PROPULSION SYSTEM DESIGN OF ELECTRIC VEIDCLES
`
`Mehrdad Ehsani
`Fellow, IEEE
`
`Khwaja M. Rahman
`Student Member, IEEE
`
`Hamid A. Toliyat
`Member, IEEE
`
`Texas Applied Power Electronics Center
`Department of Electrical Engineering
`Texas A&M University
`College Station, TX 77843-3128
`Fax: ( 409) 845-6259
`to its self starting capability. However, soon after the
`introduction of electric starter for ICE early this century,
`despite being energy efficient and nonpolluting, EV lost the
`battle completely to ICE due to its limited range and
`then ICE has evolved,
`inferior performance. Since
`improved in design, and received wide spread acceptance
`and respect. Although this essentially being the case, EV
`interest never perished completely, and whenever there has
`been any crisis regarding the operation of ICE automobiles,
`we have seen a renewed interest for EV. The early air
`the energy crisis in the
`quality concerns in the 60' s and
`70's have brought EVs back to the street again. However,
`the most recent environmental awareness and energy
`concerns have imposed. for
`the first time since its
`introduction, a serious threat to the use of ICE automobiles.
`Electric Vehicles offer
`the most promising
`solutions to reduce vehicular emission. Electric vehicles
`constitute the only commonly known group of automobiles
`that qualify as Zero Emission Vehicle (ZEV). These
`vehicles use an electric motor for propulsion, and batteries
`as electrical energy storage devices
`This paper presents the EV propulsion system
`design philosophies. The paper is organized as follows.
`Section Il describes the design constraints and the variables
`for EV system. Design philosophies of EV propulsion
`systems are presented in sections III. Section IV examines
`several most commonly used motors for EV system design.
`Section V compares our designed EV with the General
`Motors IMPACT. Summary and conclusions are presented
`in Section VI.
`II. Specifications of EV Propulsion System Design
`A. System Design Constraints
`Vehicle operation consists of three main segments.
`These are, i) the initial acceleration, ii) cruising at vehicle
`rated speed, and iii) cruising at the maximum speed. These
`three operations provide the basic design constraints for the
`EV drivetrain. A drivetrain capable of meeting these
`constraints will function adequately in the other operational
`regimes. Refinements to these basic design constraints are
`necessary for an actual conunercial product, but those are
`beyond the scope of this paper. The objective here is to
`
`Abstract:
`There is a growing interest in electric vehicles due to
`environmental concerns. Recent efforts are directed toward
`developing an improved propulsion system for electric vehicle
`applications. This paper is aimed at developing the system design
`philosophies of electric vehicle propulsion systems. The vehicle's
`dynamics are studied in an attempt to find an optimal torque(cid:173)
`speed profile for the electric propulsion system. This study
`reveals that the vehicle's operational constraints such as: initial
`acceleration and grade can be met with minimum power rating if
`the powertrain can be operated mostly i.n constant power region.
`Several examples are presented to demonstrate the importance of
`the constant power operation. Operation of several candidate
`motors in the constant power region are also examined. Their
`behaviors are compared, and conclusions are made.
`I. Introduction
`The ICE automobile at the present is a major
`source of urban pollution. According to figures released by
`(EPA),
`the US Environmental Protection Agency
`conventional ICE vehicles currently contribute 40-50% of
`ozone, 80-90% of carbon monoxide, and 50-60% of air
`toxins found in urban areas [l). Besides air pollution, the
`other main objection regarding ICE automobiles is its
`extremely low efficiency use of fossil fuel. Hence, the
`problem associated with ICE automobiles are three fold,
`environmental, economical, as well as political. These
`concerns have forced governments all over the world to
`consider alternative vehicle concepts. The California Air
`Resource Board (CARB) is among the few who acted first
`through the declaration of the Clear Air Act of September,
`1990. This act requires that 52% of all vehicles sold in that
`state be either Low Emission Vehicles (LEV's)- 48%, Ultra
`Low Emission Vehicles (ULEV's)- 2%, or Zero Emission
`Vehicles (ZEV's)- 2%, by the year of 1998 [2]. Similar
`measures are considered in other states and nations, as well.
