6 Electrical System Design

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ELECTRICAL SYSTEM DESIGN FOR A SOLAR POWERED VEHICLEDean J Patterson

Northern Territory University, Darwin, Australia

ABSTRACTTwo separate electrical system designs for avehicle built for a ra ce across Australia in November1987, ar e described, Description of a third system f ora second race in November 1990 is also included. Thedesign of motors, motor controllers, DC-DCconverters, and maximum power point trackers(MPPTs) un der a specialised set of criter ia isdiscussed.

1 INTRODUCTION. ..

Fig 1 The Desert RoseIn November of 1987 a race for solar poweredcars was held in Australia, across the continent fromNorth to South, a distance of 3000 km. The DesertRose, Fig 1, performed creditably. A second race isplanned for November 1990, and design for the nextvehicle is substantially complete.Designing an electrical system for such a vehicleoffers some interesting challenges in the use of pow er

electronic systems. The primary design criterion isthat of maximum achiev able aver age efficiency, and asecond ary issue is that of m inimum w eight.

The avera ge efficiency is the result of integrationof system component efficiency characteristics over arange of patterns of po wer use, which must take intoaccount known road gradients and surfaces, predictedmeteorological conditions, and race strategies.The estimated vehicle power requirements, forwhat appear to be achievable design parameters forthe 1990 race, are shown in Fig 2. A constantretarding force for rolling loss, due principally tohysteretic loss in the tire walls, is assumed, and theaerodynamic loss shown results from a n aerodynamicforce which. for clean flow, is assumedproportional to speed squared.

Watts1wo14aa -ima1OW - - Rollin8 loss

--

WO ,-Aerodynamic oss- -Total lossW O -400-

0 1 0 8 0 S O H ) M ) m

Speed (kph)

to be

Fig 2 Estimated vehicle power use, Cd .13, All upweight 260 kg, level ground.From Fig 2 it can be seen that if rolling loss isassumed linearly proportional to all up weight, then,at an average speed of 60 km/h, there exists a trade-off between weight and power of around 0.9watts/kg. Hence, if in the system design the availablepower can be increased by 0.9 watt, then it isreasonable to do so if the weigh t penalty is less than akilogram.The total mechanical power requirements shown

in Fig 3 result from adding potential energycalculations to the data of Fig 2.

618CH2873-8/90/0000-0618 $1.00 990 IEEE


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::Io Road Gradients

-

/ I

80-230 volt 30 kHz 1.1 kW P mDC-DC ConverterB FWM DC Motor hg ne t DC

1800 watt M a r Controller~ _ _ _ _ _

-400

o i o a o s o a o w w o a o wSpeed (kph)

Fig 3 Total mechanical power requirements on arange of road gradients.The power available depends on a number ofpredictable and unpredictable factors. However, for aphotovoltaic arra y of size specified by the ra ce rules(roughly equivalent to 8 square metres), the cost ofcells is a major determ inant.For the 1987 race an average electrical power of850 watts was assumed, whereas for the 1990 race afigure of 1200 watts is assumed.Conversion efficiency of electrical power input tomechanical power at the rear wheel of better than80% is achievable.2 INITIAL DESIGN CONSIDERATIONS

It is clear that some form of storage is necessary, andSilver-Zinc batteries are the clear choice in terms ofenergy density, (watthours/kilogram). Frommanufacturers data, optimising energy density leadsto a battery with a relatively small number of highcapacity cells, and hence a lower battery voltage.Design of electric motors for high efficiency,howeve r, generally leads t o higher voltage designs, inthe case of DC m otors for exam ple, simply tominimise the loss due to voltage drop across thebrushes. Further, commercially available inductionmotors and permanent magnet synchronous motorsgenerally require in the order of hundreds of volts.These facts, together with the fact that the operatingvoltage of A g-Zn c ells varie s by a factor of tw o fromlowest usable voltage to voltage on charge, lead tothe use of a DC-DC converter to match the load tothe battery, and to stabilize the supply for ease ofcontrol. Th e cost of DC -DC conversion (averaging2.5%) is paid for in the extra capacity carried for agiven weight.

M a x i m u m PowerPoint Tracker

80-8gO volt Induatrtal 1. 1 kW 3 PbDC-DC Converter 9 Phaae PWM Induction

80 volt 70 AhAg-Zn Batteq

Fig 4 Induction motor system

Fig 5 D C motor systemFigs 4an d 5 show two designs which werecompleted for the 1987 race and tested. They weredesigned with a maximum of compatibility andshared sub-systems, and either could have bee n usedat any time, although the second was actually usedthroughout the race. Fig 6 shows the design for the1990 race.

