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The chaotic behavior of a flexible rotor supported by active magnetic bearings is numerically investigated in this work. A statically unbalanced disk is mounted on the the shaft. The rotor is modeled by three lumped mass and 8 D.O.F. The rotor-AMB systems include many non-linear factors, such as nonlinear function of the coil current and the air gap between the rotor and the stator, nonlinearity due to geometric coupling of magnetic actuator, eddy current effect and hysteresis losses of the magnetic core material. In this work, the influence of weight parameter on nonlinear response of the system is investigated. Numerical results showed considering of weight parameter have important effect on the response of the rotor and exhibit a rich variety of nonlinear dynamical behavior including synchronous, sub-synchronous, quasi-periodic and chaotic vibrations. Bifurcation diagrams, phase planes, power spectra ,Poincar’e map and maximum lyapanov exponents are used to analyze the response of the system under different operational conditions. Chaotic vibrations should be avoided as they induce fluctuating stresses that may lead to premature failure of the machinery’s main component. It will be beneficial to the design of AMB system.