Belt grinder: motor choice and speed control

Discussion in 'The Projects Forum' started by someonesdad, Nov 27, 2009.

  1. someonesdad

    someonesdad Thread Starter Senior Member

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    I wish to design and build a belt grinder. For those of you who aren't familiar with them (they're also called belt sanders), the design I like a lot is the KMG design: http://beaumontmetalworks.com/. Click on the belt grinder and look at the videos on how they operate. You'll see that changing the tool bar can quickly give you a variety of different grinding behaviors. The main attraction of the tool for me is twofold: a) the ability to change the belt grit quickly and b) the ability to substitute a variety of platens and contact wheels to change the nature of the grind.

    My personal challenge is to build such a grinder for as low a cost as possible (I'm retired and have more time than toy money). I'll be writing up my experiences, frustrations, successes, etc. eventually and sharing them on the web.

    While I do both woodworking and metalworking in my shop, I mostly use a grinder for maintenance tasks, sharpening lathe tools, and sharpening drills. I have a hand-held belt sander that works well for the occasional woodworking task, so making this belt grinder suitable for woodworking isn't a priority.

    OK, here's where I need electrical help. From what I've read (and my own experience), I definitely wish this grinder to have a variable speed motor. I also know from experience that I want a motor rated around 1.5 to 2 hp (roughly 1 to 1.5 kW) and my preference would be the 2 hp motor. This desire partly comes from numerous posts on the web concerning knife grinders, for which this design is often used with 2 inch wide belts that are 72 inches long. If you watched the KMG videos referenced above, you'll see that a serious motor can do some serious hogging off of metal.

    My first question is:

    1. To satisfy this variable speed requirement, should I get a DC motor or an AC motor?

    My preference would be a DC motor rated to around 200-250 VDC. I could then build a PWM circuit to control the speed and run it from the numerous 240 VAC outlets I put in my shop. From what I've read and my own experience as a machinist, I will design this grinder to have belt speeds of between 15 and 30 m/s. Most operation will probably take place between 20 and 25 m/s.

    I also like the idea of the DC motor since it's easy to change direction -- this grinder will be used with the belt running mostly in one direction, but I'm considering designing and building a stand-alone "rotary power module" that I can wheel around the shop to power different tools (a future project is a fairly large bandsaw made from wood that can be used to cut both metal and wood).

    Note: I will likely have to buy the motor. It will be surplus or from a junkyard, so if I have to wait a year to find the right motor, so be it. One likely source is an old exercise treadmill (which I've read sometimes use DC motors).

    2. Should I design in closed-loop control of the speed?

    My intuition says yes. I would probably use a home-built tachometer that uses a Hall effect sensor and a magnet to count spindle rotations.

    3. If you feel I should be using an AC motor, should I look for a single phase or three phase motor?

    4. If an AC motor, how should I control the speed?

    My electronic skills are not beginner, but not terribly sophisticated (I'm a physicist, not an EE). If you recommend an AC control, remember it has to be something I build, so should be within my capabilities: I can build most things (not surface mount stuff because of my chronologically-gifted vision :p) but I'd prefer to work with an experienced person's design, as I don't feel comfortable designing something from scratch.
  2. shortbus

    shortbus Senior Member

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    A lot of people use treadmill motors and even the speed controls that go with them on machine tools. Every thing from mills to lathes.

    generally you have to gear them down with you drive belt system. they turn faster than the standard 1725 RPM.

    Cary
  3. SgtWookie

    SgtWookie Expert

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    I think a treadmill motor would be very good for your application.

    Otherwise, you'll need to find a universal-type (brushed) motor.

    If you used an AC-type (induction) motor, you would need to use a transmission or V-belts on stepped pulleys to change the speed.
    Last edited: Nov 27, 2009
  4. Duane P Wetick

    Duane P Wetick Active Member

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    Probably the treadmill junkyard (if there is such a thing) is a good place to look for everything you need. Low end torque and speed control is a good reason for selecting DC machines, although in the last 5-10 years, AC has taken over because of the cheaper motor construction and more and cheaper high power IGBT's for controls.

    Keep us informed, DPW
  5. someonesdad

    someonesdad Thread Starter Senior Member

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    Yes, I knew about the treadmill motors and a few days ago alerted my wife about finding one when she visits the second-hand stores.
  6. shortbus

    shortbus Senior Member

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    I've rescued a few from the side of the road on trash day.:D Lots of other good stuff in them like rectangular tubing.
  7. CDRIVE

    CDRIVE Senior Member

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    I've always loathed the sound of any of my universal motor machines but that may not be so in your case. On the other hand, if you find the sound of them as offensive as i do then consider a 3Ph motor driven by a Variable Frequency Drive (VFD).

    http://www.elmark.com.pl/products/rockwell/pdf/161-PP001B-EN-P.pdf

    There was a time when a HSM or woodworker would get an exceptional deal on one or more classic old machines but would back out of the purchase after realizing that the motors were 240V/3Ph. These little (VFD) jewels changed all that and many home shops use them now. Lathe operators, both wood and metal, love them!
  8. jpanhalt

    jpanhalt E-book Developer

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    Are you building a grinder or a belt sander? I have seen variable speed belt /disk sanders for wood, but not grinders. Are you sure a variable speed will be an advantage for working metal (i.e., a grinder)?

