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  • #16
    How does a VFD work with the step-on-it brake of our lathe or the little brake lever on the Bridgeport that I've instinctively reached for over the last 35 years? Does one need an EE to wire the VFD into the circuits of the lathe's Forward-Neutral-Reverse lever, brake and jog buttons?

    If my American Rotary Power brand RPC unit is delivering only 85% power, I'd be shocked. The cuts I take with the lathe and 1HP BP using 2-1/2", 3" and 4" insert face mills belie the 85% factor. If the old static converter was good for only 2/3rds (at best) of rated power, I tend to believe our RPC unit is pushing out far better than 85%.

    I've seen how nice the VFD's can work. My neighbor Rollie G. has one on his newer Clausing 15x50 lathe, and it speeds up, slows down and reverses perfect. For me, the sub-$400 I spent for the RPC motor, starter/capacitor unit, and wiring made economic sense at the time. Being instant plug-n-play and not having to delve into any machine circuits made it completely painless.

    I could be convinced to go VFD in the future, but I'm more inclined to look at a Phase Perfect as I intend to someday install CNC milling and turning machines.

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    • #17
      Originally posted by PixMan
      I wonder about that too.

      Do you set the gearing of a lathe to maximum speed and run the motor very slow, or set the gearing at low and overspeed the motor? What about the torque curves? I know that with a fixed rpm motor I always have the same Kw rating available, and depend upon gearing to make the best use if it. How does this change with an VFD?

      It seems to me that both situations are something the motors weren't designed to do, hence my fears of affecting the life of the motors.
      On a lathe I change gears often, vfd or not, and use the ranges as required. VFD then gives me 20% to 150%. On a mill - I change rarely.

      Just assume that at the "native speed", i.e, 60hz, it develops its rated hp. At 30hz, it's about half, at 90hz, maybe 60%. Above that, it's non-linear and you run out of torque really quickly. With older motors, I stay below 90hz. Sensorless vector vfd can help at the very low end but who want to 'work" down there anyhow?

      As for motor life - VFD will help. Soft starting is great for everything in lathe; I hate hearing my headstock gears thud when the 3-phase is switched directly

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      • #18
        I am still leaning towards RPC as well. Plug and play, more than one motor, all the switches on the machines work like intended.

        My plan is to wire the RPC in the corner of the shop and run the 3ph wire to outlets at what 3ph machines I have. Plug the machine in and away I go.
        Andy

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        • #19
          Originally posted by Forrest Addy
          whereas your motor will run about 85% on an RPC.
          Where did you get the number, Forrest? It looks like it belongs to a good static converter rather than to a reasonably well balanced RPC.

          All sources I ever seen quote full power when a motor is run out off a well balanced RPC. Granted that the output of an RPC is not as ideal as a 3-phase utility power, I doubt that you'll ever notice the difference in terms of your machine power. Whereas with a VFD you will for sure (mostly, closer to the extreme speed settings).
          Last edited by MichaelP; 08-21-2011, 12:10 PM.
          Mike
          WI/IL border, USA

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          • #20
            Originally posted by PixMan
            How does a VFD work with the step-on-it brake of our lathe or the little brake lever on the Bridgeport that I've instinctively reached for over the last 35 years? Does one need an EE to wire the VFD into the circuits of the lathe's Forward-Neutral-Reverse lever, brake and jog buttons?
            ......

            I could be convinced to go VFD in the future, but I'm more inclined to look at a Phase Perfect as I intend to someday install CNC milling and turning machines.

            You can't use the brake on the vfd if you have programmed deceleration; simply set the vfd to freewheel on deceleration (i.e., it just cuts the power). On your mill, just take the handle off the brake On some VFD you can feed the the lathe brake switch to the vfd and have it freewheel only when you stomp on it.

            Phase Perfect? ha, I just put one of those it, so now I'll run three phase vfds as well.

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            • #21
              Originally posted by MichaelP
              Where did you get the number, Forrest? It looks like it belongs to a good static converter rather than to a reasonably well balanced RPC.

              All sources I ever seen quote full power when a motor is run out off a well balanced RPC. Granted that the output of an RPC is not as ideal as a 3-phase utility power, I doubt that you'll ever notice the difference in terms of your machine power.

