Drop-Downs of an Outer Rotor Flywheel in Different Planetary Touch-Down Bearing Designs

With an increase in renewable energy in the electricity grid, more storage capacity for grid stabilization and energy flexibilization is necessary. Dynamic grid stabilization is one possible application for flywheels. To increase the energy density of flywheels, they can be built as highly integrated outer rotor systems. The losses of the flywheel are reduced by magnetic levitation and operation under vacuum conditions. In the case of the failure or overload of the active magnetic bearings, the system needs touch-down bearings to prevent system destruction. Planetary touch-down bearings consisting of several small bearing units circumferentially distributed around the stator are especially suited for these systems. In the literature, these planetary touch-down bearings are rarely investigated, especially the number of bearing units. Therefore, this paper investigates the influence of the number of touch-down bearing elements in simulations and experiments for an 8-element and a 6-element touch-down bearing arrangement. For the investigation, drop-downs at four different speeds were performed. Simulation and experimental results showed that, for the 6-element touch-down bearing, in contrast to the 8-element touch-down bearing, maximal velocity did not increase with the drop-down speed. Therefore, the touch-down bearing arrangement with fewer elements is preferrable

Control Strategies for Highly Gyroscopic Outer Rotors with Diametral Enlargement in Active Magnetic Bearings

Flywheels are used for peak shaving or load smoothing to generate a higher efficiency and a more stable power supply. Therefore, this paper investigates highly integrated outer rotor flywheels levitated by active magnetic bearings (AMB). Due to the highly gyroscopic behavior and the diametrical enlargement under rotation, the system behavior changes with the speed, leading to a significant decrease in the maximum force and maximum force slew rate of the AMB. Thus, the speed range in which a decentralized feedback control stabilizes the system is reduced. In the literature, there are numerous approaches for coping with gyroscopic behavior. However, there are far fewer investigations for explicit consideration of the change in the air gap in the control structure. Therefore, the goal of this work is to find a control strategy to reduce the effect of the gyroscopic behavior as well as the change of the air gap. The authors propose a control strategy combining a cross feedback control with a decentralized variable feedback control. With this combination, the drawbacks of the previously described effects are compensated, leading to a higher operating range of the system and a reduced utilization of the amplifier without overcompensation at lower rotational speeds

High speed rotor drop-downs in different planetary touch-down bearings differing in the number of bearing units

The design of touch-down bearings (TDB) for vertical, magnetically levitated outer rotor flywheels with high rotational speeds and operation under vacuum conditions is a challenging task. Conventional TDB are not suited for big diameters with high rotational speeds resulting in surface speeds of above 230 m/s. For such systems, the planetary design can be applied, consisting of several TDB units distributed circumferentially around the stator. This design has the advantages of a decoupling of the rotor diameter from the rolling element bearing diameter aswell as a whirl-suppressing characteristic due to the polygonalshaped clearance. The influence of the number of TDB units on the drop-down behavior is rarely investigated in the literature. However, it is expected that the number of TDB units influences the drop-down behavior and consequently the bearing service life heavily. Therefore, this paper investigates in simulations and experiments drop-downs with six, four, and three TDB units. The simulations are conducted with the simulation software ANEAS. The experiments are conducted with the TDB test rig, which is a robust test rig designed for rotor drop-downs in TDB. The results of both simulation and experiments indicate that the forces on the TDB are lower in configurations with less than six TDB units. However, with three units, the TDB was destroyed the fastest within less than twelve drop-downs

Simulative Investigation of Rubber Damper Elements for Planetary Touch-Down Bearings

Designing touch-down bearings (TDB) for outer rotor flywheels operated under high vacuum conditions are a challenging task. Due to the big diameters conventional TDB are not suited and a planetary design is applied consisting of a number of small rolling elements distributed around the stator. Since the amplitude of the peak loads during a drop-down lies close to the static load rating of the bearings, it is expected that the service life can be increased by reducing the maximal forces. Therefore, this paper investigates the influence of elastomer rings around the outer rings in the TDB using simulations. For this purpose, the structure and the models used for the contact force calculation in the simulation software ANEAS are presented, especially the modelling of the elastomers. Based on the requirements for a TDB in a flywheel application three different elastomers (FKM, VMQ, EPDM) are selected for the investigation. The results of the simulations show that stiffness and material strongly influence the maximum force. The best results are obtained using the material FKM. Leading to a reduction of the force amplitude in a wide stiffness range

Drop-Downs of an Outer Rotor Flywheel in Different Planetary Touch-Down Bearing Designs

