Field Controller Design
The block diagram for closed-loop operation with the field controller in
action turns out to be somewhat complex. The interaction that occurs within
a separately-excited DC motor is first presented in Fig. 8.
The block diagram in Fig.8 is now described. The filed current, marked as
IF, produces magnetic flux in the motor and the back e.m.f of the
motor is then proportional to the product of the field current and the speed
of the motor. This statement is based on the assumption that the field flux
in the motor is not saturated and that the field flux varies linearly with
the field current. If the field current is in per unit notation, where the
filed current corresponding to the rated current equals unity, then the back
emf can be shown to be equal to Km × iF ×
wR, where both Km and wR are also in per
unit representing the motor coefficient and the speed of the motor. Once the
back e.m.f and the applied voltage are known, the armature current can be
obtained as shown in Fig. 8. From the values of armature current and field
current, the torque output of motor is obtained and the speed of the motor
changes as shown.
For design of field controller, the block diagram in Fig. 8 is too complex.
The design is carried out using a simplified or a simplistic block diagram
and the performance of the controller is evaluated using the final simulation
program, which uses a model that is reasonably close to real system.
The design of field controller is based on the block diagram shown in Fig.
9.
It is easy to represent the block diagram in Fig. 9 in per unit notation.
The gain of the controlled bridge for the field circuit is KF.
Its value equals the ratio of the maximum rate of bridge output voltage to
the rated voltage of the field circuit and normally the value of KF
is likely to be near 1.2. The delay due to firing circuit is again approximated
by TD2, and it is set equal to (1/4f), where f is the frequency
of the ac source. Then the field current is obtained in per unit value and
it can be made equal to the torque, assuming that the armature circuit has
comparatively a small time constant and that the armature current stays at
the rated value. The friction coefficient, the mechanical time constant and
the time constant of the filter in the speed feedback signal are the same
signals used for design of the speed controller. The applet below can be used
to design the field controller. This applet runs somewhat slowly. The poles
and zeros are calculated for the block diagram shown in Fig.9, whereas the
step response is obtained using the block diagram in Fig. 10.
The design of field controller is somewhat difficult because both the field
circuit time constant and the mechanical time constant are relatively large.
click here to open the applet
The applet displayed below shows step response of the drive with the field
controller.
click here to open the applet
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