CHAPTER 2 SELECTION OF MOTORS FOR ELECTRIC VEHICLE PROPULSION

14 CHAPTER 2 SELECTION OF MOTORS FOR ELECTRIC VEHICLE PROPULSION 2.1 INTRODUCTION The selection of motors for electric vehicles is a major task. Since many literatures have been reported on various electric motors like DC motors, brushless DC motors, IMs, permanent magnet synchronous motors and switched reluctance motors, the selection of electric traction motors is based on the following factors. High output torque under low speed Easy speed control Reliable performance Cost effectiveness All the available AC and DC motors have been investigated and finally the best motor is selected for EV. Three types of electric motors are chosen and Finite Element Modeling analysis has been performed for all the three motors. The selections of EV motors are based on size and power rating of inverters, effects of temperature variation, both steady state and dynamic behaviour, simplicity in designing the motor and cost of the overall module. Specifically, the inverter size as small as possible with high rating should be selected and it should operate at all operating conditions with good steady

15 state and dynamic behaviour. Similarly, the construction of the motor should be simple and cost effective. By considering all these factors, the motor for EV application has been chosen. In this proposed work, aluminuim rotor IM, copper rotor IM, and Inserted Permanent Magnet (IPM) IMs are investigated for common design specifications. An IM is simple in construction, but it incurs cage loss. As compared to the aluminum rotor, the copper rotor has less cage loss. An IPM motor is an alternative for IM but the design of IPM motor involves more complex design methodologies. Moreover, the IPM motor uses rare earth magnet which increases the cost of the electric drive system nearly twice that of conventional IM. The analysis of motor construction is carried out in the proposed work by using JMAG finite element modelling software. The effect of temperature variation and losses are also analyzed and finally a motor is fabricated for the EV propulsion scheme. 2.2 PROPULSION SCHEME USING DC MOTORS The DC motors are one of the traditional types and it is suitable for a wide range of applications. These motors are easy to drive, fully controllable and readily available in all ratings and configurations Afjei et al (2010). However, the major disadvantages of the DC motors are the replacement and periodical maintenance for brushes and commutators Rahman et al (2003). The brushes on the commutator produce sparks when rotating at higher speeds called commutation, and also brush produces more noise (Electro Magnetic Interference/ Radio Frequency Interference) and carbon dusts are produced because of wear Xue et al (2008). Brushless DC (BLDC) Motor is an inside-out version of the DC motor, i.e. the rotor has permanent magnets instead of rotor core with windings which eliminate the brushes and commutator in BLDC motor. The

16 rotor windings are connected to an electronic module to control the speed, the speed of BLDC motor. The windings are energized in a pattern which rotate around the stator. The energized stator winding leads the rotor magnet and switches just as the rotor aligns with the stator. The easiest ways to know the correct moment to commutate the winding currents or by means of a position sensor. In the case of ideal brushless DC motor, flux produced by the permanent magnet cannot be adjusted due to the permanent magnet rotor. When maximum torque is required, especially at low speeds, the magnetic field strength is to be adjusted to get the maximum torque. The BLDC motor requires an electronic control for its operation which increases the complexity in designing the power controller (Yedamale 2003). The flux density of the permanent magnet will decrease due to increase in internal temperature, which leads to decrease in performance of the EV (Burwell 2012). 2.3 PERMANENT MAGNET SYNCHRONOUS MOTOR The Permanent Magnet Synchronous Motor (PMSM) uses permanent magnets to produce a magnetic field instead of electromagnets. Such motors have significant advantages such as high efficiency, smaller size, light weight, high reliability, and less maintenance (Vaclavek & Blaha 2009). Figure 2.1 depicts the block diagram which contains three-phase diode, rectifier, braking chopper, three-phase inverter, PMSM, position sensor, vector controller and speed controller. The three-phase diode bridge rectifier converts the three-phase AC supply into DC. The braking chopper block contains a DC bus capacitor and dynamic braking chopper, which is used to absorb the energy produced by the motor during deceleration. The DC supply is converted into sinusoidal AC by using the inverter and it is fed to the motor.

