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VFD & DC motors

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  • VFD & DC motors

    In a different thread I made mention of the need for slower spindle speeds thus slower carriage travel when cutting large threads. Similarly, large diameter turning seems to be an issue for me at even 65 RPM; chatter is often prevalent thus the ability to reduced spindle speeds, I think, would be of benefit. I also realize that a properly ground tool, positioned correctly, plays an important part but it seems that this is less of an issue when turning small diameter work.

    With limited computer skills I have attempted several searches on this forum to learn more about VFD vs DC motors and controllers. Having found some information, various questions still plague me and I would like to appeal to the masses for more direct and pointed discussion and information.

    I contacted a firm regarding a VFD. Though a VFD is available for single phase 220V motors, the “take home message” I got from the distributer was, a VFD is better suited for three-phase. The salesman exclaimed that torque is lost at lower speeds on single phase and changing frequencies on a single phase motor will ultimately cause damage to the motor. Naturally, cost of a VFD is appealing when compared to purchasing and installing a DC motor and controller. Conversely, it was stated that DC motors do not lose torque at low speeds and the motor is not harmed when reducing motor speed.

    Please relate your experiences with regard to using a VFD with single phase motors and while making particular mention of possible damage, torque, and absolute lowest RPM limits when using this device. In passing, I was told that a DC motor could be slowed to even ½ RPM without loss of torque or damage to the motor whereas a single phase AC motor can only be slowed within certain limits.

    I will have more questions as this thread matures.


    For those having fought for it, Freedom has a flavor the protected will never know.
    Freedom is only one generation away from extinction.

  • #2
    I am not aware of any VFD's that will run a single phase motor????

    If you only need 1hp or less, then you can get 115volt VFD's that will convert to 230volt 3phase. So this allows you to use a 230volt 3phase motor.
    I have Automation directs 1hp gs2 VFD unit that does just that. I run it with a Marathon 1hp 3ph motor.Good inverter duty motor, TEFC.
    I also have a Leeson 3/4hp 90 volt dc motor using a variable speed Minarik drive. Not a treadmill motor. This is a very nice industrial TEFC motor. Not cheap though.

    The vfd has all sorts of bells and whistles, programing options and external control options.
    The DC drive is much simpler.
    Both will run much lower than 1/2 speed with very good results for hours and not heatup. The vfd can aso more than double the motor speed.

    It's a tough call. They are both very smooth and quiet.
    I have them both running my older lathes.



    • #3

      What is TEFC? What size lathe has the 3/4 90V motor? If I go that route, I am operating a 12 X 36 lathe.

      For those having fought for it, Freedom has a flavor the protected will never know.
      Freedom is only one generation away from extinction.


      • #4
        Harold, my lathe is about the same size as yours and I too have the occasional need for a slow speed.

        But I am not considering a VFD as that would be quite expensive for me, instead I am looking to make a smaller pulley for my existing 2HP single phase motor, or maybe a pair of step pulleys.


        • #5
          Harold, TEFC= Totally enclosed , fan cooled.

          I have the 3/4hp dc motor on a South Bend 9A. Plenty of power for a 9" South Bend.
          That would be to small for your 12x36 though.
          What size motor is on it now?


          • #6
            Without going into all the particulars (Someone else may want too) for your application a VFD driving a 2hp 3ph motor will be the practical solution. This is the solution I chose for my 12x36

            A DC motor in the size needed for you is getting into the rather expensive ranges -- The usual variety of DC conversion seen among our groups in the Treadmill types and IMO wouldnt have enough oomph. Theyre great for the Chinese 9x20 & drill presses -, etc (like S J H's southbend) They will be rated in 2 & 21/2hp but they wont pull your lathe.

            For what youre talking about a VFD wont drive a single phase-- This is obviously a much broader subject than this brief bit, but I think it still sums up your best choice.
            If everything seems to be going well, you have obviously overlooked something........


            • #7
              There are Single Phase Motor VFDs...


              Brush type (Universal) motor Not required...

              They are somewhat limited, but for a 3/4 Lathe Motor would work fine


              • #8

                Are you aware of the fact that typically all modern VFD's up to 3HP will generate full 3 phase output from a single phase input?

                I'd definitely get a 3 phase motor rather than trying to mess around with speed controlling a single phase motor.

                I believe there are even some VFD's that run off 110 single phase and generate 220 3 phase output.

                Good luck-

                Paul T.


                • #9
                  I had posted some single-phase (output) vfd's a few months ago. As I remember they were very expensive compared to drives for three phase motors, four or five hundred I think. That would pay for a 3ph drive and motor right there. The 1 ph drive's usable range was less than that of a cheap 3ph setup. (which has already been stated).

                  As far as DC motors go, I just ordered a pair of 180v 1750 rpm DC motors at work, even with our discount they were over 2k apiece.

                  Yes, DC motors offer a flatter torque curve, but for applications like spindle turning, they're just not worth it. By the time you gear down from 1750rpm (I'm assuming that's your motor speed) to 65 rpm, at 1 hp you're getting about 80 lbft of torque at your spindle. That's a big number. So even if a three phase motor dropped to 50% torque output, you're still getting 40 lbft.
                  are you sure you need that much? (these are rough numbers, but I'm just trying to compare magnitudes)


                  • #10
                    Well, I've asked that "food for thought" be given. You guys have given food for thought.