`The concept of Electric Vehicle (EV) was
`conceived in the middle of previous century. After the
`introduction of internal combustion engine (ICE), EVs
`remained in existence side by side with ICE for several
`years. The energy density of gasoline is far more than what
`the electrochemical battery could offer. Despite this fact,
`the RV continued to elU.llt, egpecillll}' in the ul'him t\l'M.g due
`
`0- 7803-2775-6/96 $4.00 © 1996 IEEE
`
`7
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`meet these constraints with minimum power. The variables
`defining the above design constraints are:
`(i) Vehicle rated velocity, vrv.
`(ii) Specified time to attain this velocity, tr.
`(iii) Vehicle maximum velocity, vma><.
`,
`(iv) Vehicle mass, and other physical dimensions.
`B. System Design Variables:
`The main component of EV is its electrical
`powertrain. The electric propulsion design variables are:
`i) Electric motor power rating.
`ii) Motor rated speed.
`iii) Motor maximum speed.
`iv) The extend of constant power speed range,
`beyond the rated speed.
`v) Gear ratio between motor shaft and the wheel
`shaft (transmission).
`As mentioned earlier, the main design objective is
`to find the minimum drive weight, volume and cost that will
`meet the design constraints with minimum power.
`C. Road Load Characteristics
`The road load (Fw) consists of rolling resistance
`(fro), aerodynamic drag (f1), and climbing resistance (fn) [3].
`Fw = fro + f1 + f st
`(1)
`The rolling resistance (fro) is caused by the tire
`
`deformation on the road:
`(2)
`fro= f · m· g
`where f is the tire rolling resistance coefficient. It increases
`with vehicle velocity, and also during vehicle turning
`maneuvers. Vehicle mass is represented by m, and g is the
`gravitational acceleration constant.
`Aerodynamic drag, f1, is the viscous resistance of
`air acting upon the vehicle.
`f1 = 0.5~CwA( V + v O )2
`(3)
`where I; is the air density, Cw is the aerodynamic drag
`coefficient, A is the vehicle frontal area, v is the vehicle
`speed, and v0 is the head wind velocity.
`· The climbing
`resistance
`(fs, with positive
`operational sign) and the down grade furce (f51 with
`negative operational sign) is given by
`fst =m·g· sina.
`where ex is the grade angle.
`The following assumptions will be made in the
`analysis prsented in the following sections, unless otherwise
`specified.
`
`(4)
`
`(i) velocity independent rolling resistance
`(ii) zero head wind velocity
`(iii) level ground
`These assumptions do not change the general trend
`of the solution and can be easily relaxed.
`The motive force F available from the propulsion
`system is partially consumed in overcoming the road load,
`Fw. The net force, F-Fw, accelerates the vehicle (or
`
`8
`
`decelerates when Fw exceeds F). The acceleration is given
`by
`
`(5)
`
`F-F,
`a=
`w
`k m·m
`where km is the rotational inertia coefficient to compensate
`for the apparent increase in the vehicle's mass due to the
`on-board rotating mass.
`m. EV System Design
`The main component of EV drivetrain is its
`electric motor. The electric motor in its normal mode of
`operation can provide constant rated torque up to its base or
`rated speed. At this speed, the motor reaches its rated
`power limit The operation beyond the base speed up to the
`maximum speed is limited to this constant power region.
`The range of the constant power operation depends
`primarily on the particular motor type and its control
`strategy. However, some electric motors digress from the
`constant power operation, beyond certain speed, and enter
`the natural mode before reaching the maximum speed. The
`maximum available torque in the natural mode of operation
`decreases inversely with the square of the speed. This range
`of operation is neglected in the analysis presented in this
`section, unless otherwise specified. It is assumed that the
`electric motor operates in the constant power region beyond
`the base speed and up to the maximum speed. Nevertheless,
`for some extremely high speed motors the natural mode of
`operation is an appreciable part of its total torque-speed
`profile. Inclusion of this natural mode for such motors may
`result in a reduction of the total power requirement. Of
`course, power electronic controls allow the motor to
`operate at any point in the torque speed plane, below the
`envelop defined by the mentioned limits. However, it is the
`profile of this envelop that is important in the motor drive
`selection and design.
`In order to free up the motor speed from the
`vehicle speed, for design optimization, gearing between the
`motor shaft and the drive shaft is required. In our design,
`we will make the following assumptions.
