Abstract
Vibration characteristics of a structure on which unbalanced rigid rotors are operated are investigated experimentally and analytically. Rotor speeds cannot exceed a critical speed when drive power to each rotor is not enough. Further running speeds of rotors are entrained into a single speed just below the critical speed and phases of rotor unbalance are locked so as to excite the structure violently. On the other hand, two or three rotors operated by sufficient power supplies can run above the critical speed and unbalance forces cancel each other, that is, vibration is very small. When four rotors are operated with enough power supplies, the phases of rotor unbalance are not locked for speeds above a critical speed.
