Back EMF of Motors: Constant or Variable?

[brainyman89]

i am wondering why in some problem they are regarding back emf of a motor as a constant value that characterizes the motor although current is sometimes variable? i guess the reason is for simplifying the problem, what do u think? can this occur in reality?

thanks for helping.

 

[Antiphon]

Constant motor rpm?

Can you give more information?

 

[brainyman89]

Constant motor rpm?

Can you give more information?

they are considering that E=Ir+E' where E'(the back emf) and r are constants although the motor is supplied by a coil that has previously stored energy. so i is not constant, it is decreasing to zero, however they considering that the motor functions normally as E>E', and E' is always constant to each motor.

Anyway, i think your justification that they are using "constant motor rpm" could work here.

 

[sophiecentaur]

Could it be that they are trying to make the problem easier?