High Integrated Electric Machine for Aircraft Autonomous Taxiing

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High Itegrated Electric achie for Aircraft Autoomous Taxiig ichael Schier, Frak iderkecht, Adreas Brier, Heribert Hellster Istitute of Vehicle Cocepts, Germa Aerospace Ceter affewaldrig 38-40,

1 High Itegrated Electric achie for Aircraft Autoomous Taxiig ichael Schier, Frak iderkecht, Adreas Brier, Heribert Hellster Istitute of Vehicle Cocepts, Germa Aerospace Ceter affewaldrig 38-40, D Stuttgart, Germay Copyright 2011 C2D & ITI Abstract: Amog the geeral effort to reduce CO 2 pollutio of the atmosphere there is also the questio of whether it is possible to tax a passeger aircraft without emissios ad self-sufficiet o the taxiig field. A positive side effect is the reductio of the groud-based trasport vehicles traffic, especially the tractors, ad thus a reductio i the risk of accidets. The Germa Cetre for Aerospace has dealt with the questio ad the Istitute of Vehicle Cocepts has desiged ad built a prototype of a electric ose wheel drive for a commercial aircraft Airbus A320. The project was part of the overall research project for the itegratio of fuel cells i aircraft. The paper describes the boudary coditios, the requiremets, the desig of the electrical machie, the gear ad the test result. Keywords: aircraft applicatio, autoomous taxiig, electrical drive, permaet maget machies, sychroous machies, brushless DC-motors, plaetary gear. 1. Itroductio Passeger aircrafts are rollig from the termial to the start positio, ofte they are pushed back from the termial with a tractor by usig a towbar. After ladig, they roll back to the termial. All taxiig operatio is doe with ruig egies while they are ot very efficiet. A electric drive could save fuel, could reduce the groud-based traffic ud could make a autoomous taxig ad push back operatio possible. A critical questio still remais i the miimum towig forces, where, however, for the maximum allowable forces eough experieces are available. Based o the results of towig force measuremets a electric drive could be developed. Accordig to the reflectios i the automotive brache fuel cells ca cotribute to future eergy supply i aircraft applicatios. I this project a fuel cell system provides the eergy supply for the drive. It is positioed i the rear cargo area of the aircraft. The electric machie is operated with a iverter from the automotive sector. 2. Boudary Coditios The speed of a passeger aircraft Airbus A320 is at about 25 km/h ad the weight of the aircraft is up to 80 t. With a give rollig frictio coefficiet ad potetial slopes of the taxiig field there results a required torque i the order of 5 to 10 knm ad a wheel speed of 170 1/mi. The mai ladig gear with the brakes iside of the wheels provides ot eough space for a electric drive. Therefore, the istallatio space withi the rim of the ose ladig gear is used. A wheel of the ose ladig gear usually is mouted o a rotable cylidrical sleeve o the axle which supports the rapid acceleratio durig ladig ad which works as a redudacy i a possible bearig failure. Uder the coditio of makig o desig chages to the chassis ad the ladig gear, these sleeves serve as a iterface for the drive. So the drive

8 knm is required, for break away operatio theoretically four times of this value. To cosolidate the assumptios towig force measuremets were carried out o several plaes A319 at the airport.

