High-Speed Generator and Multilevel Converter for ... - IEEE Xplore

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Feb 10, 2012 - through a low-voltage multilevel neutral-point-clamped converter. ... permanent-magnet machines, power conversion harmonics, power.
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High-Speed Generator and Multilevel Converter for Energy Recovery in Automotive Systems Fabio Crescimbini, Member, IEEE, Alessandro Lidozzi, and Luca Solero, Member, IEEE

Abstract—This paper deals with the design solution adopted for a high-speed axial-flux permanent-magnet generator devoted to supplementing power in automotive 42 V electrical systems through a low-voltage multilevel neutral-point-clamped converter. The axial-flux permanent-magnet generator is suitably designed to be directly coupled with a radial turbo-expander which provides recovery of kinetic energy available from the exhaust gases of an internal-combustion engine. The multilevel converter is properly sized for systems having low voltage and high first-order harmonic frequency. The paper describes the proposed 4-kW power rating generating system and discusses various issues resulting from electromagnetic, thermal, and mechanical design of the high-speed axial-flux permanent-magnet generator. As well, investigations on modulation strategy, total harmonic distortion, and power losses are presented for the low-voltage neutral-point-clamped power electronic converter.

f k Fa E Sr p μ0 Vdc mf fsw f1 ΔIlp.u. Lp.u. M

Rotor disc bending. Mechanical constrain factor. Stator-rotor attractive force. Young modulus. Rotor disc cross section area. Poles number. Vacuum permeability. DC-link voltage. Frequency modulation index. Switching frequency. Fundamental harmonic frequency. Phase p.u. ripple current. Phase p.u. inductance. Amplitude modulation index.

Index Terms—AC-DC power converters, automotive applications, digital control, generators, internal-combustion engines, permanent-magnet machines, power conversion harmonics, power MOSFET, variable speed drives.

I

N OMENCLATURE Epk ke Nph Bg Ro kr Ωr Te kp ki Ji Fc mm Rm τr,max sm ρm

I. I NTRODUCTION

Peak phase EMF. Linkage flux design factor. Phase winding turns number. Air-gap flux density. Stator core outer radius. Stator core outer/inner radius ratio. Rotor speed. Electromagnetic torque. EMF shape design factor. Current shape design factor. Electric loading at stator inner radius. Centrifugal force. Permanent-magnet mass. Permanent-magnet-rotational axis average distance. Maximum radial stress. Permanent-magnet cross-section area. Permanent-magnet density.

Manuscript received December 23, 2010; revised March 18, 2011, April 26, 2011, and May 9, 2011; accepted May 16, 2011. Date of publication June 23, 2011; date of current version February 10, 2012. This work was supported primarily by the PRIN 2008 Program of the Italian Ministry for Education, University and Research (MIUR) under Award 20085BP47Z. Technical support was provided by Semikron Srl for power electronic modules and drivers. The authors are with the Mechanical and Industrial Engineering Department, University of Roma Tre, 00146 Roma, Italy (e-mail: [email protected]; [email protected]; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIE.2011.2160513

N THE LAST years, the trend in the design of automotive systems is to replace various mechanical and hydraulic components with electrical-energy-fed devices. As a result of having an increasing number of electrical components being used in automobiles for improving vehicle’s performance, comfort, convenience, as well as safety, the demand for electrical power on board vehicles is rapidly growing, and it is predicted that electrical generating systems having power rating in the range from 4 kW to 6 kW would be likely requested in the very next years to supply automobile loads such as air conditioners, electric steering systems, electric brakes, and high-energy discharge lamps. Such a new scenario is actually forcing car manufactures to explore newly conceived solutions concerning the overall electrical system being utilized on board automobiles [1], as the present generating system based on using a Lundell-type alternator would become too inefficient whenever requested to deal with higher power output. In fact, as a result of an increased electrical power requested on board the power loss in a Lundelltype alternator would be too high, and the 14 V voltage level being used in today’s cars would result in increased currents and thereby thicker wiring harnesses. As a consequence, the cost of the overall electrical system increases while the performance drops significantly. Based on the above considerations, electrical systems based on 42-V rating voltage are being widely accepted as an incoming standard for automotive applications, and various 42-V power-net architectures have been proposed since the last years [1], [2]. If a full 42-V architecture system has to be implemented in automobiles, there are many vehicle devices that would require a change of design and qualification to accept

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CRESCIMBINI et al.: HIGH-SPEED GENERATOR AND MULTILEVEL CONVERTER FOR ENERGY RECOVERY

Fig. 1.

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Architecture of automotive electrical system arranged with both turbo-expander-driven PM generator and standard Lundell-type alternator.

