Back EMF and Current Variability: A Puzzling Question

[brainyman89]

while solving an exercise, i have found that they are considering the back emf of a motor constant although the current is not constant? how comes that? shouldn't it be variable as current varies?
they are considering that as E=Ir+E' where E' and r are constants?

 

[epsilonjon]

If the magnetic field is constant then I don't see how the back emf could be constant. It will be determined by the size of the area which the magnetic field lines 'cut', and this will obviously be changing as the loop rotates.