              Balance? at what load? How many RPC are realy balanced within 5% at max load? How can the average guy at home even do this?

              Still, you may be able to get 100% out, but at the expense of your motor. Motor are supposed to be derated if the voltages are unbalanced. A 5% unbalance requires derating to 75% or so... here a useful link to explain:
              http://www.pge.com/includes/docs/pdf...lance_rev2.pdf


              Static converters? I will never use one on any motor I care about. Maybe you get 50% power before you are overloading the motor, and how do you know? If you 'hear" it slow down, it's already way into overload.
              Last edited by lakeside53; 08-21-2011, 12:18 PM.

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              • #22
                Originally posted by lakeside53
                Phase Perfect? ha, I just put one of those it, so now I'll run three phase vfds as well.
                What do you mean by this?

                A Phase Perfect Digital Phase Converter is no laughing matter in price or performance. Full power, perfectly balanced phase and voltage, and dead quiet. They are anything but cheap but if you're trying to power-up a 1990's vintage CNC mill or lathe such as I anticipate doing, a Phase Perfect DPC is the closest thing to incoming 3-phase, and probably cleaner.

                http://www.phaseperfect.com/

                Do you mean you have purchased and installed one of their products? If you have one of these, why spend even more money on 3-phase VFD's? I can see where some speed control on some machines would be nice, but it sounds like you'll be spending beaucoup bucks on power supply stuff. I try to spend it on tooling and machines.

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                • #23
                  Originally posted by macona
                  Install a VFD.
                  Agreed, this is your best bet. Yaskawa makes some real nice VFD that will run that mill without a problem.

                  JL.................

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                  • #24
                    Bridgeport Motor

                    mich 88 13

                    Sometime in the 70’s I bought a G.E. single phase ¾ Hp 115/ 230 motor from the local Bridgeport Dealer. It bolted on to my Bridgeport with no changes needed. It reversed instantly just as will as a three phase motor.

                    I do not understand what davidwdyer means by an idler motor. I ran my old 3 phase motor by hooking a capacitor to the third lag of the motor with a button hooked to one wire of the singer single phase. Turn the switch on while holding the button for a few seconds, the motor will run fine, but with some loss of power. Do not hold the button after the motor starts.

                    The ¾ Hp motor or the loss of power with the 3 phase motor has never been a problem .

                    Jcon (Jim Connell)

                    You haven’t began to learn till you learn how little you know

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                    • #25
                      Originally posted by jcon
                      mich 88 13

                      I do not understand what davidwdyer means by an idler motor. I ran my old 3 phase motor by hooking a capacitor to the third lag of the motor with a button hooked to one wire of the singer single phase. Turn the switch on while holding the button for a few seconds, the motor will run fine, but with some loss of power. Do not hold the button after the motor starts.

                      Jcon (Jim Connell)

                      You haven’t began to learn till you learn how little you know
                      The idler motor is used as you are using the normal motor with a capacitor to start it. Except that the idler motor, while idling, generates your third leg of three phase although it does not generate 100%. This is a cheap way two produce "3 phase" with only slight reduction of power. I've never noticed any lack of power. However, using an "air over" motor is slightly noisy.

                      Your motor, when running was only running on two legs, which must have noticeably reduced power. Perhaps unbeknownst to you, it was also generating a third phase of power which went unused.
                      Vitَria, Brazil

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                      • #26
                        I'm often challenged when I assert the 85% figure on HP attainable on motors run from an RPC. It's more a figure of merit than quantifiable expectation. 85% is pretty close to 100%. Without instrumentaton I doubt if I could tell if a machine motor was giving 100% or 85%. Most induction motors will 15% overload for the two or five minute duration of a cut. The motor will hum a little louder and the induction motor slip might be noticable in the note from the gear meshes. Otherwise I'd think the motor under 15% overload would do little other than warm a trifle more than normal. Keep it up for an hour and you may fry the motor. Depends on ambient temperature..

                        It stands to reason that an RPC balanced to run equal voltages at idle current will still have a weak leg at full current demand from the load motor. Whether the difference is 90% or 80% much depends on the relative rating of the idler motor, the load motor, and the circuit elements that "balance" the weak leg.

