Thermal Behavior of a Magnetically Levitated Spindle for Fatigue Testing of Fiber Reinforced Plastic

Abstract

† This paper is an extended version of our paper published in: Franz, D.; Schneider, M.; Richter, M.; Rinderknecht, S. Magnetically levitated spindle for long term testing of fiber reinforced plastic.Abstract: This article discusses the critical thermal behavior of a magnetically levitated spindle for fatigue testing of cylinders made of fiber reinforced plastic. These cylinders represent the outer-rotor of a kinetic energy storage. The system operates under vacuum conditions. Hence, even small power losses in the rotor can lead to a high rotor temperature. To find the most effective way to keep the rotor temperature under a critical limit in the existing system, first, transient electromagnetic finite element simulations are evaluated for the active magnetic bearings and the electric machine. Using these simulations, the power losses of the active components in the rotor can be derived. Second, a finite element simulation characterizes the thermal behavior of the rotor. Using the power losses calculated in the electromagnetic simulation, the thermal simulation provides the temperature of the rotor. These results are compared with measurements from an experimental spindle. One effective way to reduce rotational losses without major changes in the hardware is to reduce the bias current of the magnetic bearings. Since this also changes the characteristics of the magnetic bearings, the dynamic behavior of the rotor is also considered. active and passive components. Active magnetic bearings (AMBs) are used for radial levitation and a passive magnetic bearing is used for axial levitation. A permanent magnet synchronous machine (PMSM) accelerates and decelerates the rotor. All rotating components of the magnetic bearings and the PMSM are integrated on the inner circumference of the FRP rotor. Under rotation, these components press against the FRP, resulting in radial compressive stress transversal to the fiber orientation, which is superimposed by circumferential stress in the fiber direction (see Figure 1b). The energy density of the system increases with the radii of the rotor and its rotational speed, both factors lead to an increased stress in the FRP [7]. Consequently, high stress in the material is necessary to reach high energy densities. Charging and discharging the flywheel leads to cyclically varying mechanical stresses. To investigate the cyclic stability and lifetime of the FRP rotor, cyclic material tests, such as cyclic transverse tensile tests (derived from [8]), cyclic transverse compressive tests (derived from [9]), and cyclic four-point bending tests are performed on material samples. These samples are thin walled and relatively easy to fabricate with a high quality, whereas the rotor of the flywheel is much thicker and harder to produce in an industrial fiber winding process. Furthermore, the thermal expansion of carbon fiber and the epoxy plastic matrix used differ, leading to inner stress when a thick walled FRP structure cools down from the curing temperature. Hence, there is a much higher probabi...

Controller parameterization and bias current reduction of active magnetic bearings for a flexible and gyroscopic spindle

“…Since the upper measurement position is between the axial and the upper radial AMB, whereas the lower measurement position is below all major loss‐inducing components, the temperature rise at the upper position is bigger than at the lower position (cf. References 4,5). The rotor reaches 94°C at the upper position after 387 cycles in less than 3 h with $$$ {I}_B=3.5\ \mathrm{A} $$$.…”

“…To safely prevent overheating of the permanent magnets of the PMSM, the temperature should remain well below 100°C. To further reduce the temperature, the test cycles can be adjusted by increasing $$$ {t}_{15k} $$$, since the losses at 15,000 rpm are smaller than at 30,000 rpm 4 . This temperature reduction can be seen in Figure 16, in which the upper rotor temperature is shown during long‐term cycling with $$$ {I}_B=2.4\ \mathrm{A} $$$.…”

“…A cross section of the test rig is shown in Figure 1 on the left. The test rig is described in more detail in References 4,5. An outside view of the system is shown in Figure A1 in Appendix .…”

“…The losses on the rotor are dominated by remagnetization losses. Losses due to air friction can be neglected because of the operation in vacuum 4 . The stator is cooled using a water‐cooling system.…”

“…Hence, losses on the rotor have to be minimized. A significant portion of the remagnetization losses on the rotor are the rotational losses in the heteropolar radial AMBs 4,5 . These are largely determined by the bias current $$$ {I}_B $$$, which is used to linearize the characteristic of the AMBs (cf.…”