Current Controller Design
The transfer function G1(s) expressed as equation (11) can expressed
in terms of per unit values and then Va(s) and Ia(s)
marked in Fig. 6 would be values in per unit notation. Conversion of equation
(11) such that it conforms to the per unit notation is explained below.
Both the numerator and the denominator of expression in equation (11) can
be divided by BRa. The ratio of J/B can be represented as the mechanical
time constant, tm. The resulting expression
for G1(s) presented as equation (31). Then equations (32) and (33) explain
how equation (31) can be converted such that it is in per unit notation.
The block diagram shown in Fig. 6 conforms to per unit notation. Here G1(s)
is expressed by equation (34) and Va(s) and Ia(s) marked
in Fig. 6 are values in per unit notation. Using (34), the transfer function,
G2(s) representing the block diagram in Fig. 6 can be represented
as shown below. In this case, the controlled-rectifier is assigned to have
its highest gain, KA. It is logical to do so, because the system
designed for stability at gain KA would be stable at lower gains
too.
The second applet in this page finds the location of the poles and zeros
of the closed-loop system in Fig. 6, given the necessary data. It is seen
for a wide range of controller parameters, the zeros are located such that
they cancel almost two poles. The other two poles are located away from the
origin. It is seen that the design of current controller is fairly easy.
click here to open the applet
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