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Homework Help: Problem with induction motor

  1. Jun 17, 2014 #1
    1. The problem statement, all variables and given/known data

    A three phase cage rotor induction motor running at full load draws a stator current of 60A at a power factor of 0.8 lagging from a 415v, 50Hz Supply. Under the conditions the stator loss is 3.6kW, the rotor loss is 1.3kw and percentage slip is 4.5%. If the shaft power output is 29.4kW, calculate the shaft output torque, efficiency & mechanical friction and windage loss.

    2. Relevant equations

    Torque = Force * distance

    3. The attempt at a solution

    At first i tried using the power output 29.4kw to try to work out the torque but then realised that its not used for force.

    Now unfortunately this is were I'm stuck at, I really need help on this and I can't get my head around the question. I am uncertain of what formulas to use and would really appreciate it if someone could guide me and tell me which ones to use :confused:.
  2. jcsd
  3. Jun 17, 2014 #2


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    Staff: Mentor

    Hi Student96! http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

    They don't tell you the motor's synchronous speed?

    Does the textbook give you the answers?
    Last edited by a moderator: May 6, 2017
  4. Jun 18, 2014 #3


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    Science Advisor
    Homework Helper
    Gold Member

    Perhaps more relevant is..

    Power = Torque * angular velocity

    and as NascentOxygen said they don't give you the angular velocity.
  5. Jun 18, 2014 #4
    Thank you for replying NascentOxygen! , unfortunately it does not state synchronous speed and i haven't got a textbook containing the answer since its a homework sheet :frown:
    Last edited by a moderator: May 6, 2017
  6. Jun 18, 2014 #5


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    Staff: Mentor

    In which case, to demonstrate that you really do understand the work, you could represent synchronous speed algebraically, e.g., Ns RPM, and work through the exercise based on that in place of a numerical value.

    Alternatively, perhaps all of the induction motor exercises you have worked through in class have involved a particular synchronous speed, for convenience? If so, maybe you could assume that value for this exercise, too?

    Have you asked other students in your class how they are going to answer this?
  7. Jun 19, 2015 #6
    assume that the number of poles is 2 (or some other number) state this assumption the Ns=120f/p
  8. Jun 19, 2015 #7


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    Staff: Mentor

    Hi kevinj. :welcome:

    You may not have noticed that the thread you responded to here is over 12 months old. Most likely the poster by now has moved on (one way or another). :smile:

    ... or some other even number.
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