2 has to be desiged for moutig o this sleeve. The load torque must be liked o the strut of the ladig gear with a clamp. 3. equiremets For the desig of the drive a weight of 50 tos, a ose wheel load of 5 tos ad a maximum speed of 25 km/h were assumed. For the taxiig operatio the oly a torque of 2.8 knm is required, for break away operatio theoretically four times of this value. To cosolidate the assumptios towig force measuremets were carried out o several plaes A319 at the airport. The forces were measured with a load cell that was built ito the towbar, see fig 1. Figure 1: Towig force measuremet with a load cell mouted i a towbar The measuremets cofirmed that a aircraft A319 ca be maitaied o a plae surface with a towig force of 6 kn (fig. 2). I the figure the towig force at three break away experimets, plotted agaist time is show. The difficulty i measuremet is for the tractor to start as getly as possible to keep the acceleratio force small. The sigificat egative force peaks are clear brake cotrols so that it is easier to see the followig break away forces. egardig the radius of the wheel the equivalet of a torque of about 6 * m = 2.25 knm results. The values are relatively small i compariso to the first assumptios ad thus it is see that the requiremets are o the safe side. Figure 2: Towig force measuremet of a aircraft Airbus A Desig of the electrical machie First, it is importat to kow whether a electrical machie ca produce the required torque i the existig space withi the wheel. The limited space iside of the origial wheel does ot allow the itegratio of a electrical machie, judgig from liquid-cooled rotary machie specific forces of about 4 N/cm 2. Therefore, it is ecessary to use a gear that is a itegral part of the electric machie, of course, o the coditio of the low weight. The selected gearbox provides the ability to switch three gear stages, a free wheel fuctio to protect the electric motor with respect to high speeds, a direct traslatio for acceleratio of the wheel o ladig speed ad a traslatio stage with a ratio of 1:12 for taxiig o groud. The electric machie ad the trasmissio with three selectable gear stages are placed withi the rim. Both compoets are as small as possible. All assembly groups of the drive combie several fuctios, that meas that they have a high degree of itegratio. The electric machie is a permaet maget excited sychroous machie [1] with the advatage to provide the excitatio field without electrical power dissipatio. Compared to asychroous machies there is the advatage of the large isesibility to the width of the air gap betwee the stator ad the rotor [2]. As i the automotive idustry there is a clear tred to electroically commutated motors [3]. For the magets Neodymium iro boro material is used because of its high remaece iductio. For the selectio of the electromagetic ad electroic geometry several machie types came ito accout [4]. The umber of pole pairs is 8 ad depeds o the small available space i radial directio of the magetic yoke withi the rotor. The magets are mouted directly together i the tagetial directio to produce a rectagular form of the field i the airgap. Thus a high utiliatio ad the best possible approximatio to the field profile of a DC machie is achieved. The magets are badaged with a glass fiber reiforced plastic. For the electrical machie a wrappig techique was used i which the three-phase costructio of 24 sigle tooth coils is built out of 8 parallel coected three-phase deltacoected systems. The widig wire is woud oce ad without iterruptio through the complete machie ad each tooth coil is boud to a power rail system with 3 rails at it is show i fig. 3.

Figure 3: 3phase sigle tooth widig of the electric machie The costellatio of 24 uts ad 8 pole pairs leads to a electric agle ' of 120 degrees betwee the wires lyig i the eighboured uts because the

Oe ca see the series coectio of all followig coils, that is a coil of the phase 1 from slot 1 ad slot -2, a coil of the phase 2 from slot 2 ad slot -3 ad so o. see the widig diagram i fig. 5.

The mechaical desig of this machie is particularly compact. Figure 5: Widig diagram of the three phase widig with 24 uts ad 24 coils, cosistig of 8 parallel delta-coected systems Fig.

degrees stair-like form of the short cut curret as show i fig. 7. Such machies have already show their advatages i the automotive sector as highly dyamic drives [5].

3 Figure 3: 3phase sigle tooth widig of the electric machie The costellatio of 24 uts ad 8 pole pairs leads to a electric agle ' of 120 degrees betwee the wires lyig i the eighboured uts because the commo divisor of both, uts ad pole pairs, is 8. The electric agles of the electromagetic forces of the wires lyig i the uts ad the itercoectios of the wires to coils is show i fig. 4. Oe ca see the series coectio of all followig coils, that is a coil of the phase 1 from slot 1 ad slot -2, a coil of the phase 2 from slot 2 ad slot -3 ad so o. see the widig diagram i fig. 5. The widig ca be produced i oe process, it is woud without iterruptio ad ca be coected to the power rail system simultaeously. oreover the widig heads of the sigle tooth widig are advatageous small. The mechaical desig of this machie is particularly compact. Figure 5: Widig diagram of the three phase widig with 24 uts ad 24 coils, cosistig of 8 parallel delta-coected systems Fig. 6 shows the qualitative rectagular form of the electromagetic force i depedece of the time with a duratio of 120 electrical degrees, which is iduced i the 3phase widig ad which produces the 60 degrees stair-like form of the short cut curret as show i fig. 7. Such machies have already show their advatages i the automotive sector as highly dyamic drives [5]. electromagetic force time Figure 6: Qualitative diagram of the electromagetic forces of the three phases short cut curret Figure 4: Phase agles of the electromagetic force of the wires i eighboured uts ad combiatio to the 24 coils of the three phase widig The coils the are coected to the amed 8 parallel three-phase delta-coected systems, time Figure 7: Qualitative diagram of the short cut curret of the three phases The rectagular use of field ad curret leads to a high utilisatio of the active magetic

material ad results i a predomiatly uiform torque. 5. Gear system The gear system is itegrated betwee the axle of the strut ad the magetic yoke of the rotor.

