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.
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
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
(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