In　this　paper,　we　analyze　the　complicated　nonlinear　dynamics　of　rotor-active　magnetic　bearings　(rotor-AMB)　with　16-pole　legs　and　the　time　varying　stiffness.　The　magnetic　force　with　16-pole　legs　is　obtained　by　applying　the　electromagnetic　theory.　The　governing　equation　of　motion　for　rotor-active　magnetic　bearings　is　derived　by　using　the　Newton＇s　second　law.　The　resulting　dimensionless　equation　of　motion　for　the　rotor-AMB　system　is　expressed　as　a　two-degree-of-freedom　nonlinear　system　including　the　parametric　excitation,　quadratic　and　cubic　nonlinearities.　The　averaged　equation　of　the　rotor-AMB　system　is　obtained　by　using　the　method　of　multiple　scales　when　the　primary　parametric　resonance　and　1/2　sub　harmonic　resonance　are　taken　into　account.　From　the　frequency-response　curves,　it　is　found　that　there　exist　the　phenomena　of　the　soft-spring　type　nonlinearity　and　the　hardening-spring　type　nonlinearity　in　the　rotor-AMB　system.　The　effects　of　different　parameters　on　the　nonlinear　dynamic　behaviors　of　the　rotor-AMB　system　are　investigated.　The　numerical　results　indicate　that　the　periodic,　quasi-periodic　and　chaotic　motions　occur　alternately　in　the　rotor-AMB　system.　(C)　2017　Elsevier　Ltd.　All　rights　reserved.