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building a rpc

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  • building a rpc

    In the true homeshop machining spirit I am going to try and build a rpc.I hope to get some help from you guys because your always helpful.
    My problem is I am electronically challenged. I also live in s africa and i think this may change things a bit. So first question is are there any guys out there who live in a country with the same power as me. In my country I challenge you to find an electrical engineer who knows what a rpc is.I am being serious about this.I know pm has lots of stuff on rpc's but I hope to have some guidance from guys who don't mind if my bandsaw is Chinese.I first of all need to choose a size motor that will be lathe is 2.2kw 3 ph 220 /380v. I will like to get a slightly heavier lathe at some time but not sure what size motor it will have. Something like a sag180 graziano.Is it as simple as the following.
    1)Buy a 3 ph motor that can be changed from star to delta maybe 3kw motor. Will a 3kw motor start on 220v single phase electrics or will it pull to much current.?
    2)Buy a start cap or connect a pony motor to it to get it up to speed.What size cap would be needed?
    Check to see if the power that comes out is fairly similer to each other and if not put run caps on to try to get it similer.
    If I am on the right track I hope to ask some more questions to fine tune the set up For example I have no idea where the caps will go I dont understand electrical schematics so if someone hopefully posts some at least I can ask questions regarding it Thanks Eugene By the way a local guy(600 miles away )is the only guy who I have found who knows what a rpc is. (This excludes you Werner)He is a turner by trade and sells them for $4375.If I bought one from him I think I would also be his first customer ever and there is a high probability I would get fried by his system as he has no approved people checking his system. I would much rather fry myself at a fraction of the cost

  • #2
    I built my first one about 15yrs ago and used the files posted by Fitch Williams in the Metalworking drop box as a guide.
    See the files with the FRW prefix.
    Also see another 3phaseconv files.
    You will need an AC voltmeter and ammeter for tuning.
    Last edited by MaxHeadRoom; 12-01-2011, 02:56 PM.


    • #3
      Hi Plunger, take a look at this page

      That is an Australian woodworkers forum and is very active, as far as I know the power system in Australia is likely to be very close to yours.



      • #4
        Originally posted by plunger
        Will a 3kw motor start on 220v single phase electrics or will it pull to much current.?
        Yes it will. At least a C rated 10 A fuse (slow) handles a 3 kW motor easily, but it would be preferred to have a 16 A fuse to be on the safer side.
        Amount of experience is in direct proportion to the value of broken equipment.


        • #5
          For your 2.2kw (3hp) lathe motor, I'd choose an idler that's about 2+x that size. If not, you likely won't be able to start it in the higher speeds. My choice would be a 7.5hp motor. if your planning for a larger lathe... scale accordingly... For a 7.5, if you want to avoid the start up surge (it sounds like you are power limited) ), use a pony motor to bring it up to speed.

          You'll hear that "just start another motor with your idler, then your target machine". Too much mucking around and that approach doesn't help your power consumption.

          This is a good place to start reading :
          Last edited by lakeside53; 12-01-2011, 05:05 PM.


          • #6
            Originally posted by lakeside53
            This is a good place to start reading :
            That the Fitch Williams design referred to in the drop box link.


            • #7
              Good, so we both agree

              Plunger: your question on fusing.... it's not so simple... the power for the RPC AND the target motor need to pass though any mains breaker/fuse. Although the startup currents will likely be delayed from each other the actual current drawn is a complex question due to the power factors in use. After starting your unloaded idler will continously draw about 40-50% of it's nameplate current as will your lathe; as you increase load both it and your target motor now draw increasing current, but not in phase.

              Protect your lathe motor with the usual 3 phase motor starter. Check your local electrical codes - your rpc may also need a overload protection - it's required here, but few garage builds do this.
              Last edited by lakeside53; 12-01-2011, 05:52 PM.


              • #8
                What I am struggling to understand is that in s Africa we have a live and a neutral We also have an earth. This is at 240v
                Don"t you have to lives and a neutral or is it the same but just a different name.If you have two lives does that mean its dual phase.? If its dual phase does that mean two of the motor windings are there and the third one is generated? Does that mean I am only putting current into One winding seeing we only have single phase. Does this mean our static phase converters only produce 1/3 of the power and yours is 2/3 ? I see Fitch Williams design is a star setup(wye) Ours will only work on delta I think. Has this any bearing on the setup?Is Fitch still around? I see he hasn't visited pm for 3 months If my db board has only 20 amp plugs is it unlikely that it would start a 5.5 kw motor for example? If I had a pony motor would this change the situation. Is it really necessary to have run caps.

                I was thinking of mounting the idler on the wall with a washing machine pony motor underneath so gravity and its own weight would tighten the belt. If I had a handle on the pony motor I could tension it if necessary and when its up to speed I could just lift the handle and thus disinguage the belt.What power do they have in the UK ,New zeal and and Finland.
                Regards eugene


                • #9
                  The system in N.A. is a bit different than the rest of the world. but the results as far as 240 1ph is treated pretty much the same,still 240v 1 ph. I believe you have the 240 derived from a grounded star neutral and one phase, which as far as you are concerned is treated the same way as the 240v in the Fitch drawings.
                  The star portrayed in Fitch drawings represents the motor.
                  The 240v 1ph is fed to two of the motor windings and the motor generates the 3rd phase, in conjunction with the capacitors that when tuned, provide optimum power factor correction.


