D.C. Servomotors

Servomotor Part 3-1

      The d.c. servomotors is more or less same as normal d.c. motor. There are some minor constructional difference between the two. All d.c. servomotors are essentially separately excited type. This ensures the linear nature of torque-speed characteristics.

1.1 Basic Working Principle

       The d.c. servomotor is basically a torque transducer which converts electrical energy into the mechanical energy. The torque developed on the motor shaft is directly proportional to the field flux and the armature current.

          T_m = K_m Φ  I_a                                                                    ....... (I)

where T_m = Motor Torque

           K_m =  Proportionally torque constant 

           Φ = Field flux and I_a  = Armature current

       In addition to the torque developed, when armature conductors rotate in the field flux, they cut the flux and e.m.f. gets induced in the armature. This e.m.f. is called as back e.m.f. in case of d.c. motors. It is directly proportional to the shaft velocity ω_m  rad/sec.

            E_m = K_b Φ ω_m                                                                    ....... (II)

where   E_b = Back e.m.f. and  K_b = Back e.m.f. constant

           ω_m  = Motor angular speed in rad/sec

       As back e.m.f. opposes the supply voltage, the voltage equation of the d.c. motor is given by

          V = E_b  + I_a R_{a                                                                                              ....... (III)}

where V = Supply voltage 

          R_a = Armature resistance

       The equations (I), (II) and (III) form the basis of d.c. servomotor operation.

1.2 Basic Classification

       The d.c. servomotors are classified as

1. Variable magnetic flux motors i.e. field controlled motors.

2. Constant magnetic flux motors i.e. armature controlled motors.