Fig.3 Eigenvalue distribution of converter fed PM drive system
As will be seen from Fig. 3, there are in total 12
eigenvalues associated with the system. 6 eigenvalues are
mainly related to the drive subsystem in which two are
associated with time constants of the d- and q-axis currents,
one with the mechanical time constant and the rest are
influenced by the control bandwidths of the current and
speed loops [7]. 4 eigenvalues are related to the converter
subsystem. Although the two converter poles which are
associate with the time constant of τ = CRL and the
bandwidth of the voltage control loop are close to the origin,
their modes are quite stable as they are on the negative real
axis. The remaining two poles (a complex conjugate pair) are
closely associated with the passive RLC filter but are
influenced by both the drive and converter subsystems. It is
also evident that this is indeed a dominant pair of eigenvalues
which has the most significant influence on the system
stability.
Fig. 4 shows the influence of current control loop
bandwidth of the motor drive on the dominant pole of the
system eigenvalues which represents the interaction of the
DC-DC converter with the motor drive via the passive filter.