Tool porn / RPC question

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  • mototed
    Senior Member
    • Aug 2007
    • 385

    Tool porn / RPC question

    This big heavy girl followed me home on Friday afternoon. She's 2100 lbs. It's a real step up from the 1940s' Craftsman. This one is a 1982 model.Never heard of the brand
    My RPC runs the main motor on low speed and the lube motor all day when both are running, Turn off the main motor and the tiny little lube motor kicks a contactor out in about 2 minutes and stops
    Main motor also trips one instantly when started on high speed. This machine ran fine on power company supplied 3 phase. The RPC runs the mill fine, but it's only 1 1/2 hp and consists of a drum switch and not a lot of relays and controls like this Lathe. So, RPC too small ? When talking about balance is that voltage difference between the three legs such as L1-L2, L1-L3, L2-L3 ??
    Thanks
    Ted
  • Scishopguy
    Senior Member
    • Feb 2006
    • 948

    #2
    Re: RPC Problem

    My guess would be that you need a larger RPC. I imagine that your lathe has at least a 1hp spindle motor. The lube pump motor may be something like an 1/4 hp or less. That is still pretty close to the 1 1/2 hp converter. Converters are not 100% efficient. On a 5hp converter you can only count on about 4hp output IIRC. You may be able to remedy this by getting a larger 3ph motor for the converter to up the capacity.

    Just my $0.02 worth.
    Jim (KB4IVH)

    Only fools abuse their tools.

    Comment

    • BadDog
      Senior Member
      • May 2006
      • 3227

      #3
      What size RPC? What size lathe motor. Is the RPC "balanced" (assume no based on your balance question).

      Turn on the mill at the same time and see if it settles down or improves. If so, probably the RPC too small. My big girl has a 7.5/3.75 hp motor and low would start inconsistently on my old 3hp RPC. I built another dual idler RPC with 5hp and 10 hp idler. Low starts easily on the 5hp, but if I forget to turn on the other idler, High won't come up. With either 10hp or 15hp of idler running, high jumps right in with no hesitation.

      Mine also seems to have a balance sensing circuit since it drops out contactors when one phase is too far out. So that might be part of your problem.
      Russ
      Master Floor Sweeper

      Comment

      • mototed
        Senior Member
        • Aug 2007
        • 385

        #4
        Thanks for advice

        I really don't know what size the RPC is, No nameplate on the motor. The lathe is a 5 hp/7.5 hp. I think I better be keeping my eyes out for a replacement.
        Any body want to Swap for a 8" Kurt ?
        Too big for me.

        Comment

        • Scishopguy
          Senior Member
          • Feb 2006
          • 948

          #5
          My Bad

          Man, I need that second mug of coffee. I could have sworn that you said the converter was 1 1/2 hp. Sounds like you got it under control with Bad Dog's help.

          good luck with the "new girl." Nothing better than getting "lathed" on a Friday afternoon.
          Jim (KB4IVH)

          Only fools abuse their tools.

          Comment

          • hardtail
            Senior Member
            • Jun 2005
            • 1079

            #6
            Nice score, I love the lathes name, should start counting your money already.....LOL

            I would recommend the RPC produce at least 125% the output of your load, if theres no tags present see if you can get ahold of an ammeter and one should be able to figure it out pretty close, this might be a good situation to have 2 RPC's if you have other smaller loads in the shop.
            Opportunity knocks once, temptation leans on the doorbell.....

            Comment

            • lane
              Senior Member
              • Aug 2005
              • 2691

              #7
              This is just a guess .but looking at the lathe it should have at least a 5 HP motor .so you need at least a 7- 10 Hp RPC.
              Every Mans Work Is A Portrait of Him Self
              http://sites.google.com/site/machinistsite/TWO-BUDDIES
              http://s178.photobucket.com/user/lan...?sort=3&page=1

              Comment

              • mototed
                Senior Member
                • Aug 2007
                • 385

                #8
                After looking at dimensions of the motor on my RPC and 10hp motors, It's time to go shopping or scrounging. The old one is loud as heck anyway.Of course about three years ago I traded a perfect motor that would be nice to have about right now. Don't you hate that about junk, or as I tell the wife -potential project collecting.
                Thanks
                Ted

                Comment

                • Carld
                  Senior Member
                  • Aug 2006
                  • 6061

                  #9
                  3 ph used motors are usually cheaper than single phase. Check some motor shops or industrial salvage shops.

                  An RPC will run a motor or motors approx. 2 1/2 times it's hp. That is, a 2 hp will run a 5 hp motor.
                  It's only ink and paper

                  Comment

                  • BadDog
                    Senior Member
                    • May 2006
                    • 3227

                    #10
                    That is completely contrary to my experience and reading on RPC sizing. My 3 hp ran very well, but wouldn't reliably start the 3.75 on my lathe. My 5 hp will reliably start the 3.75, but will rarely start the 7 hp, and slowly at that. But the 10 hp idler will fire up the 7 easily. This closely matches the commonly stated ~1.5 x load motor sizing. Less if well "balanced", more if you want to run unbalanced. But I've never heard of running less than 1:1 RPC to load.

