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Homework Help: DC motor design calculations, help?

  1. May 17, 2010 #1
    1. The problem statement, all variables and given/known data
    Hi, well my task is to construct a DC motor. With 25w rotational mechanical output power. It cannot draw more then 5A current, and has to have 3 or more poles in rotation. The motor must be brushed/commutated. There is a max of 30v D.C. input. The motor needs to be built and tested within a few days, so ill need calculations done by tomorrow (brutally tough i know)

    2. Relevant equations
    I was not given relevant equations for the question, this is half my problem, id like some advise on which equations i should be using to caluclate the necessary parts to build the motor which will acomplish the goals stated above.

    3. The attempt at a solution
    My attempt came up short when i came across the term 'back emf' or 'induced emf'. i found an equation:

    Vo = IR + Ve

    Vo = power supply
    I = current
    R = terminal resistance (is this calculatable for my question or do i need to build and measure this, and what exactly is this resistance? where is it found.. i thought it was the overall resistance of the motor)
    Ve = Induced EMF (Back EMF)

    Further continuation of this formual suggest substituing Ve = w Ke
    w = angular velocity
    Ke = Induced EMF constant (how do i determine this?)

    In other attempts at solving this i came up with the following:
    Pi = VI
    Pi = power input (i assumed motor efficiency to be 50% as someone suggested this was a good idea, how does this affect other calculations, it should have more effect then just doubling the desired output getting an input power of 50w)
    V = voltage
    I = Current (as no more then 5A is allowed, i set this to 3A so im well within the boundaries)

    therefore i get voltage:
    V = Pi/I
    V = 50/3 = 16.67 Volts

    As a DC power supply is used to power the motor, this should not be a problem?

    I then attempted to find the resistance of the motor, thinking i could calculate the number of coils i needed.. evidently this led me no where as i used 'V=IR'. Apparently this resistance value is the resistance of the motor when not moving, resistance changes? I know a section of my research said that the current drawn by the motor is reduced the faster it spins. I dont know how or what im calulating from here, can someone please help?
    Last edited: May 17, 2010
  2. jcsd
  3. May 19, 2010 #2
    Can you reformulate your question to be a bit more clear and concise?
  4. May 19, 2010 #3
    I would like to work out the number of coils i need to use in a 3 armature DC brushed motor, using AWG 24 wire, 3A ( can be up to 5 if needed), the magnets being used have B=1.25 T ( id also like to know if a stack of 4 would make 5T? ) The output of the motor must be 25W of mechanical rotational power (Power = torque x angular velocity)
  5. May 20, 2010 #4
    Just updating, ive worked out how to calculate the Torque, and the Angular velocity. But to find the angular velocity i need the Back EMF, and im still unsure as to how i can find this theoretically before i find the angular velocity? Also to find torque i need a value called 'Field density at end of pole (or armature)' and i assume this is just the field density caused by the input current in the coil of the armature, but i havent been able to verify this yet.

    so i ask, how do i work out the back emf, if i dont know angular velocity? or what other method can i use to find the angular velocity?

    and when finding torque using 'T = Fr' where 'F = BIl', what is this B value and how is it calculated?

    thankyou for helping!
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