Daniel Steinert

Slotless Bearingless Disk Drive for High-Speed and High-Purity Applications

In this paper, a bearingless drive for high-speed applications with high purity and special chemical demands is introduced. To achieve high rotational speeds with low losses, a slotless bearingless disk drive with toroidal windings is used. We present the working principle of the bearingless drive as well as a model for calculating the achievable drive torque. An advantageous winding system for independent force and torque generation is proposed, which can be realized with standard inverter technology. Additionally, the winding inductances are examined to evaluate the dynamic properties of bearing and drive. The findings are verified with simulation results and the system performance is successfully demonstrated on an experimental prototype, which runs up to 20 000 rpm and is designed for an output power of 1 kW.

Ultrafast rotation of magnetically levitated macroscopic steel spheres

Pushing the limits of ultrahigh-speed electrical machines. Our world is increasingly powered by electricity, which is largely converted to or from mechanical energy using electric motors. Several applications have driven the miniaturization of these machines, resulting in high rotational speeds. Although speeds of several hundred thousand revolutions per minute have been used industrially, we report the realization of an electrical motor reaching 40 million rpm to explore the underlying physical boundaries. Millimeter-scale steel spheres, which are levitated and accelerated by magnetic fields inside a vacuum, are used as a rotor. Circumferential speeds exceeding 1000 m/s and centrifugal accelerations of more than 4 × 108 times gravity were reached. The results open up new research possibilities, such as the testing of materials under extreme centrifugal load, and provide insights into the development of future electric drive systems.

Topology Evaluation of Slotless Bearingless Motors with Toroidal Windings

In this paper, different winding and magnet topologies are analyzed and compared for a slotless bearingless disk drive with toroidal windings. Basis of the studies is a six phase motor with a diametrically magnetized one-pole-pair rotor. Due to the absence of mechanical bearings, the motor is suitable for applications with high purity and special chemical demands. Its slotless design results in low losses even at high rotational speeds. To improve the operational behavior of the rotor in different applications, the influence of higher pole pair numbers on the passive bearing stiffness is examined. Possible winding configurations for these rotors are presented and evaluated for their bearing and motor performance. Based on the results, a further prototype was built and is presented in this paper.

Active radial magnetic bearing for an ultra-high speed motor

The miniaturization trend of electric machines increases the demand for higher rotational speeds to provide a desired mechanical power level at decreased size. To push the limits of rotor miniaturization, new concepts for an ultra-high speed motor are researched, which employs sub-millimeter size rotors and is capable of achieving rotational speeds above 25 million rotations per minute (Mrpm). The rotor is supported by means of a frictionless active magnetic bearing, which counteracts the gravitational force in vertical direction. Due to the low damping of the rotor, a magnetic bearing is also required in horizontal direction for to fully stabilize it. The respective system model, position sensor system and controller design for such a magnetic bearing are outlined in this study. Experimental results demonstrate an increased horizontal damping of the rotor by a factor of more than 100.

Loss investigation of slotless bearingless disk drives

Losses are often a limiting factor for the application of bearingless motors, especially at high rotational speeds. In this paper the loss mechanisms in slotless bearingless disk drives with toroidal windings are identified and analyzed. Although the slotless topology already features comparably low losses, a detailed comprehensive analysis of the loss portion enables a further reduction of these losses. To obtain the loss composition, computationally efficient simulation and calculation methods as well as simple measurement methods are presented for each loss component, considering iron losses, eddy current losses in the coils, PWM induced losses, copper losses, windage losses, and inverter losses. This allows for optimization of the motor geometry towards minimized losses. The analysis results are compared with loss measurements of four different motors with different rotor sizes, pole pair numbers, and coil configurations.

Piezoelectric two-layer plate for position stabilization

Numerous vibrating electromechanical systems lack a rigid connection to the inertial frame. An artificial inertial frame can be generated by a shaker, which compensates for vibrations. In this article, we present an encapsulated and perforated unimorph bending plate for this purpose. Vibrations can be compensated up to the first eigenfrequency of the system. As basis for an efficient system simulation and optimization, a new three-port multi-domain network model was developed. An extension qualifies the network for the simulation of the acoustical behavior inside the capsule. Network parameters are determined using finite element simulations. The dynamic behavior of the network model agrees with the finite element simulation results up to the first resonance of the system. The network model was verified by measurements on a laboratory setup, too. Furthermore, the network model could be simplified and was applied to determine the influence of various parameters on the stabilization performance of the plate transducer. The performance of the piezoelectric bending plate for position stabilization had been in addition investigated experimentally by measurements on a macroscopic capsule.

Homopolar bearingless slice motor in temple design

This work describes a concept of a magnetically levitated homopolar bearingless slice motor in temple design. The proposed setup consists of two oppositely magnetized permanent magnets on the rotor and stator and provides high axial stiffness. The fluctuation of the air gap field for torque generation is generated with a cross-shaped rotor iron. Compared to a diametrically magnetized drive, this concept achieves three times the axial stiffness without increasing the radial stiffness and is better suited for systems with large air gaps. A drawback is the low torque capacity. Its operating principle is described and a working prototype is presented including simulation and measurement results.

A high torque, wide air gap bearingless motor with permanent magnet free rotor

This paper introduces a bearingless motor topology with a magnet free rotor, which enables higher rotor torque densities and wider air gap compared to previously published topologies. Flux density in the air gap and therefore torque capability is maximized by a stator in temple configuration, which provides large winding space. The low number of eight stator and six rotor teeth keeps stray flux low, which enables wider air gap ratios at the same time. A challenge of having a low teeth number are large angle dependent force and torque nonlinearities, which are compensated in the control algorithm. A 3D FEM optimized prototype was constructed and put into operation. Measurements of the running system are presented to confirm the feasibility of the introduced topology.

A high speed millimeter-scale slotless bearlngless slice motor

Recent developments in electrical drive systems have shown a trend toward higher power densities at increased rotational speeds with application areas in high speed spindles, turbocompressors, flywheels and reaction wheels as well as optical systems. As conventional ball bearings suffer from excessive wear and decreased reliability at such speeds, contactless magnetic levitation offers an interesting alternative. This work presents the concept, design and implementation of a millimeter-scale bearingless slice motor featuring a rotor diameter of 4 mm. To the knowledge of the authors, this is the smallest published bearingless motor to date. The slotless stator topology is optimized by means of 3D finite element method electromagnetic simulations. Initial experimental results demonstrate a rotational speed of 160 000 rpm at losses below 1W.

Improved stator design for an ultra-high speed spinning ball motor

The ongoing miniaturization trend of electric machines increases the demand for higher rotational speeds to provide a required power level at decreased size. The goal of this project is to push the limits of rotor miniaturization by researching new concepts for bearingless machines with ultra-high rotational speeds exceeding 25 million rotations per minute (Mrpm). Using a simple machine stator consisting of air coils limits the achievable rotor torque, which results in acceleration times of several hours until the aforementioned rotational speeds are reached. This study outlines the torque generation mechanisms of the machine and investigates the stator losses, from which improved stator designs, based on a ferrite core, are derived. The latter significantly increase the motor torque at decreased losses and facilitate fast acceleration of the rotor.