Touchdown Bearings Research Articles

Study on type of magnetic suspension rotor groove and wear of drop touchdown bearing

The active magnetic bearing (AMB) is widely used in the field of flywheel energy storage system (FESS) in wind power generation. This study mainly studies the magnetic suspension rotor groove (MSRG) of FESS and the wear of drop touchdown bearing (DTB). The mathematical model of air gap magnetic field waveform on stator surface and Plath equation of control in air gap region were established. Contrastive analysis of conventional, second harmonic and third harmonic ‘petal-shaped’ MSRG in AMB. The life test of DTB on magnetic suspension rotor was carried out for 24000r/min and lasted for 1500 h. The second harmonic ‘petal-shaped’ MSRG could not only generate reluctance torque and increased the torque density of the motor, but also increased the fundamental component and excitation torque component by reducing the peak value of the magnetic field. The temperature ranges of front and rear DTB were ‘30℃–46℃’ and ‘26℃–40℃’, respectively. After service, the maximum thickness of metamorphic layer in the DTB raceway was 6 μm. After service, the content of Fe in the metamorphic layer of DTB raceway decreased by 10.11% and the content of C increased by 2.41%. Raceway hardness of DTB after service was 59.67 HV higher than that before service.

Preliminary Study on Recontrol Evaluation of a Magnetic Bearing Rotor Falling on Touch-Down Bearings

The dynamic control of an active magnetic bearing (AMB) rotor after the rotor falls to its touch-down bearings has always been a difficult problem for applications such as flywheel energy storage. The rotor drop process has obvious nonlinear dynamic characteristics. This paper first discusses the structure of AMBs and the basic principles of their control. Starting from the electromagnetic forces that electromagnets can provide, the problem is simplified to the influence of an electromagnetic force with constant damping characteristics on the dynamic characteristics of a dropped rotor. A dynamic model of an AMB rotor with touch-down bearings was built and the contact force model between the rotor and the touch-down bearings was determined. A constant damping electromagnetic force was applied in two ways to verify the dynamic control feasibility of a dropped rotor through magnetic bearings. The simulation results show that the dropped rotor recovery control is feasible by applying a reasonable electromagnetic force.

Force-based feedforward control of persistent synchronous rotor/touchdown bearing contact in active magnetic bearing systems

Touchdown bearings are installed in active magnetic bearing systems to protect system components and prevent rotor contact with stator laminations. Abnormal conditions may induce a rotor dynamic response that leads to persistent contact with the touchdown bearings. Control schemes may be successful in re-levitating the rotor from contact provided the underlying contact dynamics are taken into consideration. Effective rotor/touchdown bearing contact control is opportune when active magnetic bearings remain operational and complete machine rundown is avoidable. In such cases, the active magnetic bearing forces should have the required dynamic load capacity to restore contact-free rotor levitation, eliminating the need for extra actuation in the system. This requires establishing a relationship between contact dynamics and magnetic bearing control forces. This paper presents a new contact control method based on stator strain measurement arising from rotor/touchdown bearing contact. The measurement data are utilised to evaluate frequency dependent magnetic bearing control forces and phase shifts to enable rotor recovery from contact. The control method is tested and validated experimentally on an active magnetic bearing system with a long unbalanced flexible rotor. The tests demonstrate effective rotor recovery from persistent synchronous contact over a range of rotor speeds.

Analysis of the dry whirl and dry whip response in a vertical active magnetic bearing drop test rig

A horizontal rotor is usually the research subject of rotor drop failure in active magnetic bearing (AMB) test rigs. However, research shows, that a vertical rotor drop failure is not sufficiently understood at present. The experiments in other research studies show that the vertical rotor dynamic response is not simple whirl behavior after dropping on the touchdown bearing (TDB), which concludes dry whirl and dry whip processes, where the former is synchronous forward whirl, and the latter is subsynchronous forward whirl. To predict the dynamic response of a vertical rotor drop failure, theoretical research based on a rotor-TDB model system is developed in this paper, and the effectiveness of this model is validated by an AMB-TDB rotor experiment. The conclusion is that the natural whirl speed and the resulting critical speed clearly distinguish two states of forward whirl motion. When the rotation speed is less than the critical speed, the rotor is in a dry whirl state; otherwise, it behaves as in a dry whip. The rotor unbalance is introduced in the model, which explains the change in the whirl speed. Moreover, the influencing factors on the natural whirl speed are analyzed in depth. The result shows that parameters of the TDB, such as the friction coefficient and protective clearance, and the rotor unbalance have significant effects.