42-V power supply. Hence, to start with, most car makers are planning to incorporate dual-voltage architectures being suitable to supply both 42-V and 14-V loads. Among the many ways for implementing a dual-voltage architecture, one approach relies on both retaining the existing Lundell-type alternator to supply 14-V loads and including a voltage boost converter to supply a limited number of 42-V loads from the existing 14-V battery storage. However, such an approach does not solve problems arising from the increased power demand and to date a more widely accepted 42-V/14-V architecture is arranged by means of a 42-V alternator that feeds both 42-V loads and battery storage, and it supplies a dc/dc step-down converter devoted to provide power to the conventional 14-V loads and battery storage. To date, automotive electrical systems likely draw the power required to drive the alternator from the mechanical shaft of the internal-combustion engine (ICE) and thereby the increased electrical power demand inevitably results in an increased fuel consumption. However, from recognizing that, the ICE exhaust gases still retain a significant amount of energy being usually wasted, substantial fuel saving can be achieved by using a radial turbo-expander which can provide recovery of the kinetic energy available from the ICE exhaust gases to directly drive the electrical generator [3]. Based on such a novel approach, this paper introduces a dual-voltage automotive electrical system which includes a turbo-expander driven alternator and a low voltage multilevel converter to provide the power requested by 42-V loads and battery storage. As a radial turbo-expander would likely be designed with rated speed in the range from 15 000 rpm to 30 000 rpm, a high-speed electrical generator design is sought, and in consideration of that, this paper describes the design solutions adopted to achieve the required compactness and efficiency. Direct-drive coupling is envisaged in order to take advantage from avoiding the use of a gearbox along the transmission path, which greatly affect system’s costs, reliability, and efficiency at the aforementioned high rotational speeds. However, removal of the gearbox requires high electric frequencies to be used in the electric machines, thus requiring for high fundamental frequency power electronic interface. To this purpose, the paper discusses the characteristics of the proposed low-voltage neutral-point-clamped (LV-NPC) power electronic converter.

II. D UAL -VOLTAGE AUTOMOTIVE S YSTEM To the aim of achieving a substantial reduction of the fuel consumption in automobiles, the dual-voltage automotive system depicted in Fig. 1 combines the present 14-V system based on the Lundell-type alternator with a novel 42-V generating system in which the radial turbo-expander recovers energy from the ICE exhaust gases to directly drive the electrical generator and supply the 42 V power-net through the three-phase ac-dc multilevel switching converter. The turbo-expander is a radial turbine where the component of the gas speed is perpendicular to the rotation axis. Hence, in the volute of the radial turbine, the exhaust-gas pressure is being converted into kinetic energy to move the turbine wheel, so making available mechanical power onto the alternator shaft. In the envisaged system configuration, the turbo-expander would have rating speed of 18.000 rpm and would provide output power of about 4 kW. In consideration of the huge improvements achieved in permanent-magnet (PM) materials in terms of both technical characteristics—such as high energy density and high operational temperature—and manufacturing costs, PM machines are being widely recognized to be eligible for use in 42-V automotive systems. Within the broad category of PM machines, several distinct machine arrangements can be identified, and these include the axial-flux PM (AFPM) machine topology which in the last years has drawn substantial interest concerning various applications [4]–[7]. Hence, for the turboexpander driven alternator being used in the 42-V generating system depicted in Fig. 1, the AFPM machine arrangement would be likely selected from the recognition of unique features such as high compactness and improved efficiency [8]. In fact, compactness would be achieved as result of a wider airgap surface area being made available for the electromagnetic interactions and the improved efficiency results from a winding arrangement having substantially reduced length of the end-windings. Further to that, in order to deal with the high centrifugal force being acting on the rotor magnets in highspeed machine applications [9], the AFPM machine topology allows a PM rotor arrangement that does not actually penalize the machine electromagnetic performance, whereas a much sophisticated mechanical arrangement—including either carbonfiber bandage or encapsulation in non-magnetic stainless steel

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material—that increases the magnetic air gap is needed for the PM rotor of the conventional radial-flux machines. The selection of a high value for the fundamental frequency and the low inductance of the AFPM machine ask for a suitable choice of the power electronic converter topology, which is used as power conversion interface between the turbo-expanderdriven electrical machine and the dc link. Standard configurations, such as three-legs switching inverters/rectifiers, require switching frequencies fairly higher than fundamental frequency to assure enough sinusoidal phase currents with low harmonic distortion. A multilevel configuration can be proposed for the power electronic converter in order to limit the electric motor current ripple to an acceptable value and achieve low values for the THD, which are essential requirements for high efficiency, low mechanical vibrations and acoustic noise. In fact, voltage and current harmonics in rotating machinery, as permanentmagnet synchronous machine (PMSM), increase heating due to iron and copper losses at the harmonic frequencies. The harmonic components affect the motor efficiency as well the torque developed and can produce higher audible noise emission when compared with a pure sinusoidal excitation. Furthermore, in a turbine-generator coupling, the low-frequency harmonics, such as the fifth and seventh, can create mechanical oscillations [10]. As a consequence, in these applications, particularly when a high fundamental frequency is required, the goal is to eliminate the odd non-triplen lower order harmonics (5th, 7th, 11th, 13th, etc), and then filter the remaining higher order harmonics. Hence, the THD is an evaluating key parameter, and it is defined as  ∞  Q2hrms %THDi,V =

h=2

Q1rms

× 100.

