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  • RPC noise

    i see an RPC in my future and have been reading with interest the current threads. There is a lot of complaints about how noisy they are, noisier than the machines they drive etc. I don't get it - aren't they basically a free spinning motor? How could that be noisier than a transmission, spindle & bearings etc. Motors not connected to anything mechanical don't seem that loud so I'm looking for perspective - how bad are they and what causes all the noise? Do most try to reduce it? rubber feet, insulated box etc?
    in Toronto Ontario - where are you?

  • #2
    I have a suspicion that RPCs are almost invariably created using "reclaimed" motors. Since they are usually multi-HP, they are ex-industrial, and perhaps not "at their best." This likely means growling bearings or similar complaints. As you say, they are a free-spinning motor, so they dont get an overhaul and they are noisy. Its a lot easier to complain than repair! Duffy
    Duffy, Gatineau, Quebec


    • #3

      A rotary converter does use a free spinning (unloaded) 3 phase motor as a rotary transformer,
      when I opened the case on the one I bought new, I was a bit disappointed to see a cheap looking, probably Chinese motor sitting there.

      A friend of mine also had a 3hp converter with a decent Brookes Crompton motor
      British made and very quiet

      Mine on the other hand is noisy with the motor whining sometimes ( place bad joke here )

      I looked at swapping the motor, then quickly put the cover back on as the amount of wires going in and out of the motor scared me

      As to the operation of the converter, no problems at all, starts and runs all the machines, even a little bandsaw welder, two phase, just noisy.

      The plan is to remote locate it with a r/c socket and contactor.

      I no longer own tools I had.


      • #4
        Before me and my partner got out own shop we were renting a corner of a steel building from a carpenter, he had a big 4 axis gantry router with 3 rotaries, 5 foot by 12 foot, that he made himself, it was nice.

        Anyways, we used his 25hp RPC and he had it bolted to a shelf that was welded to the building, it turned the building into a giant speaker/amplifier for any little noise that came out of it. There was also a ton of fan noise, and in the small unenclosed building, with no insulation to absorb the noise, it was miserable.

        Running a 10 and a 20hp now, while waiting for 3 phase. They stay outside in a dog kennel. I could live with the 10 being inside, but the fan noise from the 20hp is just too much.


        • #5
          I built my RPC from a used motor I bought from a building demolition salvage company. The converter worked great but was noisy, getting worse as time went along. Finally I tore the motor apart, the grease in the bearings was fried into a hard crisp mess. Cleaned the bearings in solvent, blew them clean and dry with air, repacked with very high quality pretty red #2 lube, reassembled the motor and now have very quiet RPC. JIM


          • #6
            I haven't used mine recently, but I made an RPC many years ago using a big old sleeve-bearing motor, on the recommendation of a local motor shop. They guy there said basically to use the biggest, oldest motor you can find, with plain bearings and lots of iron and copper. Since it runs unloaded, it's pretty quiet. Even so, it is not inaudible.

            But if possible, I'd try to figure out a way to put the converter either in and adjacent room, an outside shed, or an enclosure.


            • #7
              I have a 7.5hp rpc. Mine was inside a 1 car well insulated shop. The noise was pretty loud, not terrible but just grating. The frequency is high enough to annoy. My mill is cnc so turning off the rpc between cuts is painful (re-boot, re-home, re-zero)

              The other issue is heat. In winter the waste heat was welcome but in summer it turned my shop into an oven. Noticeable temp increase... easy 5-10 deg (my shop is tiny)

              I built a box to try to muffle the noise. Worked pretty good but I wound up cooking a few caps. Had pretty good ventilation too. stood on 2 2x4's so the bottom was open and the top had 4 4" hole saw cuts. Not enough I guess.

              Now it sits outside under a roof. MUCH better.


              • #8
                I bought a 7.5 hp unit from American Rotary, and it's quieter than my mill's motor. I have it sitting on a wooden platform, on a piece of sprint car tire tread, so that helps dampen any vibration and noise.

                Montezuma, IA
                David Kaiser
                “You can have peace. Or you can have freedom. Don't ever count on having both at once.”
                ― Robert A. Heinlein


                • #9
                  I built mine from what turned out to be a new surplus Hitachi motor that I got for the bargain price of $25. As such, I expect the bearings are good. Mine is a TEFC motor, and they have an external fan. I think almost all the noise is from that....spinning at 3750 RPM and mounted a half inch from the back of the motor housing and inside a shell designed to direct the air over the motor. It makes for a good bit of wind noise.

