A New Electric Accessory Drive System for Hybrid Electric Vehicles E. Mese, Y. Yasa, H. Akca, M. G. Aydeniz, M.Garip Yildiz Technical University, Istanbul,Turkey
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[email protected] Abstract— This paper presents a new electric accessory drive system for hybrid electric vehicles. The system offers cost and space advantages. In this concept, dual winding electric machine with simultaneous motor and generator functions is used. The electric machine provides power for all accessory loads such as steering pump, compressors, and 12V loads. If the engine is on, accessory loads are driven by engine through a belt like conventional vehicle. If the engine is off, electric machine’s motoring action starts and provides mechanical power to accessory loads. Simultaneously, the generator windings provide power for 12 V loads. Converter options for generator were also investigated and preliminary experimental results were presented. Keywords: Permanent Magnet Synchronous Machine, Concentrated Winding, Dual Winding Electric Machine, Electric accessory drive system, Hybrid Electrical Vehicle Application
I.
INTRODUCTION
Today’s automotive industry has tendency of manufacturing high-efficient hybrid electric vehicles (HEV). In conventional vehicles, internal combustion engine (ICE) provides power for traction of the vehicle and power for accessory loads in the vehicle. Accessory loads, such as power steering pump, air conditioner compressor, Lundell alternator increase safety and comfort level in vehicles. These loads are driven by ICE and require power whether the vehicle is in motion or not. In other words, even if the vehicle is not moving, ICE should not stop in order to provide power to accessory loads. This increases emission level in city driving unnecessarily due to frequent stop and go. Stop-start strategy is a technique to increase fuel economy in HEVs. With this strategy, ICE stops when vehicle stops and ICE restarts when the driver intends to move the vehicle. This implies that all accessory loads would be shut down when the vehicle stops moving. Some of these loads are critically important and their shut-down cannot be tolerated.
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II.
COMMON TECHNIQUES FOR ACCESSORY DRIVING
Drive-by Wire is a commonly used technique to solve this problem, in which separate independent electric motors provide driving power for each individual accessory load. Usage of several electric motors at various power levels increase cost which can be considered disadvantage [1]. A cost effective solution so called electric accessory drive system (EADS) has single electric motor and this motor drives accessory loads through belt as shown in Fig. 1. When ICE is in operation, accessory loads take their mechanical power from ICE through belt. When ICE stops, EADS starts operation and supplies power to the accessory loads through belt [2]. In conventional vehicles, electric accessories such as power window, heated seat are getting their power from low voltage 12 Volts battery, where the battery is charged by Lundell alternator. In HEVs, such loads are still powered by low voltage battery. Low voltage battery is charged by high voltage battery by using a DC/DC converter or by an alternator as in the Fig. 1. III.
PROPOSED METHOD FOR ACCESSORY DRIVE IN HEVS
In this paper, a dual winding electric machine is proposed for EADS applications in HEVs. Fig. 2 shows principle diagram of the proposed system. Fig. 3 shows dual winding electric machine and its power converters. Windings of the electric machines will be concentrated winding and electrically and magnetically independent. When the traction motors of the vehicle shut down, the proposed dual-winding electric machine runs as motor and drive accessory loads. Secondary winding set in the electric machine will operate as generator and supply low voltage needed for 12 volt battery charging and other electrical accessories. With the proposed technique, both motoring and generating operations can be implemented in a single housing of electric machine. Beside the possibility of eliminating DC/DC converter from the system, which results in lower cost, the proposed system also offers solution for packaging space problems in HEVs. Table I gives a comparison among different accessory drive solutions.
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Figure 1. Single-motor electrical accessory drive system of hybrid electric vehicles (EADS).
Figure 2. Proposed electrical accessory drive system with dual winding electric machine.
Figure 3. Concentrated dual winding machine with its power converters showing complete system. TABLE I.
