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125 VAC motor won't start

Discussion in 'Electronic Repair' started by a453, Jul 31, 2010.

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  1. a453

    a453 Guest

    Hi all I have a pump with a 2HP 125 V AC motor. All of a sudden the
    motor wont start anymore. If I switch it on it just does a humming sound
    and then thermal protection kicks in.

    If I initiate the rotation of the motor by hand when the power is on the
    motor does start working. The curious thing is that I can make it run
    clockwise or counterclockwise by simply initiating the rotation by hand.

    My guess is then that some electronic component that "tells" the motor
    the direction in which to rotate is now broken and the motor "does not
    know" any more wheter it must rotate clockwise or counterclockwise....

    What should look in the electronics to try fix the motor? Condensers?
  2. Archon

    Archon Guest

    Start capacitor most likely, they fail fairly often, if its a pool pump
    a good pool supplies shop may have them.
  3. Jamie

    Jamie Guest

    if you have a capacitor on the motor or near by in the circuit? try
    replacing it.. If you don't have a starting cap, then you may have a
    stuck centrifugal switch in side the motor, that is normally located at
    the rotor end where the wires go in..

    They do make some set ups with external time delay relays that
    connects the start winding, this done at times if the motor needs to be
    reversed. Most cheaper ones use a bi-metal relay..
  4. a453

    a453 Guest

    a453 wrote:

    Hi thanks to you all for the replies.

    I opened the motor.... there is a very rusty capacitor inside it.... the
    markings are unreadable.

    I disconnected the capacitor and tried to test if the capacitor still
    works by using a tester with different ohm settings... I always got no
    readings.... I guest that means that the capacitor is broken?

    If so, now the problem i getting a new capacitor with the right size...

    The motor data is:

    dual voltage 125/250 V

    Amps 21.0/10.5

    2.0 HP (SF 1.10 SF)

    This is a single phase Magnetek Centurion SE motor powered at normal 125
    Volt housedold power.

    My understandi is that i should look for an unpolarized motor star
    capacitor rated at 270-324 microFarad at 250 Volt

    Is this correct? Thanks so much for your help!!
  5. Archon

    Archon Guest

    Hi, I guess this a pool/spa pump? Magnatek centurion is probably an A.O.
    Smith motor.

    Are there any other identifiers on the motor plate that are readable? If
    so go to the AO Smith website

    Literature library


    Replacement Parts

    Download the parts catalog PDF, it gives all the cap values for motors
    but you need some identifier to select the correct part. If you have
    the SPA/Pump filter manufacturer it easier to find a good pool supplies
    shop and ask, my local pool outfit is pretty good.
  6. Archon

    Archon Guest

    This may help

  7. a453

    a453 Guest

    I searched but found no reference to my motor.. it's a bit old.... the
    old capacitor is all rusty and unreadable so with some approximation on
    "similar " motors I trend to install a new 220v 108-130 MFD
    capacitor.... the motor ran fine for a bit and then the capacitor
    "popped"... I guess I burned it....

    SHould I try use a capacitor with a higher or lower MFD number (the
    motor runs on ordinary houseol 115-120volt power) thanks
  8. Jack Myers

    Jack Myers Guest

    A motor starting capacitor is designed to carry AC (alternating current)
    when an alternating voltage is applied. Did you perchance substitute a
    capacitor rated for 220v DC? That would almost certainly pop.
  9. bud--

    bud-- Guest

    It is common for motors to use a capacitor to start the motor - it is
    disconnected by a switch in the motor when the motor approaches running

    Some motors have a capacitor that is permanently connected - a run

    Some motors have both a start and run capacitor.

    Since the cap died after the motor was running my guess is that the
    motor uses a run capacitor. Or the start switch is not opening?

    A capacitor that is designed for temporary starting can't be used as a
    run capacitor. Your replacement cap was maybe a start cap where a run
    cap should have been used? Does the motor have a start switch? Is it
    opening? I have never seen a motor with a cap inside.

    I think Grainger sells motor caps and you might get an idea of what is
    out there. Run caps have a lower mfd rating than start caps.
  10. CaveDave


    Aug 5, 2010
    You should check whether the motor uses a centrifugal (or thermal or magnetic) start switch, as some others have mentioned, that may be malfunctioning, or the wrong type of capacitor, but ALSO, since you mentioned age and rusted condition, check for shorted turns (difficult to measure) or shorts to ground.

    Just my 2 cents.:)



    ETA: FWIW, are there ANY other notations on the data plate such as PSC (permanent split capacitor), CS (capacitor start), etc.?
    Last edited: Aug 5, 2010
  11. Alan Douglas

    Alan Douglas Guest

    Hi all I have a pump with a 2HP 125 V AC motor. All of a sudden the
    The motor probably has a starting winding, capacitor, and centrifugal
    switch in series across the AC line. As noted, the most likely
    problem is the capacitor, but the switch could also be open, possibly
    by a bit of dirt between the contacts.

    Without the starting winding, yes, the motor will run equally well in
    either direction.
  12. shrtrnd


    Jan 15, 2010
    And an additional 2 cents to consider:
    Is the pump ITSELF sticking/seizing?
    If there's even a slight physical resistance at motor start, the motor won't have the torque to begin spinning the pump.
    Is it possible your cap is ok, and the pump is seizing?
    Just something to consider.
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