I11 SUBSYSTEM DESIGN[a) DC -DC converters

A study of expected gradients, expected racespeeds, and speed strategies, in conjunction with theinformation of Fig 3ena bles dimensioning of the DC-DC converter system. In 1987 this led to a modularsystem with three units, eac h of c apacity 600 watts

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which could be operated either in parallel to drive theDC motor, (Fig 5 ) or in series to drive the inductionmotor (Fig 4).

Fig 6 Synchronous motor systemThe units used a standard push pull transformercoupled square wave topology operating at 30 kHz.

Efficiency peaked at 9 7% and w as greater than 95%for most of the load range. T he losses we reprincipally in the leakage inductance of thetransformer secondary. The fact that the losses wereprimarily from leakage inductance had someinteresting implications for system management,ruling out the use of the available pulse widthregulation to control the DC motor directly, forexample.The design, particularly of the transformer, hasbeen extensively reworked for the new system [ l]and efficiencies consistently above 97% are beingmeasured./b) Maximum Power Point Trackers

Whilst the characte ristics of Silicon photovoltaiccells are usually presented in their V-I form asfunctions of illumination and temperature, Fig 7shows the results for a range of commerciallyavailable panels in their normal operating conditionand temperature. This indicates very clearly the valueof using a DC-DC converter to maintain the cellvoltage at the maximum power point, whilstdelivering power to a battery bus at a different, andvariable, voltage [2]. This is particularly so whenusing a battery with wide operating voltage range,such as Ag-Zn.In 1987 a substantially flat photovoltaic array,seen in Fig 1, and a single commercial MPPT wasused. However, in general, the ability to provide foran arra y of M PPT s feeding a single battery allows fordifferent illumination on different ar eas of thephotovoltaic array, granting a considerably increasedfreedom to aerodynamic designers. Further, it allowsthe use of differe nt cells in differe nt parts of thearray.A multiplicity of MPPTs of the traditional kind,measuring both voltage and current, multiplyingthem, and providing the perturbation necessary tooperate a hill seeking algorithm, leads to a substantialand complex electronic sub system, with its ownpower drain .

voltsIllumination :- 1 s u n

-D. I- Open C C t V O l b ' .82Panel output (watts)40, I

AIf I8 13 14 15 10 17

Panel voltage12.30pm. 13/5/88, Inaolation 935 W/sq mambient temp 33C

Fig 7 Initial tests of solar panels produced b y fourdifferent manufacturers

ao 4u ea BoTemperature, degrees Celsius

Fig 8 Maximum power point band as a function oftemperature, calculated from measured data, and twoopen loop algorithm s for tracking this band.An examination of measured data, in this casefrom silicon solar cells produced and tested at theUniversity of New South Wales, Sydney, Australia[3], shows that the maximum power point voltage,which is a function of bot h illumination an dtemperature, can in fact be quite accurately

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approx imated as a function of the open circuitvoltage of the cell, which varies in a very similarmanner. Fig 8 shows the effect of temperature onmaximum power point voltage at constantillumination, together with calculated tracks of thevoltage either side of the maximum power pointvoltage at which the power is 1% less than themaximum. Superimposed are two functions of theopen circuit voltage of t he cell under th ese conditions,the first being the open circuit voltage multiplied by aconstant factor, and the second with a constantvoltage subtracted.

2 -1.01111.71.8

1.41.91.2l.i

circuits, and a small number of passive components.A single module runs at close to 98%efficiency at 50kHz, and is capable of processing up to 50 watts,imposing a power drain of less than 35 mW .

/-G- ' >Y i~

............................................. .................. ...............---- :_ 5 A charge- .. :- 15 A discharge

IC) atteriesCe l l voltageL.1 r

Volts0.1 1o.( Temp:- 60 degrees CIo no iw iw BW MO JWTime in minutes

Fig 10 Charge and discharge characteristics of 70Ah, Ag-Zn cell.

Y . l0.1 0.2 0.1 0.4 0.6 0.8 0.7 0.8 0.0 I

Illumination, suns

Fig 8 Maximum power point band as a function ofillumination, calculated from measured data, and twoopen loop algorithms for tracking this band.Fig 9 shows similar calculations for the cells at aconstant temperature and variable illumination.Whilst it is clear that for these cells a constantvoltage step provides a more accurate track, it is infact slightly easier to produce a constant fraction of

the open circuit voltage. Further, this algorithm thenenables the use of identical MPPT s processing theoutputs of cell strings of varying length.The loss resulting from imperfect tracking of themaximum power point is certainly very small ineither case for most conditions that can be predicted.Further, it should be noted that even in a traditionalMPPT, the requirement to perturb the operatingpoint causes the device to dynamically traverse themaximum power point region, rather than beingcontinuously centred exactly on it.Measu rement of th e open circuit voltage is simplya matter of shutting down the DC -DC converter for ashort time, for example for 0.5 ms every second, thusrejecting abou t 0.05% of the available power.A system based on a standard boost convertercircuit has been designed using two integrated

