Motor question help

Discussion in 'Homework Help' started by amilton542, May 2, 2011.

  1. amilton542

    Thread Starter Active Member

    Nov 13, 2010
    I need a point in the right direction with the following question:

    Q: The rotor of a d.c. motor had an original diameter of 30 cm and an air gap under the poles of 2mm. During refurbishment the rotor diameter was accidentally reground and was then undersized by 0.5mm. Estime by how much the field MMF would have to be increased to restore normal performance. How might the extra MMF be provided?

    All I can think of to solve the question is ohms law (magnetic). The reluctance of the air gap I can solve but not the flux? :confused:
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
  3. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    The air gap flux is notionally

    \Phi=\mu_0 \frac{MMF}{g}A \ \ [Wb]

    g = the total air gap
    A = the rotor surface area
    μ0 = free space permeability

    Presumably we want to keep the pole tip flux the same.

    For the original condition

    A1=k*D1=k*30 where k is an arbitrary (dimensional) constant for the motor.

    For the final ground condition


    Plugging in those values will give two values of flux for a given MMF

    The ratio of the two fluxes (post grinding/original condition) will reduce to a ratio related only to g1,g2,A1 & A2. The ratio [let's call it 'r'] will be less than unity.

    The MMF must then be increased (by 1/r) so as to restore the flux ratio to unity.
    amilton542 likes this.