`(i) single gear ratio transmission operation: power
`electronic control allows instantaneous matching of the
`available motor torque with the required vehicle torque, at
`any speed. Therefore, multiple gearing in order to match
`the motor torque-speed to the vehicle torque-speed is no
`longer a necessity.
`(ii) ideal loss free gear: without loss of generality,
`the gear losses can be incorporated at the end of analysis.
`The gear ratio and size will depend on the
`maximum motor speed, maximum vehicle speed, and the
`wheel radius. Higher maximum motor speed, relative to
`vehicle speed, means a higher gear ratio and a larger gear
`size. The selection criterion for the maximum motor speed
`will be further discussed later. The torque speed diagram of
`a typical motor is drawn in Fig. l, but in terms of tractive
`force and vehicular speed for different gear ratios. Notice
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`now the electric motor base speed and maximum speed, in
`terms of the vehicle speed, depend on the gear ratio. A
`design methcxlology based on the three regions of operation
`will now be presented.
`A. Initial Acceleration:
`.
`The force-velocity profile of a typical motor is
`redrawn once again in Fig. 2. In this figure, vnn is the
`electric motor rated speed, vrv is the vehicle rated speed
`is the vehicle maximum speed. The motor
`and Vmax
`maximum speed must correspond to this Vmax. after the gear
`ratio transformation.
`
`- -~ --
`
`- ~ --~
`
`12000,~ - -~-
`1:13.2
`
`10000
`
`800()
`
`• 1:9.9
`
`~ g 6000 1:6.6_
`
`IL
`
`4000
`
`2000
`
`50
`
`100
`Vetide Speed (111)11)
`
`150
`
`200
`
`Fig. I. Torque-speed diagram of an electrical motor in tenns of
`tractive force and vehicular speed with gear site as lhe parameter.
`
`The range of operation for initial acceleration is 0-
`Vrv, For now, we will focus our attention only on this
`interval. For maximum acceleration the motor operates in
`constant rated force (torque), Fvttled::::Pn/vnn up to the motor
`rated speed Von, and in constant power (FrPulv) at speeds
`beyond the base speed, up to the vehicle rated speed Vrv,
`Here, Pm is the motor rated power. We assume vrv>vnn, The
`wisdom of this assumption will become clear, shortly. The
`differential equation describing the perfoflllance of the
`system is given by eq. (5) and is repeated here for
`convenience.
`
`dv F - Fw
`a=-=----
`dt
`km ·ffi
`Fis the motive force available from the propulsion
`system and Fw is the running resistance (road load). The
`boundary conditions are
`at t:::{}, vehicle velocity v::::O.
`at t=tt, vehicle velocity v=vrv.
`To gain insight, we will solve eq. (5) under the most
`simplifying assumptions:
`i) The vehicle is on a level ground.
`ii) The rolling resistance is zero.
`iii) Aerodynamic drag is zero.
`we will relax these assumptions later for a more realistic
`solutiolf. The above assumptions will result in a closed
`
`9
`
`form solution for the motor rated power Pm· The insight
`gained from the closed fonn solution is also valid for the
`more practical design involving running resistances.
`With these simplifying assumptions the governing
`differential equation reduces to:
`dv F
`a= - = -
`dt m
`6000---~--~-- - ------
`
`(assuming k.n=l)
`
`5000
`
`4000
`
`2000
`
`1000
`
`Constart Power
`
`O'---..._.~Vrm..,.,_~----'~V~"'--~n~,__----'VmM
`0
`20
`40
`60
`80
`100
`Velicle Speed (mph)
`
`Fig. 2. Typical torque-speed profile of electric motor in terms of
`tractive force and vehicular speed.
`This differential equation is solved with the
`previous boundary conditions and the force-speed profile of
`Fig. 2. The differential equation is integrated within the
`acceleration interval of 0-vrv in 0-tc seconds, in order to get
`a closed form solution for the rated power Pm·
`V d
`o F
`o
`The left hand side integral is broken into two parts, the 0-
`V m constant force operation and the Vnn·Vrv constant power
`operation
`
`mJ~= dt
`
`JI
`
`(6)
`
`Vnn
`
`VN
`
`dV
`dv
`mJ p /v +mj p /v=tr
`
`(7)
`
`(8)
`
`v"" m
`
`2
`2
`( V nn + V rv)
`
`rm
`O m
`Now solving for Pm, we get
`ID
`pm = -
`2tf
`For minimum motor power, differentiating Pm with
`respect to Vrm and setting it to zero gives
`V nn = 0
`(9)
`This establishes a theoretical limit for minimum
`motor power. For Vnn=O, the electric motor operates entirely
`in the constant power region. Therefore, if the motor is
`performing 0-vrv in t, seconds in ~onstant power alone, the
`power requirement is minimum. On the other hand, if the
`motor operates in the constant torque (force) region during
`the entire 0-tr period; we will have vrm=Vrv. In this case, eq.
