Power Electronics in Automotive Hybrid Drives by get11021


									                   Power Electronics in Automotive Hybrid Drives

             Čeřovský Z., Mindl P., Flígl S., Halámka Z., Hanuš P., Pavelka V.
              Czech Technical University, Faculty of Electrical Engineering
                      Technická 2, 166 27 Praha 6, Czech Republic
                    Tel : ++420 2 2435 2157, Fax: ++420 2 3333 9972
 email: cerovsky@fel.cvut.cz, xfligl@fel.cvut.cz, halamkz@fel.cvut.cz, hanus@fel.cvut.cz,
                          mindl@fel.cvut.cz, pavelkv@fel.cvut.cz
                     URL: http://bozek.cvut.cz, http://www.cvut.cz

ac machines, dc machines, Drives, Efficiency, Electric machines, Hybrid vehicles, Locomotives,

Paper deals with automotive hybrid and power splitting drives. Special attention is paid to electric
power splitting. Well-proved dc power splitting drive is described. New idea to use a similar drive
using ac machinery and power electronic converters is studied in Research Centre of internal
combustion motors and automobiles JB. An experimental working stand was projected to research, to
perform measurements and to serve for doctor students.

Main text
Expansion of automobile traffic brings new problems with gas emission and fuel consumption.
Hothouse gas emission of the cars equipped with internal combustion engines (ICE – for instance
gasoline or Diesel engine) brings ecological problems namely in big cities. Development of alternative
car drives, based on electric energy stored in battery, offers solution only for vehicles covering short
distances unfortunately. Energy density of batteries is too low in comparison with gasoline or oil for
internal combustion engines.

ICE has some specific attributes in contrast to the electric motor. Its high efficiency is accessible only
in regime of higher output and it depends on operating point choice given by torque and speed.

There are three main ways how to reduce the fuel consumption. They are: first to use power
transmission between ICE and driving wheels with high efficiency, second to adjust ICE to optimal
operating point and finally to use recuperative breaking and to save part of kinetic energy.

Possible realisation of all three methods is a hybrid vehicle which is equipped with two different
energy sources, often with fuel tank for ICE and electric battery for electric drive. ICE propulsion
system is used in the inter-city traffic and brings the advantages of high energy density and high
efficiency on higher power. On the contrary, electric propulsion system is more suitable in city traffic
where ICE would be uneconomical for low power and frequent idling.
1. Basic drive configuration

1.1 Series hybrid drive
This type of hybrid drive (Fig. 1) is very similar to the electric power transmission extended for
instance on the diesel locomotives. ICE is connected with the generator which produces electric
energy for traction motor driving the wheels. In contrast to electric power transmission, the series
hybrid drive is added with battery. So traction motor can be supplied either from the generator or from
the battery or from both together. ICE is not mechanically connected to the wheels and its speed does
not depend on the vehicle speed. It means that ICE operating point can be adjusted to the optimal one
but on the other hand there are higher losses in the electric machines than in conventional mechanical

           internal                                                                      internal
                         ICE                                                ICE        combustion
            engine                                                                        engine

          generator       G                                      box
                                             BAT             traction         TM             BAT
              traction                                        motor

              Fig. 1. Series hybrid drive                        Fig. 2. Parallel hybrid drive

1.2. Parallel hybrid drive
Parallel hybrid drive (Fig. 2) is a combination of
ICE coupled with vehicle wheels through                                                   internal
                                                                               ICE      combustion
mechanical gearbox and one electric machine
connected with the output or input gearbox shaft.
ICE can operate in optimal operating point owing to
the electric machine, which is used as a generator              generator          G
for charging the battery or as a traction motor                                                  battery
supplied from the battery. For instance when ICE                                                     BAT
operates with higher speed and torque demand is
low, then ICE can work with higher torque (which
responds to the optimal operating point) and                        motor
superfluous torque is utilised in electric machine for
battery charging. This type of hybrid drive is
advantageous for high power transmission
efficiency of mechanical gearbox but ICE cannot
operate with optimal operating point when the
battery is not able to receive or to supply electric
1.3. Combined switched hybrid drive
This type of the hybrid drive (Fig. 3) consists of two      Fig. 3. Combined switched hybrid drive
electric machines similar as the series hybrid drive
but the generator and the traction motor can be mechanically connected by the clutch which allows
regimes as serious or parallel hybrid drive. The vehicle often works with low output in the city traffic
and ICE efficiency sensibly depends on the operating point adjustment. In this case it is suitable to
disconnect mechanically both electric machines and the drive operates as serious hybrid drive.
On the contrary in the inter-city traffic with higher output the ICE efficiency is rather good and it does
not depend so much on the ICE operating point small changes. Therefore the parallel hybrid drive is
more advantageous for its better efficiency of power transmission from ICE to the drive wheels.
1.4. Power splitting systems
Propulsions with electric power splitting are based on ICE power splitting into two parts. One part is
converted in electric power in generator, which supplies traction motor mechanically connected to
drive wheels and remaining part is transmitted by electromagnetic forces in the air gap to the wheels
mechanically without losses in electric machines. The splitting device can be realised mechanically by
planetary gear or electrically by a special electric generator.

                     ICE                  G                       TM


                    Fig. 4. Power system with electric splitting generator

Electric split device (Fig. 4) consists of a special electric generator (G) with stator and rotor both
rotating. The rotor is connected to ICE and it has angular speed ωICE and torque TICE which is via air
gap electromagnetic forces transmitted to the stator and output shaft with angular speed ωout.
Mechanically transmitted power is

       Pmech = TICE ⋅ ù out                                                                            (1)

and remaining ICE power part is transformed in electric power

       Pel = TICE ⋅ (ù ICE - ù out ) .                                                                 (2)

This electric energy represents input power of electrical transmission. On higher vehicle speed when
rotor and stator speed are similar a clutch CL can be switched on. Then all ICE power is transmitted
mechanically with high efficiency to the car wheels.

