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Mechatronics, IEEE/ASME Transactions on Vol. 14 , No. 5, pp. 564-574, 2009 Megaspeed Drive Systems: Pushing Beyond 1 Million r/min Christof Zwyssig
Celeroton Ltd, Switzerland
Johann W. Kolar
ETH Zürich, Switzerland
Simon D. Round
ETH Zürich, Switzerland
This material is posted here with permission of the IEEE. Such permission of the IEEE does not in any way imply IEEE endorsement of any of Celeroton's products or services. Internal or personal use of this material is permitted. However, no recopying, reprinting, redistributing or reselling is permitted without the written consent from IEEE. By choosing to view this document, you agree to all provisions of the copyright laws protecting it. Mega-Speed Drive Systems: Pushing Beyond 1 Million RPM 1
2
C. Zwyssig , Student Member, IEEE, J.W. Kolar , Senior Member, IEEE, 2 and S.D. Round , Senior Member, IEEE 1
2
Celeroton
Power Electronic Systems Laboratory Gloriastrasse 35
ETH Zurich
CH-8092 Zurich, Switzerland
CH-8092 Zurich, Switzerland
Survey paper
Corresponding author
Christof Zwyssig
Celeroton c/o ETH Zurich
Gloriastrasse 35, ETZ J 65
8092 Zurich
SWITZERLAND zwyssig@lem.ee.ethz.ch Phone: +41 44 632 28 37
Fax:+41 44 632 12 12
Abstract
The latest research in mesoscale drive systems is targeting rotational speeds towards one million rpm for a power range of 1 W to 1 kW. Emerging applications for mega-speed drives (MegaNdrives) are to be found in future turbo compressor systems for fuel cells and heat pumps, generators/starters for portable nano-scale gas turbines, PCB drilling and machining spindles, and electric power generation from pressurized gas flow. The selection of the machine type and the challenges involved in designing a machine for mega-speed operation such as the winding concepts, a mechanical rotor design capable of 1 000 000 rpm, the selection of magnetic materials for the stator, and the optimization concerning high-frequency losses and torque density are presented. Furthermore, a review of the advantageous inverter topologies, taking into account the extremely low stator inductance and possible high-speed bearing types, such as ball bearings, air bearings, foil bearings and magnetic bearings, are given. Finally, prototypes and experimental results originating from MegaNdrive research at ETH Zurich are discussed and extreme temperature operation and Power MEMS are identified as targets for future research. Keywords
permanent-magnet machines, electric drives, turbomachinery, ultra high speed 1
(MegaNdrives). The goal is to develop new system INTRODUCTION concepts of electrical machines, inverters, and controllers Intensive research over the last two decades in the field to break through the speed barrier of one million rpm. of variable drives has resulted in a broad spectrum of This requires an extension of the performance trajectory industrial products with high performance. Therefore, of existing electrical drive systems by a factor of 10. A present research is often rather narrow and focused on main reason to strive towards the mega-speed range are topics of immediate relevance to industry, such as emerging applications in the areas of non-invasive reducing the number of sensors, improving the mains imaging techniques in medicine [1], dental technology, power quality and integrating the drive within the housing material processing, air compressors for high-compact of low power range motors. New research is focused on fuel cells and gas turbine driven portable power systems drive applications for hybrid vehicles and More Electric (see Fig. 1 and Section II). aircrafts. These applications define new requirements like MegaNdrives have very high power densities and are wide speed ranges and high utilization factors, and raise typically limited to smaller dimensions and/or power questions of the reliability and failure modes. A further levels in the range of 100 W due to the material strength important use of drives, with specific constraints and limitations and thermal limitations (losses in stator iron, research requirements, is in the interconnection of windings, bearings and air friction [2]) and/or scaling alternative energy sources such as wind energy. laws. Interestingly, for the speed target of greater than 1 Research at universities is typically undertaken in a million rpm, the machine design can be tackled from the close partnership with industry. Hence, new concepts can view point of macro-systems, i.e. through cylindrical be realized at a high technology level, with the basic geometries, or by using an extension of microsystems requirement of showing the advantages over the existing which typically have planar structures to meso scales (so products. However, this has the risk that the research called Power MEMS) [3-5] (cf. Fig. 1). Advantages of targets are adjusted to fit into the existing product employing micro fabrication are, for example, development time line. Therefore, the progress is usually manufacturing with sub micron precision to avoid the limited to incremental steps of already sophisticated drive limits of relative accuracy that occur with the downscaling systems. of macro systems [6], and the high stiffness of materials In the future, to guarantee highly innovative with high purity. However, restrictions exist in the limited development of drives technology, it is necessary for layer numbers and thicknesses and the magnetic properties universities to take on new challenges. Using this as the of the materials that can be processed. Nevertheless, background, the Power Electronics Systems Laboratory research at the intersection of macrosystems and power (PES) at ETH Zurich, in 2004, defined a new main topic MEMS offers a very high innovation potential through the of research called Mega-Speed Drive Systems use of unorthodox combinations of technology. Fig. 1. Emerging application areas and trends for MegaNdrives from Power MEMS and turbomachinery. 2
In contrast to a straight-line innovation path this can lead economically with mechanical PCB drills (spindles) (Fig. to fundamentally new concepts that could substantially 2) that operate with speeds of up to 250 000 rpm and with extend the performance space of variable speed drives. a motor power of 200 W [9]. To provide interconnections A further advantage of MegaNdrives in connection for larger pin count components requires the use of with possible applications (Section II) is the highly smaller diameter micro-vias. Presently, the smallest interdisciplinary nature of the research, e.g. there is a micro-vias have hole diameters of 25 m, although 10 m strong relationship to the field of mechanical engineering hole sizes being investigated [10]. For these hole in dealing with rotor dynamics, turbomachinery and diameters only laser drilling is possible, however the main thermodynamics. This supports a modern multidiscipline disadvantage with laser drilling is the capital cost, which training of postgraduate and Ph.D. students and aids them is over US$250k. Therefore, it is desirable to use cheaper to prepare for the transfer of the research results into mechanical drilling, but in order to maintain the same future industrial products. cutting speeds and productivity the rotational drill speed In this paper, section II presents the emerging must be increased. For 10 m hole diameters the drilling application areas with the future requirements for higher speed must be increased to over 1 million rpm [11]. speed and increased compactness. Section III gives an B. Spindles for dental drills and medical surgery tools overview of the international research landscape within the field of high-speed electrical drives, where only Majority of today's dental drill hand pieces are systems in the lower power and/or ultra-high speed ranges powered by an air turbine from a compressed air supply. are considered. The second part of the contribution Therefore, each hand piece is designed to operate at a (Sections IV-VI) presents the results of ETH Zurich's single speed and accurate speed control is not possible. A research into drives with speeds greater than 500 000 rpm, typical dentist would require up to five different speed and explains the converter topology selection and system hand pieces to cover the various tool speed ranges. By integration. Finally, a short overview is given on the replacing the air spindle with an adjustable speed planned future research that includes the consideration of electrical drive could reduce the number of hand pieces, different bearing concepts for the highest speed machines with the added benefit of accurate speed and torque and a significant extension along the temperature control. The major challenge is to reduce the size of the performance trajectory that is aiming to use MegaNdrives electrical machine to fit into a normal sized hand piece at extreme temperatures up to 550 °C. (Fig. 3).
High speed operation allows for higher performance in I. APPLICATIONS terms of cutting speed and the use of smaller diameter The small size of electrical machines operating at ultradrills. In the high-speed range, air turbine hand pieces high speeds opens up a complete new range of operate up to 400 000 rpm, with power levels between applications. These include drilling micron sized holes, a 10 W and 20 W. The currently available electrical single handheld drilling tool for dentists, electric-assisted powered hand pieces operate their electric motors up to a turbochargers for car engines, portable power generation maximum of 40 000 rpm, and then a triple-gear system units based on gas turbines and flywheel energy storage steps the speed up to a maximum of 200 000 rpm (Fig. 3) systems. Each of these applications and the main [13]. To be comparable to the speed range of the air emerging application areas of machining and dental powered hand piece, a direct drive electrical machine spindles, compressors, turbines and flywheels are requires a speed increase of a factor of 10. The increased highlighted in this section, where the present and future speed would also allow a smaller machine design, and speed requirements and output power range are indicated. therefore providing greater flexibility in the design of an ergonomic hand piece. A. Machining spindles for grinding, milling and drilling The recent trend in mechanical systems has been towards smaller sizes, which in turns requires high precision manufacturing. To accomplish this high precision requires the use of smaller and higher speed drilling, milling and grinding tools [7]. For example, notch grinding of silicon wafers requires motor speeds of up to 150 000 rpm [8].
In the electronics industry, the trend has been for reduced sized electronic packages with an ever increasing pin count. For example, fine-pitch BGAs now have over 1700 pins. The PCB has to connect all these pins to the rest of the electrical circuit and this is achieved by using multiple layers (up to 12 layers). The interconnections between the layers are provided by through-hole vias or more recently microvias. Reducing the diameter of the vias allows for more interconnections and facilitates the high pin count components. Presently, both through- and Fig. 2. An example of a PCB spindle [12]. micro-vias with diameters of 75 m can be produced 3
Fig. 4. eBooster from BorgWarner - an electrically power air compressor providing pressurized air for the support of the turbocharger at low engine speeds [19]. Fig. 3. Electric drive dental hand piece [13]. gas turbine based power generation systems. One particular application is for the modern soldier, who now C. Compressors and Turbochargers carries electrical equipment with a power consumption of Recent environmental concerns have resulted in up to 100 W. The existing heavy battery energy storage increased research activity into the improvement of system, which also needs recharging, could therefore be automotive fuel efficiency. A major thrust has been in the replaced with a fuel based gas turbine system. At these development of hydrogen based fuel cells for propulsion power levels the gas turbine system occupies a very small systems. These fuel cells require a constant supply of volume if the rotational speeds are increased to over pressurized air that is provided by an air compressor 500 000 rpm [20]. Significant challenges exist in system. To achieve a compact size, it has been reported manufacturing the gas turbine and the electrical machine. that the compressor speed has been increased to For power levels of less than 10 W the trend is for speeds 120 000 rpm at a power level of up to 12 kW [14]. to over 1 million rpm where the construction uses micro- To increase the fuel economy and reduce the CO electrical-mechanical system (MEMS) techniques [3], 2 production of the average car, there has been the trend of [21]. As an example, the MIT wafer based gas turbine developing smaller capacity internal combustion engines, prototype is shown in Fig. 5. both of the gasoline and diesel types. In order to provide a Additional applications for small portable power higher performance and improved efficiency a supplies are in unmanned surveillance vehicles, turbocharger is employed. Turbochargers do not perform autonomous robots and medical applications. Stanford well at low engine speeds and a turbo lag or a delay in the University, in cooperation with M-DOT, has been air boost exists. Electrically assisted turbochargers are developing a gas turbine (Fig. 6) with a predicted output under investigation, which provide the pressure boost at power of 200 W and a rotational speed of up to low speeds [15]. An electrical machine is mounted on the 800 000 rpm. The main application is for powering micro same shaft between the turbine and the compressor. The air vehicles [22]. electrical machine has to operate at the same rotational E. Energy Storage (Flywheels) speeds of the turbocharger (up to 200 000 rpm) and Flywheels have long been used to store energy. In order provide at least 1.5 kW to influence the acceleration to store and extract electrical energy a motor/generator is performance of the vehicle [16]. The major drawback of attached to the flywheel. The modern flywheel systems the electrically assisted turbocharger is that the electrical tend to operate in a vacuum and use magnetic bearings to machine must operate at extremely high temperatures reduce frictional losses. Two types of flywheel energy since there is a direct connection to the exhaust gas storage system exist, those with a large mass and low turbine. Therefore, there have been developments of a rotational speeds (<10 000 rpm) and those with low mass separate air compressor that operates together with the and high rotational speeds (>10 000 rpm) [24]. Special turbocharger to provide additional boost at low engine applications exist in the aerospace industry for low mass, speeds [17]. Fig. 4 shows a commercial eBooster that has high speed flywheel systems. In particular, NASA is been developed by BorgWarner for the use with their investigating their use for both attitude control and energy turbochargers. The operating speed is 86 000 rpm with a storage in satellites and the international space station. As system input power of 720 W [18]. Increasing the part of a research project a 3 kW, 40 000 rpm flywheel compressor to higher speeds would result in a reduced energy storage system has been tested (Fig. 7) that also volume and weight, which is especially important in provides attitude control. For the next generation of small, smaller engine compartments. near earth orbit satellites the power requirements are D. Portable Power Generation - Gas Turbine reduced as well as the maximum weight. Research into a Generators
100 W, up to 300 000 rpm flywheel motor/generator has been undertaken [25]. Gas turbine power generation is commonly used in large scale power generation systems up to 100s of MW, F. Other applications where the rotational speed is in the order of 10 000 rpm. A number of other applications for ultra-high-speed There are emerging applications for portable, low power electrical drives exist including those in the field of optical 4
scanning systems. To facilitate the depth scanning of II.