PART COUNT COMPARISON AMONG DIFFERENT ACCESSORY DRIVE SYSTEMS
Conventional Nonhybrid
EADS
EADS with Dual Winding (DW)
Drive by Wire
Crank Pulley
Crank Pulley
Crank Pulley
Crank Pulley
Belt
Pully Clutch
Pully Clutch
Belt
Water Pump
Belt
Belt
Water Pump
Alternator
Water Pump
Water Pump
Air Pump (Unique)
Air Pump
Alternator
DW eMotor + Inverter
eMotor for Air Pump
Power Steering Pump
eMotor + Inverter
Rectifier
EPS Pump (Unique)
AC Compressor
Air Pump
Air Pump
eMotor for EPS
Power Steering Pump
Power Steering Pump
AC Compressor (Unique)
AC Compressor
AC Compressor
eMotor for AC Comp. DC/DC Converter as Auxiliary Power Unit
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IV.
CONCENTRATED WINDING ELECTRIC MACHINE IN EADS
Dual winding electric machine brings a new approach for applications where simultaneous motor and generator operation is needed. In the proposed Permanent Magnet Synchronous Machine (PMSM), coils are concentrated around single stator tooth which has many benefits compared to distributed coils. Rotor has surface mounted type magnets. Cited advantages of concentrated winding are wider flux weakening capability due to higher direct axis inductance, higher efficiency due to shorter end winding, easy manufacturability and the last but not the least magnetic isolation between coils which allow simultaneous motor and generator operation. On the other hand, lower high speed efficiency due to high pole count, higher torque ripple are also cited disadvantages [3,4]. The new machine developed has 6kW motor output and 3kW generator output. The unique advantage of new design is that it dramatically reduces the physical dimensions of the machine. A comparison is given in Table II between the proposed concept and conventional approaches. The values in the table were obtained under the same electrical and magnetic loading conditions. It is clear from these values that the sum of the dimensions of 6kW motor and 3 kW generator is about the same as the dimensions of the proposed electric machine. A more realistic comparison could be obtained after the thermal analysis of new design. Because there is some difference between a single machine and two-machine configuration in terms of electrical loading. In other words, electrical loading values for 6 kW and 3 kW machines may be 5-10% less. Therefore, physical dimensions will be greater than what is specified in Table II.
Note that higher amplitude of the generator phase currents do not show appreciable increase or decrease in the motor average torque output as well as in the amplitude of torque ripple. The same figure also shows the motor winding current magnitude is fixed during the test to indicate no control action was taken on the motor winding to regulate the torque output. B. Dual Winding Machine’s Motor Operation Performance After showing an effective decoupling between motor and generator winding sets, motor dynamic performance is evaluated by using Matlab/Simulink model of the motor of which the parameters are given in Table III. As stated before motoring operation occurs during engine-stop period and all mechanical accessories receive their mechanical power from electric motor. Similarly, generator side of dual winding electric machine receives its mechanical power from the electric motor. From this perspective, dual winding machine runs as a rotary transformer where high voltage motor side power is converted into low voltage generator side power through dual winding electric machine. Motor operation occurs at1500 rpm constant speed. In Fig. 5 and 6, some waveforms about the performance of the motor are given. At t=0, the motor is loaded with almost full load and at t=1.25 sec., motor load is reduced to half load. Waveforms show that the motor is able to track the load variation by varying its q axis current value in a speed control loop. Motor is designed so that base speed is to be around 1500 rpm and no flux weakening is required during normal operation.
Space advantage of the proposed machine manifests itself when some unaccounted aspects of design are considered. These are end winding length, endbell thickness, one position sensor instead of two and related space reduction, less left-out space for packaging constraints. A. Magnetic Decoupling Performance of Concentrated Dual Winding Machine Finite element modeling with Maxwell 2D Transient has been used to show how concentrated winding approach is effective in decoupling two winding sets from each other. The magnitude of generator winding current is varied as in Fig. 4 to observe how motor operation is influenced by the variation of generator winding current. Increasing generator current magnitude emulates this action. TABLE II.
COMPARISON OF DIMENSIONS AND TORQUES BETWEEN THE MACHINES.
Figure 4. Motor torque variations for varying generator currents. TABLE III.
