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AC Electric Motor Running Fast Without Load But When Load is Applied Runs Slowly

Discussion in 'Power Electronics' started by zain_sh, Jun 24, 2021.

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  1. zain_sh

    zain_sh

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    Jun 24, 2021
    I have a Water pump that was running well under load but suddenly it slowed down.
    I opened up the motor and checked everything, everything was fine. Then i operated the motor without load it ran fast as it was running before the problem.
    Then i attached the load again, motor was again slow.
    I've changed the capacitor as well but to no avail
    Could anyone tell the possible problem with it.
    Motor Specs. are as follows
    Phase: 1
    RPM: 2850
    H.P: 1
    Delivery: 90
    Suction: 90
    Temp. Rise: 60°C
    Amperes: 7
    Volts: 220/230
    Capacitor: 30uF


    Thanks in Advance For Help
     
  2. Bluejets

    Bluejets

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    Oct 5, 2014
    What is the "load"..?

    More details on the installation required. e.g. how long run power cable.....what size cabling....what is supply voltage as measured both origin and destination.
    What current is the motor pulling.
    Have you megger tested the motor..?
     
  3. Harald Kapp

    Harald Kapp Moderator Moderator

    11,533
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    Nov 17, 2011
    Sounds like the problem may be with the load. A clogged pipe, or a dry bearing or ...
     
  4. zain_sh

    zain_sh

    6
    0
    Jun 24, 2021
    It is a water pump that is sucking water from the underground about 65ft deep.
    Power cable is not long it is max 4ft in length.
    Supply voltage is AC 220 - 240
    Motor was consuming around 1500KW/H when it was working properly but now it is consuming around 1100 - 1200 KW/H of power.
     
  5. zain_sh

    zain_sh

    6
    0
    Jun 24, 2021
    Bearing is fine i've checked
    It is properly lubricated and also in good condition
    As far as clogged pipe is concerned i'll check it by attaching another motor to it and check water pressure but can it be the problem??
    I'm not sure if you could explain please
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    A clogged pipe will increase the load on the pump and therefore the motor.
     
    zain_sh likes this.
  7. zain_sh

    zain_sh

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    Jun 24, 2021
    Okay i'll check and update you
    Thanks man
     
  8. Bluejets

    Bluejets

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    Oct 5, 2014
    Most motors will "thrash" under blocked condition and run at lower than normal load.
    Full load is when the pump is pushing maximum flow.
    Throttling the outlet is how we used to get pumps into run in outback areas where supply transformers were a long way from the pumps.
    Granted these were 3phase pumps and single phase motors can draw more current half loaded than full.

    BTW, a pump cannot suck more than 32ft.
    Exception is a two line setup.
    Your motor consumption is all wrong as well.
     
    Last edited: Jun 24, 2021
    zain_sh and Minder like this.
  9. Minder

    Minder

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    Apr 24, 2015
    Agree, With a typical vane radial impeller pump, air or liquid., a blockage will actually result in lower current.
     
    zain_sh likes this.
  10. zain_sh

    zain_sh

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    Jun 24, 2021
    Actually it is a double impeller pump
    Here in our area these kinds of pump are used to suck water from 100ft deep
    And single impeller pumps are normally used where water is at 50 -55 ft depth.
     
  11. Bluejets

    Bluejets

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    Oct 5, 2014
    Like I said, basic physics only allows normal suction pumps to pull up to 32 ft.
    There are 2 line pumps which force water down from the surface, through a venturi and then back up which enables greater depth.
     
  12. zain_sh

    zain_sh

    6
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    Jun 24, 2021
    ***Problem Resolved***
    Posting the solution, if someone else faces the same problem in future...

    There was no problem with the winding, Bearing nor pipelines were clogged.

    The problem was with the router of the motor. Router is normally fixed so that it doesn't displace while rotating but in my case router was loose and it was not properly rotating in magnetic field.
    So when i operated the motor without load it spun faster but when load was attached router wasn't rotating with enough speed to suck water because it wasn't properly in the magnetic field.
     
  13. Bluejets

    Bluejets

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    Oct 5, 2014
    Now that would have to be a first for the rotor to come adrift from the shaft.
    Seen many motor faults in my time but never seen nor heard of that.
    Must have been rattling around inside the stator like a rock in a tin drum....surprised it hasn't stuffed the stator from poling.
     
    zain_sh likes this.
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