Method of the unbalanced rotor critical frequency overcoming by means of the operated magnetic spring

Abstract

Abstract.When passing critical rotational frequency of a rotor with significant imbalance there can be crossoscillations with large amplitudes. The algorithm of critical rotational frequency overcoming which allows reducing these oscillations considerably is described.It is offered to use a differential magnetic spring as an elastic support with operated stiffness.When passing critical rotational frequency of a rotor with significant imbalance there can be cross-oscillations with large amplitudes. The algorithm of critical rotational frequency overcoming which allows reducing these oscillations considerably is offered. For this aim it's proposed that elastic supports have two stiffness coefficients: working and increased, with the corresponding two values of critical frequency. It is offered to use a differential magnetic spring as an elastic support with operated stiffness. The magnetic spring consists of the permanent magnets connected by magnetic conductors. The schematic diagram of a differential magnetic spring is shown in Fig. 1.Its important advantage is capability to change easily the equivalent stiffness of an elastic support. It allows to operate the value of stiffness during the rotor acceleration so that to achieve reduction of oscillations amplitude, and, therefore, stresses in the supports up to the acceptable values even at big imbalance and to provide the acceleration to the over-critical frequency of rotation. Technically it is quite possible to change (to reduce) stiffness of a magnetic spring (to increase an initial gap) quickly. If to make fastening of magnets by means of a latch, atits release magnets can be moved apart under the force of their interaction, thereby increasing an initial gap for rather small time.The magnetic support (spring) contains three permanent magnets, side magnets are fixed and the middle one, which is connected with the shaft train, can move in the vertical direction, and is itself established on the amortized support. Dependences of force ‫ܨ‬ሺ‫ݔ‬ሻ acting on a middle permanent magnet, on vertical displacement ‫ݔ‬ at various values of an initial gap ݀ are given in Fig. 2. These dependences are received by analysis of a stationary magnetic field in system of magnets with the subsequent calculation of force of their interaction. Fig.2 -Stiffness characteristic of a magnetic spring Varying the size of an initial gap between extreme magnets, it is possible to change the size of equivalent stiffness (Fig. 2), thereby changing a skeletal curve. The established oscillations of a platform at the fixed position of magnets and, therefore, amplitude-frequency characteristics can be found by solving the shaft train dynamics equations by a harmonic balance method. In Fig. 3 dependence of a platform oscillations amplitude on the rotation frequency is shown at various values of an initial gap in a magnetic spring (amplitude-frequency characteristics).