High Speed Ele Mc

2946 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 61, NO. 6, JUNE 2014

High-Speed Electrical Machines: Technologies,Trends, and Developments

David Gerada, Abdeslam Mebarki, Neil L. Brown, Chris Gerada, Member, IEEE,Andrea Cavagnino, Senior Member, IEEE, and Aldo Boglietti, Fellow, IEEE

AbstractThis paper reviews the current technologies usedin high-speed electrical machines through an extensive surveyof different topologies developed and built in the industry andacademia for several applications. Developments in materials andcomponents, including electrical steels and copper alloys, arediscussed, and their impact on the machines operating physicalboundaries is investigated. The main application areas pulling thedevelopment of high-speed machines are also reviewed to betterunderstand the typical performance requirements.

Index TermsCopper alloys, high-frequency electricalmachines, high-frequency electrical steels, high-speed electricalmachines, high-strength electrical steels, mechanical modeling,thermal modeling.

I. INTRODUCTION

H IGH-SPEED rotating mechanical machinery has beendeveloped and used for a long time, and it is now con-sidered a mature and reliable technology for a number of engi-neering applications. Such applications include turbochargers,mechanical turbo-compounding systems, aeroengine spools,helicopter engines, racing engines, and fuel pumps with typicaloperational speeds in excess of 10 000 r/min and r/min

kW in

excess of 1 105 [1].The drive toward research and development in high-speed

electrical machinery has seen a rapid growth in the last fewdecades, with a considerable application uptake in the lastdecade. It is also foreseen that this research area will be domi-nating electrical machine drive research, partly due to the cur-rent improvements in the enabling technologies and partly dueto the significant impact that the development of these machineswill make in many application areas. This is also reflected bythe large number of national and international funded researchprograms in this area. This paper will first overview some of theapplication areas applying high-speed electrical machinery, andthe resulting system benefits will be highlighted. A commonperceived advantage of high-speed machines is the reduction

Manuscript received February 6, 2013; revised July 28, 2013; acceptedSeptember 30, 2013. Date of publication October 22, 2013; date of currentversion December 20, 2013.

D. Gerada, A. Mebarki, and N. L. Brown are with the Research andTechnology Department, Cummins Generator Technologies, Stamford, PE92NB, U.K. (e-mail: [email protected]).

C. Gerada is with the Power Electronics, Machines and Control Group, TheUniversity of Nottingham, Nottingham NG7 2RD, U.K.

A. Cavagnino and A. Boglietti are with the Dipartimento di IngegneriaElettrica, Politecnico di Torino, 10129 Turin, Italy.

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TIE.2013.2286777

of system weight for a given magnitude of power conversion.This is particularly desirable in mobile applications, where anysavings in weight results directly in reduced fuel burn andemissions. The trend in electrifying future transport systems iscreating a significant pull for advancing high-speed technolo-gies. Another commonly perceived benefit in adopting high-speed machines in certain applications is the improvement inreliability as a result of the elimination of intermediate gearing(direct drives).

The research and development in this area is also fueled bythe developments in power electronic switching devices, con-verter topologies, and control methodologies that are enablingeven higher machine operating frequencies. This is in conjunc-tion with developments in soft and hard magnetic materials,which are able to withstand higher magnitudes of mechanicalstress while exhibiting low ac losses, which allow for higherrotor peripheral velocities and higher power densities. After re-viewing the application areas for high-speed electrical machin-ery, this paper will highlight significant recent advancementsin material technology related to high-speed machines. Then,different electrical machine topologies from literature will bereviewed and compared in terms of their high-speed capabilityand performance characteristics.

II. OVERVIEW OF APPLICATIONS

In some applications, high-speed electrical machines directlyreplace existing high-speed mechanical systems, whereas inother applications, high-speed electrical machines complementthe existing high-speed mechanical system. This section givesan overview of the drive behind such developments for abroad range of application spectra. The list is not exhaustivebut indicative of the application pull for high-speed electricalmachine technology.

A. High-Speed Electrical Machines for More Electric Engines(Automotive/Power Generation)

The concept of having high-performance traction machinesintegrated within hybrid drivetrains to improve fuel efficiencyand reduce emissions is now commonplace in vehicles. Increas-ingly stringent emissions and fuel efficiency requirements callfor further electrification for engines being used for automotiveand power-generating applications, primarily through the useof high-speed electrical machines. The potential applications ofhigh-speed electrical machines within a more-electric engine

0278-0046 2013 IEEE

GERADA et al.: HIGH-SPEED ELECTRICAL MACHINES 2947

Fig. 1. High-speed electrical machines for the more-electric engine(Cummins).

Fig. 2. Electrically assisted turbocharging: integrating electrical machineswithin existing high-speed machinery (Cummins).

are several, as shown in Fig. 1, which lays out a family of fourpossible high-speed electrical machines around a future engine.

In one such application, the electrical machine is placed onthe same shaft as the turbine and the compressor wheels ina turbocharger (machine M1 in Fig. 1). The function of themachine is twofold. On starting and gear shift, when there islack of energy in the exhaust gas stream, the machine is used asa motor to speed up the compressor to the required speed, thusreducing turbo lag and improving driveability. At high engineloads, when there is excess energy in the exhaust, instead ofopening a waste-gate valve to prevent shaft overspeeding, theelectrical machine is used as a generator. The typical integra-tion of the electrical machine within a turbocharger is shownin Fig. 2.

Driveline efficiency can be further improved by installing anadditional power turbine and a high-speed machine (machineM2 in Fig. 1) at the downstream of the turbocharger to extractwaste heat from the exhaust gases, which is often called tur-bocompounding. Recovered energy is then used to supply thevehicles electrical loads, including the traction machine if usedwithin a hybrid drivetrain architecture.

Fig. 3. High-speed electrical machine used within an ORC engine(Cummins).

In internal combustion engines, exhaust gas recirculation(EGR) is used to reduce NOx emissions by routing some ofthe exhaust gas back into the intake air flow. In engines wherethe exhaust backpressure is greater than the intake air pressure,a negative pressure differential exists; hence, EGR flow can berealized by simply connecting a conduit between the exhaustand intake ducts. However, in an engine having a chargedintake, an unfavorable pressure differential must be overcome[2]. One way of efficiently overcoming the aforesaid problemis by introducing an EGR compressor at the upstream of theturbine, which is driven by a high-speed motor (machine M3 inFig. 1). This drastically reduces the pumping energy requiredin comparison with more conventional EGR systems whereinexhaust gases are drawn at the downstream of the turbine.

Higher engine fuel efficiency improvements can be gained byrecovering waste heat energy from the whole engine, rather thanjust from the exhaust gas stream. This is done through an Or-ganic Rankine Cycle, whereby heat is recovered by circulatinga working fluid that is then used to drive a high-speed turbineand an electrical generator. Fuel efficiency improvement levelsof over 12% have been demonstrated [3]. Fig. 3 shows one suchengine as developed by Cummins. The range of powerspeednodes for electrical machines developed for engine applicationsis very broad, ranging from 2 kW/220 000 r/min for a passengercar application [41] up to 150 kW/35 000 r/min for a prime-power generation engine.

B. Flywheel Energy Storage Systems

Flywheel energy storage systems operate by mechanicallystoring energy in a rotating flywheel. Electrical energy is storedby using a motor that spins the flywheel, thus converting theelectric energy into mechanical energy. To recover the energy,the same motor is used to slow the flywheel down, convertingthe mechanical energy back into electrical energy. Traditionalflywheel designs have large diameters, rotate slowly, and havelow power and energy densities. More modern flywheels aredesigned to rotate at higher speeds. Such flywheels achievehigher power densities than the NiMH batteries typically usedin hybrid vehicles, albeit having lower energy densities. Forenergy storage applications requiring high peak power output


Fig. 4. High-speed composite flywheel (Williams Hybrid Power).

TABLE ITYPICAL MILLING APPLICATIONS SPEED

for a short amount of time (i.e., low energy), as is the casefor power-assist hybrid electric vehicles, high-speed flywheelstorage systems offer a number of advantages over battery tech-nologies such as a more compact solution, higher efficiency,longer lifetime, and a wider operating temperature range [68].Fig. 4 shows a flywheel developed by Williams Hybrid Power,as used within the Porsche911 GT3R. This flywheel rotates at40 000 r/min and is used to generate/motorize up to 120 kW tothe front-axle motors.

C. High-Speed Spindle Applications

The machine tool industry has also driven the developmentof high-speed electrical machines. Conventional low-cost high-speed spindles use belt drives, which are limited in maximumspeed. Increased demands for higher rotational speeds, speedcontrol, low vibration levels, and power density (due to lackof space) have led to using high-speed electrical machinesfor spindle applications. The power range and speed limits inspindle applications are very spread out, varying from 9000 to180 000 r/min, with a corresponding power level approximatelyfrom 24 down to 1 kW.