`(8) shows that the power requirement is twice that of
`
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`constant power operation. The solid line curve of Fig. 3
`shows an example of the motor power requirements
`between these two extremes. Of course, operation, entirely
`in constant power regime, is not practically realizable.
`However, this theoretical discussion demonstrates that
`longer constant power range of operation will lower the
`· motor power.
`Having · discussed the simplified resistanceless
`case, we now solve the more realistic case, involving the
`running resistance. The vehicle differential equation (5) can
`
`120...----,.---.---....----,r--~-----.
`
`110
`
`[100
`
`I 90 J 80
`I 70
`
`60 -
`
`h 1he Prasence ol Road Load /
`/
`
`/
`
`/
`
`/
`.
`
`/
`
`/
`
`/
`
`/
`
`/
`
`10
`
`40
`30
`20
`Moto.- Rated Speed Vrm (mph)
`
`50
`
`60
`
`Fig. 3. Acceleration power requirement as a function of
`motor rated speed. Solid line curve,. resistanceless case,
`dashed curve- in the presence of road load.
`
`be solved under the same boundary conditions as before
`with the presence of the running resistance Fw. In this case,
`a closed form solution is feasible. However, the result is a
`transcendental equation involving rated motor power Pm,
`rated motor velocity Vnn,
`rated vehicle velocity vrv,
`acceleration time tr, and all the other system constants, e.g.;
`vehicle · mass m,
`rolling · resistance coefficient
`f,
`aerodynamic drag coefficient Cw, etc. The resulting
`equation can be solved numerically for Pm for a specific
`motor rated velocity Vnn, using any standard root seeking
`method such as the secant method [ 4 J.
`Let's assume that it is desired to obtain Pm for the
`following case
`- 0-26.82 mis (0-60 mph) in 10 seconds.
`- vehicle mass of 1450 kg.
`- rolling resistance coefficient of 0.013.
`- aerodynamic drag coefficient of 0.29.
`- wheel radius of 0.2794 m (11 inch).
`- level ground.
`- zero head wind velocity.
`a plot of the resulting motor rated power vs. motor rated
`speed, in terms of vehicle speed, is shown in Fig. 3 (the
`dashed curve).
`·
`Examination of Fig. 3 (the dashed curve) results in
`the following conclusions:
`
`i) Rated power versus Vnn curve shows the same
`general trend of the resistanceless case.
`ii) Rated motor power requirement is minimum for
`continuous constant power operation (vrm=O).
`iii) Rated motor power is roughly twice that of
`continuous constant power operation for constant force
`(torque) operation (vrv=v.-m).
`its
`iv) Rated motor power remains close to
`minimum up to about 20 mph of rated motor speed and
`then grows rapidly.
`B. Cruising al Rilled Vehicle Velocity:
`A powertrain capable of accelerating the vehicle to
`the rated velocity vrv will always have sufficient cruising
`power at this speed. Hence, the constraint of cruising at
`rated vehicle speed is automatically met for the case of
`EV. Of course, cruising range is another issue, related to
`the battery design, which is outside the scope of this paper.
`However, minimizing power of the drive will help the
`battery size.
`C. Cruising al Maximum Vehicle Velocity:
`The power requirement to cruise at maximum
`vehicle speed can be obtained as
`
`Pvmax = (fro +fst)·Ymax +f1(v)· vmax
`(10)
`Since aerodynamic drag dominates at high speeds,
`this power requirement
`increases with
`the cube of
`maximum vehicle velocity.
