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Discussion in 'Electronic Repair' started by Smitty Two, Mar 26, 2009.

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  1. Smitty Two

    Smitty Two Guest

  2. Arfa Daily

    Arfa Daily Guest

    Selenium stack bridge, by the looks of it. Equipment it's in ?

    Arfa
     
  3. PeterD

    PeterD Guest

    Simple selenium bridge rectifier. Terminal marking shows what the
    function is (AC in, DC out) and the design is familiar to any
    old-timer... (Oh, crap, gave that one away, didn't I?)
     
  4. Franc Zabkar

    Franc Zabkar Guest

  5. Baron

    Baron Guest

    Good old Selenium rectifier ! Not seen one of those in years.
     
  6. So this far down in the thread we know by now that it's a selenium
    bridge rectifier.

    Question is, why can't he just replace it with a silicon bridge
    rectifier? Someone pointed out that Si has less voltage drop, but would
    that really be an issue? (Of course, it might help to know *something*
    about the equipment surrounding this component ...)


    --
    Made From Pears: Pretty good chance that the product is at least
    mostly pears.
    Made With Pears: Pretty good chance that pears will be detectable in
    the product.
    Contains Pears: One pear seed per multiple tons of product.

    (with apologies to Dorothy L. Sayers)
     
  7. Smitty Two

    Smitty Two Guest

  8. Smitty Two

    Smitty Two Guest

    Ancient European slide projector with rotary carousel. Trying to help a
    friend get the thing going again, via email since he lives in another
    city.

    Problem with it is that this motor:

    http://members.cox.net/prestwich/52.jpg

    turns very slowly.

    It didn't make sense to me that a rectifier would be fed by a motor
    winding, but there you go. So now it seems the motor is 240, but there's
    a secondary winding in there putting out about 17, and he's getting
    about 12 VDC out of the bridge, so that part seems OK now.

    Is there anything on that type (shaded pole?) of motor to fix, or is it
    an R & R job? (remove and replace)
     
  9. N_Cook

    N_Cook Guest


    Combined motor and "transformer" very common for "Dansette" type record
    players of the Selenium rectifier era. When higher power amps came in then
    they had to split apart.
     
  10. Baron

    Baron Guest

    Smitty Two Inscribed thus:
    That is an AC shaded pole motor with a tapped winding feeding the
    rectifier. It also means that one side of the circuit is directly
    connected to the mains.

    These commonly get glued up bearings. The cure is to strip it clean and
    lubricate the shaft on both sides. Use only a very fine oil ! Sewing
    machine oil is ideal. Reassemble. Job done.

    The shaft should rotate freely without any binding. The only other
    possible problem would be shorted turns on the coil ! But if that was
    the case I would expect smoke !
     
  11. Arfa Daily

    Arfa Daily Guest

    This used to be very common practice back in the day. Record players often
    used a motor constructed in that way. FWIW, I can't remember ever having one
    of those motors, which were used in all sorts of things from tape recorders
    to fan heaters, that had any kind of electrical problem. It used to be very
    common for the oilite type bearings to completely dry out leaving a gummy
    residue, which then made the motor run slowly. Usually, when the motor runs,
    the rotor moves forward a couple of mm in its bearings. If it is not able to
    do this, that can cause it to drag on the 'brake' pad that's sometimes
    fitted in there. They don't have a lot of power even when working correctly.
    Also, the bearings are usually 'self-centering' as in they are fixed in a
    sort of ball mount. If the motor has been knocked or jarred, the bearing
    balls can be knocked out of line, and don't re-centre if it has gone gummy
    in there. The motor in the picture looks as though it may have simple fixed
    bearings, though.

    Usually, these motors are very easy to service, being held together by two
    long screws which pass through one bearing plate, through the stator core,
    and into the other bearing plate, where the holes are threaded. In the case
    of this one, the rivets that they have used would have to be drilled out
    first, and then long screws with nuts substituted to hold it back together.
    Servicing usually involves cleaning the shaft and bearings with a solvent
    such as IPA, and soaking the bearings in a light machine oil for a while,
    before poking the rotor shaft through each in turn, and giving it a 'swing
    around' - a bit like working a joystick if you will - to make sure that the
    ball mounts are free to move. They should be tight enough to stay put
    wherever you leave them, but free enough to move fairly easily. The rotor
    and bearings are then assembled back onto the stator core, 'working' the
    ball mounts on the bearings as needed, until the rotor runs totally smoothly
    with the bearing plates refixed to the core by the screws. You should be
    able to easily spin the rotor with your fingers, and it should spin on for
    quite a while - several seconds at least.

