1. there are many types of dc motors viz series, shunt separately excited , compound etc.

2. Assuming the simplest separately excited , perhaps the simplest one can model the motor with load as follows.

 vapp= K*if w +ia*ra  +la *dia/dt

 K*if* ia = J*dw/dt+ B*w + Tload

where       vapp is applied voltage from controller,

                 K=constant from dc motor specs

                 ia=armature current

                 if= field current

                B=  coulomb type friction coefficient (mech eqn)

                J = moment of inertia of rotor

                 Tload = load torque could be a function of current

              It is good to work on per unit basis. Hope it is useful.

Cheers

Dear Narasim 

I know the basic equations of the DC motor (continuous form) 

But I cannot use these equations in a digital system like  digital signal processors 

I need a discrete form of the motor

The continuous case can be converted to digital form as follows

1. Let sampling freq =Fs then sampling time Ts= 1/Fs

2. Convert diff eqn into state space form as

    dxdt = [A]*x +u  

  x= vector [ ia w] '    A = matrix [ r/l  kif/l ; kif/J b/J }  u=[ va tl/kif ] vector

 then if t is sufficiently small Euler approx will give (using backward diff)

x(n)-x(n-1)   =Ts*A x(n) +  u(n  )(else use transition matrix exp(A*Ts)  vect eqn

so x(n) =x(n-1) +Ts*A x(n) +u(n ) , you can implement using z^-1 as delay  in

the block diagram of your choice. Note in some eqns it might be nonlinear so

 then use f(x) where f is a vector and x is a vector .

 hope it is useful ..

 Cheers

Very useful 

Do you have any resource for such thing  Narasim ? books or paper

Perhaps you can see chap 5 p202 of "Modern Digital signal processing" Roberto Cristi

or any Dsp book chapter on implementation. The suggestion for using Euler aprox

(or transition matrix) just a suggestion that occurred to me.

 Good luck

 Cheers

Dear Dr. Narasim 

It worked well, with great accuracy also.

Thanks

Great.. Cheers