`If
`this vehicle power
`requirement is greater than the motor power calculated
`previously (P v,nax>P m), then P vmax will define the motor
`power rating. However, in general Pm will dominate P vmax,
`since modern vehicles are required to exhibit a high
`acceleration performance. As mentioned before, some
`extremely high speed motors usually have three distinct
`modes of operation. The initial constant torque operation,
`followed by a range of constant power operation, then to
`the maximum speed in natural mode (see Fig. 2). For such a
`motor it may be advantageous to use the entire constant
`power range for initial acceleration of the vehicle. The
`operation beyond that would be in the natural mode. This
`would allow a longer constant power operation in the initial
`acceleration. Consequently, the motor power requirement
`will be lower. This scheme will work provided the motor
`has adequate torque in natural mode to meet the constraints
`at the maximum vehicle speed. Otherwise some part of the
`constant power operation has to be used for the vehicle
`operation beyond the rated vehicle speed.
`Natural mode of motor operation is not the
`the
`rated vehicle
`speed,
`pteferred mode beyond
`unfortunately no control algotithm exists, presently, to
`operate some high speed motors entirely in constant power
`beyond their base speed. However, the natural mode, if
`. included, can lower the overall power requirement. The
`speed at which the electric motor can enter the natural
`
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`
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`off between maximum motor speed and the gear size.
`However, this tends to be more in favor of selecting a
`medium or high speed motors. For an extremely high speed
`motor, a sophisticated gear arrangement might be necessary
`for speed reduction. Planetary gear arrangement [5) could
`be the choice, that is compact but allows high speed
`reduction. Extended constant power range, on the other
`hand, will increase drive shaft torque and stress on the
`gear. Hence, another design tradeoff is involved between
`the gear stress and the extended constant power range. It
`can be seen from the results of table I that after a certain
`point there is not any appreciable power reduction with
`extended constant power range. Any further extension of
`constant power range beyond this point will only adversely
`
`12000
`
`~ 8000
`
`!10000
`! 6000
`1400)
`'j 2000
`l2
`0
`1110
`
`9 8
`
`7 6
`
`5 ~
`
`3 2
`
`1 0 4
`
`mode and still meet the power requirement at maximum
`vehicle speed is obtained from
`
`VN =V™r~:
`
`(ll)
`
`Note that the initial acceleration power is also a function of
`VN (extended constant power range). Hence, VN and pm have
`to be solved iteratively. Also, the gear ratio between the
`drive shaft and the motor shaft
`is to be determined by
`matching vN with the motor speed at which it enters the
`natural mode. More discussion about the natural mode of
`operation appears in section IV. The rest of the analysis is
`done assuming constant power operation beyond .the base
`speed up to the maximum speed.
`The importance of extending the constant power
`speed range can be better understood by comparing the
`required motor power for different constant power speed
`ranges (as a multiple of its base speed). Table I shows an
`example of power requirement for several constant power
`ranges for the following case:
`·
`i) Maximum motor speed is l 0,000 rpm.
`ii) Maximum vehicle speed is 44.7 mis (100 mph).
`iii) Other system variables and constants are the
`same as the previous example.
`Here,
`the required gear ratio,
`the
`to match
`maximum motor speed to the maximum vehicle speed, for a
`wheel radius of 0.2794 m (11 inches), is 1:6.55. The
`results of Table I suggest an extended range of 4 to 6 times
`the base motor speed in order to significantly lower the
`motor power requirement.
`Finally, we ex.amine the effe.ct of maximum motor
`speed and the extended constant power range on the overall
`system performance. The power requirement is not a
`function of the motor maximum speed. Motor maximum
`speed only affects the gear siz.e. However, maximum speed
`
`Fig. 4. Raled motor shaft torque as a function of maximmum motor
`speed.
`affect the gearing and drive shaft appreciably without
`reducing the pow,er requirement. This will set the upper
`limit of the extended range of the constant power operation.
`Overall, the EV drive system design philosophy
`can be summarized as:
`i) Power requirement for acceleration decreases as
`Table I: EV Power requirement as a function of constant power range.
`Extended Constant HP Speed Range
`1:3
`
`Motor Rated Power (KW)
`
`110
`
`95
`
`74
`
`67
`
`64
`
`1:1
`
`1:2
`
`1:4
`
`1:5
`
`1:6
`62
`
`has a pronounced effect on the rated torque of the motor.
`An example of this is illustrated in the surface plot of Fig.
