Experimental Investigations on a High-Speed Rotor for a Switched Reluctance Machine

A 5 kW Switched Reluctance Machine (SRM) has been fabricated and tested at 10,000 rpm under the no-load condition. The SRM has a solid rotor with salient geometry. The natural/modal frequency of the rotor structure plays an important role in determining its safe operating speed. The rotor is referred to as a rigid rotor, if its first modal frequency is well beyond the operating speed of the machine. At certain speeds, the rotor experiences maximum deflections; these speeds are referred to as critical speeds. Largely, the bending mode shape of the first natural frequency of the rotor would be similar to the deformation at critical speed. Information about the modal frequencies apriori helps in deciding the speed of operation. The modal frequencies, in turn, give an estimation of the rotor stiffness. The rotor stiffness and mode shapes also provide a guideline for selection, placement and design of bearings. In this paper, impact hammer tests are performed on the 5 kW SRM rotor to evaluate its first modal frequency and the mode shape it takes at that frequency. Experiments on the rotor are performed under two supporting conditions. In the first set-up the rotor is suspended using springs, and in the second setup, the rotor is placed on a foam base. Also, a 3-dimensional structural Finite Element Analysis (FEA) of the solid rotor is performed using ANSYS Workbench software. The first natural frequency obtained from the experiment is around 925 Hz and the corresponding critical speed is 55,500 rpm, which is well beyond the operating speed of the machine. Thus, from the perspective of structural rigidity, the rotor manufactured for the SRM is safe and is adequately qualified to operate at 10,000 rpm.