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  • VFD or RPC?

    Probably the age old question...

    End user in this case is a 1.5HP Fray V/H mill



    and the operator (me) is a just about a complete newbie.

    How often will I be changing speeds, such that swapping a belt becomes a PITA? Not often I'd imagine, seeing as this thing is for my own use in my single car garage.

    Given a 2HP VFD and RPC to be within 25 or so clams of each other, I'm leaning towards the RPC. What's the verdict of those with more experience?


    Andy

  • #2
    For this single machine and the VFD superior speed versatility I'd go with a VFD, this is a low hp unit and they're pretty reasonable. The guys can recommend which model Teco etc and where the deals are...probably ebay. If you think you may one day be expanding machine of more hp of similar voltage an RPC maybe the way to go.

    Nice machine, did it come with tooling? What voltage is it?
    Opportunity knocks once, temptation leans on the doorbell.....

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    • #3
      hey fellow Andy!
      while i love RPCs, in this case a VFD would probably be the better choice. the VFD will certainly be a benefit with regards to speed changes. do you think you'd be getting any other 3-phase equipment, or is this probably the only one? if this is it, and the costs are pretty similar, then the VFD seems like a no-brainer.

      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

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      • #4
        Hey, welcome! That is the same mill we talked about on pirate, isn't it? I'm Wicked_s10 over there.

        IMO, a well built RPC is the way to go. You can run several machines when you get them, and trust me, you'll get them, and you can always run a VFD off the RPC if you want that luxury. You have to keep in mind that most of the controls in the machine may run on single phase from the 3 phase inputs. Contactors and transformers inside the machine are not going to like the VFD output much, if at all.

        Most people who go VFD rewire all of the other stuff in the machine to run off low voltage and control the VFD, it all depends on how big of a project you want. After I built my big RPC, I never worried about what the next machine I dragged home would require. In my case, my RPC has a big xformer as well and I get 460 3p as well as 220 3p. If you go with only VFD, you have to shop for a VFD for every tool you drag home...

        Later,
        Jason

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        • #5
          I'm a RPC lover as well but if any future machines are similar hp he could just plug them into a single VFD one at a time, the VFD here will solve his belt changes and speed problems and likely get breaking too......

          If he stays with 230/460 machines ok if he gets 575 then he needs more hardware. I agree more machines will likely follow you home.......LOL
          Opportunity knocks once, temptation leans on the doorbell.....

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          • #6
            :flipoff2:

            Jason -

            Yeah, this is the one I asked about on Pirate...More pics of the machine and what tooling I also got are here - http://www.pirate4x4.com/forum/showthread.php?t=699461 It's a 220V 3PH unit with the vertical motor being 1.5HP and the horizontal is 1HP. There are a couple of electrical boxes on the machine, I'll take pics tonight or tomorrow and post them. Why would some of the electrics controls be running on 110? I don't know if it's a clue, but both the vert and horiz motors still use the original drum switches.

            I was initally going to pick up this Teco model http://www.factorymation.com/s.nl/it....f?category=32 but came across a deal on a 2HP 3ph motor. The interesting this, is I was told that a 2HP motor won't work as an idler on an RPC, and I can't figure out what difference it would make. Not that it doesn't have enough HP, but a 2HP unit won't work. Is that really the case? I think for the 25 bucks that the guy wants for the motor, I'll pick it up anyway. Am I correct in thinking a wash down duty motor is a pretty solid unit?

            What other machines am I thinking of? A lathe would probably be the only other thing I can see myself getting. I already have a 4x6 bandsaw and a drill press and they both run on 110v (although I'm thinking of switching the DP to 220)


            Andy
            Last edited by murph64; 07-22-2008, 12:33 PM.

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            • #7
              I had my mill, lathe and buffer on a homebuilt RPC for a decade, but now have all but the buffer on VFD's and have added a VFD controlled drill press. I hated always having to hit the RPC before using a tool, my prototyping style work patterns don't involve long sessions at one tool. Leaving it running all day seems wasteful.

              I find I change speeds far more casually than I used to, to the benefit on my work. 2 of my VFD's were each bought at online auction for less than $70 shipped. The really sweet thing about a VFD drill is how easily you can tap with it after you set up the external controls. I rarely put on my tapping head anymore.
              Last edited by gellfex; 07-22-2008, 12:49 PM.
              Location: Jersey City NJ USA

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              • #8
                I have no idea why you woul dnot be able to use a 2hp 3p motor for an idler. I have never heard anything remotely similar. The only specific rule I have ever heard repeated over and over is that the idler has to be 50% higher HP rated than the largest motor you are going to *start first* off it. I say start first, because supposedly, each additional motor you get running adds to the amount of HP that you can start. I did not worry much about it as I built mine with a 10Hp inverter duty motor. I figured if I ever bring home a machine with a motor bigger than 7.5Hp, I will have to build a bigger shop, and the RPC will be the least of my worries.

                As for why some stuff runs on single phase, well in most cases the motor lines and other odds and ends are not switched directly, but through contactors (big relays) the contactors use a low voltage to pull in the higher voltage, keeps the higher voltage somewhat isolated from the operator. Also, coolant pumps or switched coolant circuits are typically 110, as are switched work light outlets. None of the stuff in the machine will be happy with frequencies outside of the 50-60Hz they were designed for. And VFD's themselves are not happy about having the load disconnected while they are running, unless you send feedback to the VFD to tell it to shut down as well.

