
260 Electric Motors and Drives
TRANSIENT PERFORMANCE
Step
response
It was pointed out earlier that the single-step response is
similar to that of a damped second-order system. We
can easily estimate the natural frequency (co
n
) from the
equation
2
_ slope of torque-angle curve
total inertia
Knowing co
n
, we can judge what the first part of the
response will look like, by assuming the system is undam-
ped. To refine the estimate, and to obtain the settling time,
however, we need to estimate the damping ratio, which is
much more difficult to determine, as it depends on the type
of drive circuit and mode of operation as well as on the
mechanical