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  • #31
    Originally posted by flutedchamber View Post
    Rich, did I read that chart correctly...are the input amps 132 for the 20 HP unit? It seems very high, unless they mean the starting surge amps. I could find no info as to what they recommended as the correct size 2 pole breaker for the 20 HP VFD. Do you know what it would be?

    Thank you for the input.

    If you're referring to the Polyspede link in your post, yes, that's a single phase VFD and the recommended breaker is 2X the rated input current. The fuses/breakers are sized to protect the vfd and external wiring in case of an internal vfd failure.


    You can't use a 20hp vfd to "replace" a 20hp rpc directly. VFD's drive motors - not machines with transformer, controls etc. For your WEG - forget "HP" and just look at motor current. I'd guess the 10hp single phase vfd will work fine, but you'll need to choose one set of motor windings and use those. Switching pole configuration on the fly for high/low speed is not recommended..

    Also.. is your 20hp rpc really a 20hp motor? if it's on a 60amp breaker, probably not. It may be one of these wacky "20hp capable" ratings, with different ratings for single motor startup and total connected load.
    Last edited by lakeside53; 04-21-2014, 11:54 AM.

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    • #32
      Originally posted by Rex View Post
      Good advice above. I'm dealing with the same decision right now, with a single-phase shop and several nice 3-phase lathes recently acquired. I have several good Mitsubishi VFDs and a nice loaner RPC, so I have been researching and experimenting.
      One thing about VFDs is they don't like downstream switchgear. You can't just plug the VFD into power, and then clip the plug off your machine and attach to the VFD, unless power goes straight to the motor with no switches in between. You have to use the low-voltage inputs from the VFD to control your on/off/reverse etc. On many machines that's fairly simple, but if there are contactors and such it may be complicated.
      OTOH, and RPC is basically plug and play, using all the OE controls on your machine and it's accessories. And additional machines can feed on that RPC. Additional idling motors actually make it work better.
      I'm a big fan of VFDs, but I must say walking over and plugging into that RPC is pretty nice. I think I will have to build an RPC soon.
      Rex,

      It's actually quite simple to hook a drive to a lathe and retain all the lathes factory controls..FWD, REV, JOG and of course E-stop. The drives output is connected directly to the motor while all the factory contactors on lathe are used to command the functions on the logic buss of the drive. This is very simple if the coils on the lathe mags are 120 volt and not impossible if otherwise. The big upside to this drill is it lets you retain all the factory push buttons and Fwd/Rev lever on the lathe if so equipped which results in an ergonomic machine that looks stock.

      Stuart

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      • #33
        Originally posted by lakeside53 View Post


        You can't use a 20hp vfd to "replace" a 20hp rpc directly. VFD's drive motors - not machines with transformer, controls etc. For your WEG - forget "HP" and just look at motor current. I'd guess the 10hp single phase vfd will work fine, but you'll need to choose one set of motor windings and use those. Switching pole configuration on the fly for high/low speed is not recommended..

        Also.. is your 20hp rpc really a 20hp motor? if it's on a 60amp breaker, probably not. It may be one of these wacky "20hp capable" ratings, with different ratings for single motor startup and total connected load.
        The higher power is recommended on RPCs because of inrush.... The load motor pulls so much current that the generated leg is pulled down in voltage so low it doesn't do its job.

        A higher HP motor has lower resistance and inductance so teh generated leg holds up better.

        A VFD does a ramp-up, and will drive a motor that is actually over its rating, if you do not load it heavily, and can tolerate a slower ramp-up.

        you should be able to do HP to HP directly, if the VFD is any good at all. And with the Polyspede being single phase input, all the derating is already taken care of, you don't need any more.
        CNC machines only go through the motions

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        • #34
          Originally posted by J Tiers View Post
          The higher power is recommended on RPCs because of inrush.... The load motor pulls so much current that the generated leg is pulled down in voltage so low it doesn't do its job.

          A higher HP motor has lower resistance and inductance so teh generated leg holds up better.

          A VFD does a ramp-up, and will drive a motor that is actually over its rating, if you do not load it heavily, and can tolerate a slower ramp-up.

          you should be able to do HP to HP directly, if the VFD is any good at all. And with the Polyspede being single phase input, all the derating is already taken care of, you don't need any more.
          I have a Teco FM50 1 HP vfd that is 115 input 230 3 phase out, I use it to test motors, I have ran up to 5 HP motors with it so far no problem, like J Tiers said it ramps up, nice smooth acceleration....I have even ran a lathe with a 5 HP motor to test it out, light cuts to see the threading operation....I just purchased a BP mill a guy had that never used it in the 10 years he had it due to not being able to find a single phase motor for it...his mouth dropped open when I showed up to test it with my little FM 50.............

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          • #35
            I have all kinds of stuff in my Home Shop. A home made rotary unit based off of Fitch Williams plans. Works great and has plenty of overhead. I built the 5hp unit for my bridgeport. I have several Yaskawa 5hp VFDs. A lil small for you but they make some larger units. I also have some Hitachi units in the 5hp range.

            The big bear in my garage is my American Rotary 15hp unit. Yup, its a large Baldor motor with a good panel. WHY? This stupid Emco CNC lathe I bought. I read it doesnt like VFD juice. It needs a true three phase supply according to most folks that have them. Its a small 3" lathe!!! ERRR! Dont buy a small lathe that needs so much overhead peeps. Its NOT worth the travel. Well I did and consequently I have a massive RPC in my garage. I can power up ALL my machines with this RPC. Not a great choice for a small HSM garage IMHO Learn from my mistakes so you don't have to follow them...JR

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            • #36
              JR: No reason in the world why that Emco would fail to work with VFD power. Not unless the motor is so bad that it will arc over or heat up... I have yet to see a motor that bad, but since even the chinese admit their motors are bad, there may be some.