                  • #10
                    Plunger, the big difference between the North American system and that used in most Commonwealth countries is that in NA each of the 240 legs is only 120 from ground. In our system one (the neutral) is (or should be) at ground potential and the other 240 above ground.

                    So for practical purposes it is not the voltage that is (much) different but the relationship to ground which comes important when considering the safety of the system.

                    Thats what I understand anyway......


                    • #11
                      You have been given a lot of good information. The basic RPC requires nothing but a 3 phase motor of the same voltage as the available 1 phase supply power. You just connect the power to any two of the motors leads. Once the motor is started by a pony motor, rope and pulley, kicking a pulley with your shoe, or a starting capacitor connected from either power lead to the third lead, the motor will continue to run. Any of the starting methods needs to disconnect once the motor is running.

                      When the motor is running, the three leads of the lathe motor can be connected to the three leads of the "idler" motor and it will start and continue to run.

                      All three leads of a 3 phase motor are identical so it does not matter which leads you use. It does not matter which way the "idler" motor is started. If you lathe motor starts and runs in the wrong direction, just reverse any two of the leads to the lathe.

                      Run capacitors are not necessary. They are refinements, along with relays to control the starting capacitor. Starting capacitors must not be connected any longer than necessary to get the "idler" running. I think about 100 MFD per horsepower generally works well.

                      Be careful, dangerous voltages and currents are involved. The fact that one lead of your 240 volt supply is at ground potential does not change anything about the RPC. It could be important if there are any grounds on the power circuits of the lathe. Ground the motor frame, the lathe, and any other accessible metal parts with a separate lead; do not use the neutral for this.
                      Don Young


                      • #12
                        Originally posted by Don Young
                        . The basic RPC requires nothing but a 3 phase motor of the same voltage as the available 1 phase supply power. .
                        However the majority of 3 phase motors readily available to Plunger may be 380 or 400V machines.


                        • #13
                          I'm know it's bad form, but I've never been particularly concerned about being PC.

                          For RPC information you really can't beat the dedicated RPC forum on the PM board. You'll find my plans and the whole discussion that went into making that project a reality along with MANY others over there.

                          I've built two, and now running a "CNC rated" 30hp (phase balanced, but not "Phase Perfect") that came with my current shop. The first was built with the help of our local club, we did it and 2 others as an interactive meeting topic. It was just a simple self start 3 hp unit built with a freebie 3HP GE motor. Total cost was less than $70 or so, most of that in over priced Grainger parts. My second was a much more elaborate fully balanced stepped RPC. Started on 5hp, you could then latch in a second 10hp idler (and it's separate balance circuit) for a cumulative 15hp of idler. Then you could drop or bring up either idler at will as needed, with one big red button to kill it. This one was built with salvage/scrap year parts. Both idlers are Baldor industrial and looked practically new. Seems like I paid $0.10 USD per pound for them both. I also got most other parts from the scrap yard. That includes a nice sealed NEMA enclosure, some very nice light Allen-Bradly switches (from scrap industrial control panels), commercial grade contactors, distribution bus, and 3 phase fuse block along with a 110V/24V xformer to drive the control circuit. All of it at less than $0.05 USD per pound. The only thing I had to find elsewhere were the rather large variety of run caps, and they all came from the same source on Ebay. Oh, and I did buy 3 inexpensive panel mount voltage meters so I could tell at a glance how it was doing with various loads, or if something go seriously out of whack.

                          I'm a pretty decent scrounger when I don't have to rush, but there are any number of folks who've done better for less. My main point is that this is a VERY approachable project (I'm not an electrical anything, much less engineer) that can be done very inexpensively.
                          Last edited by BadDog; 12-02-2011, 02:46 AM.
                          Master Floor Sweeper


                          • #14
                            Any you guys remember Roy, the Frugal machinist. When I was researching building my rpc I couldn't make sense of anything till Roy posted on the old Chaski list. His schematic made it simple.

                            His site is archived, although some pictures are gone, still a great site:


                            Link to his rpc on the projects page:


                            Link to the schematic:


                            Wish Roy would bring his website back and update it with all the projects he's been doing since he shut it down. May not help the OP, but may be of interest to others.
                            Last edited by Nicholas; 12-02-2011, 12:13 PM.


                            • #15
                              I am wondering if this would work. I run my emco v13 lathe on a dedicated vfd. I have an iso 40 milling machine which has a two horsepower motor. This is run off a vfd. I built a knife grinder that runs off a 1.5kw motor and I physically have to unplug the mill and then plug in the knife grinder. How would this work. I dont want to buy another vfd for my soon to be delivered chevalier surface grinder because it needs the spindle the hydraulics and the coolant all to run off vfd's . What if I made the knife grinder a rpc that is controlled by the vfd. In other words is it possible to wire the grinder to the vfd permanently so it could run my surface grinder. That would eliminate having to have start caps etc. If I just took the belt off the grinder.

                              If I dont use the vfd could I build the same motor as a rpc for the surface grinder using the knife grinder as the rpc started off by a pony motor and if I want to use the vfd for the knife grinder I still can Seems to be a waste to have a motor sitting there if it could be dual purpose.