RESEARCH LANDSCAPE human retinas through reflectometry measurements from A number of research groups are investigating the coherent light sources, a transparent cube needs to be different application areas for high speed electrical rotated at very high speeds [1]. In this reported work an air machines and drive systems. The most challenging aspects spindle operating at 580 000 rpm was used. Therefore, for the research occur when the operating speed is above good opportunities exist to replace the air spindle with a 100 000 rpm. electrical drive system with speed control. In [28], various gas turbine and compressors systems Another application of high-speed electric machines is are reported. The design target is for 240 000 rpm at a in the field of mega-gravity science. This is the study of power level of 5 kW. Currently, the system is operating at solids and liquids under high acceleration (and 180 000 rpm at no load. A fuel cell air compressor temperatures). An ultracentrifuge has been reported that operating at 120 000 rpm and 12 kW has been reported produces an acceleration of 1 million times gravity [29]. In [30], a 1 kW generator operating at 452 000 rpm through the use of a 220 000 rpm air turbine [27]. It has been reported as the world's fastest PM brushless DC describes that electrical drive centrifuges exist with motor/generator in production. At the lower power level, maximum speeds up to 120 000 rpm, although there is no MIT is developing portable power gas turbines on a micro reason why this can not be increased with the correct scale [3]. The target speed for their electric generator design. speed is 1.2 million rpm, while they have presently achieved 15 000 rpm. A micro fabricated axial-flux permanent magnetic generator has been reported in [4]. The generator has been fabricated using a combination of micro fabrication and precision machining. At a rotational speed of 120 000 rpm, the generator produced 2.5W of electrical power. [31]. For dental hand pieces, [32] reported on a design target of 150 000 rpm at a power level of 10 W. This was Fig. 5. MIT MEMS gas turbine [3]. constructed and the target achieved. In the application area of machining tools, a design target of 150 000 rpm, 5 kW is reported [7]. The achieved speed is 100 000 rpm at no load and 60 000 rpm under load. Commercially available products from ATE obtain speeds of around 200 000 rpm at power levels between 200 W and 900 W [33]. Application areas for these machines include grinding and PCB drilling. For electrically assisted turbochargers, the target speed for the electric drive is 120 000 rpm at a power level of 7.5 kW. Currently, no results have been reported in [15]. In the area of energy storage and attitude control flywheels for aerospace applications, the only reported Fig. 6. Stanford/M-DOT gas turbine [23]. system above 100 000 rpm is that in [25]. The design target is 300 000 rpm at a power level of 100 W, however only 32 000 rpm in a test run has been currently achieved. In summary, commercial drives are readily available at speeds below 100 000 rpm. Above 100 000 rpm and less than 250 000 rpm special industrial drives are available. The highest reported speed is 452 000 rpm at 1 kW [30] although very little information is available on its application. Above 500 000 rpm there are only a handful of pure research projects being undertaken, although there have not been conclusive results. One thing is for certain, there are emerging applications in the areas of portable power, drilling spindles and compressors and therefore good research opportunities exist in the field of ultra-highspeed electrical drives. Since 2004 ETH Zurich has been researching ultra-high speed machines with speeds over 500 000 rpm. A 500 000 rpm, 100 W electrical drive has been reported in [34] and [35], and recently a 500 000 rpm, 1 kW and a 1 million rpm, 100 W drive system have been realized. The design, construction and testing of these ultra-high-speed Fig. 7. NASA G2 flywheel for attitude control and energy storage electrical drive systems is not trivial. There are many (night-day). 40 000 rpm, 3 kW motor/generator [26]. 5
challenges to overcome and new solutions to be the millimeter range, a magnetic machine is the better developed. choice. The main challenges are; firstly, in the machine design The rated current of a magnetic machine scales where the reduction of the high-frequency electrical losses proportionally with the machine dimensions [6]. and the selection of a high-speed mechanical rotor Therefore, the flux density in an electrically excited construction are essential. Secondly, selection of a suitable motor, e.g. induction machines (IM) or switched compact, power electronics topology (application reluctance machines (SRM), decreases with decreasing dependent) for driving ultra-high-speed machines is size. In contrast, permanent magnet flux density remains needed. Thirdly, a sensorless rotor position detection constant for decreasing machine volume. Therefore, only method that can operate at the maximum speed has to be permanent magnet machines are considered with the aim implemented. Lastly, there are the challenges in the for a low system volume [37]. integration of the complete drive system, which include High-speed operation requires a simple and robust rotor selecting the correct bearing technology, performing a geometry and construction. The commutator employed in sophisticated thermal design and analyzing the rotor dc machines produces additional friction and limits the dynamics. speed (to typically 25 000 rpm). Therefore, the only The remainder of the paper will highlight these machine types left that meet both small size and highchallenges, the design of the electrical machine, power speed requirements are the brushless dc (BLDC) machine, electronics and controller, and will finally present the fed by square-wave currents, and the identically realized prototypes of ETH Zurich's MegaNdrives. constructed permanent magnet synchronous machine (PMSM), fed by sinusoidal currents. For these machines III. ELECTRICAL MACHINE both slotless and slotted stators could be employed. In [38] the slotless configuration is found to be the better A. Machine Scaling choice for high-speed operation because of the simpler The power
S available from an electrical machine can manufacturing of the stator core and the reduction of eddy be written as current losses in the rotor (no slotting harmonics and less 2 armature current reaction). S Cd r ln
(1)
The ETH Zurich machine design has a diametrically where dr is the rotor diameter, l the active length and n magnetized cylindrical Sm Co permanent magnet 2 17 the rotational speed (in Hz or rpm, depending on the encased in a titanium sleeve for sufficiently low definition of C ). Esson's utilization factor C is mechanical stresses on the magnet. The slotless stator core dependent on the machine type, and other various consists of high-frequency iron laminations, and the threevariables such as the cooling system and size of the phase air-gap winding is made of litz wire for low copper machine [2], [6]. Using given data for small permanentlosses. The cross-section of the machine is illustrated in magnet machines [6] and (1) the active volume of the Fig. 8. machine can be estimated for a given power and speed. The volume of a machine decreases with increasing speed, C. Machine Design and Optimization which leads to very small machines for ultra-high speeds. Furthermore, for a given speed, the diameter of a rotor For ultra-high-speed operation, the mechanical rotor is limited by the maximum allowable mechanical stresses. construction and the minimization of high-frequency With a maximal length to diameter ratio and (1) this leads losses are the main challenges. The copper losses consist to a relationship of speed and maximal available power. of the current dependent resistive losses in the stator Different relationships have been identified, for example winding, which include the influence of the skin effect, in [36] and [2]. Therefore, an ultra-high-speed drive not and of the proximity effect losses, which are mainly due to only implies high-speed operation, but a combination of the eddy currents induced by the magnetic field of the high speed and high power. permanent magnet. The iron losses in the stator can be Scaling a machine with a constant power rating and estimated with the Steinmetz equation. Air friction losses efficiency, and therefore constant losses, to higher speeds are an important part of the total losses in an ultra-highleads to increased losses per surface area, since the size of speed machine as they roughly scale proportional to the the machine decreases. This leads to lower utilization surface area and by the third power of the surface speed. factors and the need for more sophisticated thermal For simple geometries, such as cylinders and disks, air designs for ultra-high-speed machines. friction losses can be calculated analytically with friction In summary, for a given speed, there is a power limit coefficients based on empirical data. depending on the machine design, materials used, rotor The mechanical rotor design is based on a shrink-fit of dynamics and thermal constraints. With the technology the magnet into a titanium sleeve. The design is such that used in the MegaNdrives a 500 000 rpm machine is the stresses in magnet and sleeve are below the tensile limited to approximately 1 kW of shaft power. strengths of the materials. Furthermore, manufacturing for lowest eccentricity and a rotor dynamic analysis are B. Machine Selection further design aspects. There are two basic concepts of electromechanical Recently, a 100-W, 500 000-rpm permanent-magnet energy conversions, machines based either on electric or machine has been designed and investigated magnetic fields. At the required power levels of the typical experimentally [34], [35]. Based on this design, an applications and for the expected machine dimensions in optimization method for highest efficiency, considering mechanical limitations, has been developed [39]. It is 6
based on analytical models for the magnetic field, the electronics size becomes significant. Generally, the size of high-frequency copper, iron and air friction losses, and the the control electronics scales with the complexity of the mechanical stresses. Compared to a traditional motor control method selected and the complexity depends on design, this optimization procedure leads to a small rotor the topology and the modulation schemes used. diameter for low air friction losses, thin litz wire strands In order to drive the machine with a PWM inverter, a for reducing the proximity effect losses and an amorphous very high switching frequency (at least 10 times higher iron stator core for low iron losses. In Fig. 9 the than the fundamental frequency of the machine) and a optimization result of the inner dimensions of a machine is high-bandwidth current control loop are needed. However, shown, where a reduction of 37% (from 14.2 W to 9 W) the high control dynamics that can be achieved with a of the initial losses is achieved. The full loss minimization PWM inverter are not required for majority of the makes it possible to reduce the calculated losses by 63% applications. In [40], the PWM inverter is compared with as compared to a machine design not considering air two different block commutated converter topologies in friction losses, and a machine efficiency of 95% can be terms of the number of semiconductor devices, size of achieved despite the ultra-high-speed operation. passive components, control complexity, and ease of implementation of sensorless control. IV. POWER ELECTRONICS
In the literature, the selected inverter topology and In Section III.A it has been shown that scaling a commutation strategy is referred to as a variable dc link machine to higher speeds leads to a smaller volume, inverter [41], or a pulse amplitude modulation (PAM) which is advantageous in many applications. In order to inverter [42] or a voltage source inverter (VSI) with block obtain a small and lightweight total system, the power commutation [43]. It consists of a standard voltage source electronics interface must also be optimized for a low inverter topology and an additional dc-dc converter as volume and weight. shown in Fig. 10, and has a bi-directional power flow In contrast to electrical machines, the size of the power capability. The six-switch inverter is controlled with sixelectronics mainly scales with power rating and is step or block commutation, which means that each switch minimized by choosing the correct topology through is conducting for 120 electrical degrees and, therefore, efficiency improvements and the use of high switching switched with the fundamental frequency of the machine. frequencies. For systems with high power ratings, the size The dc voltage (or the dc current) can be controlled with of the control electronics is negligible compared to the the duty cycle of the dc-dc converter. Eight switch signals power electronics. However, for ultra-high-speed are needed to drive the gates of the transistors: two PWM machines with low power ratings (e.g. 100 W), the control signals for the dc-dc converter and six signals for the inverter. The dc current, for the torque controller, and the Winding
Air gap stator voltages, for the sensorless rotor position detection Stator core
Sleeve and speed controller, are measured and passed to the Permanent magnet control system. V. CONTROL SYSTEM r
R 1
The main tasks of the control system (Fig. 10Fehler! R 2
Verweisquelle konnte nicht gefunden werden.) are the commutation of the inverter switches (dependent on the R 3 rotor position), and the cascaded current (torque) and R 4 speed control loops. The control system also provides communication with an external interface setting the speed reference, for example a PC. The current reference R 5 is set by the speed controller. The rotor position directly Fig. 8.
