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  1. #1
    BobL is online now Member: Blue and white apron brigade
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    Default GMF 3P to 1P + VFD grinder conversion

    This one is not rocket science but some of you may be interested to see how I did it.

    It's an 8" GMF 3P grinder that was given to me as it was going to be thrown into a skip.
    It looked like it had been used to grind something corrosive as the grinder base and one of the wheel guards was very rusty. Sorry no photos of this.

    As usual the motor had to be converted to ∆.
    Finding the Y common point was the trickiest one I have done so far but once that was done, apart from a tight space to operate in inside the motor the conversion itself was straightforward.

    The next thing was to do a rust removal and repainting of the base.

    Then I replaced the 3P simple rotary on/off switch with a 1P NVS (fat yellow switch at the bottom of picture) and sorted out the motor inside the grinder base.

    I made up a connection/terminal block (the bits with the brass strips on it) and drilled and trapped threads into the base to hold the terminal block in place. The connections are then made in ∆ but it's also very easy to turn it back into a Y connected motor if needed.


    The grinder base sits on a sheet of 8mm plate to which is welded an 60 mm length of 1" SSS that neatly slides inside the blue SHS support

    At the back of the base I added two extra cable glands.
    1Pi - is the single phase from the mains to the the NVS.
    1Po - is the single phase to the VFD
    3Pi - is the 3P from the VFD back to the motor
    The latter two cables connect up to a VFD through the section of blue 30 x 2.5 mm SHS.

    Here's what it looks like from the front.
    The VFD is a $70 cheapie that Joe Hovel organised for a few members a while back.
    I have it set to run up to 60Hz which it seems to do very smoothly.



    I now need to pull it apart and paint all the bare steel.
    Then I need to derust and tidy up the supports.
    I'm hoping to mount a CBN wheel on one side - don't know about the other side yet.
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  2. #2
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    Hi Bob,
    Looking good so far.
    Are you just going to use the remote VSD controls or build something else?

    As for the other side, Diamond of course!(or can you grind carbide on CBN?)

    Stuart

  3. #3
    BobL is online now Member: Blue and white apron brigade
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    Quote Originally Posted by Stustoys View Post
    Hi Bob,
    Looking good so far.
    Are you just going to use the remote VSD controls or build something else?t
    I'm not sure yet.

    As for the other side, Diamond of course!(or can you grind carbide on CBN?)
    'Good idea about the diamond, except SWMBO has a nice set of diamond wheels across a range of grits she uses for her glass jewellery cutting/grinding. She also conveniently keeps the grinder in the shed and they do indeed work well on carbide.

    I don't have any specific experience with carbide on CBN but I read it's best not to do this. I did find out that grinding tool steel on diamond is not a good idea as I damaged one of SWMBO diamond wheels by repeated sharpening of my WW lathe tools on it!

  4. #4
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    Quote Originally Posted by BobL View Post
    I don't have any specific experience with carbide on CBN but I read it's best not to do this.
    From the Georgia Grinding Wheel Company:

    "Diamond wheel on steels and CBN on carbides?

    We get asked quite frequently if a diamond wheel can be used on steels and CBN on carbides. We do not recommend this as the wheel life will be greatly reduced and in some cases, the wheels may not even cut at all. A diamond wheel is specifically used for carbides, plastics and other synthetic materials. It will not grind steel well at all. CBN wheels should only be used on steels. There is a hybrid grit available that will grind both; However, it is a compromise in wheel life and grind-ability. But in cases where you must grind both materials at the same time, it can be a real time saver."

    Frank.

  5. #5
    BobL is online now Member: Blue and white apron brigade
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    For those that might be interested here is the RPM/HP curves for this grinder conversion.

    The rated HP at 415V/3P is 3/4 HP, so it looks like it's doing it's thing also at 240V/3P

    2 test runs done at 50Hz


    I don't intend running it above 60Hz so I see little point in testing above that frequency.
    As the 60Hz curve didn't tip over you can see I didn't stall the motor at 60Hz and it probably had a bit more ooomph
    Also no fan on these units so running a low frequencies should not be a problem

    Here was the test setup

    I had to rejig the rig a bit to accommodate the tall grinder stand.
    I used a shorter metal strap and had to make a bush to fit the 19 mm bore brake drum to the 5/8 grinder shaft.
    I also had to reverse the position of the floating and fixed force meters as I was using the self tightening wheel method so I had to run the motor in the usual (anti
    clockwise facing the outside of the wheel) direction to hold the drum on.

    All good fun and a way to soak up a warm saturday arvo when I didn't feel like doing much else.
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  6. #6
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    Nice work Bob. Could you explain how to decipher the figures shown on the graph please?

  7. #7
    BobL is online now Member: Blue and white apron brigade
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    Quote Originally Posted by Techo1 View Post
    Nice work Bob. Could you explain how to decipher the figures shown on the graph please?
    If you want the nitty gritty details have a read of this thread
    http://www.woodworkforums.com/showthread.php?t=182106
    This thread explains how the power testing rig works and shows the results of testing other motors

    But here is the (hopefully) short version.
    What I am doing is measuring the HP output of a motor driven by a VFD, for a range of different 3P AC frequencies (each curve on the graph is for a specific VFD frequency) to check what the motor is capable of doing.

    First some back ground.
    Electric motors are rarely set up to provide constant power irrespective of the load, so that when an electric motor is turned on, once it has reached it's running speed it draws very little current and generates very little mechanical power.

