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  • RPCs and flywheels

    Continued over from another thread to avoid a huge hijack

    https://bbs.homeshopmachinist.net/fo...e2#post1850637

    The slow speed means that the energy would be low, so the flywheel would have to be big to do much. 3450 motor as RPC should then be better, faster speed is 4x stored energy for same mass as a 1725 rpm motor..

    Voltage drops for two reasons....

    1) load is on it and so it must slow to reduce back EMF, draw more current, and so bring torque up to match the load. Slowing reduces voltage in proportion. The generated leg voltage is just the back EMF.

    If flywheel prevented slowing, it would keep that effect to a minimum.

    2) impedance in series drops more voltage when current goes up. Flywheel would have virtually no effect on that.

    The specific motor would determine which effect is the most important....
    Last edited by J Tiers; 01-23-2020, 12:38 AM. Reason: Mentioned the thread it came from
    2730

    Keep eye on ball.
    Hashim Khan

    Everything not impossible is compulsory

    Birds are NOT real, they are spying on you

  • #2
    Something I have wondered about for a while. Generators as opposed to motors often have solid rotors. By that I mean, they are still stacked up laminations, but they have no axial holes. Whereas motors often have axial (longitudinal) holes thru the rotor. I have heard this improves cooling and lessens inrush current. I have wondered about reworking a idler motor so the rotor is solid like a conventional generator (actually an alternator, but whose counting?)

    i think those cooling holes lowering the physical mass are also lowering inductance and hence magnetism that could be used to store energy. If the iron ain't there, it can't be magnetized. In other words, magnetic flywheel.
    Last edited by nickel-city-fab; 01-22-2020, 09:51 PM.
    25 miles north of Buffalo NY, USA

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    • #3
      What are you trying to accomplish? I don't see how it would save any money or time. In general, I would think that you would want the generator rotor to be as heavy as practical to minimize the affect of starting loads on it and you would want the motor rotor to be as light as practical to minimize starting current. The only motors that I can think of that actually have flywheels are the reactor coolant pumps at a nuclear station. The flywheels are there specifically to keep coolant flowing through the reactor for as long as possible in the event of a loss of power situation. They draw a god-awful amount of current on startup but they are designed for it.
      Tom - Spotsylvania, VA

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      • #4
        My RPC has a big old cast iron 7" double sheave on it, a pretty good flywheel. I have no idea what I"m going to do with that motor when I decommission the RPC, it's huge, much bigger than modern 2hp motors.
        Last edited by gellfex; 01-22-2020, 11:34 PM.
        Location: Jersey City NJ USA

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        • #5
          Originally posted by flathead4 View Post
          In general, I would think that you would want the generator rotor to be as heavy as practical to minimize the affect of starting loads on it and you would want the motor rotor to be as light as practical to minimize starting current.
          I can't speak for J Tiers but I would think that in the case of an RPC idler, the motor and the generator are both one and the same. So you want the rotor to have as much mass as possible to minimize the starting load thing as you mentioned. In my post, I was pointing out that motor manufacturers often put holes through the rotor to lighten them, and this probably reduces the amount of current that can be generated when it is being operated as an RPC. Heavy current draw slows the rotors down, hence the question.
          25 miles north of Buffalo NY, USA

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          • #6
            This thread gonna be fun to watch.

            Oddly 900 and 1200 rpm motors work best, but I only been playing this game for 40 years.

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            • #7
              OLD, Heavy Motors make the best RPC, especially if they are open frame
              New 3400 motor with Aluminum components make the worst RPC,s
              Been there, done that,
              Flywheels are nice But....you need more starting caps
              Rich
              Over 60 phase convertors built since 1962
              Green Bay, WI

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              • #8
                I built a 5hp RPC with both a flywheel and a centrifugal switch. I did this because starting old woodworking machinery is a different ballgame than starting metal working machinery. Often in WWM there is no clutch or way to disengage the motor from the load so the whole series of spinning masses must be spun up at once. This makes a stall possible.

