Electric Vehicle Performance Power and Efficiency 1 Assignment a) Examine measurement guide and electric vehicle (EV) arrangement. b) Drive the route according to teacher s instruction and download measured data. c) Plot the velocity v graph from the measured data. d) From the measured data calculate waveforms of the following powers: Power drawn from the battery P bat, converter output power P con, motor electric power P me and mechanical power at the wheel P m. e) Calculate total consumed energy W t, consumed energy without regenerative braking W c and regenerated energy W r. f) From the obtained powers calculate waveforms and average values of efficiencies: Powertrain efficiency η p, converter efficiency η con, motor electric efficiency η me and mechanical system efficiency η m. g) Obtained waveforms of powers and efficiencies plot into graphs according to the instructions at the end of the guide. h) Mark the acceleration, mixed regenerative braking and pure regenerative braking sections in the graphs. 2 Measurement Fig. 1: Block diagram of the drive. 2.1 Testing vehicle The assignment will be measured at Citroën Berlingo EV. This model is from the year 2000 and it was designated for the UK market (the steering wheel is on the right). EV powertrain is completely reconstructed from the original powertrain only the motor remained, to which we add our developed drive unit (Fig. 1) and a traction battery. The goal of the drive unit development was a device, which combines all the powertrain functions in one chassis. The drive unit comprises of these modules: - 1 -
Motor control module contains traction converter designed for DC motor with separate excitation and control unit for motor regulation according to the driver s request. Vehicle control unit evaluates the driver input through control pedals, measures vehicle velocity and manages lights, dashboard, and other peripheries. On-board DC/DC converter powers vehicle grid of 12 V DC. The DC motor with separate excitation is the main part of the drive. Its parameters are summarized in Tab. 1. The motor is accompanied with a gearbox with a constant gear ratio of 1:7.18. Gearbox output is fixed to the front axle shaft, which goes through the rotor center. Motor revolutions are measured by Hall sensor placed at the differential. Tab. 1: Motor parameters. Tag parameters Measured parameters Manufacturer Leroy-Somer Armature resistance R q (0.017 ± 0.001) Ω Type SA18 Armature inductance L q (285 ± 20) μh Power 15 kw (nom.) / 28 kw (max.) Excitation resistance R f (8.9 ± 0.2) Ω Revolutions 1650 RPM Excitation inductance L f (15 ± 2) H Armature 162 V / 110 A Common motor and 0.0906 Excitation 120 V / 9.5 A excitation constant k mφ The motor control module comprises of armature converter (Fig. 2 left), excitation converter (Fig. 2 right), which have separate power circuit, but they are regulated by the same microcontroller. Armature converter is a half-bridge, and excitation converter is a full-bridge, which provides reversion (change of motor revolution direction). Motor parameters are summarized in the Tab. 2. Fig. 2: Armature converter (left) and excitation converter (right). Tab. 2: Parameters of the armature and excitation converters. Armature voltage nominal 150 V, maximal 200 V Armature current nominal 100 A, maximal 150 A Excitation maximal 200 V, maximal 10 A Power nominal 15 kw Efficiency 90 % (v <25 km/h), 95 % A traction battery powers the drive. It consists of 12 traction lead accumulators Banner Energy Bull 955 01, with a total voltage of 150 V and a capacity of 60 Ah. Because the traction battery is made of lead accumulators, the BMS (battery management system) is not required. Fig. 3 depicts the motor control diagram. The motor is controlled by three PS regulators with anti-wind up (AWU) circuit. The PS regulators are connected to a cascade. The first regulator controls the armature current I q. The required current I q enters the regulator and the armature duty factor s q is at its output, which passes through the limiter to the armature converter. The armature current I q is measured at the converter output. The second regulator controls the armature duty factor s q. At its input is the required value s q, which is permanently set as the maximal value. Required excitation current I f is at the regulator output. Maximal possible - 2 -
value of the I f is set to 10 % I q by a limiter. This ensures behavior similar to a DC motor with serial excitation. Limiter also ensures that the calculated value of the I f ranges from 1,2 A to 10 A. Third PS regulator controls excitation current I f. Regulated value of the I f enters the regulator and the output excitation duty factor s f after limiting enters the excitation converter. Real value of the excitation current I f is measured at converter output. Fig. 3: Control diagram. Fig. 4: Simplified block diagram of the powertrain with marked measurement points of the powers and efficiency calculations. 2.2 Powertrain Schematics Fig. 4 depicts simplified block diagram with marked points of the power measurement or calculation. In the Fig. 4 bottom the efficiency calculations are illustrated. The calculation itself depends if the drive operates in the acceleration or regenerative braking regime, as explained further. Powers Power drawn from the battery P bat point A. P bat describes energy consumption or regeneration in the given moment. Corresponds with the converter input power. Converter output power P con point B. P con is a sum of the armature converter power P q and excitation converter power P f, which are measured in the same point. Corresponds with the motor input power. Motor electric power P me point C. P me represents power transferred through the motor air gap. Corresponds with the vehicle mechanical system input power. Mechanical power at the wheel P m point D. P m describes power at the output of the whole vehicle mechanical system, which consists of mechanical part of the motor, gearbox, differential, and power transmission to the wheel. - 3 -