Machine cross-section: diametrically magnetized sets the switch state of the inverter, whereas the actual cylindrical permanent magnet rotor inside a slotless stator. speed is passed directly to the speed controller. The 20 starting of the machine is achieved by applying impressed currents and a speed ramp. In contrary to standard PWM inverters, in this topology 15
) the dc link current is controlled. On average, this dc link W ( current is proportional to the phase currents, i.e. the phase d P
10 currents defining the electrical torque can be controlled s e s with a bandwidth depending on the passives in the dc link s o R = 4.5 mm
L
4 and the motor inductance. Since all of the applications for 5
R = 5.5 mm
4 the MegaNdrive require low dynamic speed control, R = 6.5 mm simple torque control via the dc link current is sufficient. 4
The advantages of this control method are the low 0
1 1.5
2 2.5
3 computation effort and only a single, non isolated current Magnet radius R (mm)
1 measurement needed.
Fig. 9. Losses of a machine with fixed outer dimensions of the stator core ( R 5 8 mm , L 15 mm ) and a shaft power of 100 W at a rotational speed of 500 000 rpm for variable magnet radius R 1 . The circle shows the value for the traditional design [34]. 7
A. Sensorless Rotor Position Estimation The displacement is depending on the stator inductance A sensorless technique is used to control the stator L , the peak stator current
S i ˆ
S and the permanent-magnet currents, in order to overcome the disadvantages of rotor position sensors, such as an increased failure probability and an axial extension of the machine. Especially in ultrahigh-speed machines, a longer rotor is unwanted because the critical speeds are lowered. Traditional sensorless control methods use model based estimation of the back EMF to calculate the rotor angle at any instant. The disadvantages of these methods are a large computation effort and the requirement of phase current measurements. For an inverter with block commutation, the back EMF can be directly measured during the off intervals of the switches in each phase. The detected zero-crossings can then be phase-shifted by 30 electrical degrees and used for the switching decisions, as described in [44] for brushless dc (BLDC) motors. The stator current is usually controlled to be approximately perpendicular to the permanentmagnet flux, thus corresponding to maximum torque-percurrent operation. In this scheme, only digital signals are processed, and the computation effort is limited. Fig. 10. Power electronics and control system for driving an ultra- Nevertheless, unwanted zero-crossings have to be digitally high-speed permanent-magnet machine (MegaNdrive). masked out, and the 30-degree phase shift implemented. The limited speed range due to the speed dependence of flux PM
. For ultra-high-speed machines, and especially the back EMF amplitude and noise sensitivity are further for slotless machines, the ratio of inductance to drawbacks. permanent-magnet flux is usually very small [2], which In the MegaNdrive sensorless control, the stator flux results in small current displacements of only a few position is estimated by integrating the terminal voltages, electrical degrees for rated current, and therefore results in resulting in signals that are in phase with the stator flux. only a small torque decrease by using stator flux and not The detected zero-crossings of these signals are then the permanent magnetic flux. directly used for switching the inverter. That means that the currents are controlled to be perpendicular to the stator Further advantages of this sensorless technique are that, flux , as can be seen in the phasor diagram in Fig. 11, due to the integrator, the terminal voltages are filtered and s instead of the permanent-magnet flux PM
. A noise is reduced, the signals are phase shifted by -90 comparison to the maximum torque per current operation degrees and the zero-crossing of the signals occur at a can be made by considering the steady-state stator current commutation instant. The integrated terminal voltages displacement lead to signals with almost constant amplitudes, due to the increase in terminal voltage, but decrease in the gain of L i ˆ s s arcsin
.
(2) the integrator with increasing speed. Similar sensorless PM techniques have been described in [45] for a BLDC motor and in [11] for a permanent-magnet synchronous motor fed by a PWM inverter or a linear amplifier. VI. SYSTEM INTEGRATION
The system integration is one of the main challenges in an ultra-high-speed drive system. All the applications are directly driven, which means the machine is directly coupled to the application. Therefore, an integrated design is required, considering mechanical stresses, rotor dynamics, bearing system and thermal design. As an example, the system integration considerations for the Solar Impulse cabin air pressurization system are described. The Solar Impulse project aims to have an airplane take off and fly autonomously, day and night, propelled uniquely by solar energy, right round the world without fuel or pollution [46]. In the Solar Impulse airplane, the solar energy collected during the day is not only stored in batteries, but also in height. For altitudes up to 12 000 m a cabin air pressurization system is needed. This should be lightweight, compact and efficient in order not to penalize the needs of the propulsion. ETH Zurich is 8
developing the cabin air pressurization system (Fig. 12) the lower torque demand (see Section II), which leads to a for the Solar Impulse airplane. smaller cooling surface. Therefore, for increasing speed, an improved cooling concept has to be implemented or the A. Bearing Technology efficiency of a machine has to be increased. For machine speeds greater than 500 000 rpm the selection of a suitable bearing is a main issue. In this C. Rotor Dynamics section the possible choices are briefly compared. For high-speed operation of rotating machinery, the High-speed ball bearings are commonly used in the rotational speed might exceed the lower critical speeds of dental industry, and bearings are available for speeds up to the rotor system. Lower speed systems are operated below 500 000 rpm. The main advantages of ball bearings are the the first critical speed, whereas ultra-high speed machines robustness and small size. The main disadvantages are the might run overcritical, which is in between two critical speeds. Then, critical speeds have to be passed as fast as possible while induced oscillations have to be sufficiently damped. The bending modes are speed dependent, although, with the MegaNdrive rotors, there is only a small change in the natural frequencies. For the Solar Impulse system, ball bearings are used. The stiffness of the bearings and the weight of the rotor mainly determine the first two bending modes. These resulting critical speeds are well damped by the bearing system. Nevertheless, the third bending mode mainly resulting from the rotor geometry and material can not be damped by the bearing system and the rotor is therefore Fig. 11. Phasor diagram for the MegaNdrive sensorless control (not designed such that the rated speed of 500 000 rpm is in drawn to scale). between the second and third critical speeds. The bending limited operating temperature and a lifetime dependent on modes of the turbocompressor rotor can be seen in Fig. lubrication, load and speed. 13.
Static air bearing, dynamic air bearings and foil VII. MEGANDRIVE PROTOTYPES AND MEASUREMENTS bearings levitate the rotor with air pressure, either generated with an external supply (static) or by spinning A. 100 W, 500 000 rpm drive system the rotor (dynamic and foil). They all demonstrate low friction losses and a long lifetime. Foil bearings are For the direct drive of miniature turbocompressors and reported for speeds up to 700 000 rpm and temperatures starter/generator in gas turbines [48] a drive system with up to 650 °C, but are not commercially available and the specifications of 100 W and 500 000 rpm has been require a complex design procedure. designed (Fig. 14) [34]. The operation of this drive has Magnetic bearings levitate the rotor using magnetic been experimentally verified [35]. With this system, the forces and have similar advantages as air bearings [47, first application to be realized is the Solar Impulse cabin 48]. However, active magnetic bearings require sensors, air pressurization system, shown in Fig. 12. actuators and control, which results in high complexity B. 1 kW, 500 000 rpm drive system and increased bearing volume. Hybrid bearings can incorporate the advantages and For a larger gas turbine, the 100 W drive system eliminate the drawbacks of different bearing types. For presented in Section VII.A has been adapted to operate at example, a combined aerodynamic and magnetic bearing an increased power level of 1 kW. This leads to a machine can eliminate the wear of the air bearing at start and stop with larger rotor diameter and length and therefore a and provide a control and stabilization possibility, whereas mechanically more critical design. Furthermore, the the air bearing can take the main load. voltage level of the power electronics is increased. This For the Solar Impulse system ball bearings are chosen system is implemented into a mesoscale gas turbine and is due to the simplicity, robustness against mechanical used as a starter/generator. The drive system is depicted in impacts, small size and avoidance of auxiliary equipment. Fig. 15.
B. Thermal design
The thermal design of an ultra-high-speed drive system strongly depends on the application, since the machines are directly integrated into the application. For example, in a turbocompressor system there is additional cooling of the rotor and casing due to the air passing impeller and inlet. In contrary, in a gas turbine there is a heat source in rotor and housing due to the close proximity to the turbine. A machining spindle running on static air bearings has forced air cooling of the rotor due to static air Fig. 12. Solar Impulse cabin air pressurization system. flow. Furthermore, for a given power, the scaling of a machine to higher speeds leads to a lower volume due to 9
VIII. CONCLUSION
176 000 rpm
Life sciences and nanotechnology are opening up numerous fascinating new challenges to young researchers and are giving new possibilities to discover groundbreaking knowledge. Therefore, traditional 275 000 rpm research into the already established areas in electrical engineering, besides answering industrial relevant questions, also must focus on radically new topics, which often can be found at the intersection of different 858 000 rpm technologies and/or lie in multi-disciplinary fields. In the electrical drives technology area, MegaNdrives represent such a field, which has a high potential to push forward Fig. 13.