`4. Low speed motors with extended constant power speed
`range have a much higher rated shaft torque. Consequently,
`they need more iron to support this higher flux and torque.
`Furthermore, higher torque is associated with higher motor
`and power electronics currents. This will also impact the
`power converter silicon siz.e and conduction
`losses.
`Extended speed range, however, is necessary for initial
`acceleration as well as for cruising intervals of operation.
`Therefore, the rated motor shaft torque can only be reduced
`through picking a high speed motor. This would however
`affect the gear rn6o. A good de1?ign i~ the rMult of II tt11de
`
`the range of constant power operation increases. More
`specifically, as the ratio of the vehicle rated speed to motor
`rated speed increases.
`ii) The gear ratio between the electric motor and
`the drive shaft is determined by the motor and · vehicle
`maximum speeds.
`iii) Power requirement for cruising at
`the
`maximum vehicle speed is obtained directly from the road
`resistance at maximum speed. In general, this power
`requirement will be
`lower than the initial acceleration
`power requirement.
`iv) High speed motors would be more favorable
`for liV 11pplic11.tion, in gi:lt:terru.
`
`11
`
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`the BLOC motor makes it appear inferior to the induction
`IV. Electric Propulsion Systems for EV Design
`motor, despite its high p-0wer factor and high efficiency.
`An electric propulsion system comprises of three
`The extremely high spee<I operation of the SRM and its
`main elements: power electronic converter, motor, and its
`relatively longer constant power range helps it to overcome
`controller. This section is devoted to examining several
`some of the difficulty associated with its ,lower power factor
`most commonly used motors
`for EV propulsion. The
`operation. Furthermore, the SRM converter is simpler and
`importance of extended speed range, under constant power
`easier to control.
`operation of electric motors in EV system design was
`Table II: Motor data.
`Maximum 5,,...,.,1 £mm)
`8750
`9000
`20000
`
`Rated Sne.ed (mm)
`1750
`4000
`4000
`
`Constant Power Ranl!e
`1:5
`1:2.25
`1:3
`Rest in Natural Mode
`
`Power Factor
`0.82
`0.93
`0.6
`
`Induction
`· Bi.DC
`SRM
`
`established in the previous section. This mode of operation
`is referred to as field weakening, from its origin in de motor
`drives. Therefore, this section will concentrate mainly on
`the field weakened extended speed operation of the EV
`motors. A more detailed study of these motors for EV
`propulsion application is presented in [6].
`Table Ill: Rated power and coaverter volt-ampere requirements for the motors of
`Table II for typical EV anolication.
`Power Ratim? (kW)
`Gear Ratio
`65
`S.7
`86
`5.9
`13
`68
`
`Induction
`BLDC
`SRM
`
`Converter Ratimz (kV A)
`79
`92
`113
`
`V. Comparison of our Designed EV and General
`Motors IMPACT.
`In this section an EV prototype is discussed. The
`actual design specifications of this vehicle are compared
`with our theoretical design of the same vehicle, b~scd on
`the ideas presented in this paper. The EV is the General
`
`Design Example
`We present a design example of some most
`commonly used motors in the constant power region. This
`example will help clarify the capabilities of these motors
`for vehicle applications.
`EV data:
`- Vehicle Rated Speed of 26.82 mis (60 mph).
`in 10
`- Required acceleration of · 26.82 mis
`seconds.
`- Vehicle maximum speed of 44.7 mis (100 mph).
`- Vehicle mass of 1450 kg.
`- Rolling resistance coefficient of 0.013.
`- Aerodynamic drag coefficient of 0.29.
`- Frontal Area of2.13 m2
`•
`- Wheel radius of0.2794 m (11 inch).
`- level ground.
`- zero head wind.
`The motor data are shown in Table II. The motor
`data chosen are for the commercially available samples of
`these motors. · Clearly, more specific motors . can be
`designed for vehicle applications, but such data were not
`available for th;s P"P"'"· ll M.id on th~ vahiclQ d 11t11, the
`powerequirement to cruise at the maximum speed is 41 kW.
`The motor power for acceleration and converter volt(cid:173)
`ampere 01 A) requirement for each motor are shown in
`Table III.
`The extended constant power range available from
`the induction motor clearly makes it highly favorable for
`vehicle application. The limited constant power range of
`
`in 8.5 .
`
`Motor corporation IMPACT car.