Efficiencies Powertrain efficiency η p from P m, P bat. Can be calculated as η p = η con η me η m. Converter efficiency η con from P con, P bat. Motor electric efficiency η me from P me, P con. Mechanical system efficiency η m from P m, P me. 2.3 Acceleration a Regenerative Braking For the correct power and efficiency calculation, it is necessary to understand vehicle behavior from the consumption perspective. Individual states are depicted in Fig. 5. An arrow from left to right corresponds with appliance behavior, and thus the power is positive. An arrow from right to left corresponds with the source behavior, and the power is negative. Following three states are important for the calculation: Acceleration (Fig. 5 a)) The powertrain accelerates the vehicle. The battery current I bat and the armature current I q must be both higher than zero at the same time. Mixed regenerative braking (Fig. 5 b)) The regenerative braking occurs; however, the regenerated power does not cover the drive s own consumption (primarily excitation input power P f ). Part of the power must be supplied from the battery. Corresponds with the situation, when the battery current I bat is higher than zero and armature current I q is smaller than zero. Pure regenerative braking (Fig. 5 c)) Regenerated power covers fully the drive s own consumption a rest of it recharges the battery. Corresponds with the situation, when the battery current I bat is smaller than zero. Fig. 5: Power flows a) acceleration, b) mixed regenerative braking, c) pure regenerative braking. 2.4 Measurement Procedure In the beginning, measure the air temperature for the aerodynamic drag F ad calculation. Each student drives the route according to the teacher instruction. The route must include longer continuous section of driving, with a portion of the pure regenerative braking. Attention! Stronger energy regeneration is set at acceleration pedal to achieve the pure regenerative braking effect easily. Therefore, expect stronger braking effect after acceleration pedal release, this effect is also proportional to the release rate. In EVs, there is usually set a mild regenerative braking at the acceleration pedal after its release and strong at the braking pedal. In our case, if the regenerative braking would be placed on the brake pedal, it would not be possible to distinguish the effect of the regenerative braking and the mechanical brake. Thus, it would not be possible to calculate efficiencies during the regenerative braking. Therefore, use the brake carefully. The measuring system is part of the powertrain electronics. A laptop is connected to the vehicle interface to obtain the measurement data. Data are measured with frequency of 5 sample per second. At the end of the - 4 -
measurement, the teacher will download measured data and distribute it to the students. Do not forget to note your measurement number to prevent any misunderstanding. 2.5 Measured Data Preparation You will obtain.log file containing measured data. Its name will consist of the measurement date and time. Before processing the measured data, verify that they are yours. Import the data into MS Excel or other suitable SW for the processing. File with measured data consists of the header, the data itself and the termination. The header and the termination contains only the information about the beginning time and ending time, respectively, and thus not important for the calculations. Measured data are organized into columns with fixed width, there is no delimiter between columns (see Fig. 6). On each line, there is at first the quantity symbol and then its value. Fig. 6: Example of a file with measured data. Following quantities are measured (measurement unit is in the square brackets): Armature current I q [ma] point B, excitation current I f [ma] point B, battery current I bat [ma] point A, battery voltage U bat [mv] point A, armature voltage U q [mv] point B, excitation voltage U f [mv] point B, vehicle velocity v [mm s 1 ] point D, revolution sensor output x [ ], mechanical braking indicator b [ ], finite state machine FSM [ ], energy drawn from the battery since last charging E [Wh] Before starting with calculation, prepare the data in following way: From the measured data, select only the longest section of the continuous driving. Remove sections with a vehicle turning or stopping. To select the suitable section, you may use the auxiliary graph of velocity in dependence on time or revolution sensor output x. Convert all the quantities except for x, b to the basic SI units (mv V). Smoothen converted values by moving average from 5 values. Remove column FSMp, it is a powertrain state quantity designated only for condition control. Column E informs the teacher about the battery condition and does not figure in the calculations. Remove it too. Add column containing time t [s]. Data were measured with the sampling period T s = 0,2 s. Convert the velocity from z m/s to km/h for the velocity graph. For the calculations use velocity v in m/s. If the excitation current I f is negative, the reversion occurs (the vehicle goes in backward). Reverse sign only marks the reversion, the current is always drawn from the battery and supplied to the excitation. - 5 -
