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Reducing speed by 40:1 using VFD?

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  • Reducing speed by 40:1 using VFD?

    My woodworking bandsaw has a 1400 rpm three phase motor. The blade speed is 1000m/min. In order to cut metal, the blade speed needs to go down to 25m/min, 50m/min and 100m/min. So at the extreme, I need a 40:1 reduction. Have seen threads where people did very nice pulley based conversions, but still wondering if a VFD can be used for this.

    The frequency will have to go down to 1.25Hz, so the motor will be pulsing and vibrating a lot. Also, heat will be a big problem, especially since metal cutting can take a long time. Is there any way to make a VFD work for this application?

  • #2
    Taydin, yes you can reduce a motor RPM over a 40 to 1 range by means of a VFD but there are a few big problems to consider. The first is stability. You will need a sensorless vector VFD one capable of compensating for the "slip" characteristic of every induction motor. The second is the mechancal power developed by an induction motor is proportional to its RPM. At 40 to 1 frequency reduction the motor will develop about 1/40 of its rated mechanical power.

    An induction motor is a constant torque device. If the motor develops a Nm torque at 1440 RPM it can develop no more than 1 Nm at 1440/40 or 36 RPM. It takes more tractive force on the blade to drag it through metal. The 1 to 3 Kg needed for wood becomes 14 or so for steel. You need a mechanical reduction.

    My suggestion is to set your saw up with two V-belt reductions. One for cutting wood and one for cutting metal. You change belts according to the material you wish to cut. Use the VFD to adjust the band speed to suit your work.
    Last edited by Forrest Addy; 05-20-2011, 05:29 PM.


    • #3
      Also one of the problems with sensorless vector is it start to lose its effectiveness a low rpm, the best way to get constant torque down to 0 rpm is with a pulse wheel or encoder option.


      • #4
        I've asked a few questions on VFD's over the last year. Usually I'm met with more confusion than answers by it all. Here's my take. I'm sure it'll get blasted down on a technical level, but it makes a good benchmark logic for the whole system in my head:

        A VFD system has three parts:
        1) Horsepower (amount of power exerted)
        2) Torque (amount of "mechanical" advantage available)
        3) Speed (in this system, a function of Hertz)

        If you change one, one of the other three will change in proportion. So, for example, if you halve the speed (i.e. Hertz setting on the drive from 60 to 30 in the USA) you will proportionally reduce the horsepower by half as well. In this example, the torque will then remain the same. That's the rule: two are affected in equal proportion for the third to stay constant. Generally speaking, the motors used on machine tools are of the constant torque type as explained by Forrest above. So it becomes a math problem of the following type:

        *How much horsepower is necessary at 35rpm?
        *Take that number and multiply by 40 for a 40:1 reduction in speed from the motor listing.

        In conclusion, no. It won't work without using a pulley reduction or speed-reducing gearbox to bring the speed into a closer realm than 40:1. Both a pulley or gear reduction system are able to retain horsepower and torque while reducing speed.
        Last edited by Arthur.Marks; 05-20-2011, 07:04 PM.


        • #5
          Not only retain torque, but torque increases by the ratio of the reduction.


          • #6
            Heck yes. If you want 1hp for metal, just put a 40hp motor and vfd on your Bandsaw

            I just traded "up" from my 14 inch Jet woodworking bandsaw to a Wilton (err... Jet) 8201. That has belts and a gearbox to cover 3300 to 39. But, that model is also 3 phase, so it's getting a $130 VFD anyhow.


            • #7
              As Forrest said, sorry but I'd suggest you need a mechanical reduction. While I have a VFD on my lathe and don't ever change speeds for normal operation, I still use the backgear when threading with dies at very slow speed, otherwise I can stall the lathe motor.



              • #8

                I have an old woodworking bandsaw that has been converted to metal cutting with a very large pulley system It works very well, but it is now a metal cutting bandsaw and not used for wood anymore.

                I had a thought, since it appears that a 40:1 VFD reduction isn't practical, what if you reduced the speed by a factor of 20:1 with a more reasonable sized pulley system and then went the next 20:1 with a VFD for a total 40:1 reduction? There would be some additional cost involved over a purely mechanical reduction, but it would not be as awkward in size.

