or by examining the Nyquist plot of Z ( jω )Y ( jω ) s l .
However, since Z ( jω )Y ( jω ) s l may contain multiple zeros,
its magnitude does not decrease monolithically with ω and its
phase plot crosses the 180o line a few times, it is difficult to
assess the stability margin based on the Nyquist plot.
Instead, the damping ratio of dominant pairs of the system
eigenvalues can be easily computed and used as the system
stability indicator.
By way of example, Fig. 3 shows the distribution of the
eigenvalues of the complete system whose parameters and
operating condition are given in Table I.
The voltage control loop bandwidth of the boost
converter is set to one twelfth of the current control loop
bandwidth. In the motor drive the speed control loop
bandwidth is a third of that of the current loop, and the gains
are determined using the symmetric optimal method [7].