17 Three Phase Diode Rectifier A PMSM B C Braking Chopper Three Phase VSI Speed Reference Speed Controller d/dt Vector Controller Position Sensor Figure 2.1 Block Diagram of PMSM control Scheme The output from the Proportional Integral (PI) speed regulator enables a torque set point, which is applied to the vector controller block. The purpose of the vector controller used in this system is to get maximum torque from the motor. A rotor position sensor is used to sense the motor speed and which is compared with the reference speed and the difference between the two speeds are given to the vector controller. In PMSM, Electro Motivate Force (EMF) will be sinusoidal and it requires sinusoidal stator current to produce constant torque. PMSM is similar to a wound rotor synchronous motor, but it does not have any damper windings. The excitation of PMSM is done by the magnets mounted on the surface of the core (Pillay & Krishnan 2009). The function of the magnets is similar to the field winding but their magnetic field is constant (Patel et al 2012). The motor requires rotor position feedback information similar to that of BLDC motor which reduces its reliability. The torque produced at lower speed are very less, hence PMSM is not suitable for EV traction drive applications (Rippel 2007).

18 2.4 SQUIRREL CAGE IM Induction Motor (IM) plays a major role in EV drive systems. These motors are mostly used in modern EV because it provides better performance compared to other types. This motor is also called asynchronous or squirrel cage motor. Speed control of IM is not as easy as the speed control of DC motor, but with the advent of power electronics if becomes a possibility. IM have many advantages such as high efficiency, long life, robustness, high initial torque and better reliability (Xue et al 2008b). Both the frequency and the voltage are varied to offer V/F speed control of IM (Dong et al 2010) as depicted in Figure 2.2. Vdc ω* Sleep current compensation + ωsl idc + Current limiting ωe* + - V*s α* P W M Figure 2.2 Block Diagram of IM control scheme 3Ø IM The block diagram shown in Figure 2.2 has a Voltage Source Inverter (VSI) fed IM drive with V/F control. In this, a VSI control scheme for controlling voltage and frequency independently in order to maintain the constant V/F ratio and the drive does not require any feedback. Based on the

Efficiency in Percentage 19 set speed a frequency command is applied to the inverter, and a phase voltage command is directly generated from the frequency command by the gain factor, then the DC voltage of the inverter will be controlled. Figure 2.3 shows the efficiency versus the speed graph for the IM which runs at higher cursing speeds. 85 Efficiency Vs Speed (Characteristics of Electric Motors) 80 75 70 65 Induction Motor Axial Flux BLDC Motor Switched Reluctance Motor 60 55 50 Radial Flux BLDC Motor DC Series Motor 45 0 2000 4000 6000 8000 10000 12000 14000 Speed in RPM Figure 2.3 Comparison of electrical motors used in EV applications The power converter should maintain constant torque at all speed. A Direct Torque Control (DTC) scheme is proposed for the EV propulsion system. A mathematical model has been developed for the IM. 2.5 CHARACTERISTICS OF EV UNDER VARIOUS DRIVING CYCLES The traction motors have been selected based on the power torque characteristics of EV system and New European Driving Cycle (NEDC). Driving cycles for urban and extra urban transport are depicted in Figure 2.4. The traction motor should deliver maximum continuous power at top cursing speed and maximum torque at low speeds.

20 Figure 2.4 New European electric vehicle driving cycle In the Figure 2.4 slopes A-B represent the flat response of torque at the lower urban speeds, B-E represents decreased torque value (square of power for urban speeds), C-D represents the maximum converter current during the transient load and the slope D-E represents the actual converter power rating. The maximum load current and load voltage will decide the converter rating (Rippel 2007). 2.6 ANALYSIS FOR MAXIMUM INVERTER CURRENT The power converters used for EV propulsion should be able to deliver both continuous current ( i c ) and maximum current ( i m ). But in practical applications, power converters have different power curves. The converter should withstand maximum current ( i m the power rating of switches it is important that the ( i m constant. ). In order to reduce ) should maintain

power(w) 21 Figure 2.5 Maximum torque per ampere characteristics The Figure 2.5 shows the Maximum Torque Per Ampere (MTPA) curve for the electric drive scheme. The point O, A, B, and C show the operation of converting at continuous power mode. The point O, A o, B o, and C show the behavior of power converter under transient condition mentioned by (Burwell 2012). 10000 9000 8000 A' B' IPM Rotor 7000 6000 5000 A' B' Copper Rotor B' 4000 3000 Aluminum Rotor 2000 1000 Vout =120v, io=180amps 0 0 5000 10000 15000 rpm 12000 Figure 2.6 Power curve for Cu. rotor, Al. rotor and IPM rotor machine