Bending modes of the Solar Impulse turbocompressor new enabling technologies in a number of applications rotor. The color indicates the area of highest mechanical stresses. ranging from medical systems to machining technology and new portable energy generation systems. In addition, this area provides an inspiration for interdisciplinary work and has potential to substantially extend the performance space of drives and therefore could function as a lighthouse project, fascinating new generations of researchers in academia. At ETH Zurich, we have shown in our work to date, an electrical drive system that is capable of speeds up to 1 000 000 rpm. To our knowledge this is a world record speed of an electrical drive system. To achieve these speeds, there have been significant challenges in Fig. 14.
100 W, 500 000 rpm drive system including electronics, determining the correct material selection, optimizing of stator and test bench rotor (two magnets: motor and generator). the machine design and undertaking high precision manufacturing, selection of the bearing technology and analyzing the rotor dynamics. In the next step, research has begun in the area of foil, air and magnetic bearings. A Fig. 16.
Low power (100 W), ultra-high-speed (1 000 000 rpm) machine. Fig. 15.
Power and control electronics, stator and rotor of the 1 kW, 500 000 rpm gas turbine starter/generator. C. 100 W, 1 000 000 rpm drive system In order to demonstrate the feasibility of an electrical drive system with speeds beyond 1 000 000 rpm, a demonstrator system with 100 W drive power has been constructed. The machine (assembled and single rotor) is shown in Fig. 16. A high-speed, no load, which shows the targeted operation at a speed of 1 000 000 rpm test, is shown in Fig. 17. The actual maximum speed achieved with the MegaNdrive, before disintegration of the ball bearings, was approximately 1 100 000 rpm, although this is not documented. The terminal phase voltage, the according phase current and half bridge switching signals are measured. To ETH Zurich's knowledge, this is a Fig. 17.
Measurement (20 µs/div) of motor phase current i ph world-record speed achieved by an electrical drive system. (5 A/div), terminal phase voltage u t (25 V/div) and half bridge switching signals T p and T n at 1 000 000 rpm no load operation. 10
hybrid bearing that consists of a magnetically stabilized [17] F. Westin, "Simulation of turbocharged SI-engines - with focus on the turbine", PhD thesis, Royal Institute of Technology, air bearing is a promising concept that is under Stockholm, Sweden, 2005. consideration. Further, investigation of machines for [18] S. Münz, M. Schier, H.P. Schmalzl, T. Bertolini, "eBooster Design operating at extreme ambient temperatures [49], such as in and performance of a innovative electrically driven charging the ultracentrifuge (mega-gravity science [27]) represent a system, http://www.turbos.bwauto.com/files/library/bwts_library_ main focus of our future research. In parallel to the 138_325.pdf research left on the macro concepts, future research will [19] http://www.turbos.bwauto.com/products/eBooster.aspx [20] S. Kang, S.J.J. Lee, F.B. Prinz, "Size Does Matter: The Pros and focus on drive systems in the meso-scale and Cons of Miniaturization," ABB Review, 2/2002, pp. 54-62, 2002. microsystems area. There, the power rating of todays' [21] S.A. Jacobson and A.H. Epstein, "An Informal Survey of Power Power MEMS, i.e. micro- to mm-scale generators and MEMS," in Proc. of Int. Symposium on Micro-Mechanical actuators need to be increased by several decades, and the Engineering, pp. 1-8, Dec. 2003. rotational speeds by a further decade. This represents [22] S. Kang, "Fabrication of Functional Mesoscopic Ceramic Parts for Micro Gas Turbine Engines," PhD Thesis, Stanford University, bright new horizons for future electric drive systems November 2001. technology!
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12
BIOGRAPHIES
Johann W. Kolar (SM) studied
Christof Zwyssig (Student industrial electronics at the Member) studied electrical
University of Technology engineering at ETH Zurich. Vienna, Austria, where he also During his studies he dealt with received the Ph.D. degree power electronics, machines and
University of Technology in
Technology in Sweden he worked Vienna, where he was teaching with wind turbines. He received and working in research in close his M.Sc. degree from ETH collaboration with the industry. Zurich in October 2004. Since He has proposed numerous novel then he has been a Ph.D. student converter topologies, e.g., the VIENNA Rectifier and the at the Power Electronic Systems Three-Phase AC-AC Sparse Matrix Converter concept. Laboratory. His research is focused on high-speed drive Dr. Kolar has published over 200 scientific papers in systems and its power electronics. international journals and conference proceedings and has filed more than 50 patents. He was appointed Professor and Head of the Power Electronics Systems Laboratory at Simon
Round
(SM'01) the Swiss Federal Institute of Technology (ETH) Zurich received the B.E. (Hons) and on Feb. 1, 2001. The focus of his current research is on Ph.D. degrees from the University ultra-compact intelligent AC-AC and DC-DC converter of Canterbury, Christchurch, New modules employing latest power semiconductor Zealand, in 1989 and 1993, technology (SiC), novel concepts for cooling and active respectively. EMI filtering, multi-disciplinary simulation, bearing-less From 1992 to 1995 he held motors, power MEMS, and wireless power transmission. positions of Research Associate in the Department of Electrical Engineering at the University of Minnesota and Research Fellow at the Norwegian Institute of Technology, Trondheim, Norway. From 1995 to 2003 he was a Lecturer/Senior Lecturer in the Department of Electrical and Electronic Engineering at the University of Canterbury where he performed research on power quality compensators, electric vehicle electronics, and cryogenic power electronics. In 2001 he received a University of Canterbury Teaching Award. He has also worked as a power electronic consultant for Vectek Electronics, where he developed a "state-of-the-art" digital controller for high-power inverter systems. From 2004 to 2008 he was a Senior Researcher in the Power Electronic Systems Laboratory at ETH Zurich, Switzerland where he researched in the areas of ultra-compact power converters, applications of silicon carbide power devices, and threephase ac-ac converters. In October 2008 he joined ABB Switzerland as the Control Platform Manager for the Power Electronics and Medium Voltage Drives Business Unit.
Dr Round has over 80 publications in journals and international conferences. He is a Senior Member of the IEEE and has been actively involved in the IEEE New Zealand South Section, where he was Vice-Chair and Chairman from 2001 to 2004.
13
Figure Captions
Fig. 1.
Emerging application areas and trends for MegaNdrives from Power MEMS and turbomachinery. Fig. 2.
PCB spindle [12].
Fig. 3.
Electric drive dental hand piece [13]. Fig. 4. eBooster from BorgWarner - an electrically power air compressor providing pressurized air for the support of the turbocharger at low engine speeds [19]. Fig. 5.
MIT MEMS gas turbine [3].
Fig. 6.
Stanford/M-DOT gas turbine [23]. Fig. 7.
NASA G2 flywheel for attitude control and energy storage (night-day). 40 000 rpm, 3 kW motor/generator [26]. Fig. 8.
Machine cross-section: diametrically magnetized cylindrical permanent magnet rotor inside a slotless stator. Fig. 9.
Losses of a machine with fixed outer dimensions of the stator core ( R 5 8 mm , L 15 mm ) and a shaft power of 100 W at a rotational speed of 500 000 rpm for variable magnet radius R 1 and various values of the inner radius R 4 of the stator core. The circle shows the value for the traditional design with R 4 5.5 mm and
R 1 2.5 mm
[34]. The loss reduction of the optimal design with R 4 5.3 mm and
R 1 1.7 mm is from 14.2 W to 9 W. Fig. 10.
Power electronics and control system for driving an ultra-high-speed permanent-magnet machine (MegaNdrive). Fig. 11.
Phasor diagram for the MegaNdrive sensorless control (not drawn to scale). Fig. 12.
Solar Impulse cabin air pressurization system. Fig. 13.
Bending modes of the Solar Impulse turbocompressor rotor. Standstill (0 kHz, 0 rpm)), first (2.94 kHz, 176 krpm), second (4.59 kHz, 275 krpm), and third bending mode (14.3 kHz, 858 krpm). The color shows the bending and therefore indicates the area of highest mechanical stresses. Fig. 14.
100 W, 500 000 rpm drive system including electronics, stator and test bench rotor (integrating two magnets for motor and generator). Fig. 15.
Power and control electronics, stator and rotor of the 1 kW, 500 000 rpm gas turbine starter/generator. Fig. 16.
Low power (100 W), ultra-high-speed (1 000 000 rpm) machine. Fig. 17.
