhiggins7
Just call me: John #26
 
Hebron, Ohio
United States of America
Ros,

If you're interested in building your own phase converter, here is a site with instructions for a DIY Single-Phase to 3-Phase "rotary" Converter.

Several years ago, I built a Phase Converter following these plans. It worked very well. I used a 7.5 HP 3-Phase Motor as the "idler" and I ran a 5 HP 3-Phase Motor on my planer. As Jim says, once you have the idler running, you can start as many 3-Phase motors as your circuits can support. The downside is that you must start the Phase Converter before operating the tool motor.

I purchased both of the used 3-Phase Motors for $75. Used large HP, 3-Phase motors are generally cheaper than equivalent HP, Single Phase motors. I purchased large capacitors and a contactor, also used. I also bought a used electrical switch box with a knife switch. My total cost was less than $125.

Regards,
John
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  #19  
Old Mon 25 May 2009, 13:03
lumberjack_jeff
Just call me: Jeff #31
 
Montesano, WA
United States of America
I gutted a large over-the-range microwave yesterday. It contained the following transformer


It weighs 11 pounds. The interesting thing about it is that the primary winding has three terminals. It appears that the center tap is intended to switch between normal and a lower voltage mode to the magnetron.

Luckily, inside the box was also an electrical diagram. I don't have a scanner, but I did take a photo of the relevant part.



James, Is this transformer suitable for rewinding? Wired unipolar, my motors are rated for 3.4mH @ 4.2a thus I need a 59v dc power supply, which in turn requires a 42v ac transformer capable of about 700va.

I think I'll need about 42 turns of 14 ga wire.

The microwave also contained a wealth of other stuff; fans, blowers, relays, switches, motors, and connectors... but that's probably another topic.
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  #20  
Old Mon 25 May 2009, 13:50
domino11
Just call me: Heath
 
Cornwall, Ontario
Canada
Jeff,
Do not assume that your transformer will give you 1V per turn as James' transformer did for him. It will depend on the core and the primary winding for each individual transformer. It would be best to put 10 test turns on your transformer and measure the voltage you get. Then figure out your volts/turn for your transformer and wind your secondary accordingly. Also a good idea is to try and find some magnet wire used for transformers to wind your secondary. That way you wont have insulation melting on you if your transformer gets warm. If you cannot get the magnet wire, use a wire with a good high temp insulation. Teflon would be a good suggestion.
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  #21  
Old Tue 26 May 2009, 06:12
lumberjack_jeff
Just call me: Jeff #31
 
Montesano, WA
United States of America
I rewound the transformer. Fun!

Electricity is interesting. If you take a piece of wire, and plug both ends into the wall, it makes great sparks but if you wrap it around iron transformer laminations first, it does not.

I knew that this is how transformers worked, but I'd never had the opportunity to test the theory by plugging those wires into the wall.

It works fine and yields 42.5 volts ac using wire with 90c insulation. No sparks.

Interesting thing though, I wrapped 10 turns and got less than 9v, but at 20 turns I had 24v. I think it worked out pretty close to 1v/turn, but the relationship was nonlinear.

So here's my question; my motors have 3.5mH inductance, which Mariss' formula calculates to a 59.87vdc maximum power supply. A DC power supply of that size requires a 42.33vac source. At first glance, my transformer has about a quarter of a winding too much.

a) my gut feel is that the calculation is a rule of thumb with some significant latitude.
b) the voltage of my transformer will drop as soon as any load is placed on it.

I think my transformer will be fine at that voltage, but I need to run it past the smart people first. Thanks in advance.

Last edited by lumberjack_jeff; Tue 26 May 2009 at 06:21..
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  #22  
Old Tue 26 May 2009, 07:15
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
If I remember correctly (no "smarts" here), the formula Mariss published will cause a temperature rise of up to 85-degrees C. I normally use 75% to 85% of the computed voltage and still get excellent results. Others use the full voltage without any problems. Depending on how hard you drive the motors, the motor's temperature may go higher or stay lower than on other CNC machines.

