Actuation System: DC Servo Motor

DC SERVO MOTOR

PRINCIPLE OF OPERATING

  A DC motor is used in a control system where an appreciable amount of shaft power is required. The DC motors are either a rmature - controlled with fixed field, or field - controlled with fixed armature current. DC motor s used in instrument employ a fixed permanent - magnet field, and the control signal is applied to the armature terminals.

In addition to the torque when conductor moves in magnetic field, voltag e is generated across its terminals which opposes the current flow and hence called as Back e.mf.

Basic Classification

Basically d.c. servo motors are classified as:

(i) Variable magnetic flux motors.

(ii)Constant magnetic flux motors

Derivation of transfer functions for

(i) Field controlled d.c. servo motor

(ii)Armature controlled d.c. servo - motors.

i. Field Controlled DC Servo motor Assumptions

(1)Constant armature current is fed into the motor.

(2)Nf % If. Flux produced is proportional to field current. Nf = Kf If

(3)Torque is proportional to product of flux and armature current.

% N Ia .

Tm = K` N

Ia = K’ Kf If Ia

Tm = Km Kf If

Where

Km = K`

Ia = constant

Now shaft torque Tm is used for driving load against the inertia and frictional torque

finding Laplace Transforms of equations Tm (s) = Km Kf If(s)

Ef (s) = (SLf + Rf) If (s)

Tm (s) = Jms 2m (s) + Bms2m (s)

Eliminate If (s) from equations (4) and (5)Input = Ef(S)

Output = Rotational displacement 2m (S

ii.Armature Controlled D.C. Servo Motor

Assumptions:

(i)Flux is directly proportional to current through field winding. Nm = Kf If = constant

(ii) Torque produced is proportional to product of flux and armature current.

T = K`m N Ia

T = K`mKf If Ia

(iii)Back e.m.f is directly proportional to shaft velocity Tm, as flux N is constant