+ Post New Thread
Results 1 to 5 of 5
  1. #1
    Newbie level 2
    Points: 20, Level: 1

    Join Date
    Aug 2019
    Posts
    2
    Helped
    0 / 0
    Points
    20
    Level
    1

    dc motor theory question??? Maths

    Hello,

    I have a few questions which I cannot be sure I'm calculating right.


    Here is what I have:

    Motor speed 500rpm
    Back emf 115v
    Supply voltage 200v
    Motor torque 40nm


    I have to work out..
    Armature current drawn
    Armature resistance
    Shaft power
    What would be the rpm if the torque increased to 60nm

    I have calculated
    18.21amp
    275.57mOhms
    2.8HP
    And by ratio 333rpm as I do not have flux.

    All assuming 100 percent efficiency.

    Can anyone find fault with my answers??

    •   AltAdvertisement

        
       

  2. #2
    Advanced Member level 5
    Points: 17,233, Level: 31
    Achievements:
    7 years registered

    Join Date
    Nov 2012
    Posts
    3,148
    Helped
    777 / 777
    Points
    17,233
    Level
    31

    Re: dc motor theory question??? Maths

    I really do not know where to start.

    But you have calculated somehow; tell us the steps.

    Once we know the steps we can comment.

    How to convert the motor torque 40nm to watts?

    Is it a DC motor with 200v DC working voltage)? Anatomically, all motors are AC- because of commutation is needed for DC- but that is a different story anyway.

    Do you assume that the load presented by the rotor is zero?

    Shaft power should be the same as the motor power because we have no idea about the losses (both copper and iron losses).



    •   AltAdvertisement

        
       

  3. #3
    Advanced Member level 4
    Points: 8,335, Level: 21
    Achievements:
    7 years registered

    Join Date
    Nov 2009
    Location
    Italy
    Posts
    1,125
    Helped
    386 / 386
    Points
    8,335
    Level
    21

    Re: dc motor theory question??? Maths

    Please, find attached my solution.

    Click image for larger version. 

Name:	20190817_225219.jpg 
Views:	14 
Size:	300.4 KB 
ID:	155048
    Click image for larger version. 

Name:	20190817_225232.jpg 
Views:	8 
Size:	278.8 KB 
ID:	155049

    About the shaft powet, it is Ps=w*T, that is Ps=52.4*40 = 2096W or 2.8HP
    if the torque is 60 Nm we will have 32.6*60=1956W or 2.6 HP

    I did the approximation that "k" is the same for both 40 Nm and 60 Nm torque
    Last edited by albbg; 17th August 2019 at 22:15.



    •   AltAdvertisement

        
       

  4. #4
    Newbie level 2
    Points: 20, Level: 1

    Join Date
    Aug 2019
    Posts
    2
    Helped
    0 / 0
    Points
    20
    Level
    1

    Re: dc motor theory question??? Maths

    Ia =( wt)÷eb (52.4x40)÷115 = 18.2amp



  5. #5
    Member level 2
    Points: 210, Level: 2

    Join Date
    Sep 2019
    Posts
    48
    Helped
    1 / 1
    Points
    210
    Level
    2

    Re: dc motor theory question??? Maths

    Quote Originally Posted by Harry.f123 View Post
    Hello,

    I have a few questions which I cannot be sure I'm calculating right.


    Here is what I have:

    Motor speed 500rpm
    Back emf 115v
    Supply voltage 200v
    Motor torque 40nm


    I have to work out..
    Armature current drawn
    Armature resistance
    Shaft power
    What would be the rpm if the torque increased to 60nm

    I have calculated
    18.21amp
    275.57mOhms
    2.8HP
    And by ratio 333rpm as I do not have flux.

    All assuming 100 percent efficiency.

    Can anyone find fault with my answers??
    I know how to DC motor calcs, no problems there, but first some questions. Where did you get the data that you "have"? Is this your measured data from a real motor? How did you measure the back emf?

    Or is this a hypothetical question that you have been given as an engineering assignment or similar?



--[[ ]]--