22 Figure 2.6 shows the MTPA curves of traction motors under transient conditions with various speed ranges. The results shows the power consumed by the traction motors was twice the rated value during starting and running. This leads to increase in cost and power rating of the converter switches. The MTPA curve shows that the Copper rotor IM and IPM rotor machine have same power rating upto 10,000 RPM as mentioned by (Burwell 2012). 2.7 DESIGN SPECIFICATIONS FOR DRIVE MOTOR The Finite Element Modelling (FEM) has been carried out to analyse the flux density, power output and torque developed in motors used in EV. All the three rotor machines (Aluminium rotor, copper rotor, and IPM rotor machines) have been designed for the same rating and it is depicted in Table 2.1 as discussed by (Burwell 2012). Table 2.1 Common design specifications for aluminium rotor, copper rotor and IPM rotor IM Parameters Values Outer diameter of the stator in mm 280 Inner diameter of the stator in mm 115 Number of pole pair 2 Number of coil turns 24 Air gap length in mm 86 Maximum speed in RPM 3000 Peak voltage (V) 120 Maximum Load current in Ampere 150 Air gap length in mm 0.73

23 Figure 2.7 JMAG Model of IPM motor with 24 stator slots and 4 rotor poles Figure 2.7 depicts the JMAG quarter model of IPM rotor, which has 24 stator slots with star connected winding. The rotor comprises of rare earth magnets designed to produce 4 poles. Figure 2.8 illustrates the circuit parameter values of the IPM motor with a supply voltage of 120 V and a current of 150 A. The winding has the resistance of 12 Ohms and 28 turns with star connected winding.

24 Figure 2.8 JMAG simulation circuit parameters Figure 2.9 JMAG model of three-phase aluminium rotor IM and copper rotor IM with 24 stator slots and 4 poles

25 Figure 2.9 depicts the JMAG half model of Aluminium rotor and Copper rotor IM, which has 24 stator slots with star connected windings. Figure 2.10 shows the JMAG circuit model of copper rotor and aluminium rotor IM fed VSI. The power converter rating of the proposed model has 120 V, 160 Amps with star connected windings. The winding has 15 Ohms with 20 turns. Figure 2.10 JMAG circuit model of aluminium rotor and copper rotor IM 2.8 EFFECT OF TEMPERATURE VARIATION IN ROTOR The temperature variation is the major issue in case of PM motor. The increase in temperature leads to decrease in stator flux which results in reduction of torque. Hence it is necessary to study the performance of EV motors under different operating temperatures. JMAG designer tool is used to perform FEM analysis to determine the stator flux distribution in Cu. Rotor, Al. Rotor and IPM Rotor Machines. Figures 2.11 (a) and (b) show the JAMG model for copper rotor and aluminium rotor IM. The maximum allowable operating temperature of 120 C is considered for the analysis and the minimum temperature is 60 C. Figure 2.12 depicts the FEM model of IPM

26 motor at 120 C. The stator and rotor flux density have been reviewed for different temperature and speed as indicated in Figure 2.14. (a) (b) Figure 2.11 Magnetic Flux Distribution (a) JMAG model of 24 slots copper rotor IM, (b) JMAG model of 24 slots aluminium rotor IM

27 Figure 2.12 JMAG model of 24 slots IPM IM (a) (b) Figure 2.13 JMAG component selection tree (a) Copper material selection for rotor, (b) Aluminium material for rotor

Flux in Percentage(%) 28 Figure 2.13 shows the Material selection of JMAG model, Figure 2.13 (a) shows the copper rotor bars and Figure 2.13 (b) shows the aluminium rotor bars for mathematical analysis. 100 Temperature Vs Flux (Cu) Rotor Induction Motor 90 (Al) Rotor Induction Motor 80 70 60 (IPM) Induction Motor 50 40 0 20 40 60 80 100 120 Temperature Degree Celsius Figure 2.14 Effect of temperature rise on magnetic flux Figure 2.14 shows the temperature variation which has very less effect in IM. The power output of the IM is uniform for the wide range of speeds. The IM is not affected by the temperature variation during normal running conditions. Maximum efficiency from an IM is obtained by maintaining constant flux to voltage ratio using Direct Torque Control (DTC) scheme as discussed by (Burwell 2012). 2.9 PERFORMANCE EVALUATION EV requires a highly reliable motor for efficient propulsion. Three different motors have been analyzed and the parameters are shown in Figure 2.15. The IM has high reliable output at all the operating conditions. The current rating of the converter is in allowable limit so the inverter has minimum voltage and current limit. This minimum power rating of the