Fig. 2.

Cross-sectional view of AFPM machine electromagnetic structure.

In consideration of the relatively high rotational speed being used, an AFPM generator having a slot-less winding arrangement [4]–[8] was selected in order to avoid cogging torque and acoustic noise, as well as to eliminate the power loss in both the stator teeth and the solid-rotor structure otherwise resulting from the use of stator slots due to high-frequency flux pulsations. Further to that, as the core is made of conventional silicon-grade iron strip, the slot-less arrangement avoids punching, and the core manufacturing process does not actually expose the magnetic material to significant mechanical stresses. A coreless design of the machine winding [12], [13] was also considered but discarded due to a much higher mass of PM material usually required.

(1)

Q1rms is the root mean square of fundamental component (either current or voltage), and Qhrms is the amplitude of the “h” order harmonic component. The THD reduction is a key issue, because it influences the efficiency of the whole system. In the generating system shown in Fig. 1, the three-phase AC/DC multilevel switching power converter is used to regulate the alternator power output at the terminals of a 42-V battery fed dc link. The power converter has a three-level neutral-pointclamp configuration arrangement in order to achieve improved alternator current waveforms and low torque ripple thereby. Further to that, the weight and size of the converter passive components are substantially reduced with respect to more common two-level converter topologies to allow a compact converter package [11]. III. AFPM G ENERATOR D ESIGN For the particular 42-V automotive system application being considered in this paper, the generator design specifications included dimensional constraints in terms of overall outer diameter lower than 200 mm and overall axial length lower than 120 mm. The alternator machine is designed to have 18 000 rpm rating speed and to provide power output of about 4 kW with efficiency better than 95%.

A. Machine Fundamentals Fundamentals of the slot-less AFPM machine topology are hereafter recalled briefly, as detailed discussion on the machine operational characteristics and design methods can be found in recent literature [14]–[18]. The machine basic layout includes a single slot-less toroidal stator being positioned between two PM rotor discs. The machine electromagnetic structure effectively comprises two independent halves, laying either side of the radial centerline. A cross-sectional view of one half of the machine is shown in Fig. 2 together with the design relevant dimensions such as iron core thickness, f , magnet thickness, m, winding thickness, w, and stator-rotor clearance, c. All these dimensions determine the active axial length, l, of the machine. The outside radius of the stator core, Ro , is the important dimension which primarily determines the machine rating torque. Either the inside radius of the stator core, Ri , or the ratio kr = Ri /Ro , is the design parameter which should be used to achieve optimum proportions with respect to given design targets. Selection of both the magnet material type and machine number of poles completes the set of variables that characterize the design of a slot-less AFPM machine. The remaining dimensions together with the electrical parameters follow from this set of design variables.

CRESCIMBINI et al.: HIGH-SPEED GENERATOR AND MULTILEVEL CONVERTER FOR ENERGY RECOVERY

In AFPM machines, the peak phase EMF, Epk , and the electromagnetic torque, Te , can be expressed as follows:   Epk = ke · Nph · Bg · Ro2 · 1 − kr2 · Ωr (2)   (3) Te = ke · kp · ki · 2 · π · Ji · Bg · Ro3 · kr 1 − kr2 where Bg is the air-gap flux density, Nph is the number of the turns of each phase, and Ωr is the rotor speed. The design factor ke is introduced in order to take into account the reduction of the linkage flux due to boundary effects at the magnet edges, and the value of such a design factor is to be obtained either through a finite-element study of flux-density distribution at the machine air gaps or through design experience. For most AFPM machine designs, a suitable value for ke is generally found in the range 0.80–0.88. On the other hand, both the design factors ki and kp used in (3) would be determined from the actual shape of both the machine phase EMF and current waveforms. In (3), the effective mean electric loading at the stator inner radius Ji should be selected by taking into account both the overall power loss produced in the machine stator when operated at rating condition and the capability of the adopted cooling system to remove heating and thereby keep the steadystate temperature of both winding and magnets within specified limits. Disregarding the mechanical losses due to both friction in the bearings and windage effect caused by the rotating discs with surface-mounted magnets, in AFPM machines, the electrical power losses include power loss in the stator magnetic core as well I 2 R and eddy-current power loss in the winding. In high-speed AFPM machines, both eddy-current loss in stator winding and iron loss in the stator core are the predominant component of the machine power losses, and a substantial reduction of the power loss in the winding can be achieved by using Litz-wire conductors. B. High-Speed Generator Design In order to develop the machine electromagnetic design and achieve prediction of machine performance, a computer program which embodies electric, magnetic, and thermal models was used. The possible machine configurations are manifold, and in order to obtain an acceptable number of design solutions, some design parameters were fixed according with specifications. Hence, the comparison was early developed by assuming for the stator core inner diameter values in the range from 70 mm to 90 mm, as lower values would be unsuitable for machine manufacturing. On the other hand, the stator core outer diameter was let to vary up to a maximum value of 150 mm in order to both deal with specification of having machine outer diameter no greater than 200 mm and achieve an acceptable value of the centrifugal force on the magnets. Moreover, for design comparison an ultimate value of overall power loss equal to 200 W was imposed in order to achieve both about 95% efficiency and winding temperature lower than 140 ◦ C. Concerning to that, it was assumed that removal of the machine heat would be accomplished through air ventilation of the machine casing and that the air temperature in the vehicle hood would likely be 80 ◦ C.