                    But I do agree on the cheaper for 3ph. Check with local commercial AC repair. They pull these big motors out when doing AC installations and upgrades.
                    Russ
                    Master Floor Sweeper

                    Comment

                    • pcarpenter
                      Senior Member
                      • Jun 2005
                      • 2283

                      #11
                      All the recommendations I have seen recommend an idler motor for an RPC that is at least as large as the largest motor to be driven. There is a very real reason for this. The winding in the idler motor for the "generated" leg will need to carry the amount of current drawn on that leg on the load motor(s). In effect, you are sizing the wire in the "generator" that way. That having been said, for stuff that runs mostly unloaded and especially starts unloaded, you are not likely to reach nameplate rated current, but why risk it. One other thing that can be done to make up for a lacking "idler" motor is to start and leave idling, other motors supplied by the same RPC. They then serve to help generate the third leg as well.

                      What I really don't understand is why that lathe has a 5/7.5 HP motor???!!! If I am seeing it correctly, its an import 13" swing lathe and mine has a 2HP motor and is not lacking. Most of the 13" imports today seem to have 2HP motors and they go to about 3HP for a 14" swing lathe.

                      One other tidbit...I did a lot of reading before building my balanced RPC....it seems that if you are lucky enough to have machines that all use motors in roughly the same HP range, you can better balance things out, and an idler close to that will balance more readily too. This means that getting a " really big idler" just in case can be counter productive. In my case, for example, I have a Bridgeport mill with a 2HP motor, a Sheldon shaper with a 1.5HP motor and another horizontal mill and a drill press both with 1.5HP motors. The only thing I have that is lower HP is a surface grinder and its 1HP as I recall. I went with a 3HP idler and was able to balance it within a few volts on each "phase" under load. If I end up with a piece of machinery with bigger needs some day, a separate RPC (or the use of a VFD) will be the way I will go.

                      I know that plenty of folks are running completely unbalanced RPC's and it works fine. However, even a balanced RPC is imperfect since two of the phases do not have correct phase relationship. My take is that the smoothest motor performance and greater torque should come from closely balanced voltages and "best tuned" phase relationships. That's one of the biggest up-sides to three phase motors anyway.


                      Paul
                      Paul Carpenter
                      Mapleton, IL

                      Comment

                      • mototed
                        Senior Member
                        • Aug 2007
                        • 385

                        #12
                        I took some voltage readings

                        Hey guys,
                        Paul, you where right. I was at work and in my old age I got the HP numbers wrong. It's 2hp/3hp and a 1/4 hp lube motor.
                        Took some readings off the RPC as follows
                        At idle L1 - L2 = 291 V
                        L1 - L3 = 242 V
                        L2 - L3 = 283 V
                        With 1.5 hp mill turned on L1 - L2 = 242 V
                        L1 - L3 = 268 V
                        L2 - L3 = 260 v
                        With 1/4 hp lube pump L1 - L2 = 239 V
                        L1 - L3 = 274 V
                        L2 - L3 = 282 V
                        With lathe on low and pump L1 - L2 = 239 V
                        L1 - L3 = 255 V
                        L2 - L3 = 240 V
                        How far off is this for a RPC? Bad capacitor maybe? It does kick out the contactors with lube pump only. I think I can see why now.
                        Thanks again,
                        Ted Lotz

                        Comment

                        • Carld
                          Senior Member
                          • Aug 2006
                          • 6061

                          #13
                          The 2 1/2 came out of a book I bought from Lindseys Tech. books. It works for me.

                          I haven't put balance caps on my RPC yet. I probably should but haven't gotten around to it for the last 15 years and the man I bought it from used it for about 8 years without the balance caps.

                          Oh, I did go to a bigger motor a few years ago but I didn't notice any difference in performance.
                          It's only ink and paper

                          Comment

                          • hardtail
                            Senior Member
                            • Jun 2005
                            • 1079

                            #14
                            It is impossible to create more energy than the idler, if you were able to you'd have a quatrillion $$$ prize sitting there. Getting 1:1 efficiences are next to impossible so I would venture that if a 2hp ran a 5hp machine it wasn't pulling any load, still tough to fathom.

                            Rule of thumb is 1.5:1 for idler to load but some get by with a lower ratio than this.
                            Opportunity knocks once, temptation leans on the doorbell.....

                            Comment

                            • hardtail
                              Senior Member
                              • Jun 2005
                              • 1079

                              #15
                              Ok back to the problem at hand.....yes balanced is trying to get your 3 phase legs as close to each other as possible, I take it by the readings your running 240V? The problem that even balancing faces is that it's only ever balanced for that specific load, so running your machine in any other configuration of speeds of lube on/off the leg voltages will change, it's kind of a pursuit of finding some middle ground thats somewhat satisfactory to all.

                              Did you find any nameplate data on either the idler suppling or the lathe consuming, namely hp and full load amps? Theres also reccomendations on which leg your control circuits are attached too when you hook it up. There are some smart fellas on here and as much as I don't like PM on occasion they have an excellent speedy RPC section over there but until we know that basic info I'm at a loss...........?

                              Edit I see it's 2-3 hp, go with a 5hp idler, I've gotten some for free from industrial motor and pump places for the scrounging.
                              Last edited by hardtail; 12-10-2008, 02:14 AM.
                              Opportunity knocks once, temptation leans on the doorbell.....

                              Comment

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