It is a two-stage plaetary gear system (fig. 8) i which the rotor directly carries the plaet gears (fig.

4 material ad results i a predomiatly uiform torque. 5. Gear system The gear system is itegrated betwee the axle of the strut ad the magetic yoke of the rotor. This type of desig allows a large diameter for the electric machie ad therefore a high torque. It also uses the space to lead the forces optimal i axial directio from the stator side to the rim side. It is a two-stage plaetary gear system (fig. 8) i which the rotor directly carries the plaet gears (fig. 9) ad i which the two plaetary gear systems ad the two su gears each are desiged with slightly differet diameters ad umbers of teeth. The two plaetary gear systems are directly coected. The rim-side su gear would rotate at a fixed plaet carrier with umber of teeth of the rim side plaet gear, umber of teeth of the rim side su to gear, speed of the rim P (2) i the opposite directio as the rotor. With the i positive directio rotatig plaet carrier, however the speed of the rotor has to be added: P (3) ad by settig equatio (1) i (3) it results to Fig. 8: Two-stage plaetary gear system By relative to the eviromet statioary positioed stator-side su gear the plaet carrier ad so the plaet gears are movig with the egie speed aroud the su gear. This results with or 1 1 (4) 1. (5) umber of teeth of the stator-side plaet gear, umber of teeth of the stator-side su gear, speed of the motor ad so of the plaet P carrier, speed of the plaet gears i a speed of the plaet gears of P (1) which have the same rotatio directio as the rotor. Fig. 9: View to the rotor with the rim-side plaetary gears 6. Clutch mechaism I additio to the previously described fuctio of the gear the gear ratio ca also be

switched to a ratio of 1:1 or a free wheel fuctio. I the first case, the pik coloured su gear o the left side i fig.

The stator-sided su gear is ot fixed to the stator. The plaet gears i this positio ca ot roll o the su gears. All rotatig parts o the same axle the rotate at the same speed.

Therefore bolts ca be discoected betwee the stator-sided su gear ad the rim-sided su gear. Thus, all rotatig parts iside the rim rotate free agaist each other. Fig.

5 switched to a ratio of 1:1 or a free wheel fuctio. I the first case, the pik coloured su gear o the left side i fig. 10, so called stator-sided su gear, is coected directly with the cya coloured su gear to the output side i fig 12, so called rim-sided su gear, ad so it is directly coected to the hub. The stator-sided su gear is ot fixed to the stator. The plaet gears i this positio ca ot roll o the su gears. All rotatig parts o the same axle the rotate at the same speed. For a free wheel fuctio the stator-sided su gear may ot be coected to the fixed stator, as it was ecessary for the ratio 1:12, ad may ot be coected to the rim as i the example described before. Therefore bolts ca be discoected betwee the stator-sided su gear ad the rim-sided su gear. Thus, all rotatig parts iside the rim rotate free agaist each other. Fig. 10 shows a assembly with the ame itermediate shaft, which ca be coupled ad decoupled to the stator ad to the hub to realie the various gear ratios. They are switched by bolts or screws. The itermediate shaft also carries the bearig of the rotor. It is mouted with two ball bearigs o the sleeve. I the evet that the itermediate shaft is coupled to the stator, it trasmits a part of the torque o the stator, whereas the other ad smaller part is directly trasferred as a coutertorque of the rotor to the stator. Fig. 11: otor of the electric machie which carries the permaet magets ad the plaet gears The hub trasmits a part of the weight of the airplae to the rim ad is therefore mouted by two large roller bearigs o the sleeve. The ose wheel load is about 10 % of the aircrafts weight. The rim-side su gear is attached directly to the hub, see fig.12. It should be oted that all parts have bee desiged so that they ca be disassembled at ay time agai, as oly a little umber of prototypes were produced. Fig. 12: im-sided parts of the gear icludig the hub 7. Test esults Fig. 10: Stator-sided parts of the gear ad the clutch, called the itermediate shaft The rotor creates as the rotatig elemet of the electric machie the torque ad forwards as the plaet carrier of the gear system the traslated torque from the wheel to the stator flage. So it is made of massive iro [6], it is stuck with 384 sigle rectagular magets for simplicity ad carries the bearigs for the plaet gears, see fig. 11. The rotor itself is mouted o the itermediate shaft. The mai features of the drive were measured at the test bech. These are the mechaical stregth of the drive ad its torque. The mechaical stregth, this meas the resistace to the static load, was tested o a large test bech (fig. 13) with a hydraulic actor by pressig oe drive wheel agaist a freely mouted wheel while the deformatio of the rim, especially the air gap betwee the stator ad the rim, was moitored. I this test, it was assumed that the ose wheel load has to be take by oly oe sigle wheel. This occurs, for example, durig corerig.

