Following possibilities

1. You can try using simplified transfer function of synchronous motor widely available in literature.(or switched reluctance motors

2. Use fundamental of input waveform and D-Q transform

3. Use fundamental equations of force = -dWstored/ dtheta

Cheers

Narasim Ramesh can you mention any link of reference where i can find simplified transfer function of stepper motor ?

I am not aware of any ref. However please see if following could be of some help.It is highly simplified though.

 1. Let us say we have supply voltage Vd  (dc

2. This produces through switching  , a balanced 2phase system of magn Eac ,

    So Eac = kVd   at some frequency w

3. Model the stepper motor as a voltage source Eb = K2* wr ( wr = rotor speed

4. Power input =  Eac*Eb sin(delta) /Zseries ...say  delta = torque angle Pinut

5. J dwr/dt +Tload = Pinput/wr

6. wr = d(delta)/dt   wr = mech

You cans see this ia nonlinear eqn. For tranfser function using laplace viz s

 linearize around steady operating point

Good Luck

 Cheers.

here is a simpler basic approach for  one coil with magnet at angle theta.

1. let flux of magnet = phim

2.max flux of coil  = lm*i  (assume or simplicity independent of theta)

3.then torqu =d(wstr/dtheta) = phim*lm*i*sin(theta)

4.toru= Jdwr/dt +Tload

5.wr= dtheta/dt

6. It is now possible to derive a linearized trf function  with init input conditions

    theta0 , i  phim

7 you can use 6 as differnt coils are switched

Cheers

Please see pdf . Hope you find it useful

Cheers