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  1. #1
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    Default 3 phase bench grinder repair

    I have this Three phase GMF Aussie made Bench Grinder.
    It is a little rusted and need a complete going over and renovation . I want to get it running and see how it goes first and maybe put a VFD to it to control speed and possibly run a CBN wheel or two . Not a 240 volt to three phase just all three phase.

    Since spraying it down with inox and wire brushing threads to take wheels of and then the rest of the disassembly I am really getting to liking this thing. Its been made so nicely and fitted together so well . The inner flanges the grinding wheels clamp to with the outer flanges had to be taken off with a puller and are one precise lovely fit . Most of its there and the tool rests and bars are pretty good . Part of the right side rest is missing and the two safety shields . Good old Australian made stuff like this must have cost a bit back when it was new.

    The exposed incoming power lines that come through the base and up into the motor have perished and copper wire is showing . Once it goes through into the Stator winding / Coil its a bit better but a twist there would still shed its insulation . I'm not game to go pulling the stator apart more than the little bit I sliced back and am wondering , Should I give it to a repair guy to take it back further and join on three new wires. Is it a big deal from the stage its at now to do that is my main question ?
    If its a big deal, I could strip the insulation off whats exposed and cover with heat shrink, possibly with two layers, from the three wires end up into the body as far as possible . From that point on the wires are stiff where they go in under all the taping and shellac .

    You can see in the last pic where I cut a bit of the tape back to expose more wire.
    IMG_3556.JPGIMG_3611.JPGIMG_3613.JPGIMG_3614.JPG

    Rob
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  3. #2
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    I wouldn't rely on just heat shrink (even 2 layers) to protect the feed through as this is the most likely place where vibe is going to again cut through the insulation

    If I was doing this I would cut off all the wire withe the perished insulation and install a new cable through a proper cable gland. Then solder the cable to the internal wires, and double insulate the join preferably with an outer layer of silicone type heat shrink. Its a bit thicker and more elastic than regular heat shrink and handles vibe better. Something else you can do is cover the heat shrink with a gobby of HT silicone. This insulates and provides good vibe protection.
    SIliconedown.jpg

    Before you do any more, have you got an ohm meter to test across each of the pairs of coils (they should be the same) and between each wire and earth should be >2MΩ. A megger would be better for the wire/earth (insulation) test. Before you do any more I would check these out otherwise you could be up for a rewind.

  4. #3
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    Thanks Bob .
    I have a new multi meter and ohm it does do. If you could tell me the basics of it that'd be great thanks

    Where are the probes put to test for ohm ? One end on a wire and the other end where ?
    You said pairs of coils? How many pairs are there ?


    So if I cut off all the tape to expose the top of the windings and where the wires are joined in ? Take a look and a pic and report back .

    If it gets to much I have a guy who I'm sure can fix it but I haven't asked yet . He's an hours drive so I'm happy to try and do it right if its possible .

    Rob

  5. #4
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    Ive got one of these .

    UntitledFluke.jpg

  6. #5
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    Nice meter.
    There are 3 coils and so 3 pair of wires . One end wire of each pair are all joined and buried in there somewhere but there is no need o worry about that. lets call the 3 wires you have A, B and C. Test resistance across A & B, A & C and B & C and all shoul be he same (low, 10s of ohms) within A few tenths of an ohm. If they are not the same this indicates a problem.
    second test is from any of ABC to earth or body of the grinder. This should be >2 Megaohms.

    There's no need to dig back into the widings to remove the badly iinsulated wires as long as you are inside the motor case and cover up the join and any remaining stub of wire back to the windings with heat shrink.

  7. #6
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    Tip for VFD installation on a grinder. Don't use any deceleration as this can loosen the wheels. The VFD stopping parameter should be set to "coast to stop"

  8. #7
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    I have a 3 Ph motor on my Thicknesser. It started to drop the circuit braker. My Electrician mate suggested I replace the bearings in the chassis of the motor. Worked a treat. Then started to drop the circuit braker again. We pulled the motor apart, again, and found the wires going to the coils had brittle insulation which arced out and caused the problem. To fix it I removed any pieces of the old insulation and slid over these wires some "spaghetti". I stripped the insulation off some wire I had and slid it over. I was going to use shrinky but I was concerned that it was too thin hence to use of the "spaghetti".

    I was very surprised at just how slim these wires were. I put the motor back together and it has been working with no problem for a few years now
    Just do it!

    Kind regards Rod

  9. #8
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    Most grinder motors have no fan so they can get quite hot which leads to insulation degradation (witness the degrading on the insulation of the OPS old grinder) and have seen this many times on old grinders .

    The extra heat means being carefully about what insulation is used. Regular heat shrink is supposed to be good to 125ºC and if your grinder gets that hot you would know about it and it would probably fail for other reasons. Despite this, in these situations I prefer to use silicone or fibre glass sheathing or spaghetti
    Like this W0854 3.5mm x 1.2m Fibreglass Sheathing - Altronics which is good to about 190ºC

    When doing any motor mod/repair I often use silicone insulated wire (usually from the inside of stoves) to bring the mains power connections from the insides of the motor to the outside eg where the mains junction. This wire is not expensive but it only requires a visits to a couple of local streets during council verge collections every few few years to keep me stocked up.

  10. #9
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    Nice Meter yes but it’s all a little confusing . Never tested resistance before and while switching it to the OHM setting is easy I wasn’t sure which range to put it on . The readings were jumping all around the place until I wrapped the wires around the probes . Then it settled down . No idea if this is right but I took pics of the three tests . Hope they loaded right way up . Or loaded at all ? Can’t quite tell with the phone .

  11. #10
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    Now in this setting it’s this , same thing by the looks . And I did a test from each lead to the body . Saw this on YTube . It’s still an ohm test isn’t it ? Nothing changed for all three wires . Screen just said OL .

    Finally got the phone picture thing sorted

    Rob

  12. #11
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    Rob, that meter is autoranging. Just set the dial to ohms and test. It will automatically adjust to the correct range. Those readings are fine. The OL just means it is outside the range of the meter. You need an insulation tester to check it properly but it is probably fine. It wouldn't cost much to have a motor rewinder put some new leads on (he would check the insulation resistance as well) and you could be assured that it was done properly and wasn't going to explode in the near future.
    Those were the droids I was looking for.
    https://autoblastgates.com.au

  13. #12
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    Quote Originally Posted by NCArcher View Post
    Rob, that meter is autoranging. Just set the dial to ohms and test. It will automatically adjust to the correct range. Those readings are fine. The OL just means it is outside the range of the meter. You need an insulation tester to check it properly but it is probably fine. It wouldn't cost much to have a motor rewinder put some new leads on (he would check the insulation resistance as well) and you could be assured that it was done properly and wasn't going to explode in the near future.
    Thanks for that NCArcher .

    Just out of interest, I set the tester to continuity and put a probe on the end of two of the three wires , it beeped , meaning the currant goes in one and comes out the other . That's right is it ? They are conected?

    I thought each one of those wires was connected to individual coils of wire that were separate from each other .

    Apart from that, I took the other end off the motor and the insides look nice and clean . Not much dust even . One dry spider is all I found in there .

    IMG_3638.JPGIMG_3640.JPG

    Rob

  14. #13
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    3 phase grinders usually have the coils connected in a “Y” or 3 armed star connection. This means one end of each of the coils are connected all together in what is called the common point. If you measure the resistance across any two wires you are measuring the resistance or continuity of two of the coils. Continuity usually means the resistance is ~<50 ohms. It’s a rough/quick way to check things without having to constantly look at the meter. It’s good for tracing faults although ultimately you should check the actual resistance values.

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