Within this work, the conventional PD-controller is proposed to suppress the lateral oscillations of a
nonlinear Jeffcott-rotor system. Time-delays in the control loop are included in the system model. The slow-flow
modulating equations governing the whole system dynamics are obtained. Influence of the controller gains and
time-delays on response curves are explored. The analyses showed that the controller efficiency has been
enhanced at specific values of time-delays; while the system may lose its stability at the other values. The
phenomenon of enhancing the controller efficiency was explained and accordingly the optimal values of the loopdelays
have been reported.