Only limited information regarding thecharacteristics of A g-Zn cells is available, particularlywhen used in a cyclic situation. Whilst these cellshave a coulomb efficiency very close to loo%, thevoltage on charge shows an unusual "plateau"behaviour. Fig 10 shows the results of trials onindividual cells at the end of the 1987 race. What caneasily be extrac ted fro m these data is a curve of cellwatthou r efficiency as a function of th e state ofcharge of the cell, which varies dramatically between75% an d 90%. This has an important effect onenergy management strategy, which must includesome risk calculations, since the most efficient use ofthe battery occurs when it is in a very low state ofcharge, shortening its life, and implying vehicleoperation with very low power reserves.Id) Motor Controllers

The industrial 3 phase induction motor controllerof Fig 4 required only minimal modification to beable to feed directly into the DC link,and required aseparate small inverter to provide the power for thecontrol circuits. The D C mo tor controller of Fig 3 isquite standard. The controller of fig 6 uses an opticalshaft position sensor, to provide the normal "DCbrushless motor" trapezoidal control. A velocitycontrol loop is added f or the eas e of control by thedriver.

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0.950 r0.925

0.900

0.875

7 0

Motor Efficiency

G r a d i e n tFig 11 Efficiency surface for 4.5" servomotor invehicle with param eters as shown in Figs 2& 3(e) Motor Choice and Design

Choosing or designing a motor for the expectedpower levels satisfying the weight and efficiencycriteria, leads fairly directly to either a high voltageDC motor, where the brush drop is negligible, or,more ideally, to the rotating field and stationaryarmature of the "permanent magnet synchronous" /"DC brushless" variety. Refining the effectiveness ofpermanent magnet material, and where necessary,iron in the m agnetic circuit, leads to the choice of arectangular flux distribution on the rotor, andrectangular/trapezoidal current waveforms on thestator.

A nine phase Nd Fe B permanent magnet rotormachine is currently being tested. Since the switchingis carried out using power MOSFETs, and efficientdesigns involve some paralleling of these devices,there is benefit to be ha d by increasing the number of"phases" (although the c oncept of phase can be ratherconfusing when the current patterns are no longersinusoidal, and phasor analysis is no longerappropriate). By increasing the number of "phases"the sam e num ber of MOS FE Ts of a given rating canbe used, the fraction of time in which arm atureconductors are not carrying current can be reduced,raising the efficiency, and the machine more nearlyapproaches an "inside out" DC machine.

Finite element analysis has been used in thedesign of this machine t o minimise the effec ts ofarmature reaction. This is not difficult given that theeffective relative permeability of Nd Fe B magnetmaterial is close to unity.The major loss in such machines using the normalstator configuration, and high flux densities of theorder of 1 Tesla, readily achievable with Nd Fe Bmagnetic material, is loss in iron magnetic pathssubjected to alternating flux. Complex structures arenecessary to reduce or remove the iron content ofthese paths.Once machine parameters have been establishedit is a simple matter to develop, by combining thisinformation with the requirements of Fig 3 , efficiencysurfaces of the type shown in fig 11, which arenecessary to develop race strategy. The data for fig11 comes from a 4.5" servomotor chosen as a back-upfor the motor under development.Although regenerative braking is appealing inprinciple, the very limited amount of steep gradienton the course and the substantial extra lossesinvolved, particularly in the battery, leads to thefitting of an over-run clutch on the motor shaft. Inover-run conditions the motor efficiency computes asunity since it can be shut down.

IV S U M M A R YA total average power loss of 6% in the electronics,being in the MPPTs, DC-DC converters and motorcontroller, 9% in the motor and 2% in a transmissionstill leaves considerable scope for improvement. Thesuggested weight powe r trade-off of .9 watts/kgencourages further sophistication in motor design andpower electronics.References[ l ] V. A. Niemela, G. R. Scut, A. M. Urling, Y.Chang, T. G. Wilson, H. A. Owen Jr., R. C.Wong, "Calculatin the Short Circuit Impedan cesof a Multiwiniing Transformer from itsGeometry," IEEE Power Electronics SpecialistsConference 89CH2721-9, 1989, pp. 607-617.[2] L. L. Buciarelli, B. L. Grossman, E. F Lyon, N. E.Rasmussen, "The Energy Balance Associated withthe Use of a MPPT m a 100 kW Peak powerS stem, 14th IEEE Photovoltaic Specialistdn fer en ce , San Diego, CA, 1980[3 ] M. A. Gre en, Electrical EngineeringDepartment, University of New South Wales,private correspondence.

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6 Electrical System Design