As reported in Table I, the maximum rotational speedachieved during the different milling applications depends onthe processed material type. For grinding applications, themachine-tool rotational speeds are higher than the typical rangereported in Table I, reaching speeds up to hundreds of thousandsrevolutions per minute in ultraprecision machining applica-tions, such as mesoscale dimension range and megaspeed drivesystems [4]. Fig. 5 shows one such machine, which is developedby Westwind Air Bearings, with a speed of 300 000 r/min usedwithin printed-circuit-board (PCB) drilling spindles.

D. Turbomolecular Pumps

Turbomolecular pumps are another application where thespeeds are increasing and where the use of very high-speedmotors can be considered a suitable choice. Currently, ro-

Fig. 5. High-speed 200-W 300 000-r/min PCB drilling spindle (Westwind).

Fig. 6. Cross section of a turbomolecular pump driven by a high-speedmotor [76].

tational speeds of up to 100 000 r/min at low power levels(a few hundred watts) are the future target for this application.The turbomolecular pumps are used to obtain and maintaina high vacuum. These pumps work on the principle that gasmolecules can be given momentum in a desired direction byrepeated collision with a moving solid surface. In a turbo pump,a rapidly spinning turbine rotor hits gas molecules from the inletof the pump toward the exhaust in order to create or maintaina vacuum. A cross section of a typical turbopump driven by ahigh-speed motor is shown in Fig. 6. These pumps are used toget a very high vacuum condition up to 1010 mbar. This typeof load demands motor performance characteristics requiring acomplex design approach, far from the classical design criteriaused for the standard motors. In particular, the rotor runs in adeep vacuum, with extreme thermal exchange problems [69].In fact, the thermal dissipation can be only done by radiation.The rotor inertia contribution on the total inertia has to be aslow as possible in order to simplify the balance process of therotating parts. The torque ripple has to be very low in order toreduce the risk of mechanical resonance in the rotating system.

E. Gas Compressor Applications

Gas compression is needed at many places in the chemical,oil, and gas industries, mainly for gathering, transmitting, andprocessing the gas downstream. Gas engines and gas turbinesare traditionally used as compressor drives. While gas-fireddrives are convenient for gas companies, they are becomingincreasingly difficult to install due to environmental restric-tions. The idea of using electric motors to drive compressorsto minimize the environmental, regulatory, and maintenanceissues is not new but progress in the field of high-speed ma-chines have made them more attractive. Oil-free compressorshave been successfully used for many years, but as long as alubrication system for the oil is still necessary for the drive or


Fig. 7. (Left) Conventional compressor and (right) integrated compressor.

Fig. 8. 10-MW 20 000 r/min high-speed IM used within an integrated com-pressor unit (GE Energy).

for a gearbox, the benefits of oil-free operation cannot be fullyexploited. Electric high-speed drives with magnetic bearingsallow the elimination of the gearbox and of the entire lubri-cation oil system, which leads to increased safety, increasedefficiency, increased availability, and reduced operation andmaintenance costs. Thus, the electric high-speed drives are themost environment-friendly compressor drives [5]. The layoutand rotor of a 10-MW 20000-r/min induction machine (IM)developed by Converteam (now GE Energy), and used withinsuch an application are shown in Figs. 7 and 8, respectively.

F. Industrial Air Compressors and Air Blowers

In many industrial applications, there is an ever-increasingdemand for higher quality and oil-free compressed air. In thefood and beverage industry, as well as in the pharmaceuticalindustry, any oil contamination can lead to unsafe products andconsumer health hazards. In the automotive industry, oil-free airis essential to achieve a high-quality finish. In the electronicsindustry, moisture can affect sensitive processes and cause oxi-dation of microterminal strips, resulting in product failure. In allthe aforementioned industries, any oil contamination can leadto expensive product recalls and plant shutdown. High-speedelectrical machines that operate at power levels of 100500 kWand speeds of 8015 000 r/min, using magnetic or air bearings,are being used in the latest generation Class-0 oil-free direct-drive industrial compressors, in the range of 49 bar.

In wastewater treatment plants, the majority of power de-mand, over 60%, is required for the delivery of air to provideoxygen for biological treatment of waste streams and mixing tosolids. Traditionally positive-displacement blowers running onvariable frequency drives or geared centrifugal units equippedwith inlet and outlet vanes are used for wastewater aeration.The last decade has seen a rapid growth in the use of turboblowers driven by high-speed motors, having higher reliabilityand durability, reduced noise, a 25% reduction in footprint and,more importantly, claimed energy savings in excess of 35% [70]with respect to conventional blowers.

Fig. 9. Typical layout of a micro gas turbine [6].

G. Microturbines

Microturbines are small combustion turbines of a size com-parable to a refrigerator, and with typical outputs of 30400 kW.They are typically used for stationary energy generation appli-cations at sites with space limitations for power production.They are fuel-flexible machines that can run on natural gas,biogas, propane, butane, diesel, and kerosene. Microturbineshave few moving parts, high efficiency, low emissions, and havewaste heat utilization opportunities. They are also lightweightand compact in size. Waste heat recovery can be used incombined heat and power (CHP) systems to achieve energyefficiency levels greater than 80% [6].

Fig. 9 shows the typical layout of a micro gas turbine. Itconsists of a compressor, a combustor, a turbine, an alternator,a recuperator (optional), and a generator. In unrecuperatedsystems, heated compressed air is mixed with fuel and burnedunder constant pressure conditions. The resulting hot gas isallowed to expand through a turbine to perform work. Simple-cycle microturbines have lower cost, higher reliability, andmore heat available for CHP applications than recuperatedunits. Recuperated units use a sheet-metal heat exchanger thatrecovers some of the heat from an exhaust stream, and transfersit to the incoming air stream. The preheated air is then usedin the combustion process. If the air is preheated, less fuelis necessary to raise its temperature to the required level atthe turbine inlet. Recuperated units have higher efficiency andthermal-to-electric ratio than unrecuperated units, and yield30%40% fuel savings from preheating [6].

Recently, there is a revisited interest in using microturbinesas a range extender within serial hybrid vehicles, as well as all-electric vehicles, as a power unit that can charge the vehiclesbatteries. Fig. 10 shows a 50-kW 80 000-r/min microturbinedeveloped by Bladon. It is claimed that such a technology canbe just 5% of the size, weight, and parts of an equivalent pistonengine [7].

III. MATERIALS

This section gives an overview of materials suitable for high-speed electrical machines, including electrical steels, copper


Fig. 10. 50-kW 80 000-r/min micro gas turbine (Bladon).

alloys, and magnets. It also describes the main developmentsthat are key in pushing the operational boundaries of high-speedelectrical machinery.

A. Electrical Steels

For the stator and rotor laminations, different siliconiron(SiFe) and cobaltiron (CoFe) alloys have been considered.CoFe ensures highest saturation magnetization, going above 2 T,thus enabling highest power densities to be achieved. Theactual value of saturation magnetization for CoFe depends onthe annealing temperature, time of annealing, and annealingatmosphere; in general, the better the mechanical characteristicsof the annealed material, the lower the saturation magnetiza-tion. However, even when annealed to the optimum mechanicalproperties, the saturation magnetization of CoFe is still signifi-cantly higher than SiFe (around 20% higher). Although CoFe issignificantly more expensive compared with SiFe laminations,it is often considered for high-speed machines as materialmass per kilowatt is intrinsically small and is thus not hugelysignificant at the system level.

Another important parameter when choosing the laminationmaterial for high-speed machines is the amount of core lossesgenerated in the lamination due to the very high fundamentaland switching frequencies. For a given frequency and flux den-sity, the core losses are primarily influenced by the laminationthickness and the final annealing method. In general, the thinnerthe laminations, the lower the core losses. Electrical steels asthin as 0.1 mm with very low core losses, which are tailoredspecifically for high-frequency applications, are commerciallyavailable [8].

Fig. 11 compares the mechanical yield-strength and core-losscharacteristics at 1 T and 400 Hz of commercially availablegrades of SiFe() and CoFe() under their respective tradenames. It should be noted that, for many of the steel grades(except those marketed specifically for their mechanical charac-teristics), the mechanical properties are normally typical indica-tive values. M270-35A and M235-35A are common 0.35-mmSiFe grades with a typical yield strength of around 350 and450 MPa, respectively. These grades are typically used forhigher performance volume-manufactured mainstream motorssuch as traction machines.

High-frequency machines typically use thinner SiFe gradesthan 0.35 mm, such as NO20 and Arnon7, which are 0.2 and

Fig. 11. Comparison of core losses and yield strength for different high-performance electrical steels.

0.17 mm thick, respectively. The core losses of the aforemen-tioned thin grades are superior, as shown in Fig. 11; however,this is at the cost of reduced yield strength, which is typically300380 MPa for such grades.

