Sunday, January 13, 2013

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.
          TKΦ  Ia                                                                    ....... (I)
where T= Motor Torque
           K=  Proportionally torque constant 
           Φ = Field flux and I = 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 rad/sec.
            EKΦ ω                                                                   ....... (II)
where   E= Back e.m.f. and  K= Back e.m.f. constant
           ω = 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 + IRa                                                                                              ....... (III)
where V = Supply voltage 
          R= 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.

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