Only during the reversion, the current flows through the opposite direction. Reversion should not occur in the evaluated section. Excitation voltage U f is a control quantity for the I f. Reversion actually occurs only if the I f is negative, not if the U f is negative. If you choose a suitable section, U f will never be negative. Round the converted currents to 1 decimal place (hundreds of ma). 2.6 Calculation of the Powers The first step of the calculation is the calculation of powers. It is followed by the consumed energy calculation and finally with the calculation of efficiencies. 2.6.1 Power Drawn from the Battery P bat Calculate the power drawn from the battery P bat from values of the voltage and current measured between traction battery and converter (section A in Fig. 3) see equation (1). If the current I bat is negative, the power flow turns (P bat is negative) and the battery is charged. P bat = U bat I bat (1) 2.6.2 Converter Output Power P con Converter output power corresponds to the sum of armature power P q and excitation power P f see equation (2). Currents at the converter output are measured (section B in Fig. 3), but armature voltage U q and excitation voltage U f is obtained from converter duty factors and voltage drops. P con = P q + P f = U q I q + U f I f (2) Regenerative braking condition is negative armature current I q. Part of the regenerated power is consumed by the excitation. If the power drawn by excitation P f is smaller than regenerated P q, excess power recharges the battery. 2.6.3 Motor Electric Power P me Motor electric power P me is considered as the power in the air gap of the electric machine (point C in Fig. 3). It can be calculated according to the (3). P me = U i I q (3) Armature current I q is constant in the whole armature circuit, therefore it is possible to use the values from the section C in Fig. 3. Nevertheless, induced voltage U i must be calculated according to equation (4) from armature resistance R q, armature current I q and armature voltage U q. This equation is based on the equation for the armature voltage U q, which is a part of the DC machine mathematical model. Voltage induced at armature inductance L q can be neglected due low value of the L q. Similarly, it is possible to omit the resistance change due the temperature, as it is lower than resistance measurement error. U i = U q R q I q (4) 2.6.4 Mechanical Power at the Wheel P m Mechanical power at the wheel P m (point D in Fig. 3) can be calculated from the vehicle velocity v a tractive force F t see the equation (5). P m = F t v (5) F t + F l = F d (6) 2.6.4.1 Tractive Force F t Polarity and Calculation To calculate the tractive force F t use the force movement equation (6). F l labels the load force and F d the dynamic force. The forces are positive if they work in the direction of the vehicle movement, and negative, if they - 6 -
work against the vehicle movement therefore, F t = 100 N accelerates the vehicle and on the contrary, F t = 100 N decelerates the vehicle. For the correct tractive force F t calculation, it is necessary understand the differences among the possible cases. If the vehicle accelerates, the tractive force value is calculated according to the equation (7) and it will have positive polarity. F t = F l + F d (7) If the vehicle decelerates, two cases can be distinguished in the dependency on the ratio between load force F l and dynamic force F d value: Provided that F l > F d, the tractive force F t works in the vehicle direction, but it is not large enough to overcome the load force F l and the vehicle decelerates. Calculate the tractive force absolute value according to the equation (8). Calculated force will be positive as in the previous case. F t = F l F d (8) Provided that F z < F d, tractive force F t decelerates the vehicle and its absolute value can be calculated according to the equation (9). In this case, the tractive power F h is negative, similarly as resulting mechanical power at the wheel P m. F t = F d F l (9) 2.6.4.2 Dynamic Force F t Calculation Dynamic force F d comprises of the linear acceleration force F dl and the rotary acceleration force F dr. Linear acceleration force F dl can be calculated according to the equation (10). F dl = m dv dt Due to the fact, that the calculation is based on the discrete values, the derivation is transformed into a difference. Index k labels the present step value and the index k 1 labels the previous step value. - 7 - (10) dv v(k) v(k 1) (11) dt T s Subsequently, calculate the linear acceleration force F dl from the equation (12). For the F dl calculation it is necessary to know the vehicle weight m, which is a