Increasingly demanding requirements on steels have pushedsteel companies metallurgical research to look beyond thetraditional boundaries in the last two decades. One such caseis the JNEX10-Core that is a 0.1-mm-thick SiFe grade and hasaround 50% of the losses at the frequency and induction levelconsidered, with respect to the other high-frequency thin-steelgrades by having 6.5% Si content [8]. This enables the designerto push further the fundamental frequencies in the stator. Con-ventional silicon steel sheets have a silicon content of 3.5% orless. It has long been known that the magnetic characteristics ofa silicon steel sheet improve as the Si content increases, peakingat 6.5% [8]. However, it had been impractical to producethin steel sheets with a Si content of over 3.5% because thesteel tends to harden. This production problem was recentlyovercome through the adoption of chemical vapor deposition.Having such a high percentage of silicon results in havinga high yield strength; however, the material is brittle, whichmakes it undesirable for high-speed rotors. The ductility canbe improved by having gradient injection of silicon, wherebya higher silicon content is injected at the edges of the sheetfor reduction of very-high-frequency losses, and a lower siliconcontent is injected at the center.

Other steel research is focused on developing very high-strength electrical steel without compromising the ductility.The drive behind this research is primarily interior-permanent-magnet (IPM) motors used in hybrid traction. Such motorsrequire the bridge to be kept as small as possible to limit short-circuiting the magnet flux (thus reducing magnet volume);however, this is limited by the yield strength of the laminationmaterial. High yield strengths of over 800 MPa can be achievedby a number of techniques such as dislocation strengthening[9], at the cost of increased iron losses. The HXT grades[10], shown in Fig. 11, can achieve a very high yield strength,comparable to high-strength carbon steel, in excess of 800 MPawith an elongation in excess of 18%.

Fig. 11 also compares iron losses for four different types ofCoFe alloys. As shown in this figure, the CoFe alloy with op-timum magnetic properties Vacoflux48 has significantly lower


Fig. 12. Alloys for rotor cage.

losses than the M235-35A material for the same thickness;however, its yield strength is only about 200 MPa (i.e., half thatof SiFe), which for the case of use in a high-speed rotor is notsufficient. In order for the CoFe to have similar yield strength(i.e., Vacodur 50 with optimum mechanical properties) as SiFe,its electromagnetic properties degrade, and the losses becomecomparable for both SiFe and CoFe. It is also important to notethat the aforementioned CoFe grades are brittle in nature, withVacoflux48 having 2% elongation and Vacodur50 having 6%elongation. The elongation is improved to 32% in Vacoflux17,which has a reduced cobalt content of 17%, albeit increasingthe core losses significantly. CoFe alloys can be strengthenedby alloying with vanadium, such as Hiperco 50 HS, whichcan achieve a yield strength of over 680 MPa with a 15%elongation.

B. Copper Alloys

For the case of IMs, the rotor-bar and end-ring materialsrequire careful selection. High yield strength is required at hightemperatures as the bars serve a mechanical function besidesthe electromagnetic function. The bars also add to the stiffnessof the rotor assembly and thus help increase the critical speedof the machine. Moreover, for IMs, keeping the rotor cage in ahealthy condition is fundamental for operation.

For high-speed high-temperature applications, pure copperis not typically used due to its low yield strength, and it softensat high temperatures. Several different types of high-strengthcopper alloys have been utilized for high-speed IMs, suchas copperzirconium (CuZr), copperberyllium (CuBe) [37],and copperaluminum oxide (CuAl2O3) [23], [24], [34], [35].Fig. 12 maps different materials that are considered for the rotorcage in terms of the two most important parameters for a high-speed induction motor application (i.e., electrical conductivityand yield strength).

Copper can be strengthened substantially by alloying it withother elements, but alloying causes a significant loss in con-ductivity. Copper can be also strengthened by incorporatingfine particles of a second phase in its matrix, causing only arelatively small loss in conductivity. The second phase can be ametal or intermetallic compound precipitated from a solid solu-tion by an aging treatment, or it can be nonmetallic particles,such as stable oxide, added to or formed within the coppermatrix. One such material, which is made using the internal

Fig. 13. Segmented magnets for rotor loss reduction (Arnold Magnets).

Fig. 14. High-temperature samariumcobalt [77].

oxidation technique, is Glidcop from North American Hgans,which retains its strength at elevated temperatures in excess of300 C.

C. PMs

The main challenges for magnets at high speed are themechanical stresses experienced and the internally generatedlosses due to flux pulsation as a result of the stator slotting,air-gap space harmonics, and asynchronous fields due to timeharmonics in the supply waveforms. Axial, radial, and circum-ferential segmentation are often used to reduce losses, as shownin Fig. 13. In addition, designing the machine winding andgeometry to minimize rotor losses and, subsequently, magnettemperature, is a critical design tradeoff [60].

High-speed machines typically use high-energy-densitymagnets from the NdFeB or SmCo families with a high workingtemperature capability. NdFeB grades alloyed with Dysprosium(Dy) meant that NdFeB magnets could go to working temper-ature limits up to 250 C, such as grade N38EH. For operationabove this temperature, samariumcobalt Sm2Co17 remains theonly suitable material. Although having slightly lower rema-nence and energy product than the NdFeB magnets, typicallySm2Co17 grades can operate up to temperatures of 350 C, withsome special grades pushing the boundary to 550 C, at the costof reduced remanence, as shown in Fig. 14.

Electrically conducting stator and rotor sleeves have alsobeen adopted to shield the magnets from asynchronous fields.PMs are very weak in tension, although they are able towithstand large compressive stress. To ensure mechanical in-tegrity of high-speed rotors, magnets are often prestressed using


sleeves made of high-strength metallic materials such as In-conel or titanium [61]. Carbon fiber is also often used as aretention mechanism due to its superior mechanical properties.This can be directly filament wound on the rotor assembly, or aprefabricated sleeve can be pressed onto the assembly.

IV. LITERATURE REVIEW ON HIGH-SPEED ELECTRICALMACHINE TECHNOLOGIES

Over the last two decades, a number of overviews onhigh-speed electrical machine technology have been published.Reichert and Pasquarella [71] present a set of analytical for-mulations relating rotor diameters to rotational speeds basedon four considerations: 1) the mechanical stresses in the rotor;2) critical speeds; 3) rotor cooling; and 4) specific power outputper rotor volume. These formulations are then used to definea design window or realization area for high-speed electricalmachines. The authors also present a powerspeed chart, whichincludes limit lines for synchronous and IMs, as well as limitareas for a number of inverter topologies. This chart is used toderive tendencies depending on the required power and speedrange. At the time of writing, the authors concluded that forlower speeds (9000 r/min) and higher power (30 MW), thesynchronous machine fed from current source inverters was thefavorable solution, whereas for higher speed (100 000 r/min)and lower power (2 MW), the IM fed from voltage source in-verters was the favored solution. PM machines were describedas suitable for low-power and high-speed applications.

Canders [72] presents the use of surface-mounted PM (SPM)machines with carbon fiber banding, as a very promisingdesign alternative for higher power and high-speed applica-tions. A suitable choice of armature current sheet is givenimportant consideration in deriving the tangential force density(in kNm/m2) limit on the rotor as a function of speed. Keepingthe design running at subcritical, the permissible rotor length iscalculated, and charts of power limits as a function of speed forPM machines are derived.

Binder and Schneider [73] discuss aspects relating to thedesign of high-speed electrical machines. The authors describehow the electromagnetic utilization factor is lower than forlow-speed machines, and through a literature review for builtPM, induction, and homopolar machines, empirically, a linearregression line relating the power limit to the design speedis derived. The bearing types used for developed high-speedmachines are also investigated.

The following make an effort to describe high-speed ma-chines in literature that have been built and tested, includingIMs, PM machines, switched reluctance (SR) machines, andsynchronous homopolar machines. A comparison between thetechnologies demonstrates the operating domain each technol-ogy has been employed at.

A. High-Speed IMs

IMs, due to their robust construction, are commonly usedfor high-speed applications. Table II, which is developed andexpanded from that presented in [19], lists high-speed IMspublished in literature, ranked in order of circumferential speed,vc (in m/s). From this table, it is noted that, for the highest

TABLE IIHIGH-SPEED IMS IN LITERATURE AND INDUSTRY, RANKED

IN ORDER OF PERIPHERAL SPEED

Fig. 15. High-speed solid-rotor topologies. (a) Smooth solid rotor. (b) Slittedsolid rotor. (c) Coated solid rotor. (d) Caged solid rotor.

peripheral speeds, a solid-rotor topology is usually preferreddue to the mechanical robustness of such a structure. Thesmooth solid rotor, as shown in Fig. 15(a), is the simplest andmost robust solid-rotor topology; however, such a design lacksa high-conductivity path for the induced rotor currents and isthus a relatively inefficient design [11], [12]. The harmonicflux components concentrate on the surface of the rotor andcause significant losses therein, which limit the overall machinepower density. Moreover, eddy currents try to push the inducingmagnetic field out of the rotor.


Fig. 16. 300-kW 60 000-r/min copper-coated solid-rotor IM (Sundyne Corpo-ration) [30].