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TABLE I AFPM G ENERATOR D ESIGN S PECIFICATIONS

TABLE II F OUR -P OLE AND S IX -P OLE M ACHINE G EOMETRIC D IMENSION

In consideration of the relatively high rating speed of the turbo-expander directly driven generator, the comparison among various design solutions was restricted to considering only either four-pole or six-pole machine configurations in order to limit the generator operating frequency. Such a design assumption comes from the desire of arranging the machine strip-iron toroidal core with commercial Fe–Si having specific power loss of 2 W/kg at 50 Hz, so that relatively low manufacturing costs would be envisaged. On the other hand, use of Fe–Si materials having both higher cost and lower specific power loss would allow an increase of the generator operating frequency and, as a consequence, a reduction of the machine axial length. However, such an apparent benefit might not be fully exploited as machine rotor discs having too low thickness would not provide the mechanical robustness required for dealing with the attraction force being exerted between PM discs and machine stator. Table I details the design goals for the investigated AFPM generator. Concerning the selected four-pole and six-pole machine configurations, Table II summarizes the geometric dimensions of the machine stator core that were taken under consideration for developing a first-tentative design of the machine parts based on generator design specifications. After having determined the dimensions of all the machine active parts as well as the power loss resulting in both iron core and winding, a comparison among the design solutions meeting the specifications was carried out by considering the axial length (Fig. 3) and the weight (Fig. 4) of the generator active parts. As expected, it is found that, in both four-pole and six-pole machine configurations, the machine axial length grows up with the increase of the stator core outer diameter as a greater area becomes available for the magnets and, for a given air-gap flux density, a thicker core is required. Thereby, the weight of the machine active parts increases accordingly. Of course, the six-pole machine configurations provide reduced both axial length and weight whenever compared with their four-pole configuration counterpart having the same core outer diameter. However, as shown in Fig. 5, at the generator rating condition, the six-pole machine configurations, due to the higher operating

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TABLE III D ESIGN C HARACTERISTICS OF F OUR -P OLE AFPM G ENERATOR

Fig. 3. Axial length of machine active parts versus stator core outer diameter.

Fig. 4. Weight of machine active parts versus the stator core outer diameter.

to difficulties of machine manufacturing, outer diameter lower than 110 mm, as well inner diameter lower than 70 mm in order to comply with the kr ratio value for machine optimum proportions, would hardly meet the desired performances for the proposed high-speed low voltage generator. Based on the above considerations, the four-pole machine configuration having stator core with 70-mm inner diameter and 110-mm outer diameter was selected for further design investigations and Table III summarizes the overall design characteristics of the selected generator arrangement. IV. AFPM G ENERATOR FEM A NALYSIS A. FE Analysis of Magnetic Circuit

Fig. 5. Power loss in the iron core and power loss in the winding versus the stator core outer diameter for four-pole and six-pole machine configurations.

frequency, have substantially higher power loss in the iron core compared with the power loss in the Litz-wire winding. Such a machine performance is deemed to be not suitable for a generating system that might be required to operate with partial load conditions depending on the turbo-expander input torque. Investigation on the four-pole machine configurations results in increase of active parts both weight and axial length versus outer diameter augment; as a consequence, also iron loss becomes higher and, even if the power loss in the winding is reduced, active parts total loss is increased. On the other side, reduction of the stator core outer diameter makes more difficult the heat exchange and the cooling process. In addition, because a smaller area becomes available for the magnets, it produces a lower emf per turn and requires higher rms current to comply with design specifications. Further to that, turns number must be an integer, and winding cross-sectional area must follows wire sizes available on the market. As a result, in addition

By using a commercially available software package, finiteelement (FE) analysis of the machine magnetic circuit was performed in order to verify that the value of air-gap flux density early used to develop a first-tentative machine design fits FE analysis results. Further to that, FE analysis of the machine magnetic circuit allows a closer investigation on the effective distribution of the flux density in both rotor discs and stator core in order to evaluate magnetic stress leading to material saturation. To this goal, a 2-D model of the AFPM generator was considered by taking into account the inherent both axial and circumferential symmetry of the machine. Hence, the FE analysis was carried out by considering only a slice of the machine magnetic circuit being taken at the mean radius of the generator geometry and restricted to a pair of poles. Fig. 6 shows both flux-density distribution and flux path resulting from the axial magnetization of Nd–Fe–B magnets having remanence of 1.2 T. As it should be expected, simulation results show that a leakage flux does exist between contiguous magnets, and that the magnetically most stressed area is found in the rotor disc at the inter-pole axis.