MAJOR PARAMETERS OF DUAL WINDING ELECTRIC MACHINE UNDER CONSIDERATION GENERATOR
MOTOR
Number of poles
22
Number of Slots
24
Rs-stator phase resistance (Ω)
0.0015
0.033
Lq-inductance (mH)
0.043
0.776
Ld-inductance (mH)
0.043
0.776
Diameter (mm)
6kW Motor 240
3kW Generator 240
9kW New Machine 240
Flux linkage by magnets (V.s)
0.0125
0.055
Length(mm)
77
43
120
Rated torque (Nm)
21.23
38.21
Torque(Nm)
38,2
19,2
57,4
Output power (W)
3000
6000
Speed(rpm)
1500
1500
1500
0.033
0.033
Parameters
2
Inertia (kg.m )
1911
C. Dual Winding Machine’s Generator Operation Performance Generator operation occurs over much wider speed range. During engine stop mode, Speed of the generator is fixed at 1500 rpm. However, during vehicle movement, engine drives dual winding machine. Hence, generator input speed depends on engine speed which is a function of vehicle’s torque and velocity demand. In this study generator speed is bounded between 1500 rpm and 6000 rpm. Given the fact that crank shaft and dual winding machine are connected with pulley and belt, this speed range corresponds to 600 rpm and 2400 rpm at the engine side due to pulley ratio. The selection of 6000 rpm as maximum speed is solely due to test dynamometer limitation. Generator operation of the proposed machine is exercised for two different options. In the first option, a controlled rectifier is used for AC to DC conversion. In the second option, an uncontrolled rectifier and a buck type DC/DC converter is used for the same purpose. Fig. 7 and 8 show the performance with controlled rectifier. In Fig. 7, motor side
load is reduced by half and this causes the speed to rise until the controller takes action. Since motor and generator are sharing the same shaft, generator sees the same increase in speed. However, closed loop voltage and current controller at the generator side takes action and voltage rise is prevented as shown in Fig. 7. In Fig. 8, steady state performance of the generator at 6000 rpm with controlled rectifier with closed loop voltage regulation is shown. In Fig. 9 and 10, the performance of the second option in which generator output voltage is processed through an uncontrolled rectifier and buck type DC/DC converter. Similar situation as in controlled rectifier case is analyzed to see the performance of the regulator. Fig. 9 shows the resulting waveforms. The performance of the generator along with the regulator is tested for 6000 rpm operating speed and performance is shown in Fig. 10. From the comparison of the related waveforms regarding controlled rectifier and DC/DC converter case, controlled rectifier causes more torque ripple at the generator side. On the other hand, high-frequency current and voltage ripple at the DC output is a bit higher for DC/DC converter case. 40 (A), speed(Rpm)
(Nm), speed(Rpm)
50 40 30
(V), I
(Nm), T
load
load
20
load
10
(Nm), V
T
T
elec
elec
n
0
/50
mech
elec
T
load
0.5
1 1.5 Time(second)
2
T
-10 0
2.5
(A) and speed(Rpm)
20 10
i
100
elec
V
load
I
-10
load
n
/10 /50
mech
-20 -30 0.5
1 1.5 Time(second)
2
2.5
Figure 7. Generator regulation performance in response to speed change around 1500 rpm for controlled rectifier case.
q
n
T
0
0
Figure 5. Variation of electromagnetic torque and speed in response to changing mechanical load for motoring.
20
/50
dref
(A), i
dref
qref
10
0
-10
load
40
load
20
V
qref
(Nm)
i
60
d
elec
i
(A), T
i
80
(V), I
mech
d
i (A), i (A), i
30
V
load
I
-20
/10
T
0
elec
q
0
load
0.5
1 1.5 Time(second)
2
-30 0
2.5
Figure 6. Variation of speed, q and d axis currents in response to changing mechanical load for motoring.
0.05
0.1 Time (second)
0.15
0.2
Figure 8. Generator performance at 6000 rpm for controlled rectifier case.
