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B&D Grinder Broken

Discussion in 'Electronic Repair' started by Chris, Jul 4, 2004.

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  1. Chris

    Chris Guest

    I have a small Black & Decker bench grinder that I picked up at a
    second-hand store. When turned on, it just buzzes loudly and will not spin,
    even when forced. I removed the rotor from the stator and sanded rust off of
    both but to no avail. Any help would be greatly appreciated.

    Chris
    www.chrisbot.com
     
  2. Wild Bill

    Wild Bill Guest

    Small bench grinders often have capacitor run motors in them (not the same
    as capacitor start split phase). Typically, these models have oilite bronze
    bearings in them.
    If the bearings are in good enough condition that you can spin the shaft
    with your finger tips, your model might have a bad capacitor. If the shaft
    doesn't spin freely with no AC power applied, you'd need to correct the
    bearing friction problem first.. polish the shaft in the areas of the
    bearings and oil the bearings).

    If you model has a capacitor, it will be an AC capacitor, probably a small
    value (uF size) and a voltage rating of at least 120VAC (maybe as high as
    370VAC).
    A DC capacitor can't be used for an AC motor operation application.

    So where can you find a suitable AC capacitor, you might be wondering. In
    several of the home improvement retail stores (the big box stores), in the
    electrical parts or fan section. The ceiling fan capacitors are the same
    type used in AC motors as run capacitors.
    You'll want to chosse a value that's the same as the original one. The
    voltage would most likely be in the range explained above.

    Larger bench grinders will have a split phase capacitor start motor with
    ball bearings. The capacitor used in these motors will have a value size of
    about 100uF per HP.
    The voltage rating will be.. same as above. These capacitors aren't
    generally stocked in retail stores, so you would probably have to get one
    from an electric motor shop or a HVAC supply dealer.
    In these fractional-HP motors there is an internal centrifugal switch used
    for start/run switching. The contacts of these switches become pitted
    (decreasing their performance) from years of usage. The contact's
    reliability can be restored by cleaning and burnishing them (not just filing
    or sanding them).
    I'd service the contacts (or replace them if they're badly burned/pitted)
    before replacing the capacitor in this type of motor.

    Cheers
    WB
    .................
     
  3. Chris

    Chris Guest

    The grinder uses a shaded pole induction motor. This kind of motor has not
    brushes or capacitor. The poles of the rotor are fabricated in such a manner
    as to mechanically alter the magnetic field for startup. The grinder will
    spin freely as long as no current is applied. When current is applied, the
    motor buzzes loudly and movement by hand is opposed with great resistance.
    The coil's DC impeadence is approximately 50 Ohms. I cannot detect a short
    to frame so I do not suspect one.

    Any ideas to resolve the problem are greatly appreciated.

    Chris
    Tampa, FL USA
     
  4. Wild Bill

    Wild Bill Guest

    The start capacitor value for fractional HP split phase capacitor start
    motors should read..
    a value size of about 500uF per HP (100uF for 1/4 HP, 500uf for 1 HP).

    Cheers
    WB
     
  5. There isn't much else besides the windings at this point. Even if the
    shading rings were damaged or open, the rotor would spin but not
    start. You could do a ring test on the coil.

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  6. Jeff Wisnia

    Jeff Wisnia Guest

    When you rotate it by hand against that "great resistance", does it
    "snap" from one pole to the next pole on the rotor, or does it feel like
    it's bound up mechanically?

    If the later, then *something* is worn or shifted enough mechanically to
    let the rotor get pulled against the stator under the attraction of the
    magnetic fields. I'd open it up again and see if you can determine by
    eye or with feeler gages whether the alignment of the rotor to the
    stator has gone out of concentricity, especially when you push or pull
    the shaft(s) "sideways".

    Last year my own little Craftsman bench grinder,which I've owned for
    maybe 45 years started "losing power". It uses a universal motor.
    (Brushes and a commutator, with a small timing belt drive to reduce the
    motor speed.)

    I opened it up and saw signs of scraping on the armature. Turned out the
    pot metal housing onto which the stator and motor bearings were mounted
    had warped over the years and moved things out of alignment. A couple of
    shims placed in the right spots put the rotor and stator concentric
    again and it's good as new again.

    Good Luck, let us know what it turns out to be.

    Jeff
    --

    Jeff Wisnia (W1BSV + Brass Rat '57 EE)

    "If you can smile when things are going wrong, you've thought of someone
    to blame it on."
     
  7. My experience is that there may be enough play in the bearings to
    allow the armature to come in contact with the stator. Also, even if
    the bearings are not worn, they frequently are mounted in a metal
    spring-loaded bearing holder, and if the mounting spring loses its
    tension, it will allow the bearing to shift a tiny amount, which is
    all that is needed to let the armature touch the stator. If you can
    move the armature radially in any direction even a fraction of a
    millimeter, it is either the bearings or the bearings holder. As much
    as I hate to throw anything out, if it is the bearing holder, I don't
    have any suggestions on how to fix the motor, as the bearing holder is
    not replaceable as far as I can tell. Let us know what you find.

    H. R. (Bob) Hofmann
     
  8. Jim Adney

    Jim Adney Guest

    Check also to see if any foreign matter might have gotten in there.
    There might be a piece of steel which is not obvious, but which might
    stand up when the magnetic field comes on, and cause a problem.

    -
     
  9. Wild Bill

    Wild Bill Guest

    One of my bench grinders is the same type you describe, 2 poles with 2
    shading rings. The stator has 2 windings connected in series and the
    resistance measures 11 ohms total, at the AC plug blades (unplugged from the
    receptacle with the switch on, of course).
    Your resistance reading is likely to be different due to differences in
    meters and the gauge of wire in the winding(s). The wire looked to be about
    22 ga in this unit.
    If your stator has 2 windings, you might be able to determine which one is
    faulty. I don't know if a growler would work for this test.. if you have
    other test instruments, you might be able to verify (and possibly isolate)
    the fault. A milliohm meter probably won't provide enough resolution.

    As Sam has said, the only fault it could have is the coil windings. I'm not
    able to describe the ringing test to you since I don't do that test with
    laminated steel cores, but I think it involves using an audio generator and
    a 'scope.
    That would only verify that the windings have shorted turns, and you'd most
    likely need to rewind the stator (or half of it).

    An alternative would be to omit the stator and use the housing as an arbor
    to be driven with a belt and an external motor (cutting a hole in the back
    or bottom of the case for a belt to connect the other motor).

    The grinder I checked is a working K-Mart 5" model, probably from the 70s.
    Model/code 30-10A T1
    115 VAC 3.2A 3500 RPM
    Rotor size about 2" x 1-1/2" diameter

    Cheers
    WB
    .............
     
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