`General Motors Electric Vehicle IMPACT
`General Motors announced the first version of its ·
`electric vehicle, IMPACT, in January, 1990. Over the years
`there have been several modifications of the IMPACT. The
`following are
`the most recent specifications of the
`IMPACT. We have included only those features which are
`pertinent to this study.
`Performance:
`0-26.82 mis (0-60 mph) acceleration
`seconds.
`Top speed of35.76 mis (80 mph).
`Dimensions:
`Frontal area 2.2578 m2
`Drag coefficient 0.19.
`Curb weight 1347.17 kg.
`Design Features:
`102.16 kW three phase induction motor.
`IGBT power inverter module- 102 kW.
`High speed rate.cl 205/50 R15 tires.
`Our propulsion system is to meet the same
`p@rfonnance specifications as that of IMPACT. In light of
`the design methodologies presented in section III and the
`electric propulsion system performance analysis presented
`in section IV, we pick an induction motor with maximum
`speed of 14000 rpm and the rated speed of 3500 rpm
`(extended constant power range of 1:4). A comparison of
`our design EV with that of General Motors IMPACT is
`presented in Table IV. The cogent result of this exercise is
`
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`that our motor power rating for this vehicle is only 73 kW,
`as compared to the 102 kW motor in the prototype. This
`demonstrates the importance of the design approach
`presented in this paper.
`VI. Conclusions
`A design methodology for EV propulsion systems
`is presented based on
`the vehicle dynamics. This
`methodology is aimed at finding the optimal torque-speed
`profile for the electric powertrain. The design is to meet the
`operational constraints with minimum power requirement
`The study reveals that the extended constant power
`operation is important for both the initial acceleration and
`
`The design methodology of this paper was applied
`to an actual EV to demonstrate its benefits. Clearly the
`detailed design of a vehicle propulsion system is more
`complicated
`than
`in our examples. However,
`this
`methodology can serve as the foundation of the detailed
`design.
`
`Refereocs
`[1) A. Alison, ''Searching for perfect fuel ... in the clean air act",
`Conference Proceedings of Environmental Vehicles, 94, pp. 61 -87,
`Dearbom, Jan, 1994.
`[2] S. Barsony, "Infrastructure needs for EV and HBV", NIST
`Worlcslwp on Advanced Components for Electric and Hybrid
`Electric Vehicles, pp. 14-23, Gaithcnsburg, MD, Oct., 1993.
`
`Table rv: Comparison of General Motor EV, lMPACT and our designed EV.
`The rotational inertia constant km= I 1 ..
`General Motor IMPACT
`Gear Ratio
`Rated Motor
`Toique(N-
`m)
`.
`
`Our Desio-"".n EV
`Gear Ratio
`RntedMotor
`Torque (N-
`m)
`198
`
`11.45
`
`Rated Drive
`Shaft Torque
`(N-m)
`2268
`
`Rat.ed Motor
`Power(kW)
`
`102
`
`Rated Drive
`Shaft Toique
`(N-m)
`-
`
`Rated Motor
`Power(kW)
`
`72.6
`
`-
`
`cru1smg intervals of operation. The more the motor can
`operate in constant power, the less the acceleration power
`requirement will be.
`Several types of motors are studied in this context.
`It is concluded
`that
`the
`induction motor has clear
`advantages for EV, at the present. Brushless de motor must
`be capable of high speeds to be competitive with the
`induction motor. The switched reluctance motor may be
`superior to both of these motors, for vehicle application,
`both in size and cost However, more design and evaluation
`data is needed to verify this possibility.
`
`[3) Automotivt Handbook, Robert Bosch Grnbh, Germany, 1986.
`[4) A. Ralston and P. Rabinowitz, A fi.rst Course in Nunurical
`Analysis, 2nd ed., New York, McGraw Hill, 1978.
`[SJ A. G. Erdman, G. N. Sandor, MechaniJm Design: Analysis and
`Synthesis, Vol. /, New Jersy, Prentice-Hall, 1984.
`(6) H. A. Toliyat, K. M. R.ahIIlllll, and M. Ehsani, "Electric machines in ·
`electric and hybrid vehicle applications," Proceedings of the 1995
`lnternalional Conference on Power Electronics, Oct. 10-14, Seoul,
`Korea..
`
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