Andreas Looser

Efficiency Optimization of a 100-W 500 000-r/min Permanent-Magnet Machine Including Air-Friction Losses

This paper proposes a method for the efficiency optimization of ultrahigh-speed permanent-magnet machines. Analytical methods are applied for the modeling of the machine that is equipped with a diametrically magnetized rotor and a slotless stator. The outer dimensions of the machine are design constraints, and the internal dimensioning is optimized for minimum losses. The air-friction losses are taken into account in addition to the usual iron, copper, and eddy-current losses. Laminated silicon-iron or laminated amorphous iron is used as the stator core material. The results show that air-friction losses influence the optimum design considerably, leading to a small rotor diameter at high speeds. The loss minimization and the amorphous iron core make it possible to reduce the calculated losses by 63% as compared to a machine design not considering air-friction losses. The resulting efficiency is 95% for a 100-W 500 000-r/min machine excluding bearing losses. Experimental results are shown to illustrate the validity of the method.

Conceptualization and Multiobjective Optimization of the Electric System of an Airborne Wind Turbine

Airborne wind turbines (AWTs) represent a radically new and fascinating concept for future harnessing of wind power. This concept consists of realizing only the blades of a conventional wind turbine (CWT) in the form of a power kite flying at high speed perpendicular to the wind. On the kite are mounted a turbine, an electrical generator, and a power electronics converter. The electric power generated is transmitted via a medium voltage cable to the ground. Because of the high flight speed of the power kite, several times the actual wind speed, only a very small swept area of the turbine is required according to Betzs Law and/or a turbine of low weight for the generation of a given electric power. Moreover, because of the high turbine rotational speed, no gear transmission is necessary and the size of the generator is also reduced. For takeoff and landing of the power kite, the turbines act as propellers and the generators as motors, i.e., electric power is supplied so that the system can be maneuvered like a helicopter. In the present work, the configuration of power electronics converters for the implementation of a 100 kW AWT is considered. The major aspect here is the trade-off between power-to-weight ratio (W/kg) and efficiency. The dependence of cable weight and cable losses on the voltage level of power transmission is investigated, and a comparison is made between low voltage (LV) and medium voltage (MV) versions of generators. Furthermore, the interdependence of the weight and efficiency of a bidirectional dual active bridge dc-dc converter for coupling the rectified output voltage of a LV generator to the MV cable is discussed. On the basis of this discussion, the concept offering the best possible compromise of weight and efficiency in the power electronics system is selected and a model of the control behavior is derived for both the power flow directions. A control structure is then proposed and dimensioned. Furthermore, questions of electromagnetic compatibility and electrical safety are treated. In conclusion, the essential results of this paper are summarized, and an outlook on future research is given. To enable the reader to make simplified calculations and a comparison of a CWT with an AWT, the aerodynamic fundamentals of both the systems are summarized in highly simplified form in an Appendix, and numerical values are given for the 100 kW system discussed in this paper.

Conceptualization and multi-objective optimization of the electric system of an Airborne Wind Turbine

Airborne Wind Turbines (AWT) represent a radically new and fascinating concept for future harnessing of wind power. This concept consists of realizing only the blades of a conventional wind turbine (CWT) in the form of a power kite flying at high speed perpendicular to the wind. On the kite are mounted a turbine, an electrical generator and a power electronics converter. The electric power generated is transmitted via a medium voltage cable to the ground. Because of the high flight speed of the power kite, several times the actual wind speed, only a very small swept area of the turbine is required according to Betzs Law and/or a turbine of low weight for the generation of a given electric power. Moreover, because of the high turbine rotational speed, no gear transmission is necessary and the size of the generator is also reduced. For takeoff and landing of the power kite, the turbines act as propellers and the generators as motors, i.e. electric power is supplied so that the system can be maneuvered like a helicopter. In the present work the configuration of power electronics converters for the implementation of a 100kW AWT is considered. The major aspect here is the trade-off between power-to-weight ratio (W/kg) and efficiency. The dependence of cable weight and cable losses on the voltage level of power transmission is investigated, and a comparison made of low voltage (LV) and medium voltage (MV) versions of generators. Furthermore, the interdependence of the weight and efficiency of a bidirectional Dual Active Bridge dc-dc converter for coupling the rectified output voltage of a LV generator to the MV cable is discussed. Based on this, the concept offering the best possible compromise of weight and efficiency in the power electronics system is selected and a model of the control behavior is derived for both power flow directions. A control structure is then proposed and dimensioned. Furthermore, questions of electromagnetic compatibility and electrical safety are treated. In conclusion, the essential results of the work are summarized and an outlook on future research is given. To enable the reader to make simplified calculations and a comparison of a CWT with an AWT, the aerodynamic fundamentals of both systems are summarized in highly simplified form in an Appendix, and numerical values are given for the 100kW system discussed in this work.