                Not sure if this might be the best of both worlds or the worst of both worlds but I thought I'd suggest it so that maybe someone more knowledgeable about VFD's could comment.

                Reno, Nevada, USA is having a Turkish Cultural Festival on Sunday and I'm looking forward to going into town for a Turkish meal.



                • #9
                  Originally posted by Langanobob
                  I had a thought, since it appears that a 40:1 VFD reduction isn't practical, what if you reduced the speed by a factor of 20:1
                  Putting what Forrest and Arthur said together:

                  Power = Torque X RPM

                  Induction motors are constant torque.

                  So if you halve the RPM with a VFD, you've halved the power:
                  what was a 1 HP motor at the rated nameplate RPM (1750 RPM), is 1/2 HP at 875 RPM, 1/4 HP at 437 RPM...

                  That's why electronic vari-speed conversions usually double the horsepower of the stock motor, which gives you a 4:1 RPM range with useful power.
                  "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."


                  • #10
                    Hmmm, 1.25HZ on a 1400 rpm 60HZ motor is not going to net you much torque, not enough to cut metal is a safe bet.

                    Some VFD's have the ability to apply full voltage at a lower frequency setting for better lower speed torque, but I have not seen one personally that would do this at such a low frequency setting (1.25 HZ).

                    It sounds to me like a mechanical speed reduction may be a better option, or at least a more usable one.


                    • #11
                      Thanks for all the responses guys. As I suspected, the VFD isn't the right solution.

                      As for using both a pulley and VFD, I think that would be overkill. Once I do the pulley system, I will already have all required speeds available, so a VFD isn't needed anymore...


                      • #12
                        ah... but if you have a vdf you don't need to change belts often (choose one mid range, and vary 5:1 with ease), and there are many advantages to VFD's other then speed control.


                        • #13
                          Closed loop control is the way to achieve very low rpms as MaxHeadRoom (my favorite TV series) wrote.

                          It works best using DC motors since a DC PM motor develops maximum torque at stall. Using PWM control and a DC motor with feedback you can dial in any speed you want down to zero.

                          Here is my mill operating with a closed loop Brushless DC motor with a 1/3 hp rating. It's geared down by about 2 to 1 which means the full speed rpm is reduce to 1500 instead of 3000. By using closed loop control it can be run at around 20 rpm if required. This was a temporary setup to test the concept.

                          Last edited by Evan; 05-21-2011, 12:01 AM.
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                          • #14
                            Evan Yabbut maybe a little more than Taydin wants to hassle with for a bandsaw drive.

                            Don't get me wrong. I like DC motors and the ease with which the controls and quatrature feedback servos can be implemented via bone simple step motion technology

                            But even the fanciest 1 HP DC motor cannot offer a constant HP over a 40 to 1 range: 2500 RPM at ratings yields 2.1 lbft at ratigs. 2500/40 = 62.5 RPM - 84 lbft. Do the math to reproportion for your favorite motor.


                            • #15
                              Horsepower is a function of rpm. It's a rate function, as in rate of material removal. That isn't what you need when you need low rpm cutting. What you need is constant torque over the entire rpm range rather than constant hp. Closed loop control of DC motors provides just that. It's torque that makes the cut regardless of the material removal rate. An induction motor can't provide high torque at very low rpm. It depends on rate of change of the stator field to induce high currents in the rotor. The slower it turns the lower the rate of change and the smaller the induced field.

                              A DC PM motor depends on the static current flowing in the windings to produce the magnetic field that interacts with the magnets. It produces maximum torque at zero rpm because there isn't any back EMF generated when the armature isn't moving. The armature in a brushless motor contains permanent magnets so the field is always present independent of the rate of change of the stator windings.

                              Low RPM torque is where permanent magnet motors shine. Pulse width modulation allows full voltage across the coils regardless of rpm. RPM sensing feedback provides accurate speed control even as the load varies.

                              The system on my mill is stupid simple. It uses a ring of permanent magnets on the motor pulley to generate an ac voltage by passing a coil as the pulley rotates. That is rectified and filtered to provide a DC level that is proportional to rpm. That DC level is mixed into the Potentiometer speed control voltage and regulates speed while still alowing for the pot to be able to control the desired rpm.

                              Total electronics parts are one coil, a diode, a capacitor and a resistor.

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