Having 60VDC instead of 59VDC will not cause any major differences (but I woud be satisfied with 55VDC if I were winding a transformer).
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  #23  
Old Mon 29 March 2010, 19:15
cordell
Just call me: cordell
 
johnson city,tn
United States of America
Hello, I have this and been messing around with it, looks like I will be able to get the voltage I need but how does one tell if it will have enough current /amps that I will need, would like to know if this may be used before wrapping with magnet wire and adding caps. here is spec sheet http://www.1000bulbs.com/images/PDF/...-specsheet.pdf I am looking to get 35vdc , 20-28amps, all comments welcome.
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  #24  
Old Tue 30 March 2010, 11:34
domino11
Just call me: Heath
 
Cornwall, Ontario
Canada
How many motors are you running to need 28Amps?
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  #25  
Old Tue 30 March 2010, 15:12
bradm
Just call me: Brad #10
 
Somerville(MA)
United States of America
I think he's building that hypothetical twin gantry, twin cars per gantry, twin cutterhead per car MM with the 16 stepper motors and 8 spindles.

The original use of that transformer as a ballast was at 1000W. 1000W / 35v = 28A, is that where you got that number? Seems like it should work without too much heating.

You'll want to make sure that whatever you wrap the secondary with can handle that amperage; likely pushing it with #12, pretty conservative with #10.

However, since most MMs run on power supplies down in the 300 to 400 watt range, be aware that you're building much more than you need. I'd fuse it down to the amperage you actually use, since the excess would only ever get used during the kind of event you wouldn't want, like a short.
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  #26  
Old Tue 30 March 2010, 16:36
domino11
Just call me: Heath
 
Cornwall, Ontario
Canada
The inrush current will probably blow a low rated fuse when the transformer is powered up. It would be better to size the transformer to the intended load.
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  #27  
Old Tue 30 March 2010, 17:09
cordell
Just call me: cordell
 
johnson city,tn
United States of America
here is the specs on my motors, 1.82v, 7.0amps, 1.2mh, this is where the 28 amps comes from, 20 would possibly work, what do you guys think?
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File Type: pdf 90HS65DE053D.pdf (149.8 KB, 21 views)
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  #28  
Old Tue 30 March 2010, 17:40
bradm
Just call me: Brad #10
 
Somerville(MA)
United States of America
Cordell, that would be 7 amps if wired parallel, and with all motors pulling full amperage all the time. Once you know the drivers you are using, you'll be able to sort out how much you'll really use ( and how you want to wire your motors ).

Regardless, you'll get more than enough out of that transformer to drive those motors. I bet you end up closer to 12 than 28.

For clarity on the fusing: 28 Amps is enough power to melt a driver and it's case and slag a motor coil.

I think Heath is right about the inrush. I think I'd be inclined to measure the incoming amperage on the transformer, and fuse the line side 10% above that with a slo-blow. I'd then be further inclined to fuse each feed to the drivers with a fast fuse right at the driver's specifications.

My concern is that as you approach 30 amps, you are getting into a range that is qualitatively different - it's enough to weld (poorly) with; it's more that the most common gauges of wire carry easily; when things go wrong, there's an impressive amount of heat, light, and smoke. That's why I'd fuse the individual output feeds.

One of my hats is as a professional lighting designer <announcer> "... featuring over a half million watts of light and sound ..." </announcer>, and over my 20+ years, I've seen ugly things happen. Circuits and devices under 10A (at any voltage) rarely create much excitement. Expense, but not excitement. Circuits and devices over 20A can cause the wrong kind of visit from the fire department faster that you'd imagine.
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  #29  
Old Tue 30 March 2010, 18:11
cordell
Just call me: cordell
 
johnson city,tn
United States of America
I plan on wiring parallel using geckos 203v drives, what might you suggest for this configuration, thanks for your input.
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