                  I can't explain what it is, but I have found the same issue to be true in my case....I expect things to go silent when I turn off the machine I am running and somehow even the quiet idler messes with my minds expectations. I suspect that in a commercial shop, everyone expects that the "noise floor" is high enough that something like this would be completely un-noticed. I have a TV and radio in my shop and somehow my expectations are different.

                  Like I posted in another thread, however, I remote mounted the motor under a work bench in a corner of the room with the motor sitting on a piece of rubber on the concrete floor and got rid of a lot of the noise.

                  Paul Carpenter
                  Mapleton, IL


                  • #10
                    my RPC is built from an old 5HP compressor motor and i have it mounted to a cheap Horror Freight two-wheeled dolly so i can move it around. it sounds like any other old electric motor just sitting there spinning free. i'm not sure what motors these guys are using that they are calling them "loud". a small Dremel tool running is louder than my RPC.

                    andy b.
                    The danger is not that computers will come to think like men - but that men will come to think like computers. - some guy on another forum not dedicated to machining


                    • #11
                      Mine is not louder than a small dremel tool running. Edit-- I take that does move more air so there is more air noise. I guess that's my point, however. I would find it annoying to work over the noise of a Dremel tool running all the time. I could not care less when another machine is running....its the fact that the noise is occuring and nothing is happening. Its like leaving the vaccuum cleaner running in your family room while you watch TV. I know its nuts. I would probably just adjust if I worked in a factory where most certainly the background noise level is high.

                      In my shop, however, the "norm" is near absolute silence or just a TV or radio on...its somehow different than a motor just doing nothing making a relatively quiet dull roar. I am sure its worse for the guys with a motor with bearing noise etc. Motor bearings are not that expensive for more typical size motors so that may be a good investment.

                      I think its the particular spectrum of the fan noise masks a lot and that means I don't hear my cell phone, the radio, etc. Its sort of white noise in that sense. Like I said, though, I reduced the issue substantially by burying the motor in the corner of the shop behind some stuff.

                      Paul Carpenter
                      Mapleton, IL


                      • #12
                        Re: RPC's

                        When I bought my mill, lathe, and surface grinder, I bought a RPC "Kit" from the dealer I purchased my machines from. The kit cost me $150 and he threw in a 5HP motor for free. It is a real industrial, made in USA, ball bearing motor and is so quiet that I sometimes forget to turn it off when I get done working. I mounted the motor and box on a frame that I attached to some pressure treated 2 X 10 scrap. It sits on the concrete floor next to the I beam upright that is part of the frame of my metal building. The breaker is clamped to the leg of the I beam and wires to the machines run up the frame and out to the machines. The whole system has worked great and I wouldn't trade it for anything. That said, the only thing I can imagine that would cause the noise would have to be bearings and fans. My motor gives off a low frequency hum that is typical of a motor idling, but it is not that noticeable if you are not listening for it.
                        Jim (KB4IVH)

                        Only fools abuse their tools.


                        • #13
                          My home made unit is quiet. I picked up a used GE 5 hp motor. Replaced the bearings. It makes a lot less noise that my shop fan that is a necessity he in Florida in the summer. I too sometimes forget to shut it off.


                          • #14
                            My 10hp one I built is far quieter than any of my machines. I have accidentally left it running in the shop over night.

                            You guys that have high pitched whines- my guess is you did something wrong and your capacitance values are way out of whack, or your starting circuit stays latched to long. It should not whine, it should not have any high frequency component.

                            As for motors, I agree, get a good one, or rebuild a surplus one. I think this is likely the biggest issue most people with noise. Just worn out bearings and sad tired motors. I was really shocked the first time I came on here and saw people saying, RPCs are noisy, I have never heard much out of mine.


                            • #15
                              I have a 5 hp GE motor and it is no louder than the mill motor or the lathe it powers. I too have left it on because it was not loud and when I turned the shop lights out the pilot lamp on the wall told me to turn it off.

                              I would suppose those that are loud have some kind of problem. It's just a motor and should not be loud.
                              It's only ink and paper