                As for washdown duty motor? I have never heard of it. Maybe someone else will chime in.

                Later,
                Jason

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                • #9
                  I have since found that a washdown duty motor can withstand high pressure water, as in a food processing plant where stuff gets hosed down/powerwashed on a regular basis. I passed on this motor as it's only 2HP, and for a 1.5 HP machine, I'd need a 2.5 idler.

                  I asked in the other VFD thread about running the VFD through the machines switches, because this mill has 2 3ph motors and I'm a little unsure how to wire them up if I'm supposed to go right from the VFD to the motor.



                  Andy

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                  • #10
                    Originally posted by murph64
                    I asked in the other VFD thread about running the VFD through the machines switches, because this mill has 2 3ph motors and I'm a little unsure how to wire them up if I'm supposed to go right from the VFD to the motor.
                    Andy
                    This mill has 2 spindle motors, a H & V? Hmm, If you went VFD you'd either need a switchover box or 2 units. I wonder if you can failsafe a switchover by using a 4 pole switch with the 4th on an E-STOP circuit, so any monkeying with the switch while the unit is running will kill the VFD (to keep it from being actually killed)
                    Location: Jersey City NJ USA

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                    • #11
                      The only way I see it working is to switch it with contactors. I don't have a lot of VFD experience, but my ABB VFD on my B'port says that it is acceptable to have a contactor, overloads, or even multiple motors on the same VFD, but ONLY if the low voltage contacts on the contactors or O/L's are wired to provide the VFD feedback of what is going on. Also, my manual made a point of specifying that there could be no R/C's or snubbers across the coils to catch the emf kickback. I currently run my motor with the original overloads because my VFD is way to oversized and the electronic overloads can't be set low enough to protect the 'ports little 2Hp motor.

                      My first two machines, a Gorton 0-16a and a Colechester 15x48 round head lathe, I never even considered a VFD for them. The Gorton is a 2 speed motor with step pulley drive, and it is not that inconvienient to adjust spindle speeds. A lot of times the difference between high and low motor speeds is enough for a lot of work. The Lathe, well it is geared head, so it is nothing to change speeds. The only reason I went VFD on my B'port, is that I wanted the CNC software to be capable of changing the speeds, that and my varispeed stuff was all shot to hell.

                      Most motors, and especially old machine motors are not meant to be used with a VFD. We have gotten in this discussion on here before, but but the root of it is that newer motors rated for "inverter duty" have a lot more iron in their cores, better cooling, and can handle the variable frequency better. Just about any 3 phase induction motor will run on a VFD, but they may not handle the lower or higher frequencies needed to get the variable speed part. Even my b'port which is relatively recent compared to my other machines, doesn't like to be run bellow about 30Hz. Fortunately I kept the back gear on it, and can get any speed I want with it from about 30-100Hz.

                      Later,
                      Jason

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                      • #12
                        Originally posted by murph64
                        The interesting this, is I was told that a 2HP motor won't work as an idler on an RPC, and I can't figure out what difference it would make. Not that it doesn't have enough HP, but a 2HP unit won't work. Is that really the case?
                        No. I use a 2 HP motor in that application. Works just fine.

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                        • #13
                          Originally posted by rantbot
                          No. I use a 2 HP motor in that application. Works just fine.

                          Interesting...


                          I was going to take the lazy way out and pick up an RPC panel from This guy on Ebay as I'm still a little sketchy on what all I'd need to make one, and he's the one that told me "a 2 HP motor won't work".


                          So I should pick up the 2 HP motor, then?

                          Originally posted by gellfex
                          This mill has 2 spindle motors, a H & V? Hmm,
                          Yup. Check out this thread on Pirate, and you can see the horizontal motor to the right of the horiz arbor.

                          Originally posted by gellfex
                          If you went VFD you'd either need a switchover box or 2 units. I wonder if you can failsafe a switchover by using a 4 pole switch with the 4th on an E-STOP circuit, so any monkeying with the switch while the unit is running will kill the VFD (to keep it from being actually killed)
                          Or something simpler - can wire the VFD to a receptical (like a 6-50R, only one for 3ph) and connect an "extension cord" to each motor? And whichever one I want to use, I'd plug in? REAL bootyfab, but it'll work.

                          Andy
                          Last edited by murph64; 07-22-2008, 10:49 PM.

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                          • #14
                            Go to Practical Machinist and look under the transformer, phase converter, vfd section, probably one of the most informative sites for this on the net and lots of knowledgable guys willing to help, drawings to build your own.
                            Opportunity knocks once, temptation leans on the doorbell.....

                            Comment


                            • #15
                              Originally posted by hardtail
                              ...lots of knowledgable guys willing to help...

                              Now that is kind of a stretch, don't you think?

                              This won't mean much to most of you, but it will to the OP:

                              I found the practical machinist board to be the Pirate4x4 of the machining world. I won't even go over there anymore after I found this place...

                              There is lots of info on building RPC's on line. I got all of my stuff surplus except the caps. I bought all of the caps at a local motor rewind shop, the owner was nice enough to let me root through his supplies until I found what values I needed. I got my basic design from a wood working web site, it came complete with some rough calculations to get you close on capacitance values. It worked out very well. If you like I can try and find the web site for you.

                              Later,
                              Jason

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