The axially slitted solid rotor, as shown in Fig. 15(b), is animprovement over the smooth solid-rotor topology by axiallyslitting the rotor surface [13], [14]. Slitting has the effect ofguiding the fundamental flux component into the rotor whilepresenting a higher impedance path to the eddy currents trav-eling on the rotor surface. However, slitting also increases theair-gap friction loss, which at the high speeds considered caneven outweigh the reduced eddy-current loss, as well as reducethe mechanical strength of the rotor.

Detailed studies on axial slitting of solid-rotor inductionmotors are presented by Aho [15] and Huppunen [16]. In thesestudies, the number of rotor slits and their geometries are inves-tigated with the aim of achieving an overall optimized design.It is noted that deep slitting provides a good torque generationcapability at the cost of reduced mechanical strength. Ahorecommends a slit depth of about 40%50% of the rotor radiusin order to achieve a compromise between rotor strength andreduced eddy-current losses. Moreover, it is noted that oddnumbers of rotor slits minimize the torque ripple and the rotorlosses at the cost of an increased unbalanced magnetic pull.

A further improvement in the rotor is achieved by coatingthe solid rotor with a copper layer, thus introducing electro-magnetic anisotropy [17], [18], as shown in Fig. 15(c). Thecopper coating acts as an infinite number of rotor bars and asend-rings. Such a design is mechanically robust and achieveshigher efficiency levels than the simple solid-rotor topology.Fig. 16 shows a 300-kW 60 000-r/min copper-coated solid-rotorIM used for an air compressor [30]. This topology is used in thehighest peripheral speed machines shown Table II; however, byhaving a coating, the air-gap from the stator iron to the rotoriron is much higher than in IMs where the copper is in bars,hence resulting in a poor power factor.

Lahteenmaki and Soitu [19], [20] investigate the use of asolid rotor with a squirrel cage, as shown in Fig. 15(d). The ideabehind this topology is to combine the mechanical strength ofa solid rotor and the electromagnetic performance of a squirrelcage rotor. Difficulties in manufacturing such a topology werereported, namely, the drilling of slots in solid steel; therefore,the designs have open rotor slots. Lahteenmaki compares thecaged solid rotor to an existing copper-coated solid rotor fora 60-kW 60 000-r/min machine and reports that squirrel cagerotors increase power density and efficiency with respect to thecoated rotor, at the cost of reduced mechanical robustness.

Lahteenmaki also compares a laminated squirrel cage rotorwith a copper-coated solid rotor [19] for a 65-kW 30 600-r/minmachine and reports higher efficiency levels and a higher uti-lization factor for the laminated-rotor topology. The laminatedrotor gave 39% more power for the same stator winding tem-perature rise.

Lateb et al. [5] also compare different types of solid-rotor constructions to a laminated-rotor topology, and it isreported that the laminated-rotor topology has 2%3% higherefficiency levels than the different solid-rotor constructions, aswell as a higher power factor. Similar results were reported byIkeda et al. [21], who experimentally compared a solid cagedrotor topology to a laminated-rotor topology for a 200-kW12 000-r/min machine.

It is commonly agreed in literature that a laminated rotorshould be used if it is mechanically possible due to the signif-icantly higher efficiency levels. The following is an overviewof what has been published related to the design of high-speedlaminated-rotor IMs in a chronological order.

The paper by Boglietti et al. [22] was one of the firstpapers to discuss the complexities associated with the designof high-speed laminated-rotor IMs. In [22], an overview isgiven of electromagnetic, thermal, and mechanical design con-siderations, noting the typically high (compared with standardIMs) rotor-bar current densities used in high-speed machines,mentioning 20 A/mm2 as a typical figure for the rotor-barcurrent density. Boglietti et al. also discuss the problem of highiron losses due to the high fundamental frequencies in high-speed machines. This often constraints the designer to choosea relatively low flux density in the stator teeth (11.1 T, incomparison to 1.51.8 T for 50-Hz IM), as well as in the statoryoke (1.11.2 T, in comparison to 1.51.7 T for 50-Hz IM).All the aforesaid issues tend to cause thermal problems giventhe overall small machine dimensions, which lead to forcedcooling systems as a requirement. The mechanical design issuesdiscussed in [22] relate mainly to bearing selection, lubrication,and balancing, noting the very high balancing grade requiredfor high-speed machines.

Soong et al. [23] give a comprehensive treatment on thedesign of high-speed IMs, describing the design and manufac-turing issues by a design case study of a 21-kW 50 000-r/minmotor for a compressor application. The considerations takenwhen choosing the rotor-bar and end-ring materials are dis-cussed, noting in particular the need for a high-yield-strengthmaterial for the rotor conductors, as well as describing thetradeoff between different lamination heat treatments in findinga compromise between the electromagnetic and mechanicalproperties. Soong et al. use SiFe laminations for both rotorand stator, and the materials are separately heat treated afterpunching to achieve desired characteristics for the rotor andthe stator (i.e., higher yield strength for rotor laminations andlower iron losses for stator laminations). The issue of stressesin rotor lamination material is highlighted as an important con-sideration, noting how the rotor bars shift the maximum stressconcentration in the lamination with respect to a rotating disk.This issue is also discussed by Kim et al. [24], who describesthe design of an 11-kW 56 000-r/min IM for a centrifugalcompressor, noting the use of a circular closed slot in order tominimize stresses in the rotor laminations.

Centner and Schafer [25], [26] discuss the steel grade usedfor the rotor laminations as an optimization parameter andtry to investigate the suitability and compare using SiFe andCoFe laminations for high-speed IMs. They compare 0.2-mmVacoflux 50, which is a CoFe alloy from Vacuumschmelze


Fig. 17. 10-kW 80 000 r/min laminated rotor with drop-shaped bars(Cummins).

GmbH, to M270-35A, which is a standard low-loss 0.35-mmSiFe sheet material. They built two machines for the sameenvelope, i.e., one made from CoFe and the other from SiFelaminations, noting that, due to the higher saturation magne-tization, a higher magnetic loading machine was possible inthe CoFe design, resulting in higher efficiency than the SiFemachine (91% versus 89% at 400 Hz).

All the aforementioned high-speed IMs use a round rotor-barshape. Gerada et al. [34], [35] describe a design methodologythat boosts the power density of laminated-rotor IMs by usingdrop-shaped bars instead of the conventionally used roundrotor bars, together with tailoring the machines electrical andmagnetic loading and the split ratio. The drop-shaped barsallow the current density in the rotor cage to be tailored tothe desired maximum rotor temperature. However, such a barshape also increases the stresses in the laminations; hence, theuse of a coupled multidomain design environment is essential.The procedure is used to increase the power density of a 10-kW80 000 r/min IM, shown in Fig. 17, for a turbochargerapplication.

The laminated-rotor IMs reported in all the aforementionedliterature have a conventional laminated-rotor assembly, i.e., asolid shaft with hollow laminations fitted onto it. In [23], itis reported that, with respect to conventional IM design, therotorID/rotorOD ratio is oversized in high-speed machines toincrease the stiffness, and in order to allow for such an increasedratio, a flux conducting shaft is utilized. In an interestingpatented approach [27], [28], a full laminated core is used, withthe end rings fixed by tie rods. This topology is composed oftwo steel shaft ends and full-depth laminations held together bya number of steel tie rods. The squirrel cage consists of coppershort-circuit rods distributed at the periphery of the core andlinked to two bronze alloy rings placed at both ends of the stack.The stack and the rings are tightened by the tie rods, which arealso distributed at the periphery of the core and screwed intothe shaft ends. This patented technology is being used commer-cially by Converteam SAS (now GE Energy) [5]. This motor iscommercially called MGV (moteur grande vitesse), in the rangeof 330 MW from 6000 to 18 000 r/min for compressors in theoil/gas industry. The mechanical/rotordynamic analysis of thistopology has been treated rigorously in [29]. Such a topologyallows very high peripheral speeds for laminated-rotor IMs tobe achieved, with speeds of 250 m/s reported [5].

Highest peripheral speeds for laminated-rotor IMs are ob-tained by using advanced end-ring design features and byusing high-strength sheet steel, which is magnetic for the rotor(i.e., not electrical steel grades). In [36] and [37], analysis

Fig. 18. 2-MW 15 000-r/min laminated IM rotor [36].

is presented of the advanced end-ring design features for a2-MW 15 000-r/min machine, shown in Fig. 18, including anonuniform cross section, hoop stress relief cuts, and an inte-grated joint boss, which reduces critical stress concentrations,allowing operation under a broad speed and temperature designrange. For the rotor laminations, high-strength aircraft-gradeAISI 4130 alloy steel is mentioned [37]. This enables the rotorperipheral speed to be increased to 290 m/s. Similarly, in [38],the high-strength magnetically permeable alloy AerMet 100is used for the rotor laminations for a 45-kW 92 500 r/minmachine with a peripheral speed of 240 m/s.

B. High-Speed PM Machines

PM machines are also popular for high-speed applicationsprimarily because of their high efficiency levels since, unlikeIMs, rotor losses can be reduced by slitting; hence, the rotortemperature can be limited to lower values for distributed-winding designs. Table III lists some of the high-speed PMmachines found in literature, again in order of peripheral speed.