Motor phase current (5 A/div), terminal phase voltage (25 V/div) and half bridge switching signals at 1 000 000 rpm no load operation. 14
Technoparkstrasse 1
8005 Zurich
Switzerland
© 2009 IEEE
Mechatronics, IEEE/ASME Transactions on Vol. 14 , No. 5, pp. 564-574, 2009 Megaspeed Drive Systems: Pushing Beyond 1 Million r/min Christof Zwyssig
Celeroton Ltd, Switzerland
Johann W. Kolar
ETH Zürich, Switzerland
Simon D. Round
ETH Zürich, Switzerland
This material is posted here with permission of the IEEE. Such permission of the IEEE does not in any way imply IEEE endorsement of any of Celeroton's products or services. Internal or personal use of this material is permitted. However, no recopying, reprinting, redistributing or reselling is permitted without the written consent from IEEE. By choosing to view this document, you agree to all provisions of the copyright laws protecting it. Mega-Speed Drive Systems: Pushing Beyond 1 Million RPM 1
2
C. Zwyssig , Student Member, IEEE, J.W. Kolar , Senior Member, IEEE, 2 and S.D. Round , Senior Member, IEEE 1
2
Celeroton
Power Electronic Systems Laboratory Gloriastrasse 35
ETH Zurich
CH-8092 Zurich, Switzerland
CH-8092 Zurich, Switzerland
Survey paper
Corresponding author
Christof Zwyssig
Celeroton c/o ETH Zurich
Gloriastrasse 35, ETZ J 65
8092 Zurich
SWITZERLAND zwyssig@lem.ee.ethz.ch Phone: +41 44 632 28 37
Fax:+41 44 632 12 12
Abstract
The latest research in mesoscale drive systems is targeting rotational speeds towards one million rpm for a power range of 1 W to 1 kW. Emerging applications for mega-speed drives (MegaNdrives) are to be found in future turbo compressor systems for fuel cells and heat pumps, generators/starters for portable nano-scale gas turbines, PCB drilling and machining spindles, and electric power generation from pressurized gas flow. The selection of the machine type and the challenges involved in designing a machine for mega-speed operation such as the winding concepts, a mechanical rotor design capable of 1 000 000 rpm, the selection of magnetic materials for the stator, and the optimization concerning high-frequency losses and torque density are presented. Furthermore, a review of the advantageous inverter topologies, taking into account the extremely low stator inductance and possible high-speed bearing types, such as ball bearings, air bearings, foil bearings and magnetic bearings, are given. Finally, prototypes and experimental results originating from MegaNdrive research at ETH Zurich are discussed and extreme temperature operation and Power MEMS are identified as targets for future research. Keywords
permanent-magnet machines, electric drives, turbomachinery, ultra high speed 1
(MegaNdrives). The goal is to develop new system INTRODUCTION concepts of electrical machines, inverters, and controllers Intensive research over the last two decades in the field to break through the speed barrier of one million rpm. of variable drives has resulted in a broad spectrum of This requires an extension of the performance trajectory industrial products with high performance. Therefore, of existing electrical drive systems by a factor of 10. A present research is often rather narrow and focused on main reason to strive towards the mega-speed range are topics of immediate relevance to industry, such as emerging applications in the areas of non-invasive reducing the number of sensors, improving the mains imaging techniques in medicine [1], dental technology, power quality and integrating the drive within the housing material processing, air compressors for high-compact of low power range motors. New research is focused on fuel cells and gas turbine driven portable power systems drive applications for hybrid vehicles and More Electric (see Fig. 1 and Section II). aircrafts. These applications define new requirements like MegaNdrives have very high power densities and are wide speed ranges and high utilization factors, and raise typically limited to smaller dimensions and/or power questions of the reliability and failure modes. A further levels in the range of 100 W due to the material strength important use of drives, with specific constraints and limitations and thermal limitations (losses in stator iron, research requirements, is in the interconnection of windings, bearings and air friction [2]) and/or scaling alternative energy sources such as wind energy. laws. Interestingly, for the speed target of greater than 1 Research at universities is typically undertaken in a million rpm, the machine design can be tackled from the close partnership with industry. Hence, new concepts can view point of macro-systems, i.e. through cylindrical be realized at a high technology level, with the basic geometries, or by using an extension of microsystems requirement of showing the advantages over the existing which typically have planar structures to meso scales (so products. However, this has the risk that the research called Power MEMS) [3-5] (cf. Fig. 1). Advantages of targets are adjusted to fit into the existing product employing micro fabrication are, for example, development time line. Therefore, the progress is usually manufacturing with sub micron precision to avoid the limited to incremental steps of already sophisticated drive limits of relative accuracy that occur with the downscaling systems. of macro systems [6], and the high stiffness of materials In the future, to guarantee highly innovative with high purity. However, restrictions exist in the limited development of drives technology, it is necessary for layer numbers and thicknesses and the magnetic properties universities to take on new challenges. Using this as the of the materials that can be processed. Nevertheless, background, the Power Electronics Systems Laboratory research at the intersection of macrosystems and power (PES) at ETH Zurich, in 2004, defined a new main topic MEMS offers a very high innovation potential through the of research called Mega-Speed Drive Systems use of unorthodox combinations of technology. Fig. 1. Emerging application areas and trends for MegaNdrives from Power MEMS and turbomachinery. 2
In contrast to a straight-line innovation path this can lead economically with mechanical PCB drills (spindles) (Fig. to fundamentally new concepts that could substantially 2) that operate with speeds of up to 250 000 rpm and with extend the performance space of variable speed drives. a motor power of 200 W [9]. To provide interconnections A further advantage of MegaNdrives in connection for larger pin count components requires the use of with possible applications (Section II) is the highly smaller diameter micro-vias. Presently, the smallest interdisciplinary nature of the research, e.g. there is a micro-vias have hole diameters of 25 m, although 10 m strong relationship to the field of mechanical engineering hole sizes being investigated [10]. For these hole in dealing with rotor dynamics, turbomachinery and diameters only laser drilling is possible, however the main thermodynamics. This supports a modern multidiscipline disadvantage with laser drilling is the capital cost, which training of postgraduate and Ph.D. students and aids them is over US$250k. Therefore, it is desirable to use cheaper to prepare for the transfer of the research results into mechanical drilling, but in order to maintain the same future industrial products. cutting speeds and productivity the rotational drill speed In this paper, section II presents the emerging must be increased. For 10 m hole diameters the drilling application areas with the future requirements for higher speed must be increased to over 1 million rpm [11]. speed and increased compactness. Section III gives an B. Spindles for dental drills and medical surgery tools overview of the international research landscape within the field of high-speed electrical drives, where only Majority of today's dental drill hand pieces are systems in the lower power and/or ultra-high speed ranges powered by an air turbine from a compressed air supply. are considered. The second part of the contribution Therefore, each hand piece is designed to operate at a (Sections IV-VI) presents the results of ETH Zurich's single speed and accurate speed control is not possible. A research into drives with speeds greater than 500 000 rpm, typical dentist would require up to five different speed and explains the converter topology selection and system hand pieces to cover the various tool speed ranges. By integration. Finally, a short overview is given on the replacing the air spindle with an adjustable speed planned future research that includes the consideration of electrical drive could reduce the number of hand pieces, different bearing concepts for the highest speed machines with the added benefit of accurate speed and torque and a significant extension along the temperature control. The major challenge is to reduce the size of the performance trajectory that is aiming to use MegaNdrives electrical machine to fit into a normal sized hand piece at extreme temperatures up to 550 °C. (Fig. 3).
High speed operation allows for higher performance in I. APPLICATIONS terms of cutting speed and the use of smaller diameter The small size of electrical machines operating at ultradrills. In the high-speed range, air turbine hand pieces high speeds opens up a complete new range of operate up to 400 000 rpm, with power levels between applications. These include drilling micron sized holes, a 10 W and 20 W. The currently available electrical single handheld drilling tool for dentists, electric-assisted powered hand pieces operate their electric motors up to a turbochargers for car engines, portable power generation maximum of 40 000 rpm, and then a triple-gear system units based on gas turbines and flywheel energy storage steps the speed up to a maximum of 200 000 rpm (Fig. 3) systems. Each of these applications and the main [13]. To be comparable to the speed range of the air emerging application areas of machining and dental powered hand piece, a direct drive electrical machine spindles, compressors, turbines and flywheels are requires a speed increase of a factor of 10. The increased highlighted in this section, where the present and future speed would also allow a smaller machine design, and speed requirements and output power range are indicated. therefore providing greater flexibility in the design of an ergonomic hand piece. A. Machining spindles for grinding, milling and drilling The recent trend in mechanical systems has been towards smaller sizes, which in turns requires high precision manufacturing. To accomplish this high precision requires the use of smaller and higher speed drilling, milling and grinding tools [7]. For example, notch grinding of silicon wafers requires motor speeds of up to 150 000 rpm [8].
In the electronics industry, the trend has been for reduced sized electronic packages with an ever increasing pin count. For example, fine-pitch BGAs now have over 1700 pins. The PCB has to connect all these pins to the rest of the electrical circuit and this is achieved by using multiple layers (up to 12 layers). The interconnections between the layers are provided by through-hole vias or more recently microvias. Reducing the diameter of the vias allows for more interconnections and facilitates the high pin count components. Presently, both through- and Fig. 2. An example of a PCB spindle [12]. micro-vias with diameters of 75 m can be produced 3
Fig. 4. eBooster from BorgWarner - an electrically power air compressor providing pressurized air for the support of the turbocharger at low engine speeds [19]. Fig. 3. Electric drive dental hand piece [13]. gas turbine based power generation systems. One particular application is for the modern soldier, who now C. Compressors and Turbochargers carries electrical equipment with a power consumption of Recent environmental concerns have resulted in up to 100 W. The existing heavy battery energy storage increased research activity into the improvement of system, which also needs recharging, could therefore be automotive fuel efficiency. A major thrust has been in the replaced with a fuel based gas turbine system. At these development of hydrogen based fuel cells for propulsion power levels the gas turbine system occupies a very small systems. These fuel cells require a constant supply of volume if the rotational speeds are increased to over pressurized air that is provided by an air compressor 500 000 rpm [20]. Significant challenges exist in system. To achieve a compact size, it has been reported manufacturing the gas turbine and the electrical machine. that the compressor speed has been increased to For power levels of less than 10 W the trend is for speeds 120 000 rpm at a power level of up to 12 kW [14]. to over 1 million rpm where the construction uses micro- To increase the fuel economy and reduce the CO electrical-mechanical system (MEMS) techniques [3], 2 production of the average car, there has been the trend of [21]. As an example, the MIT wafer based gas turbine developing smaller capacity internal combustion engines, prototype is shown in Fig. 5. both of the gasoline and diesel types. In order to provide a Additional applications for small portable power higher performance and improved efficiency a supplies are in unmanned surveillance vehicles, turbocharger is employed. Turbochargers do not perform autonomous robots and medical applications. Stanford well at low engine speeds and a turbo lag or a delay in the University, in cooperation with M-DOT, has been air boost exists. Electrically assisted turbochargers are developing a gas turbine (Fig. 6) with a predicted output under investigation, which provide the pressure boost at power of 200 W and a rotational speed of up to low speeds [15]. An electrical machine is mounted on the 800 000 rpm. The main application is for powering micro same shaft between the turbine and the compressor. The air vehicles [22]. electrical machine has to operate at the same rotational E. Energy Storage (Flywheels) speeds of the turbocharger (up to 200 000 rpm) and Flywheels have long been used to store energy. In order provide at least 1.5 kW to influence the acceleration to store and extract electrical energy a motor/generator is performance of the vehicle [16]. The major drawback of attached to the flywheel. The modern flywheel systems the electrically assisted turbocharger is that the electrical tend to operate in a vacuum and use magnetic bearings to machine must operate at extremely high temperatures reduce frictional losses. Two types of flywheel energy since there is a direct connection to the exhaust gas storage system exist, those with a large mass and low turbine. Therefore, there have been developments of a rotational speeds (<10 000 rpm) and those with low mass separate air compressor that operates together with the and high rotational speeds (>10 000 rpm) [24]. Special turbocharger to provide additional boost at low engine applications exist in the aerospace industry for low mass, speeds [17]. Fig. 4 shows a commercial eBooster that has high speed flywheel systems. In particular, NASA is been developed by BorgWarner for the use with their investigating their use for both attitude control and energy turbochargers. The operating speed is 86 000 rpm with a storage in satellites and the international space station. As system input power of 720 W [18]. Increasing the part of a research project a 3 kW, 40 000 rpm flywheel compressor to higher speeds would result in a reduced energy storage system has been tested (Fig. 7) that also volume and weight, which is especially important in provides attitude control. For the next generation of small, smaller engine compartments. near earth orbit satellites the power requirements are D. Portable Power Generation - Gas Turbine reduced as well as the maximum weight. Research into a Generators
100 W, up to 300 000 rpm flywheel motor/generator has been undertaken [25]. Gas turbine power generation is commonly used in large scale power generation systems up to 100s of MW, F. Other applications where the rotational speed is in the order of 10 000 rpm. A number of other applications for ultra-high-speed There are emerging applications for portable, low power electrical drives exist including those in the field of optical 4
scanning systems. To facilitate the depth scanning of II.
