Fig. 5 shows the variation of damping ratio of the
dominant eigenvalue pairs with the current and speed control
loop bandwidths. The speed control loop bandwidth is
related to the current control loop bandwidth fc by a dividing
factor of a which varies from 2.41 to 10. A negative damping
ratio indicates instability of the system. As will be seen,
when fc is above 500Hz the damping ratio decrease with
increase of fc. On the other hand, an increase in the speed
control bandwidth leads to a great damping ratio.