Noguchi et al. [41] designed a 2-kW 220 000-r/min PMmachine for a turbocharger application. The rotor is made up ofa hollow parallel-magnetized ring magnet, which is fitted on apermeable shaft and retained by a carbon-fiber bandage. An in-teresting feature of this motor is the rather unusual six-slottwo-pole concentrated wound slotpole configuration. This windinglayout gives a fundamental winding factor of 0.5 for the torque-producing harmonic, which is justified by the analysis. It isshown that the increase in copper losses due to the low windingfactor far outweighs the reduction in rotor losses resulting fromthe more conventionally used three-slottwo-pole fractionalslotpole combination. This pole combination is used also byShigematsu et al. [44], who investigated techniques for rotorloss reduction. Moreover, the chosen slotpole combinationresults in a sufficiently high inductance, which allows themachine to field weaken. Noguchi et al. also describe the designof a 1.5-kW 150 000-r/min PM machine for an automotivesupercharger [42], [43], which uses a similar rotor constructionand slotpole combination, but is retained by glass fiber.

Wang et al. [45] describe the rotor design features of a 22-kW120 000-r/min PM machines with the aim of improving therotor design for sensorless control. In this paper, two rotorconfigurations are presented: one rotor using a conventionalparallel-magnetized hollow ring magnet and the other rotor us-ing two parallel-magnetized segments per pole. Both magnetic


TABLE IIIHIGH-SPEED PM MACHINES IN LITERATURE AND INDUSTRY, RANKED

IN ORDER OF PERIPHERAL SPEED

assemblies are fit on a shaft and are retained by a titaniumsleeve. It is shown that, by segmenting the parallel-magnetizedmagnets, both the fundamental and third-harmonic air-gapfields improve, thus making the motor better for sensorlesscontrol.

Zwyssig et al. [46], [47] have built some of the highest rota-tional speed machines. In [46], the design, analysis, and testingof a 100-W 500 000-r/min PM generator for a mesoscale gasturbine is described. The rotor consists of a parallel-magnetizedsolid cylindrical magnet that is held within a hollow sectionof a two-part titanium shaft. The mechanical and rotordynamicconsiderations associated with such a setup are described. Thedetails of this construction are shown in Fig. 19. This machineis scaled up to a 1-kW 500 000-r/min power node [47], thus in-creasing the peripheral speed of the rotor. Both machines utilizea slotless stator configuration, with the intent of minimizing therotor losses and avoiding the use of very thin (i.e., mechanicallyweak) stator teeth. The same authors also develop a 100-W1 000 000-r/min demonstrator PM machine [4].

Zhao et al. [48], [49] present the design of a 2-kW200 000-r/min PM motor for a reverse-turbo Brayton cyclecryocooler. Similar to [47], a slotless stator is used, and a solidmagnet is fit inside a hollow shaft. Samariumcobalt is used dueto its stability at cryogenic temperatures. An elliptical magnetis used in this prototype, although no reasons are given for this.

Fig. 19. 100-W 500 000-r/min PM machine [46].

Takahashi et al. [50] discuss design considerations for a5-kW 150-000 r/min motor for a machine tool application.Important design features are investigated, particularly notingthe benefits of using a large physical air gap for high-speedmachines, in order to reduce the slot ripples and the resultingsleeve and magnet eddy-current losses. For the same electro-magnetic design, a number of sleeve thicknesses and materialsare experimentally investigated, confirming the effectiveness ofusing a relatively large air gap and the benefits of using low-conductivity fiber-reinforced plastic for retention, with the aimof keeping the rotor losses to minimum levels. The benefits ofusing a low-conductivity material for magnet retention werealso verified by Cho et al. [51], who compared the losses ofan existing 50-kW 70 000-r/min turbocompressor machine thatuses an Inconel718 sleeve, to a design using a carbon fibersleeve, noting a sixfold reduction in rotor losses.

Binder et al. [52] provide analytical formulation for design-ing a carbon-fiber retention system for high-speed machines,illustrating with a 40-kW 40 000-r/min case study. In the samepaper, using the same power/speed node, the speed limitationsof using IPM machines for magnet retention are investigated,and it is reported that, by using conventional strength SiFesteel grades, the maximum peripheral speed is limited to around80 m/s. Higher peripheral speeds for IPM machines can be onlyachieved using high-strength electrical steels, as described byHonda et al. [62] who achieved peripheral speeds in excess of230 m/s for an IPM.

Other literature has focused on the multiphysics design ap-proach and optimization of high-speed machines, such as thework done by Pfitser and Perriard [53], who designed andoptimized a 2-kW 200 000-r/min motor and the work presentedby Bianchi et al. [54][56], who investigated design optionsusing different materials for a 1-kW 25 000-r/min motor for amachine-tool application. It is noted that, for all the aforesaidmachines, a SPM machine with a high-strength retention sleevematerial (titanium/inconel/carbon fiber/glass fiber) is almostexclusively used.

C. High-Speed SR Machines

Albeit less common for high-speed applications than IMsand PM machines, some SR machines have been developedfor some niche applications. The more common applicationarea is for low-power (up to 1 kW) low-cost mass-productionmarkets such as vacuum cleaners and air blowers [65][67].These machines are typically very simple in construction witha four-slottwo-pole configuration commonly used.


TABLE IVHIGH-SPEED SR MACHINES IN LITERATURE AND INDUSTRY,

RANKED IN ORDER OF RPM

kW

Fig. 20. 30-kW-peak 100 000-r/min synchronous homopolar machinerotor [68].

Another application for high-speed SR machines is for themore-electric aircraft engine [63], [64]. Here, SR machinesare typically used as a starter/generator (S/G) system forstarting and secondary electrical power extraction. Their fail-silent capability, simple construction, and the ability to operatein the harsh operational environments (ambient temperaturesof around 400 C) make SR a suitable choice for such anapplication. These machines make use, almost exclusively, ofhigh-yield-strength vanadiumironcobalt laminations such asHiperco 50 HS. Table IV lists some of the high-speed SRmachines found in literature. In absence of rotor-dimensionaldetails, the machines are ranked in order of r/min

kW.

D. High-Speed Synchronous Homopolar Machines

Synchronous homopolar machines are similar in principleto the popular wound-field synchronous machines; however,in the case of the homopolar machines, the field winding (ormagnets) is fixed to the stator rather than the rotor. Such atopology results in a simple robust rotor structure that canbe constructed from a single piece of high-strength steel andis suitable for high-speed operation. The homopolar machinehas been investigated mainly for high-speed flywheel energystorage systems [68], [75] where it is important to have lowzero-torque spinning losses and low rotor losses due to themachine operating in vacuum. Tsao [68] designed and tested a30-kW-peak 100 000-r/min, 140-Wh machine, which is shownin Fig. 20, for an integrated flywheel. A high-strength solidrotor is used for both the electromagnetic rotor and the energystorage accumulator. In this motor, four poles are cut into boththe upper and lower parts of the rotor, with the lower polesrotated 45 with respect to the upper poles. The field windingencircles the cylindrical central portion of the rotor. The testedmachine achieved an average efficiency of 83% at an averagepower of 9.4 kW, over a speed range of 30 000 to 60 000 r/min.

Fig. 21. Powerspeed nodes and r/min

kW.

TABLE VSUMMARY OF HIGH-SPEED MACHINE LIMITS

Maas [74] also developed a synchronous homopolar machinefor a 10-kW 50 000-r/min flywheel system [75].

V. BENCHMARKING HIGH-SPEED MACHINES

With the machines listed in the literature survey, as wellas other machines from a separate industry survey, Fig. 21is derived. In this figure, the powerspeed nodes are plottedfor all machines built and tested to the authors knowledge.On the same plot, the r/min

kW lines are superimposed. The

concept of r/min

kW, as introduced and described in [1], is afigure of merit for high-speed rotating machinery. It provides areliable guide number to assess from combinations of speedand power, the likely severity of dynamic problems such ascritical speeds, high value of bearing dN , peripheral speeds andstresses, and sensitivity to good balancing [1].

In general, dynamic problems are negligible for machinerythat operates below 1 105 r/minkW and moderate for ro-tating machinery that operate between 5 105 r/minkW and1 106 r/minkW. Above this, mechanical problems becomedifficult to acute [1].

The following observations are noted for high-speed rotatingelectrical machinery and summarized in Table V.

1) The highest r/minkW, just in excess of 1 106, isachieved solely through solid-rotor IM technology. Thistechnology also achieves highest peak peripheral speedsreported, around 400 m/s.

2) Surface PM machines with a high-strength retentionmechanism (typically Inconel, titanium, or carbon fiber)are currently limited to around 8 105 r/minkW andperipheral speeds of 300 m/s.