sum of the empty vehicle weight m 0 = 1277 kg, the teachers weight m c, and the students weight m s see (13). v(k) v(k 1) (12) F dl = m T s m = m 0 + m c + m s (13) Rotary acceleration force F dr describes the energy stored in the rotary masses. For its calculation, it is necessary to know the moment of inertia of all the rotary masses in the vehicle. Since we do not know the moment of inertia value, we will estimate the rotary acceleration force F dr as 5 % of the linear acceleration force F dl : Thus, the total dynamic force can be calculated as: F dr = 0,05F dl (14) F d = F dl + F dr (15) 2.6.4.3 Load Force F l Calculation Similarly to the dynamic force F d, also the load force F l is a sum of sub-forces: Rolling resistance force F rr, aerodynamic drag F ad, hill climbing force F hc a turning force F tu. Last two forces are neglected: Hill climbing force F hc measured route is leveled, and turning force F tu all the turns are gone through slow enough. Load force F l is then a sum of F rr and F ad. Rolling resistance force F rr calculation is based on the recalculation of the value for the empty vehicle F ov0 to the value for the vehicle with the teacher and the student see equation (16). Value for the empty vehicle
F ov0 = 270 N was measured by the coast-down method and verified by a calculation. F rr is equal to zero, when the vehicle does not move. F rr = F rr0 m m 0 (16) Calculate the aerodynamic drag F ad according to the equation (17), vehicle parameters necessary for the calculation are summarized in the Tab. 3 and air density can be obtained by linear interpolation from Tab. 4. Omit the wind speed. F ad = 1 2 ρac dv 2 (17) Tab. 3: Aerodynamic drag calculation parameters. Frontal area A [m 2 ] 2.66 Aerodynamic drag coefficient C d [ ] 0.37 Tab. 4: Values for the air density calculation. Temperature T [ C] -20-10 0 5 10 15 20 25 30 35 40 Air density ρ 1.395 1.342 1.296 1.270 1.247 1.226 1.205 1.185 1.165 1.146 1.128 [kg.m -3 ] Total load force can be calculated as F l : 2.7 Energy Consumption Calculations F l = F rr + F ad (18) Calculate the total consumed energy W t (19), consumed energy without regenerative braking W c (20) and regenerated energy W r (21). Use the trapezoidal approximation. List the results in Wh. n n W t = 1 3600 P bat(k 1) + P bat (k) 2 k=2 W c = 1 3600 P bat(k 1) + P bat (k) 2 k=2 n W r = 1 3600 P bat(k 1) + P bat (k) 2 k=2 T s T s (19), (P bat (k 1) + P bat (k)) > 0 (20) T s, (P bat (k 1) + P bat (k)) < 0 (21) Tab. 5: Efficiency calculations. Acceleration Pure regenerative braking Powertrain efficiency η p Converter efficiency η con Motor electric efficiency η em Mechanical system efficiency η m η p = P m P bat η con = P con P bat η em = P me P con η m = P m P me η p = P bat P m η con = P bat P con η em = P con P me η m = P me P m 2.8 Efficiency Calculations Calculate the powertrain efficiency η p, converter efficiency η con, motor electric efficiency η me and mechanical system efficiency η m from the calculated powers. The calculation is different for the acceleration and the regenerative braking the power flow is the opposite during the regenerative braking (see ). Calculate the efficiencies if the power flows only in one direction i.e., during acceleration and pure regenerative braking. If the - 8 -
mixed regenerative braking occurs, the calculations of the powertrain efficiency η p and converter efficiency η con does not have physical meaning. Also omit the steps in which the mechanical braking occurs (b = 1). Calculate the averages of each waveform. Calculate the averages separately for acceleration and pure regenerative braking. Neglect the steps in which you did not calculate the efficiency or the efficiency is equal to zero. Consider only the steps in which the efficiency does not exceed 100 %. Note: Equation η p = η con. η em. η m is valid only for the efficiency values in the individual steps, not for the efficiency averages. 3 Measurement Protocol Besides the necessities described in Requirements for submitted protocols (available at the course website at motor.feld.cvut.cz), every protocol must be present the results in the following way. 3.1 Graphs All the graphs should be large enough for easy reading. If you use the multiple vertical axes, all the axes must use the same grid. Mark the acceleration, mixed regenerative braking and pure regenerative braking sections in the graphs. 3.1.1 Velocity and Acceleration Graph Plot the velocity v (in km/h) and the acceleration a (in m/s 2 ). 3.1.2 Graph of Powers Plot all the calculated powers (P bat, P con, P me a P m ) into a same plot. If the power is consumed (direction from the battery to the wheel acceleration, mixed regenerative braking), then depict the value as positive. If the power is regenerated (direction from the wheel to the battery mixed, pure regenerative braking), then depict the value as negative. 3.1.3 Graph of Efficiencies Plot all the calculated efficiencies (η p, η con, η em a η m ) into the same plot. Depict the efficiencies for the acceleration as positive and for the regenerative braking as negative. 3.2 Numerical Results Table In one table list the calculated energies (W t, W c, W r ) and average efficiencies (η p, η con, η em a η m, acceleration and regenerative braking separately). 24.11.2017 Michal Košík, Pavel Skarolek - 9 -