Dynamic and Thermal Investigations of the Forward Dry-Friction Whirl/Whip of a Vertical Rotor-AMB System during Touchdowns

When an active magnetic bearing (AMB) rotor drops, it impacts the touchdown bearing (TDB) and produces friction on its surface. The vertical AMB rotor has no stable support in the radial direction, and the rotor exhibits a violent whirl motion in the gap of the TDB. In this study, a complete dynamic and thermal model of the AMB-rotor-TDB system was established, and the complete drop process was simulated. When the rotor dropped, it obtained stable support after several bounces on the thrust surface of the TDB inner ring in the axial direction. In the radial direction, the rotor entered whirl motion after the initial collisions. There is a natural whirl frequency so that the drop forward whirl is divided into the dry-friction whirl and whip states. The contact force and heat generation of the TDB were monitored in the simulation and had different performancs in the two states. Both the initial collisions and the stabilized whirl motions were studied to evaluate the reliability of the TDB. Finally, a series of drop tests were performed, and the experimental results were in good agreement with the simulation.

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.

Simulative investigation of rubber damper elements for planetary touch-down bearings

Simulative investigation of rubber damper elements for planetary touch-down bearings

Shock-induced persistent contact and synchronous re-levitation control in an AMB-rotor system

Active magnetic bearings (AMBs) have limited dynamic load capacity due to magnetic saturation. Hence, large external disturbances (such as shock loads) may cause contact between the rotor and touchdown bearings (TDBs), which may evolve into complex dynamic behaviour and damage the machine. This paper considers the shock responses of a rotor and viable re-levitation control options when the AMB is still functional. Bi-stable responses and shock-induced persistent forward rubbing were observed in an experimental AMB-flexible rotor facility and its numerical model. The analytical solution for steady synchronous motions with rubbing of a general AMB-flexible rotor system was proposed. The standard control action for a contact-free rotor state would not be appropriate due to phase changes and the displacement amplitude differences in the frequency responses. To destabilise the persistent contact responses and restore contact-free levitation, open-loop phase search based synchronous compensation (PSSC) control and synchronous motion compensation (SMC) control are designed, which are activated when a persistent contact is detected. Stability of the control system and the effectiveness of these two re-levitation control methods are verified by simulation and experimental results. It is also found by comparison that the efficiency of PSSC depends on the phase difference (incorrect phases may degrade rotor response), while the SMC consumes more computing effort.

Shock Resistance Design of High-Speed Maglev Motor–Stability Considerations and Quantitative Shock Tests

Large external disturbances (such as shock loads) can cause contact between the rotor and touchdown bearings (TDBs). Hence, maintaining the stability of systems with active magnetic bearings (AMBs) is a major challenge for mobile applications such as on-board steam turbines or vehicle turbochargers. In this paper, two key factors (power bandwidth and bi-stable characteristic) that affect the shock stability of AMB-rotor systems are considered in the design of a high-speed maglev motor. Insufficient power bandwidth can induce current saturation leading to destabilization, while a bi-stable characteristic can cause persistent contact between the rotor and TDBs under external disturbances. Theoretical analyzes and criteria are provided, and the influence of these two factors is investigated by base shock experiments of a high-speed maglev motor. These experiments involved mounting a high-speed maglev motor on a shock table then subjecting it to shock loads of different amplitudes. The designed motor continued to operate stably under shock loads up to 20 G, verifying the correctness of the stability considerations.

Investigations on the dynamic behaviour of an on-board rotor-AMB system with touchdown bearing contacts: modelling and experimentation

Active magnetic bearing (AMB) technology provides to rotating machinery, high operating speeds, direct couplings, almost frictionless support and a reduced footprint. In case of strong base excitations, the rotor may contact its touchdown bearings (TDBs), which are used as landing and safety bearings, while AMBs still operate. The paper presents, experimentally and numerically, this particular operating conditions where sliding friction-induced vibrations or AMB instabilities might be triggered. For this purpose, the developed finite-element model of the rotor-AMB-TDB system is experimentally validated using a lab-scale rotor test rig mounted on shaker. The validation is first conducted with no base excitation by analysing the mass unbalance responses and drop onto TDBs. In the presence of harmonic then impulse base excitations, the mass unbalance response are analysed in time and frequency domains. Considering harmonic tests, particular combinations of base motions and rotordynamics are observed depending on the frequency ratio between the base motion and the speed of rotation. TDB interactions produce rotor orbits flattening. The impulse tests highlight the friction effects and short periods of backward whirl are observed just after the contact. Despite some transient instabilities, the controller was always able to maintain the system stable by bringing damping. At each of these steps, the model reliability is assessed and predictions are close to the measured phenomena.