RESEARCH LANDSCAPE human retinas through reflectometry measurements from A number of research groups are investigating the coherent light sources, a transparent cube needs to be different application areas for high speed electrical rotated at very high speeds [1]. In this reported work an air machines and drive systems. The most challenging aspects spindle operating at 580 000 rpm was used. Therefore, for the research occur when the operating speed is above good opportunities exist to replace the air spindle with a 100 000 rpm. electrical drive system with speed control. In [28], various gas turbine and compressors systems Another application of high-speed electric machines is are reported. The design target is for 240 000 rpm at a in the field of mega-gravity science. This is the study of power level of 5 kW. Currently, the system is operating at solids and liquids under high acceleration (and 180 000 rpm at no load. A fuel cell air compressor temperatures). An ultracentrifuge has been reported that operating at 120 000 rpm and 12 kW has been reported produces an acceleration of 1 million times gravity [29]. In [30], a 1 kW generator operating at 452 000 rpm through the use of a 220 000 rpm air turbine [27]. It has been reported as the world's fastest PM brushless DC describes that electrical drive centrifuges exist with motor/generator in production. At the lower power level, maximum speeds up to 120 000 rpm, although there is no MIT is developing portable power gas turbines on a micro reason why this can not be increased with the correct scale [3]. The target speed for their electric generator design. speed is 1.2 million rpm, while they have presently achieved 15 000 rpm. A micro fabricated axial-flux permanent magnetic generator has been reported in [4]. The generator has been fabricated using a combination of micro fabrication and precision machining. At a rotational speed of 120 000 rpm, the generator produced 2.5W of electrical power. [31]. For dental hand pieces, [32] reported on a design target of 150 000 rpm at a power level of 10 W. This was Fig. 5. MIT MEMS gas turbine [3]. constructed and the target achieved. In the application area of machining tools, a design target of 150 000 rpm, 5 kW is reported [7]. The achieved speed is 100 000 rpm at no load and 60 000 rpm under load. Commercially available products from ATE obtain speeds of around 200 000 rpm at power levels between 200 W and 900 W [33]. Application areas for these machines include grinding and PCB drilling. For electrically assisted turbochargers, the target speed for the electric drive is 120 000 rpm at a power level of 7.5 kW. Currently, no results have been reported in [15]. In the area of energy storage and attitude control flywheels for aerospace applications, the only reported Fig. 6. Stanford/M-DOT gas turbine [23]. system above 100 000 rpm is that in [25]. The design target is 300 000 rpm at a power level of 100 W, however only 32 000 rpm in a test run has been currently achieved. In summary, commercial drives are readily available at speeds below 100 000 rpm. Above 100 000 rpm and less than 250 000 rpm special industrial drives are available. The highest reported speed is 452 000 rpm at 1 kW [30] although very little information is available on its application. Above 500 000 rpm there are only a handful of pure research projects being undertaken, although there have not been conclusive results. One thing is for certain, there are emerging applications in the areas of portable power, drilling spindles and compressors and therefore good research opportunities exist in the field of ultra-highspeed electrical drives. Since 2004 ETH Zurich has been researching ultra-high speed machines with speeds over 500 000 rpm. A 500 000 rpm, 100 W electrical drive has been reported in [34] and [35], and recently a 500 000 rpm, 1 kW and a 1 million rpm, 100 W drive system have been realized. The design, construction and testing of these ultra-high-speed Fig. 7. NASA G2 flywheel for attitude control and energy storage electrical drive systems is not trivial. There are many (night-day). 40 000 rpm, 3 kW motor/generator [26]. 5
challenges to overcome and new solutions to be the millimeter range, a magnetic machine is the better developed. choice. The main challenges are; firstly, in the machine design The rated current of a magnetic machine scales where the reduction of the high-frequency electrical losses proportionally with the machine dimensions [6]. and the selection of a high-speed mechanical rotor Therefore, the flux density in an electrically excited construction are essential. Secondly, selection of a suitable motor, e.g. induction machines (IM) or switched compact, power electronics topology (application reluctance machines (SRM), decreases with decreasing dependent) for driving ultra-high-speed machines is size. In contrast, permanent magnet flux density remains needed. Thirdly, a sensorless rotor position detection constant for decreasing machine volume. Therefore, only method that can operate at the maximum speed has to be permanent magnet machines are considered with the aim implemented. Lastly, there are the challenges in the for a low system volume [37]. integration of the complete drive system, which include High-speed operation requires a simple and robust rotor selecting the correct bearing technology, performing a geometry and construction. The commutator employed in sophisticated thermal design and analyzing the rotor dc machines produces additional friction and limits the dynamics. speed (to typically 25 000 rpm). Therefore, the only The remainder of the paper will highlight these machine types left that meet both small size and highchallenges, the design of the electrical machine, power speed requirements are the brushless dc (BLDC) machine, electronics and controller, and will finally present the fed by square-wave currents, and the identically realized prototypes of ETH Zurich's MegaNdrives. constructed permanent magnet synchronous machine (PMSM), fed by sinusoidal currents. For these machines III. ELECTRICAL MACHINE both slotless and slotted stators could be employed. In [38] the slotless configuration is found to be the better A. Machine Scaling choice for high-speed operation because of the simpler The power
S available from an electrical machine can manufacturing of the stator core and the reduction of eddy be written as current losses in the rotor (no slotting harmonics and less 2 armature current reaction). S Cd r ln
(1)
The ETH Zurich machine design has a diametrically where dr is the rotor diameter, l the active length and n magnetized cylindrical Sm Co permanent magnet 2 17 the rotational speed (in Hz or rpm, depending on the encased in a titanium sleeve for sufficiently low definition of C ). Esson's utilization factor C is mechanical stresses on the magnet. The slotless stator core dependent on the machine type, and other various consists of high-frequency iron laminations, and the threevariables such as the cooling system and size of the phase air-gap winding is made of litz wire for low copper machine [2], [6]. Using given data for small permanentlosses. The cross-section of the machine is illustrated in magnet machines [6] and (1) the active volume of the Fig. 8. machine can be estimated for a given power and speed. The volume of a machine decreases with increasing speed, C. Machine Design and Optimization which leads to very small machines for ultra-high speeds. Furthermore, for a given speed, the diameter of a rotor For ultra-high-speed operation, the mechanical rotor is limited by the maximum allowable mechanical stresses. construction and the minimization of high-frequency With a maximal length to diameter ratio and (1) this leads losses are the main challenges. The copper losses consist to a relationship of speed and maximal available power. of the current dependent resistive losses in the stator Different relationships have been identified, for example winding, which include the influence of the skin effect, in [36] and [2]. Therefore, an ultra-high-speed drive not and of the proximity effect losses, which are mainly due to only implies high-speed operation, but a combination of the eddy currents induced by the magnetic field of the high speed and high power. permanent magnet. The iron losses in the stator can be Scaling a machine with a constant power rating and estimated with the Steinmetz equation. Air friction losses efficiency, and therefore constant losses, to higher speeds are an important part of the total losses in an ultra-highleads to increased losses per surface area, since the size of speed machine as they roughly scale proportional to the the machine decreases. This leads to lower utilization surface area and by the third power of the surface speed. factors and the need for more sophisticated thermal For simple geometries, such as cylinders and disks, air designs for ultra-high-speed machines. friction losses can be calculated analytically with friction In summary, for a given speed, there is a power limit coefficients based on empirical data. depending on the machine design, materials used, rotor The mechanical rotor design is based on a shrink-fit of dynamics and thermal constraints. With the technology the magnet into a titanium sleeve. The design is such that used in the MegaNdrives a 500 000 rpm machine is the stresses in magnet and sleeve are below the tensile limited to approximately 1 kW of shaft power. strengths of the materials. Furthermore, manufacturing for lowest eccentricity and a rotor dynamic analysis are B. Machine Selection further design aspects. There are two basic concepts of electromechanical Recently, a 100-W, 500 000-rpm permanent-magnet energy conversions, machines based either on electric or machine has been designed and investigated magnetic fields. At the required power levels of the typical experimentally [34], [35]. Based on this design, an applications and for the expected machine dimensions in optimization method for highest efficiency, considering mechanical limitations, has been developed [39]. It is 6
based on analytical models for the magnetic field, the electronics size becomes significant. Generally, the size of high-frequency copper, iron and air friction losses, and the the control electronics scales with the complexity of the mechanical stresses. Compared to a traditional motor control method selected and the complexity depends on design, this optimization procedure leads to a small rotor the topology and the modulation schemes used. diameter for low air friction losses, thin litz wire strands In order to drive the machine with a PWM inverter, a for reducing the proximity effect losses and an amorphous very high switching frequency (at least 10 times higher iron stator core for low iron losses. In Fig. 9 the than the fundamental frequency of the machine) and a optimization result of the inner dimensions of a machine is high-bandwidth current control loop are needed. However, shown, where a reduction of 37% (from 14.2 W to 9 W) the high control dynamics that can be achieved with a of the initial losses is achieved. The full loss minimization PWM inverter are not required for majority of the makes it possible to reduce the calculated losses by 63% applications. In [40], the PWM inverter is compared with as compared to a machine design not considering air two different block commutated converter topologies in friction losses, and a machine efficiency of 95% can be terms of the number of semiconductor devices, size of achieved despite the ultra-high-speed operation. passive components, control complexity, and ease of implementation of sensorless control. IV. POWER ELECTRONICS
In the literature, the selected inverter topology and In Section III.A it has been shown that scaling a commutation strategy is referred to as a variable dc link machine to higher speeds leads to a smaller volume, inverter [41], or a pulse amplitude modulation (PAM) which is advantageous in many applications. In order to inverter [42] or a voltage source inverter (VSI) with block obtain a small and lightweight total system, the power commutation [43]. It consists of a standard voltage source electronics interface must also be optimized for a low inverter topology and an additional dc-dc converter as volume and weight. shown in Fig. 10, and has a bi-directional power flow In contrast to electrical machines, the size of the power capability. The six-switch inverter is controlled with sixelectronics mainly scales with power rating and is step or block commutation, which means that each switch minimized by choosing the correct topology through is conducting for 120 electrical degrees and, therefore, efficiency improvements and the use of high switching switched with the fundamental frequency of the machine. frequencies. For systems with high power ratings, the size The dc voltage (or the dc current) can be controlled with of the control electronics is negligible compared to the the duty cycle of the dc-dc converter. Eight switch signals power electronics. However, for ultra-high-speed are needed to drive the gates of the transistors: two PWM machines with low power ratings (e.g. 100 W), the control signals for the dc-dc converter and six signals for the inverter. The dc current, for the torque controller, and the Winding
Air gap stator voltages, for the sensorless rotor position detection Stator core
Sleeve and speed controller, are measured and passed to the Permanent magnet control system. V. CONTROL SYSTEM r
R 1
The main tasks of the control system (Fig. 10Fehler! R 2
Verweisquelle konnte nicht gefunden werden.) are the commutation of the inverter switches (dependent on the R 3 rotor position), and the cascaded current (torque) and R 4 speed control loops. The control system also provides communication with an external interface setting the speed reference, for example a PC. The current reference R 5 is set by the speed controller. The rotor position directly Fig. 8.
