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  • Yet another rpc question

    Some time ago I saw something on here about adding an idler motor adjacent to the rpc.
    I have a two hp converter. I just acquired a five horse 3ph motor and I have a six belt pulley about sixteen inches diameter that fits. I am thinking it would add mass.
    Is there any advantage to that setup?

  • #2
    Originally posted by 1-800miner View Post
    Some time ago I saw something on here about adding an idler motor adjacent to the rpc.
    I have a two hp converter. I just acquired a five horse 3ph motor and I have a six belt pulley about sixteen inches diameter that fits. I am thinking it would add mass.
    Is there any advantage to that setup?
    I have heard that there is some electrical advantage to using a cascaded motor with an RPC but I don't have the scientific reason for this. I have read on another site that running a lathe motor will assist a hard starting motor like a compressor if connected to the same RPC driven 3 phase circuit.

    That said, a better choice for you might be to swap the 5 hp 3 ph motor into your 2 hp RPC. My 5 hp RPC starts on 500 mfd and uses 50 mfd on each line to the generated line to get good balance. All you would need might be some additional capacitors.

    Comment


    • #3
      Adding your 5hp motor to your converter SHOULD give it the ability to start loads approaching 7hp, depending on the load. The two motors running together should help smooth out your phases. They do not have to be connected by belts, the one should start the other (although it is iffy starting the 5hp with the 2hp, but it might work- you can swap them, but it may disturb your capacitor balancing).

      I am presently constructing a rotary phase converter with two 5hp idler motors. Don't have it together yet, but in process (mostly acquiring parts on the cheap). There is a whole section on phase converters on the Practical Machinist web site.

      Comment


      • #4
        An idler motor can simplify the RPC, RPC startup with simple RPC can be problematic with high current draws by the RPC during windup of the 3ph motor being used as a generator. The motor is
        a relative short circuit across the 1 ph power line until it gets up to some speed and the generated magnetic field begins to induce the back emf that limits current draw. By using an auxiliary 'pony'
        motor the generator motor is rapidly ramped upto speed reducing the current surge. Some primitive RPC designs use a rope wrapped around a pulley on the shaft of the generator motor with the user pulling
        on the rope to assist the generator motor rampup. As an example a 5hp single phase motor on 220V has a start surge of ~65A with a run current max load in the 27A range though the idle current at speed
        (no load) will be much less. A 3ph motor with a capacitor start for 1ph operation may take seconds instead of milliseconds to get upto speed, depends on design of the RPC.
        Last edited by sch; 03-05-2013, 03:27 PM.
        Steve

        Comment


        • #5
          Originally posted by sch View Post
          An idler motor can simplify the RPC, RPC startup with simple RPC can be problematic with high current draws by the RPC during windup of the 3ph motor being used as a generator. The motor is
          a relative short circuit across the 1 ph power line until it gets up to some speed and the generated magnetic field begins to induce the back emf that limits current draw. By using an auxiliary 'pony'
          motor the generator motor is rapidly ramped upto speed reducing the current surge. Some primitive RPC designs use a rope wrapped around a pulley on the shaft of the generator motor with the user pulling
          on the rope to assist the generator motor rampup. As an example a 5hp single phase motor on 220V has a start surge of ~65A with a run current max load in the 27A range though the idle current at speed
          (no load) will be much less. A 3ph motor with a capacitor start for 1ph operation may take seconds instead of milliseconds to get upto speed, depends on design of the RPC.

          You are correct that a rope or pony start RPC would take much less current to start. I think your estimate of startup current on a typical 5 hp RPC is high, however. I have two RPC's presently. One is a 5 hp version with a Boston Gear motor that idles at 8 amps on each line measured with a known good Craftsman clamp meter. It's connected to a 20 amp line and has never tripped a breaker. It will even start when my 3 hp Webb Mill is turned on. Total draw with the mill is about 14 amps. Startup is instantaneous on 500 mfd. start capacitors.

          The other RPC is a 7.5 hp version with a Baldor motor that I just finished. It draws only 11 amps on a 30 amp circuit and it idles at 17 amps with the mill idling. Startup is also instantaneous with 800 mfd start caps and it won't trip the breaker either.

          I've heard that startup current is high on RPC's but I've yet to experience it. It's possible that the current is high at startup but the units are starting so quickly that the breakers don't heat up enough to trip. I have no way to measure this, however, since none of my meters are capable of reacting quickly enough to give a reading during startup.

          EDIT: Just an aside, but I noticed recently that the voltage in my shop, read on four different meters, is 252 VAC. That would give a slightly lower current reading given on each RPC, but not very much. My house voltge measures 126 VAC. I was concerned and I recently contacted the utility company and they verified that this is what they are sending nowadays. The last time I checked was a few years ago when I built the 5 hp RPC and it was 242 then. It doesn't seen to hurt anything.

          Both RPC's are autostarters with Steveco potential relays.
          Last edited by GNM109; 03-05-2013, 04:10 PM.

          Comment


          • #6
            I don't think you want to add mass to the RPC.
            It sounds nice, in that it would smooth load handling when a slave motor starts/runs,
            but the offset is higher amperages at start, allong with more Caps required to rotate to speed.

