# build phase converter



## Honestly (Feb 3, 2011)

Any good reason to drop 5k on a RPC to power a 3ph 50hp sawdust machine from a 1ph service, rather than just getting a used 3ph motor and capacitors to make my own? This is for family, so cheap is good. Rural area, no inspections.


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## backstay (Feb 3, 2011)

Do you need to power a 50 Hp motor or are you going to use a 50 Hp motor with caps. Then you're going get about 2/3 of that(35 Hp). 

And what's a sawdust machine?


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## Honestly (Feb 3, 2011)

Powering a 50hp machine, so I know I will need something bigger to start it. Its for agricultural use. I haven't seen it yet, but the way he explained it, it just makes sawdust out of small wood scraps or branches. I think they may use the sawdust in poultry houses.


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## denny3992 (Jul 12, 2010)

Honestly said:


> Powering a 50hp machine, so I know I will need something bigger to start it. Its for agricultural use. I haven't seen it yet, but the way he explained it, it just makes sawdust out of small wood scraps or branches. I think they may use the sawdust in poultry houses.


Maybe a vfd....


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## backstay (Feb 3, 2011)

denny3992 said:


> Maybe a vfd....


I don't know if they make one that big to do what he wants. I've installed some, but they were under 5 Hp. The price is going to be around 3 grand.


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## Honestly (Feb 3, 2011)

Yeah, that would be one massive VFD, and it probably be as much as the RPC (if I could find one). I was planning on running the machine 230V to keep from having to buy a transformer. Again, this is for family, so cheap (and safe) is best. He will have to get a new service, but it would be outrageous for 3phase, as he is miles away from the closest 3ph lines. 

I have heard of and read of many machine shop guys and farmers and such making their own RPC, and it seems it would be WAY cheaper this route. Aren't the RPC's that you buy just 3ph motors and caps? I hooked up one in the past, but didn't pay as much attention as I should to how it was made. Just wanted to see if any of the motor gurus around here had any advice. I do not have much motor experience, so I look forward to learning some more with this one.


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## backstay (Feb 3, 2011)

Honestly said:


> Yeah, that would be one massive VFD, and it probably be as much as the RPC (if I could find one). I was planning on running the machine 230V to keep from having to buy a transformer. Again, this is for family, so cheap (and safe) is best. He will have to get a new service, but it would be outrageous for 3phase, as he is miles away from the closest 3ph lines.
> 
> I have heard of and read of many machine shop guys and farmers and such making their own RPC, and it seems it would be WAY cheaper this route. Aren't the RPC's that you buy just 3ph motors and caps? I hooked up one in the past, but didn't pay as much attention as I should to how it was made. Just wanted to see if any of the motor gurus around here had any advice. I do not have much motor experience, so I look forward to learning some more with this one.


They make much bigger VFD than 50 Hp, but I don't know if they make on that big that will take 1 ph in and 3 ph out. 

The rotary phase converters are just that, 3 phase motor with caps, sometimes with no shaft sticking out or a guard over it. You're taking about needing 50 Hp, so that converter is going to have to be a 75 Hp motor plus caps. Then you need to figure out how to size the caps.


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## ampman (Apr 2, 2009)

no three phase available from poco


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## Honestly (Feb 3, 2011)

ampman said:


> no three phase available from poco


A Long way away= $$$$


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## wendon (Sep 27, 2010)

I would look on line for a kit to convert a 3 phase motor to a Roto-Phase. I think the Tork company that builds start switches makes one or at least used to. Some of the new converters just use a simple one shot timer for the start switch and then you adjust it until the unit starts correctly. There's everything imaginable out there. One friend of mine had one that used a single phase motor to get it started but it was rather hack. The other problem with the big converters is the spike on the power companies primary. I think they actually build them with a soft start system. Sounds like you're probably looking at a hammer mill and it'll be interesting to start something like that with a roto-phase!!


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## hardworkingstiff (Jan 22, 2007)

How much are you going to save by building it yourself?


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## JRaef (Mar 23, 2009)

What backstay was referring to is that when you "roll your own" RPC, you do not get the full HP out of the motor, you get roughly the equivalent of 2/3 HP. So if you NEED 50HP then, you will have to start with a 75HP 3phase motor.

You also have to consider how you will start that motor from a 1 phase source. Remember, the 1 phase current will already be 1.732 x the 3 phase current. So assuming you 75HP 230V 3phase motor is 208A FLC, that is around 350A at 230V single phase. On top of that, the starting current will be 600% of that, so the surge capacity of your single phase transformer had better be up to the task. The rule of thumb is that the transformer kVA must be 3x the HP rating of the motor. So unless you have a 225kVA single phase transformer from the utility, it isn't likely to work.

They may want to consider a 3phase DG set for this.


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## Honestly (Feb 3, 2011)

*hmmm*

I had looked at FLC and thought a 400A service would work, but I did not consider the size of the poco transformer. Sounds like I might need to talk with them. Thanks

hardworkingstiff, used three phase motor for a few hundred, plus cost of caps vs. around 5k for a RPC.


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