# Converting a Miller CP-200(3 phase) to single phase



## cathead (Nov 5, 2019)

The older 3 phase commercial welders are ending up at scrap yards and on Craig's list and can be converted to operate on single phase. 
The Miller CP-200 is one such unit that has 100 percent duty cycle and is a DC(direct current) 200 amp welder.  The conversion involves
adding several capacitors and a bit of rewiring and modification along with a new single phase power cable.  I presently have a CP-200
in the shop and yesterday was a day for a massive cleanup of the unit to remove as much welding blackness as possible.  I went through
a roll of paper towels and a bottle of 409 and after major disassembly, cleanup and reassembly it is beginning to look fairly respectable.  
I tested all six of the power diodes and reapplied thermal conductivity goo under the diodes on reassembly.  The fan was the biggest nightmare
as it took me an hour to remove it and another hour to clean all the black off the blades.  I ended up using laundry soap and a Scotch Brite pad
to get down to the aluminum.  On the web are step by step conversion instructions.   The power control dial assembly was completely
stuck so obviously this machine has been sitting in some unused corner for a long time.  With some cleaning and a bit of oil, the control dial
works nicely and in good shape.  The infinitely adjustable power control is a great feature.   I expect I will have to add some wheels to the unit
and paint the cabinet Miller blue as it is looking a bit shop worn after 50 years of hard use.  One of the side panel covers is missing so that
will be another project after it is up and running.  


My camera quit so I was not able to take any photos but I did extract a photo off the web so you can see what a CP-200
 machine looks like.  





DC welding will be a new experience to me so looking forward to a bit of learning curve with something new to me.  These old units are
more dependable than the new light weight IGBT welders and still have their place in the shop as being very dependable.  I don't know if anyone else
has any interest in saving old 3 phase welders from doom but certainly worth doing in my opinion.  I will make another post as to how
the project turned out for anyone interested in this sort of activity.


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## DangerZone (Nov 5, 2019)

I have definitely been looking into this a bit, but I can only handle so many projects at a time. Interested to see what you do.


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## jwmay (Nov 5, 2019)

Highly interested as well. How about plasma cutters? Any conversion possibilities there?


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## cathead (Nov 5, 2019)

jwmay said:


> Highly interested as well. How about plasma cutters? Any conversion possibilities there?




Plasma cutters are a whole different animal.  I would have to say that one is entering into the realm of solid state electronics
for them.  I have a great interest in a plasma cutters too.  Generally speaking, they are in the realm of solid state devices
using a  MOSFET device or two as well and or IGBT transistors.  I can see a plasma cutter in my shop but will have to
accept the new technology and go with the flow as they say.  I expect I will purchase one soon but not without a lot of trepidation
on my part.  Presently I am looking seriously at an Everlast 80 plasma machine but am holding off presently on the trigger.  
Sooner or later I will give in and order one to see what it will do.


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## pontiac428 (Nov 5, 2019)

Nice job with the welder.  I have seen a few of those machines going for less than the copper in them is worth.  I still run a Dialarc HF from the late 1960s for my TIG, but I've had dreams of going solid state and upgrading to a Dynasty series...

You are right to be cautious about plasma cutters.  The technology has gotten better, more efficient, and cheaper, meaning that the merits of buying an old battle ax of a machine are diminishing as cheaper, more sophisticated machines come out.  I have a mid-1990's Thermal Dynamics Stak Pak with 70A and 1" gouge capacity.  It's a big commercial machine that had a $14k price tag at one time.  I also got the sales rep's collection of consumables and spares when TD discontinued that model.  I know it is a possibility that some day I'll hear a board go pop while I'm using the machine, and that will be all she wrote- even Thermal Dynamics doesn't have spares for it.  Everybody has moved on from those old machines.  Now, TD makes the same cutting capacity in a machine that can be loaded in a car's trunk by one person at a third of the price.  Like you said, a plasma cutter is a solid state appliance, it would be hard to justify such a mod.


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## cathead (Nov 6, 2019)

Just a little follow up...

The rewiring is complete  with the exception of the two capacitors  that are on order.  Presently The cabinet is getting a new
cote of blue paint so expect it won't be long before I can report how it welds.


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## MikeInOr (Nov 6, 2019)

Could you post a link to the wiring conversion instructions you are using?  Are the capacitors just to smooth out the DC current after switching to single phase?


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## cathead (Nov 6, 2019)

MikeInOr said:


> Could you post a link to the wiring conversion instructions you are using?  Are the capacitors just to smooth out the DC current after switching to single phase?



The capacitors are used on the primary side to change the phase of the middle transformer so that all three sections of the transformer are used.   
On the secondary side, I think it is more important to have an inductor in line to smooth out the current flow for better welding.  A capacitor
would have to be really huge in the secondary circuit to do much good I think.  You can find some conversion information on You Tube and also
on Practical Machinist.  I ended up not using any of the conversion instructions for my welder because it had different voltage requirements
than the PM instructions.  Are you going to modify a Miller CP-200 or something else?  If you are going to modify a CP-200, I can help you
along with it since I have done it.  You can cook book your way through the modification only if you have the same exact unit in the conversion
instructions.  If it isn't the same, you will have to do what I did and tear off all the wires and rewire it completely.   If you need some help, I would
be happy to work you through the conversion.


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## MikeInOr (Nov 6, 2019)

cathead said:


> The capacitors are used on the primary side to change the phase of the middle transformer so that all three sections of the transformer are used.
> On the secondary side, I think it is more important to have an inductor in line to smooth out the current flow for better welding.  A capacitor
> would have to be really huge in the secondary circuit to do much good I think.  You can find some conversion information on You Tube and also
> on Practical Machinist.  I ended up not using any of the conversion instructions for my welder because it had different voltage requirements
> ...