CRESCIMBINI et al.: HIGH-SPEED GENERATOR AND MULTILEVEL CONVERTER FOR ENERGY RECOVERY

Fig. 6.

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Flux-density distribution and flux path in the magnetic circuit. Fig. 8. Distribution of the air flow speed (indicated by an arrow field) on the generator casing.

Fig. 7.

Flux-density distribution at the machine air gap.

As shown in Fig. 7, FE simulation results show that the airgap flux density is close to 0.6 T, as early assumed for firsttentative design. Further to that, for the flux density in stator core, a peak value of 1.58 T is found, being this value an acceptable compromise in consideration of the relatively highspeed machine application. Therefore, results of the FE analysis were deemed to substantially validate the predictions achieved from the earlier design. B. FE Analysis of Thermal Behavior For automotive applications of PM machines, the machine cooling is one critical aspect to deal with, as the operating temperature of the magnets significantly influences the machine electromagnetic performance. In order to provide effective heat removal in the AFPM generator being the subject matter of this paper, an aluminum ring having cooling fins would be used to both serve as machine casing and remove heating due to power loss in the machine stator. To this goal, the stator active parts comprising both winding and toroidal core would be both completely encapsulated and joined with the aluminum casing by means of an epoxy resin which provides a high thermal

conductivity path for heat removal. Due to the high rotational speed, further contribution to the machine cooling should be expected from the rotor discs having surface-mounted magnets which naturally act as fans in sustaining air streams in the radial direction within the machine air gaps. A steady-state analysis of the machine thermal behavior was carried out by means of FE 3-D model which takes into account the machine circumferential symmetry. Hence, the toroidal stator was split into 12 slices, being each one including both a single coil of the core winding and the related portions of the PM rotor discs. Thermal loads were applied in both the stator core and the winding coil according with a predicted overall power loss of 145 W. For heat removal from the machine casing, it was assumed that air available in the vehicle hood at temperature of 80 ◦ C would be used to provide an air flow of 8 m/s. In order to simulate the distribution of the air flow speed on the machine casing, a weakly compressible fluid was modeled by NavierStokes equations, as shown in Fig. 8. FE simulation of the machine thermal behavior under the generator rating condition resulted in steady-state distribution of temperature within the machine as shown in Fig. 9. Simulation results indicate that generator operation with input power of about 4 kW at 18 000 r/min would be achieved with hot spot in the stator core having a temperature of about 120 ◦ C. In such a steady-state thermal condition, the winding temperature would be 115 ◦ C, whereas the magnets would operate with a temperature of about 95 ◦ C. These results are deemed to be completely satisfactory leaving a large enough temperature margin for safe operation of the machine magnets. C. Mechanical Analysis In the rotor discs of AFPM machines, the magnets are usually bonded with a bi-component glue being distributed on the overall magnet-disc contact area and having a thickness of about 0.1 mm. The bi-component glue has suitable characteristics to keep the magnets secured in the right position for the expected working temperature distribution of Fig. 9. However, in case

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Fig. 10. ERGAL ring used to withstand the centrifugal force on the rotor disc surface-mounted magnets. Fig. 9. Steady-state temperature distribution within the machine resulting from nominal load operation with vehicle hood temperature of 80 ◦ C.

of high rotational speeds, the centrifugal force being acting on the magnets would cause a high radial stress component which would harm the glue and might force the magnet to come loose from the rotor disc in radial direction [19]. In the envisaged AFPM generator arrangement, both the centrifugal force (Fc ) and the maximum radial stress being acting on the glue (τr,max ) at the machine rating speed can be calculated by the following: Fc = mm · Ω2r · Rm

(4)

τr,max = sm · ρm · Ω2r · Rm

(5)

where mm is the magnet mass, sm is the surface area of the magnet, ρm is the magnet density, Ωr is the rotor speed, and Rm is the average distance of the magnet from the rotational axis. By using (4) and (5), centrifugal force of about 13 kN is found, whereas the glue would have stress of about 10 MPa. In order to withstand the centrifugal force acting on the magnets and thereby avoid significant stress on the magnet glue, an ERGAL (i.e., Aluminium70-75T6) ring having 3-mm thickness would be fixed on the rotor discs to embody the magnets, as schematically shown in the Fig. 10. In order to evaluate the mechanical strength of the envisaged arrangement, a FE mechanical stress analysis was developed by considering the centrifugal force being a distributed load on the internal surface of the ERGAL ring, whereas the glue contribution was neglected to ameliorate safety. Fig. 11 shows analysis results in terms of both distribution of Von Mises stress and resulting ring deformation. A maximum stress of about 280 MPa is being focused in the contact edge between the ring and the rotor disc. Such a stress value is far below of the tensile strength of the selected material. A maximum deformation of about of 0.1 mm, that actually does not pose concerns, is found at the ring edge being in contact with the magnet outer diameter surface. Further than the centrifugal force being acting on the magnets, attractive force being exerted between each PM rotor disc and the machine stator would cause bending of the rotor discs toward the stator, so that the mechanical air gap would be

Fig. 11. FE mechanical analysis of the ERGAL ring used to withstand the centrifugal force on the rotor magnets: (a) distribution of the Von Mises stress and (b) the resulting shape deformation.