                        You really can't make these distinctions without instrumenting each individual case. Once upon a time I had time and resources to do just this. Results varied so I picked 85% as a fair approximation and went with it. YMMV

                        A VFD is not a universal solution. A little experience and strategy is needed to take full advantage of its benefits and features. A plain one motor manual machine benefits out of proportion to the investment from a VFD.

                        If you have single phase power and a complex machine combining motors, digital controls, and other wierdness, a Phase Perfect is the way to go. The output of a Phase Perfect can be treated just like three phase from the utilty company. Your machine won't know the difference except the power is a little cleaner on the synthetic leg.

                        Rant mode: The witless will install a VFD on a mill, set the step pully at 1700 RPM, dial the spindle speed down to 450, stall the mill with a 4" carbide shell mill, and forever after condemn a VFD as "having no torque". Idiot assertion.

                        People shouldn't use big words like "torque" if they don't understand what they mean. Torque is twisting force not power. You wouldn't lever up a rock by placing the fulcrum near your hands because of the mechanical disadvange. Similarly, you don't belt or gear a machine for high RPM, dial it down and expect it to develop full rated motor power at low RPM. An induction motor is a constant torque device. Dial it down in RPM and the developed mechanical power drops in proportion.

                        In these situations it's the operator setting conditions that's at fault not the VFD "reducing torque".

                        When ignorance of physics doesn't kill, it humiliates you or costs you money.

                        OK, rant off. The topic was a Bridgeport with a three phase motor. Yes, a VFD will do the job of running its three phase motor from single phase power and the features incorporated in a VFD will greatly enhance the operabiliy and performance of the machine. Thousands have made the conversion on similar amachines but very few were dissatisfied.
                        Last edited by Forrest Addy; 08-21-2011, 05:38 PM.

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                        • #27
                          As you can see, I don't have a VFD, but it sounds like someone would need a different one for every machine. Is this the case or can I use one for the whole shop but only use one machine at a time?

                          Sorry if this is a dumb question, but I never have worked with VDF before.
                          Vitَria, Brazil

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                          • #28
                            You'll need a separate one for each machine. Sorry!

                            Lakeside53,

                            Thank you for the very interesting link.

                            Now, some measurements and calculations demonstrate that my 3-phase RPC-based line, when fully unloaded, is 3.4% unbalanced. This is only slightly above the recommended 3% max. for the utility lines, but well below the 5% max. permitted by Nema MG-1 for motors.

                            As soon as I turn on one of my machines, the unbalance drops to 1.8-2%. Based on the table presented in the article, I can derate my motor to about 95-97% of its full capacity to be very safe. And it's not because the motor loses power, but in order to keep it at specs at full power (in terms of heat). Anything to avoid an extra 10% of temperature rise if I decide to take the motor to its limit.

                            How often, if ever, we operate our motors at their full capacity is, really, a rhetorical question.
                            Last edited by MichaelP; 08-21-2011, 06:56 PM.
                            Mike
                            WI/IL border, USA

                            Comment


                            • #29
                              Originally posted by MichaelP
                              You'll need a separate one for each machine. Sorry!
                              Well, that settles that question for me anyway. Everything is already working great without one.
                              Vitَria, Brazil

                              Comment


                              • #30
                                Well, IDEALLY you need a VFD for each three phase motor (single phase motors cannot be made to work from a VFD).

                                BUT - I went for years swapping plugs on my lathe and mill on a single VFD. I set the VFD in a central location, added a drag around control station with a magnet mounted on the bottom, made a long cord from control station to the VFD, and used good three phase rated plugs on the power connectios. I used this set-up from 1993 to 1998 when I lucked on to more VFD's.

                                A lot of home shop owners mounted their VFD's on a little stand they took from machine to machine. A VFD may run only one motor but it will run any three phase motor in a range from a little larger to 1/3 its rating - depending on the motor and how the VFD responds to it. You may have to jigger the parameters if the motor range is too extreme but that takes seconds once you're practice. Post a how-to cheat sheet on the VFD.

                                In the mid '90's, my VFD was a MagneTek GPD333. My mill motor is still 2 HP and Chinese. The lathe motor at the time was 5 HP and German. No parameter changes required. YMMV
                                Last edited by Forrest Addy; 08-21-2011, 05:55 PM.

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