    John
  9. CDRIVE

    CDRIVE Senior Member

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    That type of belt sander is popular with wood workers and metal workers alike. Many prefer them (over grinders) for sharpening. For wood working with it I see no benefit of variable speed at all. In fact, it would be undesirable to have a very slow speed belt sander.
  10. awright

    awright Well-Known Member

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    I won't opine about the virtue of variable speed in a belt grinder, but I have very good experience with a VFD in my home shop. My single 3 HP VFD bought from a surplus dealer several decades ago is used to drive a geared-head drilling machine with integral 3-phase motor on which it would have been impossible to change to a DC motor, two lathes, a 26" bandsaw, a bench grinder, a universal tool grinder, and a Bridgeport vertical mill. This illustrates the versatility that a VFD can bring to your shop for future expansion. I changed out the motor on my Rockwell vertical mill before getting the VFD but wish I had left the 3-phase motor in place and gotten the VFD earlier. I am a BIG advocate of VFDs.

    In addition to being well suited to varying the speed of your belt grinder, a VFD gives you the ability to bid on 3-phase machines on auction that other pass up or underbid on because they are not able to provide 3-phase power. All of my machines were purchased at auction.

    3-phase motors are inherently reliable, rugged, and inexpensive. Speed is controllable without use of a tachometer because the motor is (loosely) locked to the frequency generated by the VFD, not the voltage. The voltage is varied with the frequency to avoid magnetic saturation of the motor by low frequency power but voltage is not the main speed control mechanism. This control tends to maintain relatively constant torque as speed is reduced below line frequency but this is somewhat dependent upon the design and programming of the VFD. Such constant torque is well suited to your belt grinder application for which the torque requirement would be relatively independent of speed.

    A 3 phase motor is MUCH cheaper than a DC motor of equivalent power however buying salvage or junk or finding at the side of the road changes this equation unpredictably. I purchased my surplus 3 HP VFD for a little over US$100 several decades ago but I believe you can get one new for around that price now with careful shopping or on ebay because VFDs are very common these days.

    I do not think it is feasible/practical for your average home shop hobbiest to build a VFD because they are fairly sophisticated and because they use somewhat costly power semiconductors. I also don't think you could beat the price of a commercially-built VFD by building at home unless you already had appropriate power components in your junk box. I say this as a dedicated do-it-yourselfer and electronics hobbiest with an enormous junk box.

    Just my biased opinion.

    awright
  11. CDRIVE

    CDRIVE Senior Member

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    Let's not forget these neat features: Ramp Up & Ramp Down. ;)
  12. someonesdad

    someonesdad Thread Starter Senior Member

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    As I mentioned in my OP, I will be using this tool for both metal and wood. There is no fundamental difference between a belt grinder and belt sander -- they both move an abrasive belt past an object with the intent of removing material.

    Yes, variable speed can be quite important in cutting metal -- the canonical example is to avoid heating a hardened and tempered high carbon steel enough to change its temper.

    I have a hand-held belt sander with a variable speed that I used on numerous woodworking projects. The variable speed is definitely an asset -- it gives finer control over the removal rate. Thus, I consider variable speed important for both metalworking and woodworking. However, if you were doing a repetitive dedicated task like sanding door panels, the variable speed feature would be much less important, as you could tune the setup to the application.
  13. someonesdad

    someonesdad Thread Starter Senior Member

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    You make some good points, awright. I've seen that VFDs sell for new at approximately $0.5k-$1k, so that's well beyond my budget. As I mentioned, my goal is to build a cheap system. I was figuring that a 200-250 volt DC motor could be found for $50-$100 surplus and that it wouldn't be terribly difficult to build a PWM circuit that operated using the 240 VAC line rectified. The diodes and MOSFETs should be relatively cheap and I can machine some heat sinks from some scrap aluminum plate I have.

    20-25 years ago I put an Electrocraft DC servo motor on my mill (I got it and its big, heavy controller on an employee purchase for a song). The motor was only around 1 hp or so, so it eventually burned up the controller. But it was superb while it lasted. :)

    However, you've brought up some good points about a VFD, so one will go on the potential shopping list. I agree about the desirability of the 3 phase motors and that they tend to be cheaper than the equivalent single phase motors. Most homeowners don't consider them because we don't have three phase power and don't want to spring for a phase converter.

    Can you recommend any specific VFDs to look for, either by brand or features?
  14. CDRIVE

    CDRIVE Senior Member

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