I am just interested in what it takes to do the conversion.  If I know what all it entails I might pick up a big old transformer based welder someday if the conversion to 1 ph isn't too involved.  So I was asking out curiosity.


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## cathead (Nov 6, 2019)

MikeInOr said:


> I am just interested in what it takes to do the conversion.  If I know what all it entails I might pick up a big old transformer based welder someday if the conversion to 1 ph isn't too involved.  So I was asking out curiosity.



Before you buy one, it would be wise to see if the model you are interested in has been converted over before by someone else.  
On my CP-200, it involved some rewiring and adding two capacitors and resistors as well as installing a single phase power plug.  
I wouldn't call it difficult but it does require some experience in electrical wiring and knowledge of the basics of three phase power.


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## cathead (Nov 8, 2019)

Another followup while I wait for the capacitors...................


I started to assemble the cabinet and the screw holes(1/4x20 I think they were) were mostly stripped out.
Some 1/4 inch thick aluminum stock worked really well for making some nut plates and affixing each one
to the cabinet with two 1/8 inch AN426 rivets.  The cabinet is starting to look pretty nice now so happy
about that.  I need to make a right side cabinet door with louvers or at least a screened ventilation inlet
so that will be the next project.  Before I do that, I will make a trip to the scrap yard and see if possibly
that side cover might be there just waiting for me to haul home......


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## Barncat (Nov 8, 2019)

General question about buying old industrial welders. A lot of the auction ones I see say 70-100 amps input. If I only have a 50 amp circuit, could they still be used, just not at max power, or will they not power on?


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## cathead (Nov 8, 2019)

Barncat said:


> General question about buying old industrial welders. A lot of the auction ones I see say 70-100 amps input. If I only have a 50 amp circuit, could they still be used, just not at max power, or will they not power on?



Sure, it would work, however the breaker would trip if you got into some really heavy welding.


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## pontiac428 (Nov 8, 2019)

Could they mean 70-100 amps total, so would it be 25 per phase at 3 phases, or 35-50 amps per phase on single 230v?  Or are you looking at welding output current, which is at low voltage and (therefore) high amperage?  I scratch my head over stuff like that more often than I feel I should...


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## Barncat (Nov 8, 2019)

The recent one at auction said the input was 3 phase, 230v, 70 amps. Does that mean 70 amps each phase? I have no idea


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## cathead (Nov 9, 2019)

Barncat said:


> The recent one at auction said the input was 3 phase, 230v, 70 amps. Does that mean 70 amps each phase? I have no idea



It would be the total amps on the primary I would think.  Otherwise you would have to use at least 1/2 inch rebar for welding rod!


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## Tim9 (Nov 9, 2019)

Not sure on specifics but remember that most arc welding is @ 40-60 amps....

  so using ohms law...you’re putting in 70 amps at 240v and converting it to “X” amps at say 60 volts. 
Of course there’s some heat loss too.
200 amps less the heat loss would be my rough guess
60 x 240 = 12000
12000. / 60 = 200


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## cathead (Nov 11, 2019)

Another update for anyone interested...

I'm beginning to think that the 3 phase transformer has a short in it somewhere so the plot thickens.  I think I will have
to remove all the secondary connections and power each part of the transformer separately to try to isolate the problem.  
Likely I will power each section with 120 volts to try and find out where the difficulty might be.  This may not be an 
easy thing to do because all three transformers are on the same core to confuse the issue.  If I can prove that the
transformer is defective, I will save the good parts and scrap the remainder.   

If it ends up parted out, at least I will have a bunch of good parts and learned a lot about the intricacies of three phase power.


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## cathead (Nov 12, 2019)

Egads!  I think I have found the problem!   I applied 120 volts from the center tap primary circuit of the transformer to each outer winding  of the
coil that was seemingly drawing a lot of current.  You can tell by the louder hum on that coil.
One side was fine and the other side dumped a 20 ampere breaker instantly.  I suppose I could disassemble the transformer
and have a go at rewinding the primary or repairing the primary but I will have to give this a bit of cogitating to decide what to do for the next
course of action.  I will have to scrap it if I don't fix it so might as well disassemble the transformer and see if I can find the problem
before junking it.  Some transformer cores separate easily and not so for others so will have to see what happens.  The transformer looks like
it weighs 100 pounds or so so expect it will be fun to just get out of the box, not to mention disconnecting all the heavy wiring.  So anyhow
that where this project sits for the moment.  If anyone has any great ideas, I would love to hear them.


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## cathead (Dec 26, 2019)

Well, the CP200 Miller welder has worn out it's welcome in the shop so am going to part it out I think.  There are 
a lot of good parts to save and the base I think I can use to make a stand for my new Everast 210EX TIG machine.  
It's kind of a shame but it's time has come.  I will save the meters, heavy 200 amp wires,  the reactor(inductor), 
and all the fittings and otherwise good stuff.  I may be able to use the transformer for a stout battery charger possibly.
I can't part out the transformer for the copper and sell it to my scrapper friend because he gave me the machine free. 
If I do scrap it for copper I will have to present my friend with a bunch of copper transformer wire to recycle, maybe a
new year's present for him.


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## FOMOGO (Dec 26, 2019)

At least you gave it a go, and learned a few things. Cheers, Mike


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## cathead (Dec 27, 2019)

FOMOGO said:


> At least you gave it a go, and learned a few things. Cheers, Mike



Thanks Mike, I like your thinking.


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