CRESCIMBINI et al.: HIGH-SPEED GENERATOR AND MULTILEVEL CONVERTER FOR ENERGY RECOVERY

somewhat reduced. A first-tentative estimation of the rotor disc bending resulting from rotor-stator attraction can be attempted by the following expression f=

k · Fa · Ro2 E · Sr3

(6)

where Sr is the cross section of the rotor disc, E is the Young modulus, Fa is the attractive force between magnets and stator, and k is a coefficient which takes into account the particular mechanical constrain resulting from fixing the rotor disc to the rotor hub. The attractive force between the PM rotor disc and the stator can be evaluated as follows Fa =

p · sm · Bg2 μ0

(7)

where p is the poles number and μ0 is the vacuum permeability. For the AFPM generator being herewith discussed, the use of both (6) and (7) gives a bending deformation of about 3 m. Actually, further investigation on this matter was carried out by means of a FE mechanical stress simulation that considered the rotor-stator attraction force being fully applied at the rotor outer diameter. By such FE analysis, a bending deformation of about 5 m was obtained as the holes used in the rotor disc for the machine cooling result in a decreased flexural rigidity. However, disregarding the calculation method being used, it clearly appears that in the envisaged AFPM generator arrangement, the rotor disc bending deformation due to rotor-stator attraction is completely negligible compared with the selected 2-mm mechanical air gap. Such an observation indicates that rotor discs having lower thickness could eventually be arranged, and this may lead to further investigation of machine configurations having slightly higher number of poles, provided that a stator core magnetic material having suitably low specific power loss is used. V. LV-NPC P OWER E LECTRONIC C ONVERTER In the proposed application of energy recovering in automotive systems, the power converter is conceived to interface a PMSM, having a rated fundamental frequency of 600 Hz up to 1 kHz as future development, to a single 42-V dc link. As a consequence, it can be considered a low-voltage-fed and highfrequency application. By using a NPC converter, the Vdc link can be split into two equal sources having Vdc /2, which allows to use lower voltage rating and smaller semiconductor devices, such as MOSFETs. This solution is very attractive since it could bring to an increase of switching frequency. Furthermore, the NPC converter properties [20], [21] allow improving current waveform and reducing torque ripple with respect to two-level solution, so a significant reduction of weight and size can be achieved also in passive components. In the proposed application, due to the very low inductance of the PMSM, in the order of some μH, which is a valuable characteristic for achieving low voltage drops in generation units, the goal is to minimize the current ripple which is responsible for deteriorating the whole system efficiency, because of the significant torque ripple. In fact, AFPM machines with slotless

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winding arrangement show a p.u. inductance (Lp.u. ) usually lower than 0.1; for a conventional two-level three-phase acdc power electronic converter, the following relationship can be found between the p.u. phase current ripple (ΔI1p.u. ) and the Lp.u.

ΔIlp.u.

√ 4π · 2 (1 − M ) · M √ · = mf · Lp.u. 3

(8)

where M = V11pk /Vdc is the amplitude modulation index, defined as the ratio between the peak line-to-line ac voltage and the dc link voltage, whereas mf = fsw /f1 is the frequency modulation index, defined as the ratio between the switching frequency and the ac fundamental harmonic frequency. Considering 15 kHz as fsw , current ripple of the same magnitude of the rated rms phase current is quite common in slotless AFPM generator with characteristics as in the investigated application. It can be roughly calculated that such amplitude for the current ripple is responsible for almost 30% increase in power loss, and consequent efficiency reduction of at least 1.5% at rated power, both in the AFPM generator and in the power electronic converter. Use of NPC configuration as power electronic converter reduces current ripple nearly at 1/2; further, keeping fixed the switching losses, NPC converters can operate with double switching frequency with respect to conventional two-level acdc switching topologies. As a consequence of the above considerations, the phase current ripple results reduced at roughly 1/4 and the power loss increases in the order of 2% with respect to negligible current ripple condition, insignificantly affecting efficiency of both AFPM generator and power electronic converter. Additional value is the reduced dynamic stress and thermal hot spots in power electronic devices junctions, which are direct outcome of current ripple amplitude at switching frequency. The carried out approximate cost analysis shows that the proposed LV-NPC configuration has no significant economic drawbacks when compared to conventional twolevel converter, which requires significant devices oversize to withstand to transients, related to the current ripple, as well to thermal stress and to assure enough reliability. As proved in literature, the phase disposition (PD), compared with the others multicarrier PWM, allows achieving better THD and reduces switching losses, particularly for diode-clamped topology. As well, it achieves superior line-to-line harmonic performances [22], [23]. For these reasons, the phase disposition PWM (PD-PWM) has been chosen as modulation technique to control the investigated multilevel converter, whereas the benchmark two-level converter is modulated with conventional PWM. Multilevel MOSFET power converter solution allows to increase switching frequency so that frequency modulation index can be increased. In this case, the lower frequency harmonics can be shifted to higher frequency region. As voltage and current harmonics in rotating machinery significantly reduce system efficiency, increasing of the order of the lower harmonics can be advantageous, as a consequence, the amplitude of their components is reduced.