                    Someone asked, "What HP is your motor"? I don't know but if all goes as planned I will be able to provide an answer this evening (AK time).

                    So, if I am hearing correctly, I should really consider purchasing a 3ph and VFD and "trash" the idea of going DC. Again I would like to say that my main goal for "chasing change" is to have the option of slowing my lathe **way down** in order to cut large threads, e.g., 4 TPI. 65 RPM is simply too fast for these old eyes and reaction time.

                    For those having fought for it, Freedom has a flavor the protected will never know.
                    Freedom is only one generation away from extinction.


                    • #11
                      The disadvantage of slowing via the VFD is the lowering of the power output.

                      Even if the torque is maintained, or slightly increased, power (which is what cuts the metal) is reduced in proportion to speed. Torque cannot be drastically increased, because the current would have to increase beyond the capability of the motor to handle the increased heating, even with helper fans.

                      If you have 65 rpm, half speed is quite reasonable, and should be fine for you. 33 rpm is sufficiently slow for most folks. Even 20 RPM would be possible without drastic power problems, given that you have a somewhat oversized motor to begin with, and drive to match.

                      A pulley solution might be practical, and quite considerably cheaper
                      1601 2137 5683 1002 1437

                      Keep eye on ball.
                      Hashim Khan

                      If you look closely at a digital signal, you find out it is really analog......


                      • #12
                        Originally posted by J Tiers
                        A pulley solution might be practical, and quite considerably cheaper
                        When changing a pulley is mentioned, or "pulley solution", I have a mental picture of having to stop whatever I am doing, getting out the pulley-puller, and removing a pulley and replacing it with another. Surely this is not what individuals have in mind.

                        Maybe there is a way that additional pulleys are added and the only requirment is to loosen a bolt and rearrange a belt. On the other hand, I rarely use the highest speed so maybe a compromise could be made. Wish there was a way to install back gears like those on my previous South Bend.

                        Regarding torque, I thought an increase in torque would mean an increase in "hogging ability". How can you have increased torque with decrease in "power"?

                        For those having fought for it, Freedom has a flavor the protected will never know.
                        Freedom is only one generation away from extinction.


                        • #13
                          Harold, does your lathe have 9 or 18 spindle speeds?

                          Mine, which appears practically identical to yours, has double pulleys for using two belts but I know some of these same lathes have a set of pulleys where each pulley has two grooves but not the same diameter so if your lathe claims 18 speeds you would have to move the belt across, am I right?

                          Anyway, to gain lower speeds you could make a pair of pulleys, just like the ones you have but different ratios so as to get the lower range.

                          Changing to your low speed would just be a matter of moving the belt, just as it is now to get those other 9 speeds.

                          I think there is plenty of room at the back of our lathes to drop the motor several inches on some sort of outriger frame and to introduce another idler shaft with different sized pulleys to get the required reduction. This is something I have considered doing myself.
                          Last edited by The Artful Bodger; 04-08-2009, 09:33 PM.


                          • #14
                            power, or in this case horsepower, is torque x rpm x 5250

                            if you maintain constant torque at lower rpm's then you're producing (or consuming) less horsepower.

                            installing a smaller drive pulley will indeed drop your rpm's across the board, and give you a slightly better torque multiplier, but remember there's a limit. Belt drives can only handle so much load per rpm... too much torque at too low a surface speed and you'll exceed what the belt can transmit, and you'll slip. Which is probably not a big deal in most cases, I mean, you probably won't blow anything up...but slipping belts on a regular basis isn't good for surface finish quality.

                            If you have a gear head lathe then you need'nt worryabout this,just undersize yoour drive pulley and recalculate your spindle speeds. If you have a 100% belt drive then there's a limit to how much you can 'gear' down with belts.


                            • #15
                              I also have a lathe that offered a minimum spindle speed of 65 rpm, way too fast for my threading ability and eyes. Conversion to 3 phase and VFD has made threading a much easier process, especially when threading to a shoulder.

                              I suggest you give some thought as to the physical attributes/model of motor you plan to use in your conversion, before proceeding. Cabinet and black splash sheet metal (without modification) may restrict your choices, as will shaft size, shaft placement and pulley size.

                              I initially attempted to install a new US series 3 phase Baldor motor (got a great deal on it) and after several hours of fooling around, came to the conclusion I would be left with a real hack job attempting to get the 'great deal' motor to work. Biggest problem was that the US series motors have the connection junction box on the opposite side as compared to the metric series (I have a Jet lathe (aka: "Leakin' Lena")). The connection box failed to clear the enclosure sheet metal by a mile. Cutting out clearance meant many chips out through the motor area into the motor fan intake. Even though the motor was TEFC, unacceptable.

                              I bit the bullet and went for the metric 3 phase equivalent. Bolted right in, drive pulley fit, same drive belts could be used and the pulley alignment was perfect. No sheet metal mods were required. Right motor for the application. Some differences though, the new motor was MUCH heavier than the old, dictating a realignment of the lathe and all lathe speeds are reduced overall as a function of the 3 phase motors offered for the application, since they run at a slightly reduced shaft speed as compared to the single phase previously installed.

                              Bottom line, spent WAY more than I had planned to and have a nice new Baldor sitting on the shelf looking pretty, but I'm very happy with the VFD set up.