The　nonlinear　dynamics　of　rotors　suspending　in　the　active　magnetic　bearings　(AMBs)　with　eight　pole　pairs　are　investigated.　The　nonlinear　governing　equations　of　two　degree-of-freedom　(2DOF)　are　obtained　considering　the　rotor　with　proportional-derivative　(PD)　controller.　By　studying　the　conservative　free　vibrations　of　the　general　2DOF　nonlinear　systems,　three　types　of　motions　are　found,　namely,　in-unison　modal　motions,　elliptic　modal　motions,　and　quasi-periodic　motions.　The　method　of　multiple　scales　is　used　to　obtain　the　amplitude-phase　portrait　to　demonstrate　the　three　types　of　motions　by　introducing　the　energy　ratios　and　phase　differences.　It　is　found　that　in-unison　modal　motions　do　not　exist　due　to　the　symmetry　feature　of　the　rotors　in　the　AMBs.　The　effect　of　the　PD　controller　and　damping　to　the　vibration　suppression　is　discussed　by　the　idea　of　the　rolled-up　amplitude-phase　portrait.