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Fig. 12. Assembled 3-phase LV-NPC converter.

However, a key problem with diode-clamped topology is to control the neutral-point voltage at one half of the dc-link voltage. In fact, under certain operating conditions [24], a low-frequency voltage oscillation appears in the neutral point. Therefore, current is drawn from neutral point, causing one dc link capacitor to be charged, while the other is discharged. Several methods to balance the neutral-point voltage have been proposed in recent literature [25]–[33]. In the proposed application, an improved version of the neutral-point control proposed in [33] has been chosen. The basic procedure is to measure the error between the capacitor voltages and use a simple controller to calculate an offset to be added to each of the PWM modulation waves, which modifies the neutral-point current and produce a charge balancing on the capacitor. A prototypal model of the NPC converter, being mounted on a power printed circuit board (PCB), has been designed. The phase-leg has been achieved by using devices from Semikron. The three phase-legs have been achieved by using devices from Semikron. Each NPC phase-leg has been realized through a power module, which can be used for low-medium power applications. Switch current is rated 150 A (Tj = 25 ◦ C) and the drain-source breakdown voltage, with gate-source shortcircuited, is 55 V. To drive the four semiconductors, two SKHI22AH4 semidrivers have been used. Drivers can achieve switching frequency up to about 100 kHz, and all the devices necessary for driving, voltage supplying, error monitoring, deadtime generation, and isolation between I/O signals are integrated in the driver itself. The designed 1Phase-3Levels PCB includes all the devices required to interface the phase-leg converter to a control platform, as voltage and current transducers, voltage translator for PWM signals, connectors, and the dc/dc converters for the necessary voltage supply. The power module has been mounted on the heat sink and has been placed under the PCB, while the two drivers have been mounted on the PCB top layer. The dc-link, composed by four capacitors, is located on a separate PCB, and is mounted on the top of NPC converter phaseleg prototype. The three-phase NPC converter is achieved by assembling three 1Phase-3levels PCB, as shown in Fig. 12. The investigated LV-NPC converter is controlled by a DSP (Digital Signal Processor) platform. The digital control platform has been designed for general purpose applications. It includes the 32-bit Texas Instruments TMS320F28335 digital processor, as well the necessary devices to access DSP utilities.

Fig. 13.

Block diagram of the laboratory test setup.

An important feature of this DSP is the enhanced pulse width modulator module that can generate 12 PWM signals, which can be easily used to control a three-phase NPC converter or even two conventional converters at the same time. The DSP has a 16 channels 12 bit ADC module that allows the acquisition of 16 measures. Eight of these channels are already equipped with second-order active Butterworth filters, placed on to the PCB, while the other eight channels are available on a connector and can be easily used through a suitable expansion PCB. Furthermore, the control platform includes two isolated controller area network interfaces, a resolver to digital converter which is used to acquire the AFPM generator speed/position measurement, and a digital to analog converter. The LV-NPC converter prototype has been used in order to carry out a preliminary experimental investigation. Due to the fact that the previously designed high fundamental frequency electric machine is not yet available in our laboratory, first experimental tests have been carried out by using the NPC converter prototype in three-phase inverter mode of operation with a RL load (R = 0.75 Ω and L = 15 μH). The chosen load allows to produce similar operative conditions of the expected final electric drive for automotive energy recovery at partial load. Experimental tests have been performed in order to evaluate primarily waveforms and harmonic contents. In fact, as previously mentioned, the very low inductance of the AFPM generators, which is a valuable characteristic for achieving low voltage drops in generation units, requires the minimization of the current distortion, which is responsible for deteriorating the whole system efficiency. Testing has been achieved with amplitude modulation index of 0.9, 1 kHz as fundamental frequency, and 15 kHz as switching frequency, which are congruent with the expected values for the final electric drive prototype. Total harmonic distortion evaluations have been achieved by using the Voltech PM3000A power analyzer, which allows to evaluate, according to (1), THD until 99th harmonic. Waveforms have been recorded by means of the Yokogawa DL1540 digital oscilloscope. Fig. 13 shows the block diagram of the laboratory test setup, whereas in Fig. 14, the LV-NPC converter prototype

CRESCIMBINI et al.: HIGH-SPEED GENERATOR AND MULTILEVEL CONVERTER FOR ENERGY RECOVERY

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and harmonic contents have been compared with the LV-NPC converter results. It has been found that, the two-level converter voltage THD is definitely worse than in NPC case. In fact, it results almost doubled. Further, also current THD is rather higher than in experimental LV-NPC configuration. As a consequence, in case of two-level configuration, a definitely higher torque ripple would deteriorate the whole system performances. VI. C ONCLUSION

Fig. 14. LV-NPC converter experimental voltage (top 20 V/div) and current (bottom 10 A/div) waveforms.