3) Laminated-rotor IMs using conventional electrical steelgrades achieve an r/min

kW of around 2.5

105 r/min

kW and peripheral speeds of 185 m/s.4) Laminated-rotor IMs that use high-strength sheet steels

for laminations, such as AerMet 100 or AISI 4130 canachieve around 6 105 r/minkW. This can be alsoachieved using newly introduced high-strength electricalsteel grades with yield strength in excess of 800 MPa,such as 35HXT780T. Typical peripheral speeds are on theorder of 280 m/s.

5) SR machines with high saturation magnetizationand high-yield-strength VCoFe laminations achiever/min

kW of around 3.5 105 and peripheral speeds

over 200 m/s.6) Development of high-strength steel is an enabler for IPM

machines to be used for higher speeds, with over 1.5105 r/min

kW and 230 m/s achievable [62].

VI. CONCLUSION

The selection and design of electrical machine topology forhigh-speed applications is often a complex issue that mustbe decided by taking into close consideration the sciencesinvolved, namely the electromagnetic, mechanical, thermal, andpower electronics. This paper has presented the main applica-tions that drive the development of the technology, and throughan extensive literature survey and in-house design experience,this paper has determined the achievable r/min

kW and speed

for the considered topologies. Recent commercial availabilityof high-strength electrical steel grades will inevitably be anenabler for laminated-rotor IMs and IPM machines to find moreuses in high-speed applications.

REFERENCES[1] R. D. van Millingen and J. D. van Millingen, Phase shift torquemeters

for gas turbine development and monitoring, in Proc. Int. Gas TurbineAeroengine Congr. Expo., Jun. 1991, pp. 110.

[2] J. S. Shao, L. K. Hwang, P. R. Miller, and S. J. Charlton, EGR deliveryand control system using dedicated full authority compressor, U.S. Patent6 216 460, Apr. 17, 2001.

[3] C. Nelson, Exhaust Energy Recovery, Jun. 2010. [Online]. Available:http://www1.eere.energy.gov/vehiclesandfuels/pdfs/merit_review_2010/high-eff_engine_tech/ace041_nelson_2010_o.pdf

[4] C. Zwyssig, J. W. Kolar, and S. D. Round, Megaspeed drive systems:Pushing beyond 1 million r/min, IEEE/ASME Trans. Mechatronics,vol. 14, no. 5, pp. 564574, Oct. 2009.

[5] R. Lateb, J. Enon, and L. Durantay, High speed, high power electrical in-duction motor technologies for integrated compressors, in Proc. ICEMS,Nov. 2009, pp. 14381442.

[6] L. Goldstein, B. Hedman, D. Knowles, S. I. Freedman, R. Woods, andT. Schweizer, Gas-Fired Distributed Energy Resource Technology Char-acterizations, Nat. Renewable Energy Lab., Golden, CO, USA, Tech.Rep. NREL/TP-620-34783, Nov. 2003.

[7] Bladon Jets Gas Turbines. [Online]. Available: http://www.bladonjets.com/technology/gas-turbines

[8] K. Senda, M. Namikawa, and Y. Hayakawa, Electrical Steels for Ad-vanced AutomobilesCore Materials for Motors, Generators and High-Frequency Reactors, JFE Steel Res. Dept., Tokyo, Japan, Tech. Rep. 4,Nov. 2004.

[9] I. Tanaka and H. Yashiki, Magnetic and mechanical properties of newlydeveloped high-strength nonoriented electrical steel, IEEE Trans. Magn.,vol. 46, no. 2, pp. 290293, Feb. 2010.

[10] M. Yabumoto, T. Wakisaka, N. Suzuki, C. Kaido, and T. Kubota, Electri-cal Steel Sheet for Traction Motors of Hybrid/Electric Vehicles, NipponSteel Res. Lab., Tokyo, Japan, Tech. Rep. 87, Jul. 2003.

[11] J. R. Bumby, E. Spooner, and M. Jagiela, Equivalent circuit analysis ofsolid-rotor induction machines with reference to turbocharger acceleratorapplications, Proc. IEE Elect. Power Appl., vol. 153, no. 3, pp. 3139,Jan. 2006.

[12] J. R. Bumby, E. Spooner, and M. Jagiela, Solid rotor induction machinesfor use in electrically-assisted turbochargers, in Proc. Int. Conf. PEMD,Apr. 2006, vol. 1, pp. 341345.

[13] J. Pyrhonen, J. Nerg, P. Kurronen, and U. Lauber, High-speed, high-output, solid-rotor induction motor technology for gas compression,IEEE Trans. Ind. Electron., vol. 57, no. 1, pp. 272280, Jan. 2010.

[14] J. Pyrhonen and J. Huppunen, A new medium speed solid rotor inductionmotor for a high speed milling machine, in Proc. SPEEDAM, 1996,pp. B5-1B5-8.

[15] T. Aho, Electromagnetic design of a solid steel rotor motor for de-manding operation environments, Ph.D. dissertation, Lappeenranta Univ.Technol., Lappeenranta, Finland, 2007.

[16] J. Huppunen, High-Speed solid-rotor induction machineElectromagnetic calculation and design, Ph.D. dissertation,Lappeenranta Univ. Technol., Lappeenranta, Finland, 2004.

[17] J. Saari and A. Arkkio, Losses in high speed asynchronous motors, inProc. ICEM, Sep. 1994, vol. 3, pp. 704708.

[18] N. D. Sharma, R. Anbarasu, J. Nataraj, A. Y. Dangore, andB. Bhattacharjee, Experimental investigations on high speed solid andcomposite rotor induction motor, in Proc. Int. Conf. Power Electron.,Drives Energy Syst. Ind. Growth, Jan. 1996, vol. 2, pp. 913919.

[19] J. Lahteenmaki, Design and voltage supply of high-speed inductionmachines, Ph.D. dissertation, Helsinki Univ. Technol., Espoo, Finland,2002.

[20] J. Lahteenmaki and V. Soitu, Comparison of solid steel rotors with a cop-per coating or with a copper cage for a 60kW 60 000 RPM compressor,in Proc. ICEM, Aug. 2000, vol. 2, pp. 623626.

[21] M. Ikeda, S. Sakabe, and K. Higashi, Experimental study of high speedinduction motor varying rotor core construction, IEEE Trans. EnergyConvers., vol. 5, no. 1, pp. 98103, Mar. 1990.

[22] A. Boglietti, P. Ferraris, M. Lazzari, and F. Profumo, About the design ofvery high frequency induction motors for spindle applications, in Conf.Rec. IEEE IAS Annu. Meeting, Oct. 1992, vol. 1, pp. 2532.

[23] W. L. Soong, G. B. Kliman, R. N. Johnson, R. A. White, andJ. E. Miller, Novel high-speed induction motor for a commercial cen-trifugal compressor, IEEE Trans. Ind. App., vol. 36, no. 3, pp. 706713,May 2000.

[24] Y. K. Kim, M. C. Choi, K. H. Suh, Y. C. Ji, and D. S. Wang, High-speedinduction motor development for small centrifugal compressor, in Proc.ICEMS, Aug. 2001, vol. 2, pp. 891894.

[25] M. Centner and U. Schafer, Optimised design of high-speed inductionmotors in respect of the electrical steel grade, IEEE Trans. Ind. Electron.,vol. 57, no. 1, pp. 288295, Jan. 2010.

[26] M. Centner, Entwurf und Erprobung Schnelldrehender Asynchron-maschinen unter Besonderer Bercksichtigung der Magnetisch AktivenMaterialien, Ph.D. dissertation, Berlin Tech. Univ., Berlin, Germany,2009.

[27] K. Velloso Rodrigues, J. F. Pradurat, N. Barras, and E. Thibaut, Designof high-speed induction motors and associate inverter for direct drivecentrifugal machine, in Proc. ICEM, Sep. 2008, pp. 15.

[28] P. Bawin, R. Botte, and J. M. Edebouw, Electric motor with high powerand rotational speed, U.S. Patent 5 512 792, Apr. 30, 1996.

[29] G. Mogenier, R. Dufour, G. Ferraris Besso, L. Durantay, and N. Barras,Identification of lamination stack properties: Application to high speedinduction motors, IEEE Trans. Ind. Electron., vol. 57, no. 1, pp. 281287,Jan. 2010.

[30] J. F. Gieras and J. Saari, Performance calculation for a high-speedsolid-rotor induction motor, IEEE Trans. Ind. Electron., vol. 59, no. 6,pp. 26892700, Jun. 2012.

[31] B. M. Wood, C. L. Olsen, G. D. Hartzo, J. C. Rama, and F. R. Szenasi, De-velopment of an 11000-r/min 3500-HP induction motor and adjustable-speed drive for refinery service, IEEE Trans. Ind. Appl., vol. 33, no. 3,pp. 815825, May 1997.

[32] F. Viggiano and G. Schweitzer, Active magnetic support and design ofhigh speed rotors for powerful electric drives, in Proc. Int. Symp. Magn.Bearings, Jul. 1992, vol. 1, pp. 549558.

