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Rotary converter for a welder

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#1
Hi folks

I need to build a rotary phase converter to power a three phase welder .
I think I have a decent understanding of how to do a basic build but I do have some questions.
I already have a 5hp converter with a phase Kraft panel powering my shop so I'd like to keep this one very simple ( and cheap ) .

So , I have a 15hp leeson 1750 rpm motor I am going to use for the idler and a 1hp single phase 1750 to use for a pony motor to start it .
I have a bunch of 46uf 530v caps and a 60 amp knife switch .

Being as for right now this is going to be single purpose ( welder ) and not daily use I don't plan on using any contractors or anything fancy , just simply get the idler spun up to speed with the pony and use the knife switch to power the idler .

Now here's my questions .

First is wire sizes and such .
What size wire should I use from my breaker panel to the motor .
Total run lengh is less then 8 feet . Is 10 gauge enough for that short a run ?
And do I need to use the same size wire for wiring in the capaciters?
And terminal blocks , with a separate block for each leg do I need to use blocks rated the same as my breaker or can I use say a 15 or 20 amp block for each leg .

Balancing capacitance .
Using L1 and L3 as my powered legs and L2 as my generated leg
If I add say 3 of my 46uf caps from L1 to L2 do I need to run a like amount of capacitance from L3 to L2 to properly balance things or would a single 46uf cap be enough . Or should I maybe run two caps from each powered leg .
I have no idea how sensitive a older transformer welder is to voltage
And unfortunately I don't have a clamp on meter to checks things properly so I'm just kinda winging it .

Thanks in advance for the help .
 

markba633csi

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#2
Firstly, you left out one critical spec and that is how much does the welder draw?
That will help determine wire size.
Secondly, you can run it without any caps to start with (except for those required by the pony motor) until such time that you can get a helper to run a bead while you check current and voltage on the generated leg. My hunch is it might work well enough without any caps at all.
Mark
 
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#3
To be honest I hadn't considered the welders power use , I was looking more at the capacity of the generator itself .
The welder is a 150 amp welder , so without going out and actualy looking at the tag I'd say power draw at full load will be 36 amps or Maybe slightly less .
 

markba633csi

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#4
Oh that's nuthin'. Flea power LOL
I'd use 8ga wire to the rotary and 10 ga to the welder. If you are planning more loads on the rotary then go to 6 ga or even 4 to rotary input
 
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#5
Flea power , for now maybe ;)
Small welders tend to breed Into bigger welders . Much like our other machinery.
Eventually I'd like to set it up a little more user and safety friendly for daily use so I don't have to have two seperate phase converters.
But for right now simple and cheap is what's for dinner :)
 
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#6
I had a couple hours so I gathered up some scrap metal and started on a mount for the pony motor .
I got it mostly done , except for the hinge , I'll finish that tomarow afternoon .

I think I have plenty of capacitance ;)
 

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markba633csi

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#7
That's a bucket full of caps all right :)
Charge 'em up and practice your juggling (you'll learn fast)
 
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#8
Another question .

I don't remember for sure if my pony motor is wired to run clockwise or counter clockwise .
Does this matter ? The idler should run whichever way I start it being as it's only running on two legs right ?
 

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#9
Another question .

I don't remember for sure if my pony motor is wired to run clockwise or counter clockwise .
Does this matter ? The idler should run whichever way I start it being as it's only running on two legs right ?

As near as I could tell when I built my RPC, rotation makes no difference. I tried it in both directions, just to see what would happen.
 

markba633csi

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#10
Agreed, don't think it matters
 

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#11
What welder do you have?
 
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#12
I didnt think rotation direction would matter , but it doesn't hurt to ask :)

The welder is a Miller Sr-150-32 Cc 150 amps dc only .
I had to go to Cleveland last week and decided to stop at hgr surplus.
And couldn't pass it up for the huge price of $49 +tax
It didn't come with any of the tig welding stuff , but it did have 25 Feet of leads for stick welding with it .
 
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#13
Almost finished , I need to wire up an on/off switch for the pony motor and add a few cable clamps and I'll be all finished .

I did test it today , with no load I have 223v on L1. 246v on L2 and 233v on L3
I only added capacitance between L1 and L2 ( 138 uf )
I'm still unsure if I need to add it between L3 and L2 along with how much . I guess I'll see how the welder works before I get to worried about it .

Total cost so far using junk I've scavenged over the years .. maybe $130-$140 .

A bit ugly and mounted on a skid because I don't have a permanent spot for it yet
 

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