29 converter increases its reliability with reduced cost as discussed by (Burwell 2012). Figure 2.15 Transient characteristics of converter current 2.10 SPEED-TORQUE CHARACTERISTICS OF IMS The torque speed characteristics should be considered while selecting the motor for EV applications. The EV motor should deliver high torque at low speed range and high power at cursing speeds. An analysis has been carried out using JMAG for the three motors. The results are shown in the Figure 2.16. The comparison study shows that the IPM motor can deliver high torque at low speeds than Cu. Rotor and Al. Rotor IM. However, when considering the cost of overall system and reliability, Cu. Rotor IM replaces the IPM motor because of the requirement of rare earth magnetic material for IPM motor.

Torque in N-M 30 120 110 100 90 80 70 60 50 40 30 20 10 IPM Motor Cu Rotor IM Al Rotor IM 0 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 Speed in Rpm Figure 2.16 Speed-torque characteristics of IM and IPM motor 2.11 COST COMPARISON OF IM AND IPM MOTOR The IPM motor is widely used for traction purposes (James Goss 2013). However, it has a permanent magnet mounted on the rotor to produce the magnetic flux/field. These magnets are Neodymium and Dysprosium Oxide (rare earth magnets) magnets and their cost is very high. Figure 2.17 shows the cost variation of rare earth materials versus copper. Figure 2.17 Cost comparison chart of rare earth material versus copper

31 The cost of copper rotor is higher than the aluminium rotor, but it saves 40% of energy (James Goss 2013). In the case of IPM motor, the efficiency is around 92% and the efficiency of copper rotor is nearly 88%. Table 2.2 gives the cost comparison between permanent magnet motor and copper rotor IM. The amount of copper material required for stator is 4.5 kg for IPM and 9.1 Kg for Cu. Rotor IM as discussed by (Burwell 2012). Table 2.2 Cost comparison of 25 kw motors Materials Copper required for Stator winding Cu rotor For 25 KW motor Price in Rupees IPM Motor Price in Rupees 9.1kg 3966.72 4.5 kg 1921.38 Steel laminations 24kg 1487.5 24kg 1487.5 Permanent magnet 1.3kg 21073.20 Rotor copper materials 8.4 kg 3656.82 Rotor laminations 10.1kg 619.80 10.1kg 619.80 Total Cost in Rupees 9730.84 25101.88 Aluminium rotor IM reduces the overall efficiency by 4% as compared to that of copper rotor (Malcolm Burwell 2013). Since aluminum has only 56% conductivity when compared to copper, which leads inferior performance. The copper rotor IM consumes 10-15% more current than an IPM rotor machine to attain maximum torque. 2.12 FABRICATION OF COPPER ROTOR IM From the analysis of three motors, Copper rotor IM is the best motor to that of Permanent Magnet (PM) motor. The Cu. rotor IM is

32 fabricated with the following specifications. 25 kw, 120 Volts, 2 Poles, and 3000 RPM. 24 Slots Air Cooled IM Figure 2.18 Stator view of the copper rotor IM Figure 2.18 shows the 24 slot IM. The conventional 3 Phase, 415 Volts, 1500 RPM, 5 HP motor has been reconstructed with 3 Phase, 25 kw. Class F Lamination Sheets Figure 2.19 Class F insulation inserted in the stator slots

33 Figure 2.19 shows the insertion of Class F insulation in stator slots in order to withstand a current rating of 150 Ampere with the maximum allowable temperature of 110 C. R Phase Coil B Phase Coil Figure 2.20 Three-phase stator coils Y- Phase Coil Figure 2.20 depicts the stator winding with 24 Slots, 2 pole per phase, the pole pitch as 4 with double layer winding with the phase split of 60. Copper Conductors Copper End rings Figure 2.21 Proposed copper rotor

