Modal excitation of circular rotating structures using an innovative electromagnetic device

Abstract

Metallic structures like circular plates or turbine discs feature special mode shapes that can be classified by nodal diameters and nodal circles. It is often desirable to experimentally examine just one mode shape with its distinct nodal diameter and nodal circle. To excite just one mode shape at a certain eigenfrequency the circumferential spectrum of the excitation force must not have other frequency content than the desired frequency and additionally have to resemble the mode shape itself. A device capable of creating such a single mode shape excitation has not existed yet. Here, a patented device is presented which is able to excite a single distinct mode shape of a rotating metallic circular structure. To achieve this the device features a toroidal horseshoe electromagnet with a variable gap geometry. By using the variable gap geometry and variable inductor current any modeshape of a metallic circular rotating structure can be excited. It is also possible to excite a combination of modeshapes by linear superposition of the necessary gap geometries. A brief explanation of the device is given followed by experimental results.