Machine cross-section: diametrically magnetized sets the switch state of the inverter, whereas the actual cylindrical permanent magnet rotor inside a slotless stator. speed is passed directly to the speed controller. The 20 starting of the machine is achieved by applying impressed currents and a speed ramp. In contrary to standard PWM inverters, in this topology 15
) the dc link current is controlled. On average, this dc link W ( current is proportional to the phase currents, i.e. the phase d P
10 currents defining the electrical torque can be controlled s e s with a bandwidth depending on the passives in the dc link s o R = 4.5 mm
L
4 and the motor inductance. Since all of the applications for 5
R = 5.5 mm
4 the MegaNdrive require low dynamic speed control, R = 6.5 mm simple torque control via the dc link current is sufficient. 4
The advantages of this control method are the low 0
1 1.5
2 2.5
3 computation effort and only a single, non isolated current Magnet radius R (mm)
1 measurement needed.
Fig. 9. Losses of a machine with fixed outer dimensions of the stator core ( R 5 8 mm , L 15 mm ) and a shaft power of 100 W at a rotational speed of 500 000 rpm for variable magnet radius R 1 . The circle shows the value for the traditional design [34]. 7
A. Sensorless Rotor Position Estimation The displacement is depending on the stator inductance A sensorless technique is used to control the stator L , the peak stator current
S i ˆ
S and the permanent-magnet currents, in order to overcome the disadvantages of rotor position sensors, such as an increased failure probability and an axial extension of the machine. Especially in ultrahigh-speed machines, a longer rotor is unwanted because the critical speeds are lowered. Traditional sensorless control methods use model based estimation of the back EMF to calculate the rotor angle at any instant. The disadvantages of these methods are a large computation effort and the requirement of phase current measurements. For an inverter with block commutation, the back EMF can be directly measured during the off intervals of the switches in each phase. The detected zero-crossings can then be phase-shifted by 30 electrical degrees and used for the switching decisions, as described in [44] for brushless dc (BLDC) motors. The stator current is usually controlled to be approximately perpendicular to the permanentmagnet flux, thus corresponding to maximum torque-percurrent operation. In this scheme, only digital signals are processed, and the computation effort is limited. Fig. 10. Power electronics and control system for driving an ultra- Nevertheless, unwanted zero-crossings have to be digitally high-speed permanent-magnet machine (MegaNdrive). masked out, and the 30-degree phase shift implemented. The limited speed range due to the speed dependence of flux PM
. For ultra-high-speed machines, and especially the back EMF amplitude and noise sensitivity are further for slotless machines, the ratio of inductance to drawbacks. permanent-magnet flux is usually very small [2], which In the MegaNdrive sensorless control, the stator flux results in small current displacements of only a few position is estimated by integrating the terminal voltages, electrical degrees for rated current, and therefore results in resulting in signals that are in phase with the stator flux. only a small torque decrease by using stator flux and not The detected zero-crossings of these signals are then the permanent magnetic flux. directly used for switching the inverter. That means that the currents are controlled to be perpendicular to the stator Further advantages of this sensorless technique are that, flux , as can be seen in the phasor diagram in Fig. 11, due to the integrator, the terminal voltages are filtered and s instead of the permanent-magnet flux PM
. A noise is reduced, the signals are phase shifted by -90 comparison to the maximum torque per current operation degrees and the zero-crossing of the signals occur at a can be made by considering the steady-state stator current commutation instant. The integrated terminal voltages displacement lead to signals with almost constant amplitudes, due to the increase in terminal voltage, but decrease in the gain of L i ˆ s s arcsin
.
(2) the integrator with increasing speed. Similar sensorless PM techniques have been described in [45] for a BLDC motor and in [11] for a permanent-magnet synchronous motor fed by a PWM inverter or a linear amplifier. VI. SYSTEM INTEGRATION
The system integration is one of the main challenges in an ultra-high-speed drive system. All the applications are directly driven, which means the machine is directly coupled to the application. Therefore, an integrated design is required, considering mechanical stresses, rotor dynamics, bearing system and thermal design. As an example, the system integration considerations for the Solar Impulse cabin air pressurization system are described. The Solar Impulse project aims to have an airplane take off and fly autonomously, day and night, propelled uniquely by solar energy, right round the world without fuel or pollution [46]. In the Solar Impulse airplane, the solar energy collected during the day is not only stored in batteries, but also in height. For altitudes up to 12 000 m a cabin air pressurization system is needed. This should be lightweight, compact and efficient in order not to penalize the needs of the propulsion. ETH Zurich is 8
developing the cabin air pressurization system (Fig. 12) the lower torque demand (see Section II), which leads to a for the Solar Impulse airplane. smaller cooling surface. Therefore, for increasing speed, an improved cooling concept has to be implemented or the A. Bearing Technology efficiency of a machine has to be increased. For machine speeds greater than 500 000 rpm the selection of a suitable bearing is a main issue. In this C. Rotor Dynamics section the possible choices are briefly compared. For high-speed operation of rotating machinery, the High-speed ball bearings are commonly used in the rotational speed might exceed the lower critical speeds of dental industry, and bearings are available for speeds up to the rotor system. Lower speed systems are operated below 500 000 rpm. The main advantages of ball bearings are the the first critical speed, whereas ultra-high speed machines robustness and small size. The main disadvantages are the might run overcritical, which is in between two critical speeds. Then, critical speeds have to be passed as fast as possible while induced oscillations have to be sufficiently damped. The bending modes are speed dependent, although, with the MegaNdrive rotors, there is only a small change in the natural frequencies. For the Solar Impulse system, ball bearings are used. The stiffness of the bearings and the weight of the rotor mainly determine the first two bending modes. These resulting critical speeds are well damped by the bearing system. Nevertheless, the third bending mode mainly resulting from the rotor geometry and material can not be damped by the bearing system and the rotor is therefore Fig. 11. Phasor diagram for the MegaNdrive sensorless control (not designed such that the rated speed of 500 000 rpm is in drawn to scale). between the second and third critical speeds. The bending limited operating temperature and a lifetime dependent on modes of the turbocompressor rotor can be seen in Fig. lubrication, load and speed. 13.
Static air bearing, dynamic air bearings and foil VII. MEGANDRIVE PROTOTYPES AND MEASUREMENTS bearings levitate the rotor with air pressure, either generated with an external supply (static) or by spinning A. 100 W, 500 000 rpm drive system the rotor (dynamic and foil). They all demonstrate low friction losses and a long lifetime. Foil bearings are For the direct drive of miniature turbocompressors and reported for speeds up to 700 000 rpm and temperatures starter/generator in gas turbines [48] a drive system with up to 650 °C, but are not commercially available and the specifications of 100 W and 500 000 rpm has been require a complex design procedure. designed (Fig. 14) [34]. The operation of this drive has Magnetic bearings levitate the rotor using magnetic been experimentally verified [35]. With this system, the forces and have similar advantages as air bearings [47, first application to be realized is the Solar Impulse cabin 48]. However, active magnetic bearings require sensors, air pressurization system, shown in Fig. 12. actuators and control, which results in high complexity B. 1 kW, 500 000 rpm drive system and increased bearing volume. Hybrid bearings can incorporate the advantages and For a larger gas turbine, the 100 W drive system eliminate the drawbacks of different bearing types. For presented in Section VII.A has been adapted to operate at example, a combined aerodynamic and magnetic bearing an increased power level of 1 kW. This leads to a machine can eliminate the wear of the air bearing at start and stop with larger rotor diameter and length and therefore a and provide a control and stabilization possibility, whereas mechanically more critical design. Furthermore, the the air bearing can take the main load. voltage level of the power electronics is increased. This For the Solar Impulse system ball bearings are chosen system is implemented into a mesoscale gas turbine and is due to the simplicity, robustness against mechanical used as a starter/generator. The drive system is depicted in impacts, small size and avoidance of auxiliary equipment. Fig. 15.
B. Thermal design
The thermal design of an ultra-high-speed drive system strongly depends on the application, since the machines are directly integrated into the application. For example, in a turbocompressor system there is additional cooling of the rotor and casing due to the air passing impeller and inlet. In contrary, in a gas turbine there is a heat source in rotor and housing due to the close proximity to the turbine. A machining spindle running on static air bearings has forced air cooling of the rotor due to static air Fig. 12. Solar Impulse cabin air pressurization system. flow. Furthermore, for a given power, the scaling of a machine to higher speeds leads to a lower volume due to 9
VIII. CONCLUSION
176 000 rpm
Life sciences and nanotechnology are opening up numerous fascinating new challenges to young researchers and are giving new possibilities to discover groundbreaking knowledge. Therefore, traditional 275 000 rpm research into the already established areas in electrical engineering, besides answering industrial relevant questions, also must focus on radically new topics, which often can be found at the intersection of different 858 000 rpm technologies and/or lie in multi-disciplinary fields. In the electrical drives technology area, MegaNdrives represent such a field, which has a high potential to push forward Fig. 13.
Bending modes of the Solar Impulse turbocompressor new enabling technologies in a number of applications rotor. The color indicates the area of highest mechanical stresses. ranging from medical systems to machining technology and new portable energy generation systems. In addition, this area provides an inspiration for interdisciplinary work and has potential to substantially extend the performance space of drives and therefore could function as a lighthouse project, fascinating new generations of researchers in academia. At ETH Zurich, we have shown in our work to date, an electrical drive system that is capable of speeds up to 1 000 000 rpm. To our knowledge this is a world record speed of an electrical drive system. To achieve these speeds, there have been significant challenges in Fig. 14.
100 W, 500 000 rpm drive system including electronics, determining the correct material selection, optimizing of stator and test bench rotor (two magnets: motor and generator). the machine design and undertaking high precision manufacturing, selection of the bearing technology and analyzing the rotor dynamics. In the next step, research has begun in the area of foil, air and magnetic bearings. A Fig. 16.
Low power (100 W), ultra-high-speed (1 000 000 rpm) machine. Fig. 15.
Power and control electronics, stator and rotor of the 1 kW, 500 000 rpm gas turbine starter/generator. C. 100 W, 1 000 000 rpm drive system In order to demonstrate the feasibility of an electrical drive system with speeds beyond 1 000 000 rpm, a demonstrator system with 100 W drive power has been constructed. The machine (assembled and single rotor) is shown in Fig. 16. A high-speed, no load, which shows the targeted operation at a speed of 1 000 000 rpm test, is shown in Fig. 17. The actual maximum speed achieved with the MegaNdrive, before disintegration of the ball bearings, was approximately 1 100 000 rpm, although this is not documented. The terminal phase voltage, the according phase current and half bridge switching signals are measured. To ETH Zurich's knowledge, this is a Fig. 17.