              Maybe the folks who claim the Emco won't work have not changed the controls to match..... Very few VFDs will start an equal rated motor without a ramp-up, so if they just "hit the switch" they will get an overcurrent trip. You need to either ignore the machine controls entirely, or convert them to send signals to the VFD.

              .
              .


              If you DO have a number of machines that you need to run, a large RPC can easily make sense.... It would not "be a mistake".

              Considering that you may pay $150 per each motor for 1 HP VFDs, and considerably more for larger ones, you can pay for a lot of electricity with the difference if you DIY an RPC instead of buying a whole slew of various sized VFDs.

              Machines that have multiple 3 phase motors may cause even more economic havoc to power with VFDs, since each one may require its own VFD. One machine might take a large VFD for the main motor, and at the least one more for feeds and coolant pump, if they are 3 phase.

              You may be able to run both feed and pump from a single oversized VFD, or use a static converter on each, but you will still be paying for extra stuff.
              CNC machines only go through the motions

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              • #37
                I'm not surprised the Emco won't work. Neither will many other "cnc" lathes or even manual lathes. Manual lathes are relatively easy to convert, but the CNC Emco isn't just a lathe with a 3 phase motor -it's steppers, electronics, spindle control (?) etc etc... and would take some significant restructuring of the internals to make it work on single phase and "a vfd". An RPC is a no-brainer in this case, but it could have been quite small.

                For those who think it's a choice of rpc verse vfd, it's not - both have their places and overlap. In addition to many VFD's, I also have an Phase Perfect in my primary shop -bought it used at a great price and sold my RPC(s). I still have an RPC in my second shop, but it's just to power my shear; one day that will be vfd'd, but low priority. Unfortunately PP is not an option for many as they almost never appear used on Ebay or CL. An RPC is almost invisible in the background and is a good complement to vfds.

                Nice thing about the PP is that it uses about the same power as a 100w light bulb when powered up and no load.

                Maybe the PP patent (? - something is suppressing the clone market) will expires soon and Hyang will start cranking them out
                Last edited by lakeside53; 04-22-2014, 11:37 AM.

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                • #38
                  Not sure why some folks were having problems powering the Emco 120 CNC lathe. Might be cause its a 380V 3 phase machine. The VFD would feed a transformer. Would that be an issue with a VFD? JR

                  Some stuff I wrote on the PM site re: the emco.

                  http://www.practicalmachinist.com/vb...winded-102474/
                  Last edited by JRouche; 04-22-2014, 02:05 PM.

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                  • #39
                    Generally... you don't use a vfd to feed a transformer for "general purpose" 3 phase. VFD's are variable voltage variable frequency devices and all about motors. I have seen some doing this to feed a motor, but there are issues even with this.
                    Last edited by lakeside53; 04-22-2014, 03:07 PM.

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                    • #40
                      Originally posted by lakeside53 View Post
                      If you're referring to the Polyspede link in your post, yes, that's a single phase VFD and the recommended breaker is 2X the rated input current. The fuses/breakers are sized to protect the vfd and external wiring in case of an internal vfd failure.


                      You can't use a 20hp vfd to "replace" a 20hp rpc directly. VFD's drive motors - not machines with transformer, controls etc. For your WEG - forget "HP" and just look at motor current. I'd guess the 10hp single phase vfd will work fine, but you'll need to choose one set of motor windings and use those. Switching pole configuration on the fly for high/low speed is not recommended..

                      Also.. is your 20hp rpc really a 20hp motor? if it's on a 60amp breaker, probably not. It may be one of these wacky "20hp capable" ratings, with different ratings for single motor startup and total connected load.
                      My phase converter is a true 20 HP unit, made by American Rotary. It is a soft start unit, rated at 50 amps and is on a 60 amp dedicated circuit. Start current for the first two seconds is 125 amps, but then it drops to a few amps.

                      I had contacted a few VFD manufacturers when I was shopping for 3 phase power for my shop and they all said I should "upsize" for safety because of the motor. As you mentioned, the two phase motor presented it's own set of problems.

                      I'm happy with my America Rotary phase converter. It easily runs my lathe and milling machine and whatever else I may choose to buy in the future. Plus there is the fact that I would need a VFD for the lathe and a separate one for the milling machine.
                      Last edited by flutedchamber; 04-22-2014, 11:40 PM.

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                      • #41
                        Originally posted by flutedchamber View Post
                        As you mentioned, the two phase motor presented it's own set of problems.

                        Single phase. Two phase is not used any more.

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                        • #42
                          If the Emco is intended to work from 3 phase, it will work from a VFD, BUT...... to power general electronics, the PWM needs a "sine filter" or "reconstruction filter" to take the pulses and produce a clean sine wave.

                          A Phase Perfect includes a sine filter on the output, so it should be fine, despite being *exactly like* a VFD internally.

                          We have used VFDs (with a new control program) as grid-tie inverters. Works fine, but again, you need a sine filter.

                          A motor acts as it's own filter, converting the "volt-seconds" of the pulses into current. Other things may not do that as efficiently, and need the help of a filter.

                          You would set the VFD to a constant 60 (or 50) Hz, and use a sine filter. Electronics don't care much for raw PWM as a source voltage, if the pulses get through as interference.

                          Originally posted by lakeside53 View Post
                          Generally... you don't use a vfd to feed a transformer for "general purpose" 3 phase. VFD's are variable voltage variable frequency devices and all about motors. I have seen some doing this to feed a motor, but there are issues even with this.
                          It can be done.... but usually one would prefer to step up the voltage and then use a higher voltage VFD. That will work without issues.
                          CNC machines only go through the motions

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