34 with end rings. Figure 2.21 depicts the copper rotor IM with copper conductors 2.13 MATHEMATICAL MODELING OF IM Modeling of IM is a complicated task because the IM variables are time-varying and multi-variable non-linear system. For ease of analysis, the following assumptions have been made. Iron losses have been eliminated Air gap flux density distribution is sinusoidal and air gap is radial Slot effect and saturation effects are neglected Symmetrical two poles and three-phase windings with edge effects are eliminated Linear Magnetic circuit From the physical model of IM, the three-phase stator is fixed on A, B, and C axis. The three-phase rotors are fixed on the b and the c axis as shown in Figure 2.22 Dilmi & Yurkovich (2005)

35 B Vb ia b i2b u2a a Va A Uc i2c Vc c C Figure 2.22 Mathematical model of 3-Phase AC IM where ABCis,, the stationary reference frame, a,b,c is the rotor reference frame, d (2.1) i L u Ri L r i dt t T T i i TL 2 0 r 1 1 1 T L 2 2 L t t J J 2 0 (2.2) Where, u u, u, u, u, u, u A B C a b c T T, vectors of stator and rotor voltages i i, i, i, i, i, i, vectors of stator and rotor current; A B C a b c d dt 0 r (2.3)

36 Where, J = is the total moment of inertia T L =is the load torque R diag R1 R1, R1 R2, R2 R2 Where, R1 is the resistance of the stator winding, R2 is the resistance of the rotor winding, L L L L 11 12 L 21 22 (2.4) Where, L L L L11 L L L L L L A AB AB AB A AB AB AB A, (2.5) La Lab Lab L22 Lab La L ab, (2.6) L ab Lab La cos cos( 120 ) cos( 120 ) T L12 L21 M cos( 120 ) cos cos( 120 ) cos( 120 ) cos( 120 ) cos (2.7)

37 where s v as is added as the zero sequence component, which may or may not be present. Note that in the above equations, voltage was considered as the variable. Similarly, the current and flux linkage equations can also be transformed into similar equations. Note that if θ is set to zero, the q-axis will be aligned with the a-axis. Once the zero sequence components are ignored, the transformation equations can be simplified as v as v (2.8) s qs 1 s 3 v v v 2 2 s bs qs ds (2.9) 1 s 3 s vcs vqs vds (2.10) 2 2 The inverse equations are obtained as s 1 1 vds vbs vcs (2.11) 3 3 s 2 1 1 vqs vas vbs vcs vas (2.12) 3 3 3 The synchronously rotating de qe axes rotate at synchronous speed ωe with respect to the ds qs axes and the angle θe is equal to ωe. The 2-phase ds qs windings are transformed into the hypothetical windings mounted on the de qe axes. The ds qs axes voltages can be converted or resolved into the de qe frame as follows: s s v v cos v sin (2.13) qs qs e ds e s s v v sin v cos (2.14) ds qs e ds e

38 The superscript e has been dropped henceforth from the synchronously rotating frame parameters. Resolving the rotating frame parameters into a stationary frame, Equations (2.13) and (2.14) can be written as s v v cos v sin (2.15) qs qs e ds e s v v sin v cos (2.16) ds qs e ds e Let us assume that the 3-phase stator voltages are balanced and are given by v V cos( t ) (2.17) as m e v cos( t 2 ds Vm e ) (2.18) 3 v cos( t 2 cs Vm e ) (2.19) 3 Substituting equations (2.15) (2.16) in (2.17) (2.19) yields s v V cos( t ) (2.20) qs m e s v V sin( t ) (2.21) ds m e Substituting equations (2.20) and (2.21) v V cos (2.22) qs m v sin ds Vm (2.23) Equations (2.20) and (2.21) show that s v qs and s v ds are balanced 2-phase voltages of equal peak values and the latter is at 90 angle phase lead

39 with respect to the other component. Equations (2.22) and (2.23) verify that sinusoidal variables in a stationary frame appear as DC quantities in a synchronously rotating reference frame. 2.14 SUMMARY A large number of today's electric vehicles are using AC motor controller systems because of their improved efficiency and lesser weight. These AC motors are commonly used in home appliances, machine tools, and are relatively inexpensive and robust. Compared with the different types of motors, the IMs performance will meet the required factors like torque, speeds, volume, control, reliability, and cost. So, an IM is proposed in this study for EV applications. An FEM model has been developed to analyze the effects in magnetic flux due to temperature variation. The cost of the overall EV scheme is less when compared to an IPM driving scheme. Cu. rotor IM has been selected in the proposed work and it is fabricated with the power rating of 25 KW with 120 Volts and 3000 RPM.