Measurement (20 µs/div) of motor phase current i ph world-record speed achieved by an electrical drive system. (5 A/div), terminal phase voltage u t (25 V/div) and half bridge switching signals T p and T n at 1 000 000 rpm no load operation. 10
hybrid bearing that consists of a magnetically stabilized [17] F. Westin, "Simulation of turbocharged SI-engines - with focus on the turbine", PhD thesis, Royal Institute of Technology, air bearing is a promising concept that is under Stockholm, Sweden, 2005. consideration. Further, investigation of machines for [18] S. Münz, M. Schier, H.P. Schmalzl, T. Bertolini, "eBooster Design operating at extreme ambient temperatures [49], such as in and performance of a innovative electrically driven charging the ultracentrifuge (mega-gravity science [27]) represent a system, http://www.turbos.bwauto.com/files/library/bwts_library_ main focus of our future research. In parallel to the 138_325.pdf research left on the macro concepts, future research will [19] http://www.turbos.bwauto.com/products/eBooster.aspx [20] S. Kang, S.J.J. Lee, F.B. Prinz, "Size Does Matter: The Pros and focus on drive systems in the meso-scale and Cons of Miniaturization," ABB Review, 2/2002, pp. 54-62, 2002. microsystems area. There, the power rating of todays' [21] S.A. Jacobson and A.H. Epstein, "An Informal Survey of Power Power MEMS, i.e. micro- to mm-scale generators and MEMS," in Proc. of Int. Symposium on Micro-Mechanical actuators need to be increased by several decades, and the Engineering, pp. 1-8, Dec. 2003. rotational speeds by a further decade. This represents [22] S. Kang, "Fabrication of Functional Mesoscopic Ceramic Parts for Micro Gas Turbine Engines," PhD Thesis, Stanford University, bright new horizons for future electric drive systems November 2001. technology!
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[31] D.P. Arnold, S. Das, J.W. Park, I. Zana, J.H. Lang, M.G. Allen, [5]
K. Komori and T. Yaman, "Magnetically levitated micro PM "Microfabricated High-Speed Axial-Flux Multiwatt Permanentmotors by two types of active magnetic bearings," IEEE/ASME Magnet Generators - Part II: Design, Fabrication, and Testing", Trans. Mechatronics, vol. 6, no. 1, pp. 43-49, 2001. Jour. of Microelectromechanical Systems, vol. 15, no. 5, pp. 1351- [6] U. Kafader and J. Schulze, "Similarity relations in electromagnetic 1363, Oct. 2006. motors - limitations and consequences for the design of small dc [32] D.E. Hesmondhalgh and D. Tipping, "Design of a High-Speed motors," ACTUATOR 2004, 9th International Conference on New Permanent Magnet Motor," IEE Colloquium on Permanent Actuators, Bremen, Germany, pp. 309-312, June 14-16, 2004. Magnet Machines, pp. 6/1-6/2, 15 Jun 1988. [7]
I. Takahashi, T. Koganezawa, G. Su and K. Ohyama, "A Super [33] http://www.ate-system.de/ High Speed PM Motor Drive System by a Quasi-Current Source [34] C. Zwyssig, J.W. Kolar, W. Thaler, M. Vohrer, "Design of a 100 Inverter", IEEE Trans. Industry Applications, vol. 30, no. 3, pp. W, 500000 rpm permanent-magnet generator for mesoscale gas 683-690, 1994. turbines," in IEEE Industry Applications Conference 2005, Hong [8] http://www.westwind-airbearings.com/specialist/ Kong, vol. 1, pp. 253-260, Oct. 2-6, 2005. waferGrinding.html [35] C. Zwyssig, S.D. Round, and J.W. Kolar, "Analytical and [9] http://www.westwind-airbearings.com/pcb/overview.html experimental investigation of a low torque, ultra-high speed drive [10] M. Kauf , "Microvia Formation: Technology and Cost system," in IEEE Industry Applications Conference 2006, Tampa, Comparison", URL: http://www.circuitree.com, , Jan. 2002. FL, vol. 3, pp. 1507-1513, Oct. 8-12, 2006. [11] J. Thur, "Antriebssystem für höchste Geschwindigkeiten zur [36] M.A. Rahman, A. Chiba, and T. Fukao, "Super high speed feldorientierten Regelung von permanenterregten electrical machines - summary," in 2004 IEEE Power Hochfrequenzspindeln ohne Drehgeber und ohne Signalrechner," Engineering Society General Meeting, Vol. 2, pp. 1272-1275, June PhD thesis, Bergische Universität Wuppertal, Germany, 2006. 6-10, 2004.
[12] http://www.westwind-airbearings.com/pcb/index.html [37] P. L. Chapman and P. T. Krein, "Micromotor technology: electric [13] http://www.sirona.com/ecomaXL/index.php?site=SIRONA_COM drive designer's perspective," IEEE Industry Applications _s_and_c_handpieces
Conference 2001, Chicago, USA, vol. 3, pp. 1978-1983, Sept 30- [14] http://www.miti.cc/ Oct 4, 2001.
[15] J.R. Bumpy, E.S. Spooner, J. Carter, H. Tennant, G. Ganio Mego, [38] N. Bianchi, S. Bolognani, and F. Luise, "Potentials and limits of G. Dellora, W. Gstrein, H. Sutter, J. Wagner, "Electrical Machines high-speed PM motors," IEEE Trans. Industry Applications, vol. for use in Electrically Assisted Turbochargers", in Proc. of Power 40, no. 6, pp. 1570-1578, Nov.-Dec. 2004. Electronics, Machines and Drives Conf., vol.1, no., pp. 344-349, [39] J. Luomi, C. Zwyssig, A. Looser, and J.W. Kolar, "Efficiency Apr. 2004.
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[45] K. Iizuka, H. Uzuhashi, M. Kano, T. Endo, and K. Mohri, Microcomputer control for sensorless brushless motor," IEEE Trans. Ind. Applicat., vol. IA-21, pp. 595-601, May/June 1985. [46] http://www.solarimpulse.com/ [47] H. Kanebako and Y. Okada, "New design of hybrid-type selfbearing motor for small, high-speed spindle," IEEE/ASME Trans. Mechatronics, vol. 8, no. 1, pp. 111-119, 2003. [48] B. Schneider, M. Bruderer, D. Dyntar, C. Zwyssig, M. Diener, K. Boulouchos, R. S. Abhari, L. Guzzella and J. W. Kolar, "Ultra- High-Energy-Density Converter for Portable Power," Int. Workshop on Micro and Nanotechnology for Power Generation and Energy Conversion Applications, Tokyo, Japan, pp. 81-84, Nov. 28-30, 2005.
[49] John H. Glenn Research Center, "High-temperature Switched- Reluctance Electric Motor," http://www.techbriefs.com/Briefs/ Feb03/ LEW17287.html
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BIOGRAPHIES
Johann W. Kolar (SM) studied
Christof Zwyssig (Student industrial electronics at the Member) studied electrical
University of Technology engineering at ETH Zurich. Vienna, Austria, where he also During his studies he dealt with received the Ph.D. degree power electronics, machines and
- (summa cum laude). From 1984 magnetic bearings. At the to 2001 he was with the
- Chalmers
University of Technology in
Technology in Sweden he worked Vienna, where he was teaching with wind turbines. He received and working in research in close his M.Sc. degree from ETH collaboration with the industry. Zurich in October 2004. Since He has proposed numerous novel then he has been a Ph.D. student converter topologies, e.g., the VIENNA Rectifier and the at the Power Electronic Systems Three-Phase AC-AC Sparse Matrix Converter concept. Laboratory. His research is focused on high-speed drive Dr. Kolar has published over 200 scientific papers in systems and its power electronics. international journals and conference proceedings and has filed more than 50 patents. He was appointed Professor and Head of the Power Electronics Systems Laboratory at Simon
Round
(SM'01) the Swiss Federal Institute of Technology (ETH) Zurich received the B.E. (Hons) and on Feb. 1, 2001. The focus of his current research is on Ph.D. degrees from the University ultra-compact intelligent AC-AC and DC-DC converter of Canterbury, Christchurch, New modules employing latest power semiconductor Zealand, in 1989 and 1993, technology (SiC), novel concepts for cooling and active respectively. EMI filtering, multi-disciplinary simulation, bearing-less From 1992 to 1995 he held motors, power MEMS, and wireless power transmission. positions of Research Associate in the Department of Electrical Engineering at the University of Minnesota and Research Fellow at the Norwegian Institute of Technology, Trondheim, Norway. From 1995 to 2003 he was a Lecturer/Senior Lecturer in the Department of Electrical and Electronic Engineering at the University of Canterbury where he performed research on power quality compensators, electric vehicle electronics, and cryogenic power electronics. In 2001 he received a University of Canterbury Teaching Award. He has also worked as a power electronic consultant for Vectek Electronics, where he developed a "state-of-the-art" digital controller for high-power inverter systems. From 2004 to 2008 he was a Senior Researcher in the Power Electronic Systems Laboratory at ETH Zurich, Switzerland where he researched in the areas of ultra-compact power converters, applications of silicon carbide power devices, and threephase ac-ac converters. In October 2008 he joined ABB Switzerland as the Control Platform Manager for the Power Electronics and Medium Voltage Drives Business Unit.
Dr Round has over 80 publications in journals and international conferences. He is a Senior Member of the IEEE and has been actively involved in the IEEE New Zealand South Section, where he was Vice-Chair and Chairman from 2001 to 2004.
13
Figure Captions
Fig. 1.
Emerging application areas and trends for MegaNdrives from Power MEMS and turbomachinery. Fig. 2.
PCB spindle [12].
Fig. 3.
Electric drive dental hand piece [13]. Fig. 4. eBooster from BorgWarner - an electrically power air compressor providing pressurized air for the support of the turbocharger at low engine speeds [19]. Fig. 5.
MIT MEMS gas turbine [3].
Fig. 6.
Stanford/M-DOT gas turbine [23]. Fig. 7.
NASA G2 flywheel for attitude control and energy storage (night-day). 40 000 rpm, 3 kW motor/generator [26]. Fig. 8.
Machine cross-section: diametrically magnetized cylindrical permanent magnet rotor inside a slotless stator. Fig. 9.
Losses of a machine with fixed outer dimensions of the stator core ( R 5 8 mm , L 15 mm ) and a shaft power of 100 W at a rotational speed of 500 000 rpm for variable magnet radius R 1 and various values of the inner radius R 4 of the stator core. The circle shows the value for the traditional design with R 4 5.5 mm and
R 1 2.5 mm
[34]. The loss reduction of the optimal design with R 4 5.3 mm and
R 1 1.7 mm is from 14.2 W to 9 W. Fig. 10.
Power electronics and control system for driving an ultra-high-speed permanent-magnet machine (MegaNdrive). Fig. 11.
Phasor diagram for the MegaNdrive sensorless control (not drawn to scale). Fig. 12.
Solar Impulse cabin air pressurization system. Fig. 13.
Bending modes of the Solar Impulse turbocompressor rotor. Standstill (0 kHz, 0 rpm)), first (2.94 kHz, 176 krpm), second (4.59 kHz, 275 krpm), and third bending mode (14.3 kHz, 858 krpm). The color shows the bending and therefore indicates the area of highest mechanical stresses. Fig. 14.
100 W, 500 000 rpm drive system including electronics, stator and test bench rotor (integrating two magnets for motor and generator). Fig. 15.
Power and control electronics, stator and rotor of the 1 kW, 500 000 rpm gas turbine starter/generator. Fig. 16.
Low power (100 W), ultra-high-speed (1 000 000 rpm) machine. Fig. 17.
Motor phase current (5 A/div), terminal phase voltage (25 V/div) and half bridge switching signals at 1 000 000 rpm no load operation. 14
About the company