[33] R. Siegwart, R. Larsonneur, and A. Traxler, Design and performance ofhigh speed milling spindle in digitally controlled AMBs, in Proc. Int.Symp. Magn. Bearings, Aug. 1990, vol. 1, pp. 197204.

[34] D. Gerada, A. Mebarki, N. L. Brown, K. J. Bradley, and C. Gerada, De-sign aspects of high-speed high-power-density laminated-rotor inductionmachines, IEEE Trans. Ind. Electron., vol. 58, no. 9, pp. 40394047,Sep. 2011.


[35] D. Gerada, A. Mebarki, N. L. Brown, H. Zhang, and C. Gerada, Design,modelling and testing of a high speed induction machine drive, in Proc.ECCE, Sep. 2012, pp. 46494655.

[36] M. Caprio, V. Lelos, J. Herbst, and J. Upshaw, Advanced induction motorendring design features for high speed applications, in Proc. IEMDC,May 2005, vol. 1, pp. 993998.

[37] M. Caprio, V. Lelos, J. Herbst, S. Manifold, and H. Jordon, High strengthinduction machine, rotor, rotor cage end ring and bar joint, rotor end ring,and related methods, U.S. Patent 7 504 756, Mar. 17, 2009.

[38] M. Mekhiche, J. L. Kirtley, M. Tolikas, E. Ognibene, J. Kiley,E. Holmansky, and F. Nimblett, High speed motor drive development forindustrial applications, in Proc. IEMDC, May 1999, vol. 1, pp. 244248.

[39] M. Larsson, M. Johansson, L. Naslund, and J. Hylander, Design andevalution of high-speed induction machine, in Proc. IEMDC, Jun. 2003,vol. 1, pp. 7782.

[40] R. Anbarasu, R. K. Gupta, N. D. Sharma, G. Gauthaman, A. K. Wankhede,P. H. Chavda, J. Nataraj, and B. Bhattacharjee, Design and experimentalinvestigation of high speed squirrel cage induction motor, in Proc. Int.Conf. Power Electron., Drives Energy Syst. Ind. Growth, Jan. 1996, vol. 2,pp. 920926.

[41] T. Noguchi, Y. Takata, Y. Yamashita, Y. Komatsu, and S. Ibaraki,220000 r/min 2kW PM motor drive for turocharger, IEEJ Trans. Ind.Appl., vol. 125, no. 9, pp. 854861, Sep. 2005.

[42] T. Noguchi and M. Kono, Development of 150000r/min 1.5kW perma-nent magnet motor for automotive supercharger, in Proc. IEEE Conf.PEDS, Nov. 2007, pp. 183188.

[43] T. Noguchi and T. Wada, 1.5kW, 150000r/min ultra high-speed PMmotor fed by 12V power supply for automotive supercharger, in Proc.EPE, Sep. 2009, pp. 19.

[44] K. Shigematsu, J. Oyama, T. Higuchi, T. Abe, and Y. Ueno, The study ofeddy current in rotor and circuit coupling analysis for small size and ultahigh speed motor, in Proc. IPEMC, Aug. 2004, pp. 275279.

[45] K. Wang, M. J. Jin, J. X. Shen, and H. Hao, Study on rotor struc-ture with different magnet assembly in high-speed sensorless brushlessDC Motors, IET Proc. Elect. Power Appl., vol. 4, no. 4, pp. 241248,Sep. 2010.

[46] C. Zwyssig, J. W. Kolar, W. Thaler, and M. Vohrer, Design of a 100W,500000 rpm permanent-magnet generator for mesoscale gas turbines, inConf. Rec. IEEE IAS Annu. Meeting, Sep. 2005, pp. 253260.

[47] C. Zwyssig, M. Duerr, D. Hassler, and J. W. Kolar, An ultra-high speed,500000 rpm, 1 kW electrical drive system, in Proc. PCC, Apr. 2007,pp. 15771583.

[48] L. Zhao, C. Ham, L. Zheng, T. Wu, K. Sundaram, J. Kapat, and L. Chow,A highly efficient 200,000RPM permanent magnet motor system, IEEETrans. Magn., vol. 43, no. 6, pp. 25282530, Jun. 2007.

[49] L. Zheng, T. X. Wu, D. Acharya, K. B. Sundaram, J. Vaidya, L. Zhao,L. Zhou, C. H. Ham, N. Arakere, J. Kapat, and L. Chow, Design ofa super high speed cryogenic permanent magnet motor, IEEE Trans.Magn., vol. 41, no. 10, pp. 38233825, Oct. 2005.

[50] I. Takahashi, T. Koganezawa, G. Su, and K. Ohyama, A super high speedmotor drive system by a quasi-current source inverter, IEEE Trans. Ind.Appl., vol. 30, no. 3, pp. 683690, May/Jun. 1994.

[51] H. W. Cho, S. M. Jang, and S. K. Choi, A design approach to reduce rotorlosses in high-speed permanent magnet machine for turbo-compressor,IEEE Trans. Magn., vol. 42, no. 10, pp. 35213523, Oct. 2006.

[52] A. Binder, T. Schneider, and M. Klohr, Fixation of buried andsurface-mounted magnets in high-speed permanent-magnet synchronousmachines, IEEE Trans. Ind. Appl., vol. 42, no. 4, pp. 10311037,Jul./Aug. 2006.

[53] P. D. Pfitser and Y. Perriard, Very high speed slotless pm motors : Analyt-ical modelling, optimization, design and torque measurement methods,IEEE Trans. Ind. Electron., vol. 57, no. 1, pp. 296303, Jan. 2010.

[54] N. Bianchi, S. Bolognani, and F. Luise, Potentials and limits of high-speed PM motors, IEEE Trans. Ind. Appl., vol. 40, no. 6, pp. 15701578,Nov. 2004.

[55] N. Bianchi, S. Bolognani, and F. Luise, Analysis and design of a PMbrushless motor for high-speed operations, IEEE Trans. Energy Con-vers., vol. 20, no. 3, pp. 629637, Sep. 2005.

[56] N. Bianchi, S. Bolognani, and F. Luise, High speed drive using a slotlessPM motor, IEEE Trans. Power Electron., vol. 21, no. 4, pp. 10831090,Jul. 2006.

[57] G. Munteanu, A. Binder, T. Schneider, and B. Funieru, No-load tests of a40kW high-speed bearingless permanent magnet synchronous motor, inProc. Int. SPEEDAM, Jun. 2010, pp. 14601465.

[58] T. Schneider, J. Petersen, and A. Binder, Influence of pole pair combina-tions on high-speed bearingless permanent magnet motor performance,in Proc. PEMD, Apr. 2008, pp. 707711.

[59] J. D. Ede, Z. Q. Zhu, and D. Howe, Rotor resonances of high-speedpermanent-magnet brushless machines, IEEE Trans. Ind. Appl., vol. 38,no. 6, pp. 15421548, Nov. 2002.

[60] D. Gerada, A. Mebarki, N. L. Brown, and C. Gerada, Optimal design of ahigh speed concentrated-wound PMSM, in Proc. ICEMS, Tokyo, Japan,Nov. 2009, pp. 16.

[61] T. Wang, F. Wang, H. Bai, and J. Xing, Optimization design of rotorstructure for high speed permanent magnet machines, in Proc. IEEEICEMS, Oct. 2007, pp. 14381442.

[62] Y. Honda, S. Yokote, T. Higaki, and Y. Takeda, Using the halbach magnetarray to develop an ultrahigh-speed spindle motor for machine tools, inConf. Rec. IEEE IAS Annu. Meeting, Oct. 1997, vol. 1, pp. 5660.

[63] E. Richter and C. Ferreira, Performance evaluation of a 250 kW switchedreluctance starter generator, in Conf. Rec. IEEE IAS Annu. Meeting,Oct. 1997, vol. 1, pp. 434440.

[64] C. A. Ferreira, S. R. Jones, W. S. Heglund, and W. D. Jones, De-tailed design of a 30 kW switched reluctance S/G system for a gasturbine application, IEEE Trans. Ind. Appl., vol. 31, no. 3, pp. 553561,May 1995.

[65] H. J. Brauer and R. W. deDoncker, Thermal modeling of a high speedSRM for vacuum cleaners, in Proc. EPE, Sep. 2011, pp. 110.

[66] J. Kim and R. Krishnan, High efficiency single pulse controlled SRMdrive for high speed application, in Conf. Rec. IEEE IAS Annu. Meeting,Oct. 2008, vol. 1, pp. 18.

[67] D. H. Lee, T. H. Pham, and J. W. Ahn, Design and operation characteris-tics of four-two pole high speed SRM for torque ripple reduction, IEEETrans. Ind. Electron., vol. 60, no. 9, pp. 36373643, Sep. 2013.

[68] P. I. Tsao, An integrated flywheel energy storage system with a homopo-lar inductor motor/generator and high-frequency drive, Ph.D. disserta-tion, Univ. California, Berkeley, CA, USA, 2003.

