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Miller CP-250TS converted to single-phase

DaveKamp

Titanium
Joined
Oct 3, 2004
Location
LeClaire, Ia
Hi All-

The saga continues... now entering my evil laboratory, is a 1969 Miller CP-250TS welding supply, and following the same process as the CP200 and SRH-333, will become a single-phase 240v machine.

Follow along as I post pics, math, and wiring solution for this fine piece'a iron!
 
Electrical details

Okay, so the CP250TS's specs indicate:

33A at 208v, or 30A at 230v. We can estimate roughly 29A at 240v... and that'll be close enough. Total load is around 11.4kw.

Looking at the jumper and wiring diagram for the main transformer primary, it's obvious we'll be using the 230v wiring taps. The transformer wiring scheme at that voltage is delta, which (as noted from the SRH-333 project) works out great.

Here's the capacitance formula.

-------------------------------------------------------
I = 2 * π * F * C * V

where:
Current (I) is in Amperes,
Pi (π) is 3.14,
Frequency (F) is 60,
Capacitance (C) in Farads (NOT microfarads)
Potential (V) is in Volts.
---------------------------------------------------------

Solve for C, and here's what we got on the CP-200:

C = (18.6 / 3) / 2 * π * 60 * 240
C = 6.2 / 90477.87 F
C = 68.53 µF

Now, The CP250's input is 11kw... You'd think that if I were to take the time to do intense math, we'd find that the capacitance value required to yield similar phase-shift, would be slightly greater than the CP200's CALCULATED 68.53 UF/side. In reality, it worked wonderfully at 60uf, BUT...

The CP-200's transformer was originally wired in Y, so the individual coil voltages were substantially different (277 across each, when wired for 480v supply). Can't use the CP-200's value as the 'seed' here, 'cause the applied coil voltage is totally different.

The SRH-333, however, was delta, practically identical to the 250TS, we'll just try whatever Surplus Center has that'll mechanically fit well and be in a similar range.I used 200uf caps for the SRH, so betting that 140-160uf @ 300+v ought'a do it. Looks like the big square 120's are all gone, so I'll hafta either use a group of other form factor/size units, or go a-hunting for more options. Of course, selecting a different package means knowing what I have for available space inside the unit... since I haven't opened it up yet, I don't know, so....Stay tuned!
 
nah
i say you mount the caps on the hood with duck tape and big curely-q orange and red wires. couple of those blue led indicator lights on a flasher circuit and a door buzzer bolted to the back that goes off when ever you pick up the gun from hook
:D
 
Pix BEFORE

Here's what it looks like BEFORE:
 

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And here it is 'cleaned up'...

meaning, I removed all the irrelevant cross-connects, jumpers, etc., so we can clearly see that it's wired in delta... in 230v mode... that means, 230v across each coil winding. I indicated the phases with A, B, and C to clarify the delta connections.

I'll run it with 240v across each winding... that's under a 10% change, so it'll be fine.
 

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Schem AFTER

Here's the schematic AFTER conversion.

Shows capacitors shifting center coil's phase, and the third coil is reversed 180-degrees.
Assume bleeder resistors across the capacitors, and note that I've nixed all irrelevant jumpers, as well as the control-power transformer section on the left.

The main control contactor (W) MAY NOT be able to carry full single-phase current through just two contacts- I won't know 'till I have it open, and maybe not even then... but if it welds itself shut, I'll know when it does!
 

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yup
one of the few times blue rustoleum is appropriate
those round decals are available on e-bay for about 8 bucks
i can't wait till we ( ;) ) try this on a miller deltaweld
there are a bunch of them out there
 
Yeah...

it certainly ain't pretty... I think one of the 'other' company shops has a soda-blasting setup... or some brightener in the pressure-washer. That control handle is a piece'a conduit, they notched it a little in one spot, and used a piece of 1/4-20 allthread to hold it in place... super-cheezy. I'll find a piece'a aluminum roundstock that fits in the lathe, and turn it into something more appropriate. Might even set it up with a pilot light and thermostatically-controlled fan, so I don't hafta listen to it howl when I'm only workin' it 6% of the time.
 
A few interior pix

I had a minute to pop the cover off for a look inside. Plenty of room, but plenty of dirt. Looks like this one will get hosed out first, and once cleaned, I'll lube the control leadscrews and the brush-holders.

The plenty-of-room note is fortunate- Looks like the big square-can 120uf/450v motor run cap supply has sold out, so I'll be using some other combination, probably some 60's in parallel, to give phase shift to the center coil.

This one is set up so that the main power switch doesn't carry main power- that's a good thing- makes the concern of overload easier to contend with.

Stay tuned!
 

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Just got back...

Just got back from a grueling road assignment, had a minute to get online and order a quartet of combination 55/4 uf 460v capacitors (and a pressure relief valve for a backhoe project).

Over the next six-eight days, I'll be cleaning out this dirty little piggie, wiring it up for single, and powering it up... also hafta mount a wire-feeder, find appropriate wiring connections, a gun, and adapter kit for it... Stay tuned!
 
Got Capacitors!

55uf and 4uf @ 370v. Got 4 of 'em- two for each leg.

That means I should be able to get 110uF, 114uF, and 118uF- should fall right about middle of the adjustment range.

Next- to make an appropriate mounting setup for 'em....
 

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Got it cleaned out...

This is what it looks like after a gentle garden-hosing and a visit from a scrub-brush:
 

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And a few more

And a few more photos. I placed the capacitors in there to illustrate about-how-much-space was actually necessary for this reconfiguration.
 

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Capacitor bracket

Here's the capacitor bracket in-progress. The piece is a scrap of UHMW... about 1/2" thick.

I used a hole saw to open up four holes just big enough for the caps to drop in flush, but not enough for the cap's crimp-lip to clear through, then used some small sheet metal screws with washers.

Gotta cut it down a bit, and fasten it in 'the right spot'... and start the wiring process.
 

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Changed my mind

Okay, I changed my mind... I like THIS arrangement better...
 

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And the last few...

And the last few pix...
 

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