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electrical torque at 1821 W generator output power. This is slightly higher than 50 % of rated generator power. Generator’s currents are shown in Fig. 13.
30
As seen in Fig. 13 generator phase currents are non sinusoidal and cause significant torque ripple generation. However due to high pole count number, ripple frequency is very high. For this reason mechanical impact of ripple should be low in the application.
20 10 T
elec
0
V
load
I
-10
/10
Similar analyses are performed for the second option which includes an uncontrolled rectifier and a buck converter. Fig. 14 shows generator phase currents and Fig. 15 shows induced torque variation at 1800 W generator output power.
load
n
/50
mech
-20 -30
T
elec
(Nm), V
load
(V), I
load
/10(A), Speed(Rpm)
40
0
0.5
1 1.5 Time(second)
2
2.5
Idc
ANSOFT
125.00
Figure 9. Generator regulation performance in response to speed change around 1500 rpm for uncontrolled rectifier and DC/DC converter case.
Idc [A]
100.00 75.00 50.00
20
10
0.00 0.00
-10
20.00
30.00 Time [ms]
40.00
50.00
Torque
1.05
V
ANSOFT
load
I
-20
/10
-2.50
load
Torque [NewtonMeter]
T
elec
-30 0
0.05
0.1
0.15 0.2 Time (second)
0.25
0.3
0.35
-7.50
-12.50
Figure 10. Generator performance at 6000 rpm for uncontrolled rectifier and DC/DC converter case.
V.
10.00
Figure 11. Output current for controlled rectifier case at 1821 W output power.
0
-17.50
COUPLED FINITE ELEMENT-CIRCUIT ANALYSIS OF THE PROPOSED ELECTRIC MACHINE
Circuit coupled finite element analysis has been conducted in Maxwell 2D and Simplorer environment. Main goal of the analysis is to estimate efficiencies of electric machine, converter and overall system at different operating conditions. Analyses are performed at 1500 rpm shaft speed because it is most critical speed where both motoring and generating operations occurs in the intended applications. Analyses are carried out for the following operating conditions; half load motor / no load generator operation, full load motor / no load generator operation, half load generator / no load motor operation, full load generator / no load motor operation, full load motor / half load generator operation, full load motor / full load generator operation. Input and output power, all losses, and efficiencies are given in Table V, Table VI and Table VII. For the thyristor controlled rectifier case, Fig. 11 shows output current of the rectifier and Fig. 12 shows induced
1913
-22.50
0.00
20.00
40.00 60.00 Time [ms]
80.00
100.00
Figure 12. Induced generator torque for controlled rectifier case at 1821 W output power. Generator Currents
150.00
ANSOFT
100.00 Ias, Ibs, Ics [A]
V
load
(V), I
load
/10(A), T
elec
(Nm)
25.00
50.00 0.00 -50.00 Curve Info
Current(PhaseA) Current(PhaseB) Current(PhaseC)
-100.00 -150.00 30.00
32.00
34.00
Time [ms]
36.00
38.00
40.00
Figure 13. Generator’s currents for controlled rectifier case at 1821 W output power.
nonzero trigger angle creates phase shift in the motor current during half load operation. This is not only the cause for some extra reactive power demand out of the generator but also significant torque ripple generation. In particular, it is very striking that controlled rectifier draws current with less harmonic content compared to uncontrolled rectifier with DC/DC converter. But its torque ripple is much higher than its counterpart. This can be interpreted as phase angle of generator current play more vital role than current harmonic content in torque ripple generation. Another interesting observation is made about the magnitude of generator phase current for two converter options. As seen in Fig. 13 and Fig. 14, controlled rectifier draws current with much higher magnitude. This is interpreted as extra reactive power demand when thyristor controlled rectifier is used. In Table VII, apparent power and power factor values are given for two converters and load level cases. Extra reactive power could be tied to trigger angle of thyristors as well as nonequivalent passive component sizing in two converter types as shown in Table IV.
Figure 14. Generator phse currents for uncontrolled rectifier with DC/DC converter case at 1800 W output power.
TABLE IV.
