Discussion of Induction Motor Effect on Rotordynamics

In this dissertation, the influence of using induction motor in machinery train on rotordynamics is discussed. Two areas are considered – the use of variable frequency drives (VFDs), which control and drive the induction motor; and the unstable forces due motor eccentricity, which is resulted from motor rotor lateral motion. VFDs – The dissertation documents fatigue related mechanical failures in VFD motor machinery due to mechanical vibrations excited by drive torque harmonics which are created by PWM switching. Present effort models the coupled system with full electrical system including DC bus, inverter, motor, and an industrial mechanical system including flexible couplings, gearboxes and multiple inertias. The approach extends failure prediction beyond simple occurrence of resonance, to fatigue life evaluation based on Rain-flow algorithm, which is suitable for both steady state and transient startup mechanical response. The use of multilevel inverters is demonstrated having the possibility actually exacerbate resonance and fatigue failure. The model is also compared to an industrial test case, which provides good agreement. Motor eccentricity – In this dissertation, a MEC modeling method is proposed to calculate both the radial and tangential motor eccentric force. The proposed model is also coupled with the motor electric circuit model to provide capability of transient simulation. FEM (Ansys Maxwell) is used to verify the proposed model. Parametric study is performed on the motor radial and tangential eccentric forces. Also a Jeffcott rotor model is used to study the influence of the motor eccentric force on mechanical stability. A stability criteria of the bearing damping is calculated. The motor radial and tangential eccentric forces are all curved fitted to catch their nonlinearity, which are used in time domain simulation. Nonlinear motions are observed, including limit cycle and jumping phenomena. The results of this dissertation show that both the use of VFDs and the motor lateral motion (motor eccentricity) can cause severe mechanical vibration problem in a rotating machinery train. Both of these two problems need to be carefully concerned in design stage