Assumptions :
1. Constant armature current is fed into the motor.
2. ɸf α If . Flux produced is proportional to field current.
3. Torque is proportional to product of flux and armature current.
Apply Kirchhoff's law to field current.
Now shaft torque Tm is used for driving load against the inertia and frictional torque.
Finding Laplace transforms of equations (1), (2) and (3) we get,
Eliminate If (s) from equations (4) and (5),
Eliminate Tm (s) from equations (6) and (7),
Input = Ef (s)
Output = Rotational displacement θm (s)
Block diagram for field controlled d.c. motor is shown in Fig.2.