With an increase in renewable energy in the electricity grid, more storage capacity for grid stabilization and energy flexibilization is necessary. Dynamic grid stabilization is one possible application for flywheels. To increase the energy density of flywheels, they can be built as highly integrated outer rotor systems. The losses of the flywheel are reduced by magnetic levitation and operation under vacuum conditions. In the case of the failure or overload of the active magnetic bearings, the system needs touch-down bearings to prevent system destruction. Planetary touch-down bearings consisting of several small bearing units circumferentially distributed around the stator are especially suited for these systems. In the literature, these planetary touch-down bearings are rarely investigated, especially the number of bearing units. Therefore, this paper investigates the influence of the number of touch-down bearing elements in simulations and experiments for an 8-element and a 6-element touch-down bearing arrangement. For the investigation, drop-downs at four different speeds were performed. Simulation and experimental results showed that, for the 6-element touch-down bearing, in contrast to the 8-element touch-down bearing, maximal velocity did not increase with the drop-down speed. Therefore, the touch-down bearing arrangement with fewer elements is preferrable

Adapting the control of the magnetic bearings of a highly flexible and gyroscopic rotor to the excitations by the motor

A test rig was built to perform fatigue tests on thick-walled cylinders made of fibre reinforced plastic (FRP). During the fatigue test, the rotational speed of an FRP cylinder is periodically varied until it fails. The FRP cylinder is connected to a drive spindle that accelerates and decelerates it using a permanent magnet synchronous machine (PMSM). To avoid excessive wear, the rotor is supported by active magnetic bearings (AMB). After the fatigue test was finished with the first cylinder, a new cylinder was attached to the test stand. With this new specimen, previously uncritical radial vibrations became more severe. For high accelerations, these vibrations led to instability of the rotor. However, high accelerations are desirable to perform the fatigue tests in the shortest possible time. Hence, the AMB control should be made insensitive to these vibrations. Since the vibrations depend on the acceleration of the rotor, it is reasonable to assume that they are induced by the PMSM. To reduce the vibrations, these excitations from the PMSM are included in the model-based controller parametrization process for the radial AMB, in which the parameters are adjusted via optimization. With the adjusted control, the amplitude of the vibration was significantly reduced and higher accelerations were possible. The described parameter tuning process can easily be adapted to different AMB systems with disturbances and changes in the system

Asymmetries in planetary touch-down bearings

For certain applications, magnetic bearings are preferred over rolling element bearings, for example, due to the absence of mechanical friction leading to high efficiency and low maintenance needs. However, they also require touch- down bearings (TDB) to bear thpe rotor in case of a malfunction or overload. For vertical systems with high DN numbers, like outer rotor flywheels, the design of the TDB becomes a challenging task. For such systems, the planetary TDB can be applied. The suitability of this design has already been shown in the literature (Quurck et al., 2017; Quurck et al., 2018). However, there are multiple parameters influencing the performance of the planetary TDB. For example, if an asymmetric planetary TDB should be preferred over a symmetric one, and if this is the case which asymmetries increase the performance of the planetary TDB. Therefore, this paper investigates asymmetries in planetary TDB in a simulation study performed with the Matlab-based simulation environment ANEAS. The results of this study indicate that the performance of planetary TDB can be increased when the air gap of the individual bearing units differs. If the air gap from one bearing unit is reduced by 20 % the maximum force acting on the TDB was reduced, in the best case up to 52 %

Data-driven modelling of touch-down bearing forces

To reduce CO2 emissions, the share of renewable energies in the grid increases. At the same time, many sectors like the transport and the building sector are changing to be powered by electricity. Especially electric cars demand high peaks in current from the grid. Storage is needed to balance demand and supply of electric energy. Flywheels can be part of the solution as they can be charged and discharged with high power and do not suffer from losing significant capacity even after thousands of cycles. Minimum loss of energy is crucial for a flywheel therefor active magnetic bearings (AMB) are used. If a malfunction of the AMB occurs the rotor falls into a touch-down bearing (TDB). To decide whether further maintenance in case of a drop-down event is needed information about the forces stressing the TDB is important. To avoid costs for physical sensors soft sensors are a suitable solution. In this research, a data-driven soft sensor based on recurrent neural networks is created to calculate the forces during the drop-down event. As input data only the position of the rotor is used. A test rig with physical sensors applied to every TDB supplies the force data to train, validate, and test the soft sensor model. Three different network architectures are compared. The results show that the sensor can calculate whether the rotor hits a TDB and is also capable of predicting the peaks in the force signal