PASSIVE COMPONENTS OF CONTROLLED RECTIFIER AND UNCONTROLLED RECTIFIER WITH DC/DC CONVERTER
Figure 15. Generator induced torque for uncontrolled rectifier with DC/DC converter case at 1800 W output power.
Converter Type
Inductance
Capacitance
Thyristor Controlled Rectifier
225 µH
1700 µF
Uncontrolled Rectifier with DC/DC Converter
100 µH
4400 µF
Torque ripple performance of two different converter usages is worth discussing. In uncontrolled rectifier case,
TABLE V. Motor Operation (Generator without Load)
Input Power (W)
Output Power (W)
Copper Loss (W)
Core Loss (W)
Efficiency (%η)
19.822
3378
3112
36.42
163.95
92.12
Full Load
38.734
6515
6075
146
190
93.24
LOAD CONDITIONS AND EFFICIENCIES FOR SIMULTANEOUS MOTOR AND GENERATOR OPERATION.
Motor Operation & Generator Operation DC-DC Converter Model
Shaft Torque (Nm)
Half Load
TABLE VI.
Controlled Rectifier Model
LOAD CONDITIONS AND EFFICIENCIES FOR MOTOR ONLY OPERATION.
Shaft Torque (Nm)
Machine Input Power (W)
Motor Output Power (W)
Generator Input Power (W)
Mechanical Load Power (W)
Total Copper Loss (W)
Total Core Loss (W)
Machine Efficiency (%η)
Motor
Generator
Full Load
Half Load
26.85
6530
6080
1865
4215
156.68
184.32
93.10
Full Load
Full Load
16.05
6563
6080
3560
2520
192.49
174.55
92.64
Motor
Generator
Full Load
Half Load
25.23
6525
6080
2121
3961
184.68
235
93.18
Full Load
Full Load
11.23
6671
6080
4320
1763
270.68
225
91.14
1914
TABLE VII.
LOAD CONDITIONS AND EFFICIENCIES FOR GENERATOR ONLY OPERATION.
Generator Operation (Motor Winding Unexcited) Shaft Torque (Nm)
VI.
Uncontrolled Rectifier & DC-DC Converter
Controlled Rectifier Half Load
Full Load
Half Load
Full Load
13.500
27.500
12.096
23.035
Mechanical Input Power (W)
2121
4320
1899
3616
Generator Output Power (W)
1821
3874
1701
3383
Generator Output Power (VA)
3839
4638
1857
3574
Generator Power Factor
0.474
0.835
0.916
0.946
DC Output Power (W)
1675
3250
1485
2820
Generator Copper Loss (W)
39
125
11.064
47.47
Generator Core Loss (W)
235
225
137.66
132.10
Converter Loss (W)
146
624
216
563
Generator Efficiency (%)
85.85
89.67
89.62
93.55
Converter Efficiency (%)
87.73
83.9
87.30
83.35
EXPERIMENTAL RESULTS
An experimental setup was built to test performance of the proposed electric machine. Fig. 16 shows preliminary setup of the experimental system. Dual winding PMSM was driven by a squirrel cage induction motor. A rotary torque sensor between induction motor and dual winding PMSM measures torque level dynamically. Four-quadrant adjustable speed drive system controls induction machine so that both motoring and generating operation would become possible. Induction machine in the experimental setup represents mechanical load in accessory drive system as well as internal combustion engine of the vehicle. There also exists a resistive load bank which imitates electrical accessory loads in the vehicle. Generator side of the dual winding PMSM will be ultimately connected to resistive load bank through either a thyristor controlled rectifier or uncontrolled rectifier/DC-DC converter set. However, at this stage of the preliminary setup, generator side of the proposed PMSM is loaded with an uncontrolled rectifier and resistive load bank. A bulk DC link capacitor also exists at the output of the rectifier.