Concerning a novel 42-V generating system which uses a radial turbo-expander to recover energy from the ICE exhaust gases and directly drive an electrical generator, this paper discussed various issues related to the design of an AFPM generator having rating power of about 4 kW at 18 000 r/min nominal speed. For such an application, a four-pole machine design was selected to meet specifications such as overall power loss lower than 200 W and machine outer diameter lower than 200 mm. Simulations based on suitable FE models validated the selected electromagnetic design and proved that effective heat removal from the machine stator can be arranged by means of air cooling of the generator casing. Mechanical aspects concerning the forces acting on the rotor disc magnets were also analyzed to find out a suitable mechanical arrangement of the machine rotor discs. The paper investigated also the proposed configuration of LV-NPC MOSFET converter in order to comply with the high first-order harmonic frequency of the direct-driven electric generator. Preliminary experimental results assessed the superior performance in terms of harmonic content of the LV-NPC topology with respect to three-phase two level MOSFET converter, thus improving electrical machine torque ripple reduction. The results described in this paper will be followed in order to develop a full-size machine prototype and validate the design methodology with experimental results. As well, the final electric drive prototype is intended to be mounted on board an actual ICE for recovering exhaust gases energy. R EFERENCES

Fig. 15. LV-NPC experimental line-to-line voltage THD (a) and current THD (b).

measured voltage and current waveforms are depicted. It can be noted that experimental voltage waveform shows some distortion [34], due to the presence of the 3.25 μs dead time imposed by the semidrivers. Fig. 15 shows the LV-NPC converter prototype experimental voltage and current THD, for the aforementioned operating conditions. In order to evaluate the achieved results, a conventional twolevel MOSFET converter has been also tested for the same operating conditions and achieved experimental waveforms

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Fabio Crescimbini (M’90) received the degree in electrical engineering and the Ph.D. degree from the University of Rome “La Sapienza,” Rome, Italy, in 1982 and 1987, respectively. From 1989 to 1998, he was with the Department of Electrical Engineering, University of Rome “La Sapienza,” being there in charge as Director of the Electrical Machines and Drives Laboratory. In 1998, he joined the University Roma Tre, Department of Mechanical and Industrial Engineering, where he currently is Full Professor of electrical machines and drives. Since 2006, he serves as Head of Department. His research interests include newly conceived permanent-magnet machines and power converter topologies for unconventional applications such as electric vehicle motor drives and renewable energy generating systems. Prof. Crescimbini is an active member of the IEEE. From 2001 to 2004, he served as a member of the Executive Board of the IEEE Industry Applications Society (IAS). In the 2000, he served as Co-chairman of the IEEE-IAS “World Conference on Industrial Applications of Electric Energy,” and in 2010, he served as Co-chairman of the 2010 International Conference on Electrical Machines. He was a recipient of IEEE-IAS Electrical Machines Committee awards, such as Third Prize Paper in the 2000 and First Prize Paper in the 2004.

Alessandro Lidozzi received the Electronic Engineering and Ph.D. degrees from the University Roma Tre, Rome, Italy, in 2003 and 2007, respectively. During 2005–2006, he was Visiting Scholar at the Center for Power Electronics Systems, Virginia Polytechnic Institute and State University, Blacksburg. From 2007 to 2010, he worked at University Roma Tre as a Laboratory Technician. Since 2010, he has been Assistant Professor with the Department of Mechanical and Industrial Engineering, University Roma Tre. His research interests are mainly focused in multi-converter based applications, dc-dc power converters modeling and control, control of permanent-magnet motor drives, and control aspects for power electronics in diesel-electric units. Dr. Lidozzi was a recipient of Student Award and Travel Grant at ISIE 2004—International Symposium on Industrial Electronics.

Luca Solero (M’98) received the Electrical Engineering degree from the University of Rome “La Sapienza,” Rome, Italy, in 1994. Since 1996, he has been with the Department of Mechanical and Industrial Engineering, University Roma Tre, Rome, Italy, where he currently is an Associate Professor in charge of teaching courses in the fields of Power Electronics and Industrial Electric Applications. During 2002, he was a Visiting Scholar with the Center for Power Electronics Systems, Virginia Polytechnic Institute and University, Blacksburg. He has authored or coauthored more than 100 technical published papers. His current research interests include power electronic applications to electric and hybrid vehicles as well to distributed power and renewable energy generation units. Prof. Solero is a member of the IEEE Power Electronics and IEEE Industrial Electronics Societies.