Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Torque equation of an electric motor

  1. Jan 29, 2012 #1
    The example I am looking at in my text book starts by considering and area of the rotor surface of width w and length L.

    Then the axial current flowing in the width w is equal to I=wA which is exposed to a radial flux density B

    So from the Lorentz force F=IBxL the equation becomes

    F= wABxL

    so the force per unit area is F/wL which becomes:


    Then to obtain the torque the force per area is multiplied by the entire area of the rotor (2∏rL) then multplied by the radius of the rotor

    So the overall torque equation becomes:

    T=BA x 2∏rL x r

    What doesn't make sense is how can the current be equal to wA? by the Lorentz equation the force on a current carrying conductor is IBxL so the width and area of the conductor carrying the current I does not matter? so why does it apply here?

  2. jcsd
  3. Jan 29, 2012 #2
    I agree with you. Without seeing any diagram I would say the force on the side of the coil of length L will be BIL. The torque produced by the coil will be BILd where d is the separation of the sides of length L. Ld is the area of the coil.
    If the coil is in a uniform field then the torque will be BIASinθ where θ is the angle between the field and the normal to the plane of the coil.
    If the coil has N turns then the torqe is BNIASinθ
    I don't understand what seems to be in your book !!!!!
    hope this is some help
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook