Just a question to make you think a bit

Discussion in 'General Electronics Chat' started by mik3, Aug 3, 2009.

  1. mik3

    Thread Starter Senior Member

    Feb 4, 2008
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    An oil pump is driven by a 3-phase asynchronous motor. It starts normally and when the oil pressure rises to a value the motor reverses direction of rotation.

    Why this happens?
     
  2. Externet

    AAC Fanatic!

    Nov 29, 2005
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    Because the pressure switch opens at the rated value, de-energizes the motor and the hydraulic pressure turns the hydraulic pump backwards as if it was a hydraulic motor. The electric motor turns with it as they are coupled.

    The hydraulic reservoir has to have a expansion chamber, bladder, spring or similar accumulating fluid for that to happen.
     
  3. yourownfree

    Active Member

    Jul 16, 2008
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    whats the question? Here is my thought. It builds up pressure, pressure switch activates a DPDT relay to reverse two of the wires and the motor now runs in reverse. basically I don't know whats going on.
     
  4. mik3

    Thread Starter Senior Member

    Feb 4, 2008
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    The motor starts and the pump develops pressure. After the pressure passes a value the motor starts to rotate in reverse. There are no pressure sensors which activate a relay and reverse the motor.
     
  5. mik3

    Thread Starter Senior Member

    Feb 4, 2008
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    To clear things up.

    The output of the pump is closed and the oil cannot flow.
    The motor is started and when the pressure of the oil increases over a value the motor starts to rotate in reverse. There is no pressure sensor to sense this pressure value or something to control the motor. It just changes direction of rotation.

    Why this happens?
     
  6. Externet

    AAC Fanatic!

    Nov 29, 2005
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    Is it on a healthy motor and supply condition ?
    If the motor is energized when reversing happens... as it is asynchronous, lost phasing with itself due to severe stalling .
     
  7. mik3

    Thread Starter Senior Member

    Feb 4, 2008
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    The motor is healthy.
    The power supply may be not healthy.
    The motor is started when the oil pressure is low.
     
  8. b.shahvir

    Active Member

    Jan 6, 2009
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    If the discharge of pump is blocked, then the accumulated oil pressure might force the impeller and hence motor, to rotate backwards. This might have something to do with density of oil (as oil is heavier than water).

    Reversing of motor with supply connected is kind of 'plugging' phenomenon. This condition is dangerous for motor if it persists for long, as the back EMF now changes polarity and the motor now absorbs extra amps from the supply to which it is connected. This is unhealthy for the installed system and should blow some fuses. I suggest you monitor the motor amps during speed reversal to get a clear picture.

    Best regards,
    Shahvir
     
  9. eblc1388

    Senior Member

    Nov 28, 2008
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    The oil companies should hire you as their spoke person.

    According to your definition, it is impossible to have oil spill on water. :p
     
  10. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    if the output of the oil pump was blocked, there would be no flow, hence no rotation. You would be in a locked rotor condition.

    If you had some oil accumulation taking place, you could rotate to stall, blow a phase fuse, causing the motor's torque to fall below the accumulated oil head, which would reverse the motor, and then single phase in reverse.
     
  11. mik3

    Thread Starter Senior Member

    Feb 4, 2008
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    GetDeviceInfo and Alberto found the answer, a phase goes off. In my mind the answer was a faulty phase from the beginning.
     
  12. b.shahvir

    Active Member

    Jan 6, 2009
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    Oops Sorry!! i think i got out of the wrong side of bed today! :p
     
  13. BillB3857

    Senior Member

    Feb 28, 2009
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    When I was doing the technical interviews for a large aircraft manufacturer, I would give this scenario to an applicant: A large area within the plant has multiple machines running. Normally, when break time comes, all of the machines shut down for break. Yesterday, one of the machines was left running in order to meet a tight schedule. All of the other machines were shut down. When break was over, only the smallest machine would restart. (remember, one machine had been running during break) What is the problem??
     
  14. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Because the machines all run from a battery bank, and one of the machines that was shut down in the break was the battery charging generator. After the second break the batteries were pretty flat and only the smallest machine would run. That problem would not occur when ALL machines were shut down for a break.
     
  15. mik3

    Thread Starter Senior Member

    Feb 4, 2008
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    What kind of machines are (electric motors, petrol engine, jet engine)?
     
  16. BillB3857

    Senior Member

    Feb 28, 2009
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    Details of what the machines were being powered by were not mentioned because the applicant had walked through the plant on the way to the interview. He could easily see that there were no petrol engines, jet engines or steam engines powering the machines. Lighting was not provided from kerosene lamps, either.
     
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