The measuremets cofirmed the previous calculatios ad showed that the motor icludig the wheel will withstad the load. for a short break away operatio ad should be demostrated.

14 the torque of oe motor at a speed of 50 rpm is show, as it was measured o the motor test bech, workig agaist a secod electric load machie. A torque up to 2.

The iverter itself was supplied by a fuel cell system, icludig DC/DC-coverters to trasform the voltage of the fuel cell from a DC level of 50 V to a level of 300 V for the test equipmet.

6 The measuremets cofirmed the previous calculatios ad showed that the motor icludig the wheel will withstad the load. for a short break away operatio ad should be demostrated T [Nm] Fig. 13: Static load test of the drive by pressig a drive wheel agaist a freely mouted wheel I fig. 14 the torque of oe motor at a speed of 50 rpm is show, as it was measured o the motor test bech, workig agaist a secod electric load machie. A torque up to 2.5 knm was reached withi the acceptable temperature rage ad with the give iverter. The drive was fed by a iverter which was developed for automotive applicatios. The iverter itself was supplied by a fuel cell system, icludig DC/DC-coverters to trasform the voltage of the fuel cell from a DC level of 50 V to a level of 300 V for the test equipmet. The motor also may work with a DC-lik-voltage up to 600 V which allows the use of iverters which are available from the idustrial brach I [A] Fig. 15: Torque of oe electric machie 8. Coclusio Startig from the questio of whether a passeger aircraft ca move idepedetly o the groud, a electric hub motor has bee desiged which ca be mouted easy o the axle of the ose ladig gear of a Airbus A320. The purpose of the developmet was to demostrate the feasibility of such a drive. The drive is characteried by its compactess, which was possible because the rotor of the electric machie, which ormally forms with its material the magetic yoke of the permaet maget excitatio, also forms the plaet carrier of a two-stage plaetary gear. The electric machie has a relatively high umber of pole pairs ad is desiged as a rig machie which provides the space withi the rotor for the gear ad the clutches. The two motors of the ose ladig gear may brake away the aircraft ad may move the aircraft o a taxiig way with a slope up to 1.5 %. Fially a picture of the complete drive is show i fig. 16. Tmotor Fig. 14: Torque at the rim of oe motor Fig. 15 is the torque diagram i depedece of the phase curret of a sigle electric machie as the machie works with a gear ratio of 1:1 i short term operatio, what meas i a time period less tha oe miute. The egie is so overloaded for a short time. It is see that there is the capability to produce a torque up to 400 Nm, what meas a torque of 5 knm if the gear ratio is switched to 1:12. This is a total torque of 10 knm for the complete ose wheel drive existig of two motors. This is eough Fig. 16: Oe of the two ose wheel hub drives with the rim for a passeger aircraft Airbus A320

7 Ackowledgemet This work was fiacially supported by the iistry of Ecoomic Affairs of the Federal epublic of Germay. A special thaks goes to the compaies Goldhofer ad LufthasaTechik. efereces [1] T. Bödefeld ad H. Seque, Elektrische aschie, Spriger Verlag, Wie, Germay, [2] A. Lelkes ad T. Bertolii, Eergy savig motor to substitute iductio motor, AD&C Nr.5/2000, pp [3] H. Scheeweiß, Usig Brushless DC- otors for Automotive Applicatios, Performace up to 1000 W, Iteratioal Coferece o Fas, pp , Nov. 2004, Lodo. [4]. Schier ad A. Lelkes, Low-Noise EC otor with Exteral otor for Automotive Applicatio, 10 th Iteratioal Power Electroics ad otio Cotrol Coferece (EPE-PEC), Sep. 2002, Cavtat & Dubrovik [5]. Schier ad. Schwabe ad S. ü, Higly Dyamic Drive for Automotive Applicatio, Iteratioal Coferece o Electrical Drives ad Power Electroics (EDPE 2005), Sep. 2005, Dubrovik & Cavtat [6] T. Lu ad A. Bider ad. eiffer, otor Losses i Permaet aget High-Speed achies, Proceedigs of the Iteratioal Coferece o Electrical achies (ICE), p , Aug. 2000, Helsiki

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