[69] A. Boglietti, R. I. Bojoi, A. Cavagnino, P. Guglielmi, and A. Miotto,Analysis and modeling of rotor slot enclosure effects in high-speed in-duction motors, IEEE Trans. Ind. Appl., vol. 48, no. 4, pp. 12791287,Jul. 2012.

[70] K. Y. Bell and S. Abel, Optimization of WWTP aeration processupgrades for energy efficiency, Water Practice Technol., vol. 6, no. 2,pp. 110, Jul. 2011.

[71] K. Reichert and G. Pasquarella, High speed electric machines, sta-tus, trends, and problems, in Proc. IEEE/KTH Stockholm Tech. Conf.,Stockholm, Sweden, Jun. 1995, pp. 4149.

[72] W. Canders, High speed machines on magnetic bearingsdesign con-cepts and power limits, in Proc. ICEM, Sep. 1998, pp. 2025.

[73] A. Binder and T. Schneider, High-speed inverter-fed AC drives, in Proc.Int. ACEMP, Sep. 2007, pp. 916.

[74] C. Maas, Elektrischer Antrieb fr supraleitend-magnetisch gelagerteSchwungrad-Energiespeicher, Ph.D. dissertation, Univ. Stuttgart,Stuttgart, Germany, 1996.

[75] R. Koch, R. Wagner, M. Sander, and H. J. Gutt, Development and test ofa 300Wh/10kW flywheel energy storage system, in Proc. Appl. Super-cond., Sep. 1999, pp. 10551058.

[76] Bladon Jets, Turbo-Molecular Pumps, May 2012. [Online]. Available:http://www.bladonjets.com/applications/turbo-molecular-pumps/

[77] Electron Energy Corporation, Jun. 2013. [Online]. Available:http://www.electronenergy.com/products/demagnetization-curves/SmCo217-16-T550C.pdf

David Gerada received the B.Eng.(Hons.) degree inelectrical engineering from the University of Malta,Msida, Malta, in 2007 and the Ph.D. degree in high-speed electrical machines from The University ofNottingham, Nottingham, U.K., in 2012.

In 2007, he joined the Research and Technol-ogy Department, Cummins Generator Technologies,Stamford, U.K., as an Electromagnetic Design En-gineer. Since 2012, he is also a Technical Leaderwith the Cummins Innovation Centre in ElectricalMachines, Nottingham, U.K. His research interests

include high-speed machines, traction machines, use of novel materials, andmultiphysics-based optimization of electrical machines.

Dr. Gerada is a Chartered Engineer in the U.K. and a member of theInstitution of Engineering and Technology.


Abdeslam (Salem) Mebarki received the IngenieurdEtat degree in electrical power engineering fromthe University of Annaba, Annaba, Algeria, and thePh.D. degree in electrical and electronic engineeringfrom the University of Bath, Bath, U.K.

From 1996 to 2000, he was with Westwind AirBearings, Ltd., where he was responsible for thedesign and development of high-speed induction andbrushless dc motors used in printed-circuit-boarddrilling spindles and laser scanning applications. In2000, he joined the Research and Technology De-

partment, Cummins Generator Technologies, Stamford, U.K., as an Electro-magnetic Design Engineer. He is currently electrical machines technical projectleader with the Research and Technology Department, Cummins GeneratorTechnologies, working on the design and development of novel electricalmotors/generators. His research interests include electrical machine design,development, and control.

Neil L. Brown received the degree in electrical andelectronic engineering from Nottingham Trent Uni-versity, Nottingham, U.K., in 1991 and the Ph.D. de-gree in electrical machines from Durham University,Durham, U.K., in 2002.

Since 1995, he has been with the Research andTechnology Department, Cummins Generator Tech-nologies, Stamford, U.K., where he is currentlythe Director of advanced electrical machines re-search and technology. He is also a Visiting Pro-fessor of electrical machines with The University of

Nottingham, Nottingham, U.K. While at Cummins Generator Technologies,he has held various roles, including Applications Engineer, ElectromagneticDesign Engineer, Research Manager, Chief Engineer for Stamford Products,and Chief Engineer Research and Technology. He is the author of over 60published papers and a holder of 11 patents.

Dr. Brown is a Chartered Engineer in the U.K. and a Fellow of the Institutionof Engineering and Technology.

Chris Gerada (M05) received the Ph.D. degreein numerical modeling of electrical machines fromThe University of Nottingham, Nottingham, U.K.,in 2005.

He subsequently worked as a Researcher withThe University of Nottingham on high-performanceelectrical drives and on the design and modeling ofelectromagnetic actuators for aerospace applications.Since 2006, he has been the Project Manager of theGE Aviation Strategic Partnership. In 2008, he wasappointed as a Lecturer in electrical machines; in

2011, as an Associate Professor; and in 2013, as a Professor at The Universityof Nottingham. His main research interests include the design and modeling ofhigh-performance electric drives and machines.

Prof. Gerada serves as an Executive Member of the Management Board ofthe U.K. Magnetic Society and the Institution of Engineering and TechnologyAerospace Technical and Professional Network. He serves as an Associate Edi-tor for the IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS. He receivedthe Royal Academy of Engineering Senior Research Fellowship supported byCummins Generator Technologies in 2011.

Andrea Cavagnino (M04SM10) was born inAsti, Italy, in 1970. He received the M.Sc. andPh.D. degrees in electrical engineering from thePolitecnico di Torino, Turin, Italy, in 1995 and 1999,respectively.

He is currently an Associate Professor with theDipartimento Energia, Politecnico di Torino. He isthe author of more than 120 papers published intechnical journals and conference proceedings. Hisresearch interests include electromagnetic design,thermal design, and energetic behavior of electric

machines.Prof. Cavagnino is currently the Chair of the Electrical Machines Committee

of the IEEE Industrial Electronics Society. He serves as an Associate Editor forthe IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS and for the IEEETRANSACTIONS ON INDUSTRY APPLICATIONS and as a Reviewer for severalIEEE TRANSACTIONS and other international journals.

Aldo Boglietti (M04SM06F12) was born inRome, Italy. He received the Laurea degree in electri-cal engineering from the Politecnico di Torino, Turin,Italy, in 1981.

He is currently a Full Professor with the Politec-nico di Torino. He is the author of more than 150papers in the field of energetic problems in electricalmachines and drives.

Prof. Boglietti was the Chair of the Electrical Ma-chines Technical Committee of the IEEE IndustrialElectronics Society. He is currently the Chair of the

Electric Machines Committee of the IEEE Industry Applications Society. Heserves as an Associate Editor for the IEEE TRANSACTIONS ON INDUSTRIALELECTRONICS and a Reviewer for several IEEE TRANSACTIONS and otherinternational journals.

/ColorImageDict > /JPEG2000ColorACSImageDict > /JPEG2000ColorImageDict > /AntiAliasGrayImages false /CropGrayImages true /GrayImageMinResolution 300 /GrayImageMinResolutionPolicy /OK /DownsampleGrayImages true /GrayImageDownsampleType /Bicubic /GrayImageResolution 300 /GrayImageDepth -1 /GrayImageMinDownsampleDepth 2 /GrayImageDownsampleThreshold 1.50000 /EncodeGrayImages true /GrayImageFilter /DCTEncode /AutoFilterGrayImages false /GrayImageAutoFilterStrategy /JPEG /GrayACSImageDict > /GrayImageDict > /JPEG2000GrayACSImageDict > /JPEG2000GrayImageDict > /AntiAliasMonoImages false /CropMonoImages true /MonoImageMinResolution 1200 /MonoImageMinResolutionPolicy /OK /DownsampleMonoImages true /MonoImageDownsampleType /Bicubic /MonoImageResolution 600 /MonoImageDepth -1 /MonoImageDownsampleThreshold 1.50000 /EncodeMonoImages true /MonoImageFilter /CCITTFaxEncode /MonoImageDict > /AllowPSXObjects false /CheckCompliance [ /None ] /PDFX1aCheck false /PDFX3Check false /PDFXCompliantPDFOnly false /PDFXNoTrimBoxError true /PDFXTrimBoxToMediaBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXSetBleedBoxToMediaBox true /PDFXBleedBoxToTrimBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXOutputIntentProfile (None) /PDFXOutputConditionIdentifier () /PDFXOutputCondition () /PDFXRegistryName () /PDFXTrapped /False

/Description > /Namespace [ (Adobe) (Common) (1.0) ] /OtherNamespaces [ > /FormElements false /GenerateStructure false /IncludeBookmarks false /IncludeHyperlinks false /IncludeInteractive false /IncludeLayers false /IncludeProfiles false /MultimediaHandling /UseObjectSettings /Namespace [ (Adobe) (CreativeSuite) (2.0) ] /PDFXOutputIntentProfileSelector /DocumentCMYK /PreserveEditing true /UntaggedCMYKHandling /LeaveUntagged /UntaggedRGBHandling /UseDocumentProfile /UseDocumentBleed false >> ]>> setdistillerparams> setpagedevice

Documents

High Speed Ele Mc