# Back EMF of DC motor

#### khanh

Joined Jan 6, 2006
28
I got info in some web kept telling me that the back EMF of a simple motor is:
Eemf = Ke*w*If

Where If is field current of the motor, w is the angular velocity and Ke is a constant. But from my understanding,
Emf = K*d(BA)/d(t).

But if B is constant under the magnatic field, then as the motor rotates the change in the area field is: A = Area*cos( Φ ) = Area*cos(wt)
So taking the derivative of this equation
Emf = K*B*Area*w*Sin(wt); which is not a constant.:mellow:
Can anyone explain this???

#### khanh

Joined Jan 6, 2006
28
No answers? Would my question be too dull or difficutlt to understand 'cause of my poor english? or is it simply considered not related to electricity ? Can anyone tell me?

Joined Jan 19, 2004
220
the result u hav arrived at is true..the armature current is varying..which is why a dc motor uses a commutator

#### khanh

Joined Jan 6, 2006
28
Originally posted by haditya@Feb 16 2006, 01:30 PM
the result u hav arrived at is true..the armature current is varying..which is why a dc motor uses a commutator
[post=14076]Quoted post[/post]​
you misunderstood my point, I mean EMF = Ke*If*tw? but my proof shows that:
EMF = K*If*w*sin(wt) so Ke = k*sin(wt) which is not a constant.

If you google phares like "dc motor emf equation" or anything I am sure you will end up with the same result above which I believe they are right so that why I doubt at my derivation, it must be wrong somewhere else.

For your convenience, here is one of that website: http://www.ecircuitcenter.com/Circuits/dc_...motor_model.htm
(this one says EMF = K*w (K is constant as long as If is a constant) ????????)

Joined Jan 19, 2004
220
wat is considered is the rms value of emf