Nonlinear dynamic simulation and parametric analysis of a rotor-AMB-TDB system experiencing strong base shock excitations

The application of active magnetic bearings (AMBs) enables turbomachinery to increase power density, controllability and resilience to external disturbances. However, due to the limited load capacity of AMBs, base shock excitations may result in contact between the rotor and the touchdown bearings (TDBs), which can seriously damage the machine. This paper presents a dynamic analysis of a rotor-AMB-TDB system experiencing strong base shocks while the AMBs are operating. The contacts between the rotor and the TDBs, and the rolling-ball contacts within the TDBs are modeled based on the Hertzian contact theory. A PD controller is designed, considering saturation. The dynamic equations were solved numerically, and the results showed three different rotor motion modes after base shocks. FFT spectra, STFT spectra, Poincaré maps and bifurcation diagrams were used for the dynamic analysis. Unsuitable system parameters, including unbalance eccentricity, radial clearance, equivalent stiffness and damping ratio, may lead to complex behaviors including periodic, KT-periodic, and quasi-periodic responses as well as jump phenomena. This study reveals the mechanism of nonlinear responses, provides a method for predictions, and helps with controller designs for rotor re-levitation.

Performance evaluation of touchdown bearing using model-based approach

In the active magnetic bearings (AMBs) supported rotating machinery, touchdown bearings are considered as safety devices to support the rotor in the deficiency of electromagnetic field. Generally, the industrial AMB machines do not have force sensors for touchdown bearings and the system is only equipped with the position sensors to monitor the rotor displacement that disables the opportunity to measure the forces during dropdown events that might be destructive for the safe operation of the rotor system. This study explores the relative severity of dropdowns that are evaluated from a computational rotor model using the rotor displacement data recorded from the position sensors installed in the machine as an input. The model for double-row angular contact ball bearing type touchdown bearings is integrated with the rotor model. The simulation model is verified by comparing the simulated rotor orbits against measured orbits at four different dropdowns. The Fast Fourier transform (FFT) is used to observe the studied dropdown events in frequency domain revealing from the rotor response the following details: harmonics of the operation speed, rub-impact frequencies, pendulum motion of the rotor and the first two bending frequencies of the rotor settled on the touchdown bearings. The critical speed map also verifies the bending frequencies and identified support properties. The model-based approach can be used to evaluate and compare a single dropdown event with respect to previous events providing an insight for decision making whether touchdown bearing should be replaced.

Dynamic responses after rotor drops onto a new-type active eliminating protective clearance touchdown bearing

A new-type active touchdown bearing (TDB) system integrated with a centering device to eliminate the protective clearance after rotor drop in an active magnetic bearing (AMB) system is proposed. A rotor dynamic model is established on the basis of rigid rotor theory. A collision model between the support base and outer race of the ball bearing and a real-time ball bearing elastic supporting force model are established on the basis of Hertz contact theory. The dynamic model of the new-type TDB is established on the basis of vibration theory. Using these established theoretical models, the dynamic responses after rotor drops onto the new-type TDB are simulated and compared with the responses after rotor drops onto traditional TDB. Relevant experiments are conducted on an AMB test platform to verify the simulation results. The results show that the proposed new-type TDB can rapidly eliminate the protective clearance after rotor drop and can thus effectively reduce the following rotor vibrations and impacts.

Stability of Rotating Machinery Supported on Active Magnetic Bearings Subjected to Base Excitation

Abstract The stability of rotating machinery is a major challenge for the floating production storage and offloading (FPSO) units such as steam turbines or centrifugal compressors. The use of active magnetic bearings (AMBs) in turbomachines enables high operating speeds, active mechatronic system for the diagnostics, and the control and enables downsizing of the whole installation footprint. In case of strong base motions, the rotor can contact its touchdown bearings (TDBs) which are used as emergency and landing bearings. The aim of this study is to assess the stability of a rotating machine supported on AMBs during severe foundation excitation. The combined effect of unbalance forces, base motion excitation, and contact non-linearity on a rotor–AMB system response is analyzed focusing on the capacity of an augmented proportional-integral-derivative controller to maintain the system stable. An academic scale test rig was used for the experimental investigations. The controller was efficient and able to maintain the system stable during and after the application of the excitation, but the dynamic capacity of the AMBs was largely oversized with respect to the studied system. In order to check the capacity of the AMBs, when they are designed as a function of the rotor weight and expected excitation, numerical simulations were carried out (downsized). A finite element (FE) model was developed to model the on-board rotor–AMB system. Predicted and measured responses due to impulse excitation applied on the foundations were compared. The capacity of the controller to maintain the system stability is then discussed.

