| There are many kinds of fault types of large induction motors, but it is also very important to analyze the dynamic characteristics of rotating machinery. Large induction motors are inevitable in the operation of the unbalanced magnetic pull, the unbalanced magnetic pull on the rotor, will inevitably have adverse effects on the dynamic performance of the rotor system.In this paper, a currently is the world's largest 20 pole induction motor as an example. First of all to finite element and classical electromagnetic theory based established two-dimensional finite element model of the prototype of rotor eccentricity, rotor eccentricity on the no-load and load state of the air gap flux density distribution was calculated and analyzed. By using Fourier series to expand the air gap magnetic conductance, the nonlinear analytical expression of unbalanced magnetic pull of large induction motor is derived, and the result is calculated.Secondly, the finite element model of rotor bearing system is established by analyzing the finite element modeling theory of rotor system. The influences of bearing stiffness and unbalanced magnetic force on the natural frequency of the rotor system are analyzed, and the first three order modes of the prototype are analyzed.At last, the theoretical basis for the unbalance response and stability of rotor system is derived. According to the model of the prototype rotor system, the unbalance response is analyzed, and the amplitude of the rotor system under steady state is solved. To jefcott rotor model as an example analysis the causes of instability of rotor system, rotor system stability criterion is derived to solve prototype rotor system critical damping curve and logarithmic decay rate curve, and obtains the speed of losing stability. |
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