    When it's all back together, it doesn't hurt to add a little more machine
    oil to the bearings, particularly if there are felt 'resevoir' pads in them
    or by them. On some motors of this type that I have seen, there is actually
    a little oil hole in the bearing housing. Good luck to your friend. Hope he
    succeeds in rescuing it !

    Arfa
     
  12. N_Cook

    N_Cook Guest


    Are shaded pole motors synchronous to the mains frequency unless overloaded
    ?
    I was always surprised how much axial movement there was of the rotor
    between the bearings, is this a necessary part of the design ? maybe to
    assist startup as relatively low torque even when rotating.

    In the present case could low revs be a consequence of a fault in the
    electronics, via back emf? is the motor speed the same when the Se
    rectifiers are disconnected , not just the electronics disabled?
     
  13. Baron

    Baron Guest

    N_Cook Inscribed thus:
    Yes !
    Generally it allows the rotor to align itself in the magnetic field.
    Very unlikely ! Even if you reduce the voltage the speed tries to stay
    constant. Smoke is more likely particularly if stalled or shorted
    turns.
     
  14. Smitty Two

    Smitty Two Guest

    Thanks. My friend says the motor spins freely by hand, and since the
    secondary voltage seems correct, giving 12 VDC on the other side of the
    rectifier, I'm doubtful about shorted windings. But, he now says that
    the mechanism jammed for a few seconds (not longer) just prior to
    failure. That seems like it ought to be a good clue, but I haven't been
    able to fathom it yet.
     
  15. Smitty Two

    Smitty Two Guest

    Thanks for your usual comprehensive treatise! I forwarded your comments
    to my friend. I'm off for a weekend of work and play at "her" house in
    another city, so he's on his own for a bit now, and maybe by the time I
    get back he'll have it sorted out. The machine is critical to his
    business, not just a hobby, so he's anxious to have it operational.
     
  16. Baron

    Baron Guest

    In that case it is possible that there is some sizing in some other part
    of the mechanism.

    It looks like a belt drive. If the motor still runs slow without the
    belt it could be that the bearings have worn oval allowing the rotor to
    catch the frame ! For that matter there could be debris in the gap
    that could cause the problem.
     
  17. Smitty Two

    Smitty Two Guest


    All right then, here's what happened with this. (Recall that the motor
    is 240VAC and has a secondary winding to power some simple DC stuff
    through the bridge) My friend disconnected the AC supply to the
    rectifier, and presto, the motor returned to normal speed.

    Thinking something downstream could be drawing too much current, he
    reconnected the rectifier supply wires and then disconnected the DC side
    of the rectifier. Motor slowed down. So he replaced the rectifier, and
    all is well.

    But I'm still puzzled, and since I was never on-site I didn't do any of
    the tests myself. If the rectifier had some fault that was drawing too
    much current and it pulled the voltage down, how could he have measured
    240 on the slow-turning motor, and 17VAC / 12VDC on the rectifier?
     
  18. Arfa Daily

    Arfa Daily Guest

    As it was a selenium stack, I would suspect that one arm was leaky. Enough
    to make it draw excess current, but not enough to represent a 'serious'
    failure that would load up the supply really hard - such as happens when one
    arm of a silicon bridge fails short circuit. With the other three arms
    functioning normally, the result may well have been a DC output sufficient
    for the rest of the circuitry to work.

    Out of interest, did your friend replace with a silicon bridge, and
    re-measure the AC in / DC out voltages ? Looking again, assuming that a
    resevoir cap follows the rectifier, with 17v AC in, you would expect to see
    around 24v DC at the output, rather than the 12v that was apparently
    measured with the defective bridge in place.

    Arfa
     
  19. Smitty Two

    Smitty Two Guest

    Arfa, are you sure about that? I thought you were supposed to divide by
    the square root of 2, not multiply by it. Oddly, my friend says there is
    no smoothing cap in the circuit. I did not ask him whether he checked
    voltages again, but I will.
     
  20. Baron

    Baron Guest

    Arfa is spot on ! 24v as near as makes no difference.
     
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