# Back EMF of DC motor

Discussion in 'General Electronics Chat' started by khanh, Feb 12, 2006.

1. ### khanh Thread Starter Member

Jan 6, 2006
28
0
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???

2. ### khanh Thread Starter Member

Jan 6, 2006
28
0
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?

3. ### haditya Senior Member

Jan 19, 2004
220
0
the result u hav arrived at is true..the armature current is varying..which is why a dc motor uses a commutator

4. ### khanh Thread Starter Member

Jan 6, 2006
28
0
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) ????????)

5. ### haditya Senior Member

Jan 19, 2004
220
0
wat is considered is the rms value of emf

6. ### khanh Thread Starter Member

Jan 6, 2006
28
0
I duno, but they use it to solve the equations ?
No point of finding RMS value!!! That's weird!!!!
Anyways, how many of you find my proof of EMF true or only little of you insterested in this topic ???