Novel high-speed, Lorentz-type, slotless self-bearing motor

Active magnetic bearings are a preferred choice for supporting rotors spinning at high-speed due to low friction losses and no wear. However, the rotational speed in previous bearing topologies has been limited by complex rotor constructions, high rotor losses, or position control instabilities at high speed. This paper presents a novel Lorentz-type, slotless self-bearing motor concept which overcomes most limitations of previously presented high-speed AMBs. An analytical model for motor torque and bearing forces is presented and a design for 500 000 rpm is verified with FE simulations, showing exceptionally low negative stiffness cross coupling. Finally, a prototype system is described.

Analysis and Measurement of Three-Dimensional Torque and Forces for Slotless Permanent-Magnet Motors

Slotless windings, both skewed and rhombic, are widely used in industry. In addition to the drive torque, possibly undesirable transverse torques and forces are generated. An analytical derivation of the torque and force components in all three directions is detailed in this paper for the skewed and the rhombic windings. It is shown that, for some winding configurations, alternating transverse torque components are generated, which may compromise stable operation in applications where, for example, magnetic or gas bearings are involved. Finite-element method results, which enable the transverse torque for various winding geometries to be quantified, are also included. Finally, the theoretical results are verified by measurements.

An Active Magnetic Damper Concept for Stabilization of Gas Bearings in High-Speed Permanent-Magnet Machines

The successful application of ultrahigh-speed electrical-drive systems in industrial products is currently limited by lacking high-speed bearing technologies permitting high reliability and long lifetime. Promising bearing technologies for high rotational speeds are contactless bearing concepts such as active magnetic bearings or gas bearings. While magnetic bearings usually are major electromechanical systems with substantial complexity, gas bearings allow compact realizations with high load capacity and stiffness; however, poor dynamic stability has been limiting their use at high rotational speeds. Following a hybrid bearing approach with an aerodynamic gas bearing for load support, a small-sized active magnetic damper concept is proposed to enable the stable high-speed operation of the gas bearing with a minimum of additional complexity and costs. As for the effective stabilization of the gas bearing, a high-quality displacement measurement is essential, and a new eddy-current-based rotor-displacement self-sensing concept employing an auxiliary signal injection and rotor displacement measurement circuit is presented. A hardware implementation of the proposed concept is shown providing high-resolution measurement signals.

Analysis of rotary transformer concepts for high-speed applications

In many applications electrical energy has to be transferred to rotating parts. Usually a cylindrical transformer with a rotating and a stationary part is used, which are separated by a small air gap. In order to achieve a high magnetic coupling, on both parts a highly permeable core material is employed. In high-speed applications the diameter of the rotary transformer should be small since the mechanical stresses can be high. Therefore, a high electrical frequency has to be selected. This high switching frequency would result in high core losses; ferrite would be the best suited material. However, ferrite is brittle and has a limited mechanical strength. Therefore, in this paper two concepts of rotary transformers are analyzed, where no core material is used on the rotating part. The major advantage of these concepts is a simple and mechanically robust design with a lightweight construction resulting in very small unbalanced mass.

A hybrid bearing concept for high-speed applications employing aerodynamic gas-bearings and a self-sensing active magnetic damper

Successful application of ultra high-speed electrical drive systems in industrial products is currently limited by lacking high-speed bearing technologies permitting high reliability and long lifetime. Promising bearing technologies for high rotational speeds are contact-less bearing concepts such as active magnetic bearings or gas bearings. While magnetic bearings usually are major electromechanical systems with substantial complexity, gas bearings allow compact realizations with high load capacity and stiffness; however poor dynamic stability has been limiting their use at high rotational speeds. For a new hybrid bearing concept employing an aerodynamic gas bearing for load support, a small-sized self-sensing active magnetic damper is proposed allowing to effectively counteract the self-excited whirl instability of the gas bearing and therewith enabling high speed operation with a minimum of additional complexity and costs.

Analysis and measurement of 3D torque and forces for permanent magnet motors with slotless windings

Slotless windings, skewed and rhombic, are widely used in industry. Beside the drive torque, possibly undesired transverse torques and forces are generated, which have not been analyzed previously. An analytical derivation of the torque and force components in all three directions is detailed in this paper for the skewed winding. It is shown that for some winding configurations alternating transverse torque components are generated, which may compromise stable operation in applications where for example magnetic or gas bearings are involved. Moreover the windings are analyzed with regard to a potential use as active magnetic radial bearings in high-speed applications. Finally, measurements are presented to verify the theoretical results.

Novel miniature motors with lateral stator for a wide torque and speed range

For various drilling applications where the space in the tool head is limited, two motor topologies with the stator outsourced from the head are proposed, allowing for compact design and direct drive. In order to meet the high torque requirements at low speed and operate at high speed up to 200 000 rpm a finite element analysis (FEA) is carried out and the machine design is optimized for maximum torque while considering the space limitations and the loss constraints at the critical operating points. As an example, for the specifications of a high-speed micro-machining spindle covering a wide application range, FEA results are presented. Moreover, a torque ripple compensation method based on phase current profiles obtained from FEA is proposed to eliminate the torque ripple caused by the stator asymmetry and slotting.