Transformer help please

Discussion in 'Homework Help' started by elecstudent, Nov 17, 2011.

  1. elecstudent

    Thread Starter New Member

    Nov 11, 2010
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    This is the questions that I have been given;

    1a) A 4 pole synchronous motor rotates at 3000 RPM. The torque on the motor is 20Nm and the efficiency is 90%. What is the frequency of the electrical supply? (Hz)

    1b)
    What is the load on the electrical supply? (Watts)

    Can anyone help to start me off?
     
  2. crutschow

    Expert

    Mar 14, 2008
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    There is a direct relationship between motor speed, the number of poles, and the supply frequency. Google synchronous motors and you will find that equation.

    The power generated by the motor is torgue times RPM. From that and the motor efficiency you can determine the motor input power.
     
  3. elecstudent

    Thread Starter New Member

    Nov 11, 2010
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    I calculated question one to be 100..

    am I right?
     
  4. crutschow

    Expert

    Mar 14, 2008
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    That is correct.
     
  5. elecstudent

    Thread Starter New Member

    Nov 11, 2010
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    using this formula (if it's the correct 1)
    HP = rpm x T(torque)
    5252(constant)
    I got;

    11.9HP = 8519W


    Is this correct? as the W value looks huge..
     
  6. elecstudent

    Thread Starter New Member

    Nov 11, 2010
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    anyone? a bit of advice please would be appreciated
     
  7. crutschow

    Expert

    Mar 14, 2008
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    I believe your constant is not correct. That looks like the constant for conversion of torque in lb-ft to HP, not Nm.

    So you either need a new constant or you must convert Nm to lb-ft.
     
  8. elecstudent

    Thread Starter New Member

    Nov 11, 2010
    26
    0
    thanks this was really appreciated, as my calulator in build conversion can't handle this, I only realised.

    cheers
     
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