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Discussion in 'Electronic Repair' started by [email protected], Mar 7, 2007.

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  1. Guest

    The machine will start very slow. There is only one motor on this
    machine. Initially there is very little torque and low speed. After
    about 30 seconds or so the speed and torque increase however it never
    reaches full speed. The tape runs slow. I have lubricated it and there
    doesn't seem to be excessive play in the bearings nor is it binding.
    Does anyone know what would cause an AC motor such as this to exhibit
    these symptoms? This motor was used in an old Ampex audio tape
    recorder manufactured in the late 1950's. I'm in desperate need of a
    replacement or repair information for this motor. The label reads:
    Howard Industries Inc.
    Festus Missouri
    Loyd scruggs Div.
    115V 60cy.
    Model: 28125-2903 .7A 55W Code 1061
    Does anyone have any information on it such as RPM.? By chance, does
    anyone know who can possibly supply this part? Any information would
    be most sincerely appreciated. Thanks.
    Lenny Stein, CET.
    Barlen Electronics.
    465 Derry Road
    Chester, N.H. 03036
  2. Franc Zabkar

    Franc Zabkar Guest

    I'd suggest you use a micrometer to measure the diameter of the
    capstan shaft and then calculate the flywheel RPM by dividing the tape
    speed by the shaft circumference.

    Then calculate the motor RPM as follows:

    motor RPM = capstan RPM x (flywheel diameter)/(motor pulley diameter)

    - Franc Zabkar
  3. is this a synchronous motor with capacitor? at 60 Hz, a 4 pole motor
    gives you 1800 RPM, 6 pole motor gives 1200. A common failure is a blown
    capacitor, resulting weak torque and starting problems.

    Sometimes they used capacitors with PCB. be careful, this is poisonous.

    Hope this helps
    Matti Adolfsen
  4. Guest

    I disassembled the motor cleaned it and lubricated it with ATF. It
    appears to be in very good condition. There is no end play in the
    bushings and very little apparent wear. It starts better now and comes
    right up to full speed, however the speed is 1720RPM either loaded or
    running free. Same as before the cleaning. There seems to be four
    poles which would suggest 1800RPM as you mentioned. Is it
    theoretically possible for a four pole motor to operate at 1720RPM?
    Varying the line voltage around 120V has very little effect on speed
    so it appears that it is running synchronously. There is no capacitor
    externally or internally. I just can't imagine what could have
    possibly failed (if something did), to cause this. Lenny
  5. bz

    bz Guest

    I don't suppose your powerline frequency is running at 57.333 Hz, but that
    would do it.

    bz 73 de N5BZ k

    please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
    infinite set.

    remove ch100-5 to avoid spam trap
  6. Jerry Peters

    Jerry Peters Guest

    If it's not really a synchronous motor, it's normal for it to run
    slightly slower than 1800 rpm. How much slower depends upon the actual
    motor design.

  7. Franc Zabkar

    Franc Zabkar Guest

    Induction motors need "slip" to operate properly. A figure of 5% or
    less is typical.

    "Synchronous speed is the absolute upper limit of motor speed. At
    synchronous speed, there is no difference between rotor speed and
    rotating field speed, so no voltage is induced in the rotor bars,
    hence no torque is developed. Therefore, when running, the rotor must
    rotate slower than the magnetic field. The rotor speed is just slow
    enough to cause the proper amount of rotor current to flow, so that
    the resulting torque is sufficient to overcome windage and friction
    losses, and drive the load. This speed difference between the rotor
    and magnetic field, called slip, is normally referred to as a
    percentage of synchronous speed."

    - Franc Zabkar
  8. You'd have to pull it apart to see, but it is possible that the rotor bars
    have melted the solder. I doubt it though.

    IME, it is the bearings. I would try to strip the oil with a solvent, check
    for ANY play, and relubricate with Oilite lubricant.

    Oilite: oil impregnated bronze bearings
  9. Guest

    no there doesn't seem to be any reason for this . Lenny
  10. Then I'd consider all electrical problems, low voltage, shorted turn,
    defective rotor or stator.
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