M.Sc Thesis


M.Sc StudentNoam Barak
SubjectInvestigating the Controlled Dynamics of an
Electromagnetically Damped Rotating System
DepartmentDepartment of Mechanical Engineering
Supervisors Full Professors Bucher Izhak
Dr. Sela Naftali
Full Thesis textFull thesis text - English Version


Abstract

Rotating systems often experience vibrations that can cause degradation in performance or failure. In reality, even a small imperfection can bring about large vibration levels and therefore a supplemental damping mechanism, which restrains the vibrations, needs to be implemented. Typical damping mechanisms are: an oil based damper or an eddy current damper which is discussed here.

Electromagnetic dampers have an obvious advantage compared to other methods of damping because it is clean, does not induce wear and can be controlled electronically. In particular, we propose an eddy current damper which is simple and mechanically robust, therefore it requires little maintenance throughout its life. Because the system is non-contacting, the damper can be configured so that the mass loading and added stiffness common with other damping schemes can be avoided. This allows the system to have a significant increase in damping while avoiding changes in the natural frequencies and mode shapes.

However the mechanism of magnetic dampers, in its traditional form, may cause instable behavior while operating in the super-critical regime. This research examines a novel approach to enable stability and vibrations reduction for the entire operation regime. The basic model, introduced here, includes a nonmagnetic disk fixed to the rotating shaft; a ring shaped magnetic field is rotated perpendicular to the disk plane.

The damping mechanism remains the same. Eddy currents are created in the disk due to magnetic flux change, which leads to the creation of Lorentz forces. These forces in the radial direction oppose the disk lateral vibrations and dissipate energy from the system. To prevent the instability phenomena in the super-critical operation regime, the stator of the proposed damper will have the capability to rotate a magnetic field. The rotation speed of the magnetic field will be dynamically controlled in closed-loop in such a way that at every moment (in the super-critical zone), opposing force component will be created to eliminate the inherent instable force component.

In the limits of this dissertation, an analytical electromagnetic model is introduced, closed loop rotating electromagnetic damper control law is suggested and the investigation of the dynamic behavior is examined in simulation.

 It is shown that while the magnetic field rotation speed equals the disk whirling speed, the system remains stable for the entire operational regime. Beyond the critical speed, as the spin speed raises, the overall vibrations amplitude grow. This is expected due to the growing imbalance forces which are proportional to the square of the rotor spin speed. In the sub-critical speed regime the lateral vibrations damping was less effective compared to a traditional eddy current damper. Lastly, parametric tests took place in order to investigate the dynamic system behavior while: A. supplying a saturation limit for the magnetic field speed. B. Implying a non-linear switched controller which was operated at different spin speeds.