                I had built several RPC's using centrifugal switches mounted to the motor shaft for starting. They worked perfect since once the RPC was up to speed they dropped out. I also used them to start 3 phase air compressors automatically off the pressure switch which also worked well.

                The RPC with the flywheel was built to start a small four side moulder up.It had four six knife cutter heads that all spun up at once on a 3hp motor. The flywheel I used came off of a 3hp 1800rpm DC motor salvaged from a conveyor line. It was there because the DC motor had a pneumatic clutch and brake between it and the gear reducer it was running. The flywheel itself was 8" in diameter and 1-3/4" thick if I recall and had a 1-1/8" keyed bore.

                The RPC worked well, so well it was able to start a 7-1/2hp motor on a wide belt sander with no issues. I don't know whatever happened to that RPC, the guy I built it for gave it back to me when he retired and I traded it to a buddy when he brought home a 3hp b-port mill he needed to run. I may check and see if he still has it.
                I just need one more tool,just one!

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                • #9
                  I just ripped out my RPC tonight!! God, looking at it I have no idea how it worked with 2 contactors and a pile of capacitors, I guess I built it following an internet recipe and didn't need to think about it again for 22 years. It used a momentary 3 pole switch to start, I'd just hold it down till the flywheel motor was far enough up to speed. 4 vfds now and I won't look back, less sketchy looking electrical equipment in my basement now....
                  Location: Jersey City NJ USA

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                  • #10
                    The original issue was starting nasty loads, from the K&T mill thread linked above.

                    You would need to have as much energy stored in the flywheel as you need to start the other motor and load.... plus losses. Maybe not that much, since the electric input will still be there as well, but as close as possible to avoid much slow-down.

                    AMD, that energy must be available between the normal speed, and the final speed that you do not want to go below (same proportion as voltage you do not want to go below). That's a tougher spec, because normally you want those two speeds closer together, so the square of one less the square of the other is what you get your energy from. When they are close, you have little to work with.

                    There is an advantage to be had, but it is not easy to get there.
                    Last edited by J Tiers; 01-23-2020, 02:40 AM.
                    2730

                    Keep eye on ball.
                    Hashim Khan

                    Everything not impossible is compulsory

                    Birds are NOT real, they are spying on you

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                    • #11
                      There was a multi-page discussion on this topic on PM a couple years ago. I haven't read it all, but there was some doubt about the efficacy of a flywheel on an RPC idler motor. There is also the question of ideal pole count for the motor.

                      https://www.practicalmachinist.com/v...-idler-349112/
                      http://pauleschoen.com/pix/PM08_P76_P54.png
                      Paul , P S Technology, Inc. and MrTibbs
                      USA Maryland 21030

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                      • #12
                        Originally posted by Franz© View Post
                        This thread gonna be fun to watch.

                        Oddly 900 and 1200 rpm motors work best, but I only been playing this game for 40 years.
                        What is this sorcery anyways?
                        Never even seen rotary converter but I want one since everybody is talking about them constantly.
                        Location: Helsinki, Finland, Europe

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                        • #13
                          No you don't; you want a phase perfect! (assuming you don't have three phase to your shop). I bought one of the only used PP I've ever seen locally; best thing I ever did other than discover IPA decades ago.

                          RPC threads are like oil threads on motor- head sites. Can't ever truly kill them.

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                          • #14
                            Something interesting I noticed in the Wiki was that the generated third leg of the RPC is at 90 degrees phase from the input, but I suspect it will shift toward 60 degrees (and lower voltage) under load. If this is true, it would be a reason for added capacitors or inductors.
                            http://pauleschoen.com/pix/PM08_P76_P54.png
                            Paul , P S Technology, Inc. and MrTibbs
                            USA Maryland 21030

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                            • #15
                              Originally posted by MattiJ View Post

                              What is this sorcery anyways?
                              Never even seen rotary converter but I want one since everybody is talking about them constantly.
                              3600 - 1 set of poles /Ø
                              1800 - 2 set of poles /Ø
                              1200 - 3 set of poles /Ø
                              900 - 4 set of poles /Ø

                              More poles & more iron works better.

                              Best solution - buy a Ronk

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