            Rich
            Green Bay, WI

            Comment


            • #7
              Are you talking about starting under load or no load?
              I was thinking of starting the rpc first, then kick in the five horse idler motor,then applying the load (lathe or mill).
              Sort of like a pony engine starting the diesel engine.
              Right now the rpc starts instantly,not even a flicker in the shop lights.
              Tell me if I am confused.
              Last edited by 1-800miner; 03-05-2013, 09:36 PM.

              Comment


              • #8
                I you have ever seen an old blanchard machine (if you haven't, think HUGE grinder) they use an amp meter to let you monitor how the machine is grinding. One thing you may have noticed is that the amp guage will peg all the way when the grinder is started, and will stay that way until the grinder motor comes up to full rpms. Then it settles into a no load amperage that is just a fraction of what it took to start the motor.

                So a lot of the talk is about starting what is generally thought of as no load, but the "inrush" current required to get the motor started.

                I had a buddy whose 5hp rpc was plenty for running his lathe, until he starting doing some heavy tapping with it; the lathe would stall. He had a larger lathe on the same circuit, so he just started the larger lathe motor and let it idle while he was using the smaller lathe. He was then able to do his tapping without stalling. This has the same effect as adding a second idler to a rotary phase converter.

                In your case, your 2 hp rpc has been "tuned" with start caps and run caps for the 2hp motor. Merely switching motors would probably change that. You MAY have enough power there anyway to start your second motor electrically (not sure if you were thinking of connecting the two motors by belts in your first post).

                Hope this helps.
                Last edited by toolmaker76; 03-06-2013, 08:52 AM. Reason: clarification

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                • #9
                  My rpc is sealed with no shaft out of the housing.So I would have to start the second one electrically.
                  Is there some way to "soft start" the five horse electrically? It would be easy if I had the belt option.

                  Comment


                  • #10
                    Originally posted by GNM109 View Post
                    EDIT: Just an aside, but I noticed recently that the voltage in my shop, read on four different meters, is 252 VAC. That would give a slightly lower current reading given on each RPC, but not very much.
                    I agree with the general sentiment of the post, my 10hp rpc is on 40 amp breaker, starts instantly and never has popped the breaker...but why do you think if volts have gone up, amps will go down slightly? You increase the volts, for a given resistance, the amps will increase.
                    in Toronto Ontario - where are you?

                    Comment


                    • #11
                      Originally posted by Mcgyver View Post
                      I agree with the general sentiment of the post, my 10hp rpc is on 40 amp breaker, starts instantly and never has popped the breaker...but why do you think if volts have gone up, amps will go down slightly? You increase the volts, for a given resistance, the amps will increase.

                      Correct. I misspoke. Can't work my calculator.

                      One thing that dropped the current about 2 amps on both the 5 hp and 7 hp units that I built was to add a power factor correction capacitor across L1 and L2. It was recommended to me by a guru on another site, which shall go unnamed. He said to use about 5 mfd per hp. I did that and it works!

                      Comment


                      • #12
                        Originally posted by 1-800miner View Post
                        My rpc is sealed with no shaft out of the housing.So I would have to start the second one electrically.
                        Is there some way to "soft start" the five horse electrically? It would be easy if I had the belt option.
                        Okay, I think I get what you mean by "adding mass"- sorry, getting kind of slow at this age. You are wanting to use the pulley on the shaft of the other motor as a sort of flywheel. I remember reading that did help some kind of problem, but I don't remember what.

                        Depending on what kind of circuit you have, you may not need to "soft start." I suppose you could rig up a pony motor to the second motor to lower inrush current, but my thinking is you probably won't need to.

                        Comment


                        • #13
                          The time that a pony motor or rope start really works nicely is with a 20 hp or greater Rotary Phase Converter. Those motors do require a lot to get them going, especially with regard to capacitance if they were to be built as automatic starting units. It's good to have them moving before turning the power on and they can be made much simpler.

                          When I was testing my motors before installing them as idlers, I used rope start on each of them and found that they would start very easily. If I were to build say a 50 hp RPC, I would certainly use either rope or pony start.

                          Obviously, if you buy a ready made unit with the shaft cut off, you are limited to capacitance start. I don't like cutting up motors. Mine are unmolested.

                          Comment


                          • #14
                            I built a pony start 30Hp RPC, at the time the 30Hp motor was the smallest motor I could get. I had picked it up when I had the chance to and do not regret that decision.
                            I start the 30Hp with a 2Hp motor through a belt with a clutch mechanism, I spin the 2Hp up and then engage the clutch to spin up the 30Hp motor, once the idler is up to speed I energize it and then kill the power to the pony motor.
                            It has started a 10Hp 3phase motor on an American Pacemaker lathe, that had I not had the 30Hp RPC it probably would have went to the scrapyard. We started the lathe and ran it through the its paces, he has it now set up in his shop and uses it regularly, not bad for a piece of "free" equipment.
                            Dan.

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                            • #15
                              Apparently using two motors in tandem for a 'staged' RPC is fairly common as I have read of quite a few. No mechanical connection is required. There seems to be little agreement on the value of flywheels but I believe it would help the surge capability while making starting more difficult. With the two motors running in parallel, the second motor works exactly like the first one. Optimum capacitance would change but it changes with varying loads anyhow and is certainly not critical. Many people use no capacitors at all. Each motor can have its own capacitors and either motor could be run independently or both together. Most commonly the smaller motor is started first, of course.
                              Last edited by Don Young; 03-06-2013, 10:22 PM.
                              Don Young

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