As seen from waveform, there is no difference between no load back EMF waveform and rectifier loaded back EMF waveform of the motor winding. Experimental results show that there is no magnetic coupling between motor and generator winding sets with uncontrolled rectifier application. Proposed PMSM was designed to operate as motor at 1500 rpm shaft speed and as generator between 1500 rpm and 6000 rpm shaft speed. In this preliminary experimental work, limited generator operation was tested. Motor operation as well as more extensive generator operation results will be given in a future publication with more details. Table VIII shows efficiency values of generator side for two different load levels at 1500 rpm shaft speed. By recalling speed range (1500 rpm-6000 rpm) and rated power level (3 kW) of the generator, the values of in Table VI can be considered as low-speed and partial-load operation data. TABLE VIII.
In order to show magnetic decoupling between motor and generator winding sets of the proposed PMSM, some tests were conducted with experimental setup in Fig. 16. An uncontrolled rectifier is connected to the generator output for observing the effect on generator winding set and similar tests were performed at 1500 rpm shaft speed. In Fig. 17, it can be seen that distorted current drawn by the load (green trace) causes distortion at the voltage output of the machine (pink trace). Motor winding back EMF waveform during no load operation of the generator winding is also shown in the Fig. 17 with white trace. Yellow trace also shows motor winding back emf during loaded generator operation with uncontrolled rectifier having 1516 Watt resistive load at its output.
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EXPERIMENTALLY OBTAINED DATA FOR GENERATOR ONLY OPERATION WITH UNCONTROLLED RECTIFIER (NO DC/DC CONVERTER)
Shaft Speed (rpm) 1500 1500
Rectifier Load (Ω) 1,728 0,864
Input Power (W) 1196 1965
Output Power (W) 938 1516
Shaft Torque (Nm) 7.62 12.52
System Efficiency (%η) %78 %77
Figure 16. Experimental set up for EADS
VII. CONCLUSION This paper proposes a new solution for accessory load driving problem during auto stop intervals of HEVs. Proposed solution is to combine motor and generator functions in the same electric machine so that it acts both as motor and as generator simultaneously. This allows the elimination of the conventional Lundell alternator (or DC/DC Converter). A new dual winding electric machine has been designed for this purpose. Finite element analysis and experimental results show that proposed electric machine is capable of providing complete isolation between motor and generator operations while maintaining simultaneous motor and generator actions. On the generator side power processing, two options are investigated by circuit coupled finite element analysis. These options are controlled rectifier and uncontrolled rectifier with DC/DC converter. Analyses show that due to extra reactive power demand in the controlled rectifier case, power factor is poorer than DC/DC converter option. This ultimately yields poorer utilization of electric machine. As far as efficiency is concerned, efficiency numbers of two options are comparable. The study shows that proposed approach can be considered as technically viable and cost effective alternatives to the existing methods. Preliminary data from experimental study shows that the proposed system offers technically feasible solution.
Figure 17. Various waveforms for motor and generator windings: Motor Back EMF waveform with no load (yellow curve), Motor Back EMF waveform with load (white curve), Generator Back EMF waveform with load (pink curve), Generator Output current waveform (green curve)
ACKNOWLEDGMENT This work is being supported by The Scientific and Technological Research Council of Turkey under contract number 110E111. Also Joel M. Maguire and Gary E. McGee from General Motors Corporation are greatly appreciated for their inspiration and support during problem definition of accessory drive systems in hybrid electric vehicles. REFERENCES [1]
[2] [3]
[4]
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N. R. Trevett, “X-by-Wire-New Technologies for 42V Bus Automobile of Future”, Msc. Thesis, The South Carolina Honors College, 2002. Serrels, R.K., “Accesory Drive System”, US Patent Application, Patent Number 20080020875, 2008. Z. Q. Zhu, “Fractional Slot Permanent Magnet Brushless Machines and Drives for Electric and Hybrid Propulsion Systems”, Ecological Vehicles and Renewable Energies (EVER 09) - 26-29 March 2009, Monaco. J. Cros, and P. Viarouge, “Synthesis of High Performance PM Machines with Concentrated Windings”, IEEE Trans. on Energy Conversion, vol. 17, no. 2, pp. 248–253, 2002.