    It's only by putting a load on the motor that it draws more current and generates more mechanical power. As the motor is increasingly loaded the RPMs will drop but it generates more torque and more power up to a point where it starts to stall and very rapidly lose RPM (that's where the tops of the curves on the graph kick over)

    Just before the motor stalls is usually the max HP that the motor can generate but that is not normally the rated HP of the motor - rather the rated power is some HP value below the maximum, and is an HP that the motor can usually continuously reliably run at.

    To accurately determine the rated HP of a motor at a specific frequency, it has to increasingly loaded and the (1) load or Force applied, and (2) the RPM of the motor measured. (1) multiplied by the radius of the drum gives the torque, which combined with the RPM equals the Power.

    The load is applied by hand using a brake handle from a bicycle which pulls on a cable attached to a steel strap that wraps half way around the drum.

    The reason I'm doing this is that I'm converting this grinder from 3P 415V to 3P 240V with variable speed so that I can put a CBN wheel on it and run it at slower RPM.

    What I want to make sure of is that I still have sufficient HP at say 30Hz (~1700 RPM) to use for grinding.
    Theory says the motor should generate 60% of the rated motor HP at 30Hz, 80% of the rated motor power at 40 Hz, and 100% of the rated power at 50Hz

    60% of 3/4 HP = 0.45 HP.
    Following the graph for the 31.2Hz (red squares) you can see that the motor generates 0.45HP well before the stall point so I should be able to easily obtain the rated power.
    0.45HP doesn't sound like much but it should be more than enough for basic HSS tool grinding etc.

    At 40 Hz, 80% of 3/4HP = 0.6HP (Green triangles) is also easily achieved before the stall point at this frequency.

    Same at 50Hz and 60Hz where it easily generates 3/4HP well before the stall point

    To be sure of the 50Hz data I repeated the test twice i.e. two curves (50 and 50.5 Hz).
    I repeat many of the tests - I just don't show them all on the graphs otherwise it would get too confusing.

    If anything, the given rating of this motor is a touch conservative but what it means is that (provided the bearings hold out) it will last for a long time without being stressed.
    Remember unlike a regular motor these motors have no fans but then grinders are not normally expected to run for very long periods anyway.

    I hope that is of some use.

  8. #8
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    Thanks Bob.

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    is the point where those cables enter the RHS protected with non metalic substance.

    mmm I cant see any...

    but I am going to copy your idea with an 8 inch I have..maybe not a true identical copy tho


  10. #10
    BobL is online now Member: Blue and white apron brigade
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    Quote Originally Posted by eskimo View Post
    is the point where those cables enter the RHS protected with non metalic substance.
    mmm I cant see any...
    That picture was pre painting, now there is a length of flexy conduit covering the entry point. Same at the exit point

    but I am going to copy your idea with an 8 inch I have..maybe not a true identical copy tho
    I fitted the grinder to my grinder turntable last night, once it's sone I will take a couple of pics.

  11. #11
    BobL is online now Member: Blue and white apron brigade
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    I finally got the grinder up and turning a wheel.

    I had to make new tool post rest support posts (the shiny 19mm horizontal rods) as the originals were too rusty to recover


    I had some probs with the super budget VFD (more about that in a later post) so I swapped to a 1.5kW HY unit and it runs sweet as.
    I have it set up for a 60Hz max speed and 10s Acceleration and Deceleration time. (if the motor coasts to a stop it takes about 90s) so that is a useful improvement.
    One thing about the VFD up on a pole like that and sitting on my grinder turntable is vibe, but the unit itself runs smoothly all the way up to 60Hz.
    I could have put the VFD underneath the grinder turntable but that is where heaps of grinder wheel and metal dust falls so I though it would be better above the grinder.

    Here's the protection around the wiring for EJ


    Anyway I'm pretty happy with it and spent an hour on the green wheel tidying up my lathe cutters.
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  12. #12
    BobL is online now Member: Blue and white apron brigade
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    Finally got the CBN wheel for this and though some might like to see this.

    Thats an 8" CBN and a 6" diamond and wheel on one side, and a now possibly redundant green wheel on the other.



    I used a 6" instead of an 8" diamond wheel as I didn't want the edges of the two wheels to conflict.

    The diamond wheel is mounted backed by a 20 mm thick by 75 mm diameter steel spacer - see photo below.
    This helps stiffen the diamond wheel and allows for adequate clearance at the edges.

    The spacer has a 5/8" bore to suit the grinder shaft but as the diamond wheel would then be riding on the grinder shaft thread the bore on the diamond wheel has been opened up to 19mm and it rides on a 1 mm raised 19 mm lip turned onto the spacer.

    A couple of issues.
    Speed must be changed slowly ~5 Hz/s or it will trip out the VFD.
    DON'T set up the VFD to apply any breaking as the VFD cannot cope AND this will loosen the wheels
    Still need to make suitable tool rests.
    Might make up a bit of a top cover for the wheel as I'd rather not have something hard fall onto the CBN wheel.

  13. #13
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    I might have to get to work on my GMFs one of these days if you keep this up Bob.

    Quote Originally Posted by BobL View Post
    Speed must be changed slowly ~5 Hz/s or it will trip out the VFD.
    You know there is a setting for that.

    Stuart

  14. #14
    BobL is online now Member: Blue and white apron brigade
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    Quote Originally Posted by Stustoys View Post
    . . . . .You know there is a setting for that.t
    There is? I will take a butchers.

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    Quote Originally Posted by BobL View Post
    There is? I will take a butchers.
    Pretty sure its PD071(well in my manual at least) Analog filtering Constant.


    Stuart

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