Dynamic Simulation of Active Magnetic Bearing Supporting Rotor Centrifugal Compressor During Drop On Touchdown Bearings

Dynamic Simulation of Active Magnetic Bearing Supporting Rotor Centrifugal Compressor During Drop On Touchdown Bearings

Design and Prototype Test Data for a 300 kW Active Magnetic Bearings-Supported Turbine Generator for Natural Gas Pressure Letdown

Abstract A 300 kW integrated and fully sealed turboexpander-generator for natural gas pressure letdown (PLD) was developed by Baker Hughes, a GE company (BHGE), in conjunction with Calnetix Technologies. This paper describes the design and analysis of the generator, magnetic bearings, and touchdown bearings, with a focus on the dynamic performance and key characteristics of the machine. The permanent magnet (PM) synchronous generator is supported by PM-biased, homopolar magnetic bearings and has a maximum continuous operating speed (MCOS) of 31.5 krpm. A touchdown bearing system is implemented using rolling element bearings with soft mount supports. Also described is a thrust load balancing scheme that uses the thrust bearing coil current for reference. A time transient simulation showing the effect of process conditions on the AMB dynamics is shown. Preliminary data from the prototype mechanical run test are shown, including transfer functions measured using the magnetic bearings, Campbell diagram, and touchdown bearing drop test results.

Key Technologies of the Superconducting Flywheel Energy Storage Demonstration Machine

The superconducting flywheel energy storage systems (FESS) can stabilize the fluctuation of the output from solar photovoltaic power generation systems. The FESS has been developed as a joint project of five enterprises subsidized by the New Energy and Industrial Technology Development Organization. Four key technology of the FESS are the magnetic fluid seal (MFS) technology for vacuum seal using a multi-material shaft, the active magnetic bearing (AMB) technology for non-contact levitated support, the dynamic break resistor (DBR) technology for allowing storage energy to be consumed as heat energy, and the touch-down bearing (TDB) technology for the safety device in case of the superconducting magnetic bearing’s failure. In this paper, the principles and features of those technologies are explained, and the validity of the design is also presented.

Measurement and calibration of rotor/touchdown bearing contact in active magnetic bearing systems

Abstract Active magnetic bearing systems provide contact-free rotor levitation, which allows near frictionless rotation and high rotational speeds when they operate normally. Under certain operational conditions, a transient fault, overload condition, or disturbance may occur. Touchdown bearings or bushings are therefore implemented in such systems to prevent contact between rotor and stator laminations. If the rotor makes contact with a touchdown bearing, the rotor dynamics may become transient or persistent in contact. Appropriate control strategies through the magnetic bearings to restore contact-free rotor operation may extend the life of touchdown bearings, and minimise operational downtime. To achieve this, an understanding of the contact dynamics is required, together with the relationship between contact and magnetic bearing forces. In this paper, rotor/touchdown bearing contact conditions are investigated experimentally using an active magnetic bearing system with a flexible rotor. Design methodology is presented for a measurement system capable of providing rotor/touchdown bearing contact related data, based on strain measurement. Strain induced contact signals are calibrated against applied magnetic bearing forces. The frequency dependent behaviour of the active magnetic bearing system is considered using evaluated force and phase measurements. The measurement data represent system identification for the potential of active magnetic bearing force-based contact control.

Influence of surface waviness in the heat generation and thermal expansion of the touchdown bearing

In an active magnetic bearings (AMBs) supported rotor system, the touchdown bearings have been proposed to support the rotor during an electromagnetic field shortage. The heat generation due to the high impact and collision of the rotor and touchdown bearing, in addition to the internal friction in the bearing, raises the temperature of the touchdown bearing. In this work, a numerical model is applied to simulate the rotor dropdown where the FE-model of the rotor is integrated with the dynamic and thermal model of the touchdown bearing. The rotor that is used as a case study is supported by a deep groove ball bearing and a pair of angular contact ball bearings installed in an X-configuration. The present study focuses on evaluating different orders of surface waviness in a pair of angular contact ball bearings. The equivalent electrical circuit model is implemented for computing the thermal behavior of the touchdown bearing. Results indicated that the dynamic friction coefficient between the rotor and inner race has a considerable impact on bounce height and the whirling motion of the rotor. Furthermore, for a certain amplitude of surface waviness, the effect of various orders of waviness is investigated and it has been found that surface waviness of the bearing alters the contact force and friction loss in the touchdown bearing. In addition, the effect of a higher amplitude of surface waviness on thermal expansion of the touchdown bearing is studied.

Touchdown bearing models for rotor-AMB systems

This paper investigates numerically and experimentally the rotor drop dynamics when unexpected Active Magnetic Bearings (AMBs) shut down occurs. In such an event, the rotor behaviour drops on two touchdown bearings (TDBs) composed of a ball bearing and a ribbon damper providing stiffness and damping to the overall system. The aim of this paper is to establish and to validate the rotor-drop system model. A first experimental set up is used to identify the dynamic characteristics of the corrugated ribbon damper and to test the Kelvin-Voigt model and the generalized Dahl model. Then, three TDB models are proposed including either the first or the second ribbon damper models. The second experimental set-up is devoted to the rotor drop response of an industrial scale rotor-AMB system equipped with two TDBs. Rotor drop simulations in the time domain are carried out by using the Finite Element method and the three TDB models. Predicted and measured rotor drop responses are compared regarding the displacements as well as transmitted loads and permits evaluating the model efficiency.