                              control unit


                                Fig. 5. Power splitting system with planetary gear

Mechanical split device (Fig. 5) uses a planetary gear PG. Instead of the special generator a classic one
with fixed stator is used. The rotor of generator has a cavity for passage of ICE output shaft.
Fig. 6 shows the propulsion configuration of two Czechoslovak express railcars „Slovenská Strela“
from the year 1936. Each of its two two-axle bogies had a 120 kW gasoline engine with electric power
splitting dc generator and one traction motor. When the clutch was switched, gasoline engine was





                    clutch                 rotor                          shaft

                      Fig. 6. Electric power splitting drive of railcar “Slovenska Strela”

directly coupled with the driving axle. the traction motor was disconnected from output shaft by the
idle wheel to limit mechanical losses in the electric machines. Both these machines were serious


                                LG              LTM


      ICE                  G                             TM

      Fig. 7 Traction circuit of railcar “Slovenska Strela”                Fig. 8 Railcars “Slovenská Strela”

excited dc machines as shows Fig. 7. A resistor with adjustable rider was connected parallel to
exciting winding of both machines. ICE speed governor Rn controlled ICE torque by means of
mentioned rider. When the ICE speed was lower than required value then the rider was moved to the
left, the generator exciting was reduced and the traction motor exciting was increased. The speed,

                         Fig. 9. Waveforms of propulsion used in “Slovenská Strela”

current and voltage waveforms in dependence of the vehicle velocity shows Fig. 9. From point O to T
the differential speed between the rotor and stator of electric splitting generator is kept constant.

                                     =                      ~
                                                     ~ =
                                         el. power
                          ICE            splitting

                   Fig. 10. Power electric splitting drive using ac machines
Between T and K the ICE power is constant because its speed and torque are also constant and the
differential speed decreases. The clutch is switched on in the point K and the ICE speed decreases to
the value of the output shaft speed. From point K to M the ICE speed again increases and all ICE
power is transmitted mechanically from the clutch to the rotating stator and to the output shaft.

         M [Nm]

                                                    output shaft

                                                   ICE torque

                                                           traction motor

                   0             1000              2000                 3000              -1
                                                                                     n [min ]

                           Fig. 11. Torque curves of ac power splitting drive

         P [W]
          2000                                                                 4


                   0              1000                 2000                        3000         -1
                                                                                           [min ]
              1 - ICE power                               4 - generator input power
              2 - output TM power                         5 - mechanically transmitted power
              3 - output shaft power
                           Fig. 12. Power curves of ac power splitting drive

Systems with power splitting have the same advantages as drives with electric power transmission.
They can adjust the optimal ICE operating point easily. They have lower losses in electric machines,
in contrast to the electric power transmission, because only part of ICE power is transmitted by
electrical way.
A modified power splitting system equipped with ac machines is shown Fig. 10. Ac/dc/ac converter is
added which connects the ac generator and the ac traction motor with different voltage and frequency
levels. Fig. 11 shows torque of ICE, traction motor and output shaft torque as a function of the output
speed. In the speed range 0 – 1000 min-1 the output shaft torque is adjusted to the constant value not to
exceed adhesion. The ICE torque and speed increase with the increasing output shaft speed. Power
curves of all machines are demonstrated in Fig. 12. Fig. 13 shows electric values curves (voltage,
current and frequency) as function of the output speed.



                                         6                                    3

             0       500     1000            1500         2000      2500  3000 speed
             1 - 10 x AM current                        4 - AM voltage         [min ]
             2 - 10 x GEN current                       5 - GEN voltage
             3 - AM frequency                           6 - GEN frequency

                             Fig. 13. Torque curves of ac power splitting drive

2. Experimental working stand
Experimental working stand was projected in the Research Centre JB. The aim was to project a
working place where as many variants as possible of the described automotive hybrid drives could be
physically modelled. Only electrical problems and power converters control of such drives should be
studied on the experimental working stand. The scheme is depicted in Fig. 14. The ICE is simulated by
a controlled electric ac motor ACM1 supplied by the power converter EC1. Rotor of special
synchronous permanent generator SGPM is firmly coupled with the ACM1 shaft. Stator is firmly
coupled with car wheels and rotates with the same speed. On this shaft is inserted traction motor TM.
Generator SGPM and traction motor TM are electrically connected through EC3 power electronic
converter set. Traction resistance is simulated with controlled ac motor ACM2 supplied by the power
converter EC2. A battery, may be a super condenser in the future, will be connected with power
converter set EC3. It acts as an energy buffer.
2.1. Parameters of the experimental set
The output of both ACM1 and ACM2 motors is the same 7.5 kW, 3 x 400 V, 0 – 6000 min-1,
maximum torque 31 Nm.

The output of the special permanent magnet synchronous generator SGPM with rotating stator and
rotor is 5 kW, 4000 min-1, maximum speed 6000 min-1, 267 Hz, 400 V.
The output of the traction motor TM is 3 kW, 1420 min-1, max speed 6000 min-1, 400 V.
2.2. Description of the function
During start of the car when ACM2 speed is zero the ACM1 torque is transmitted via SGPM
synchronous generator air gap to the SGPM stator. Conventional fixed stator fastens this torque by the
foot but electric split device transmits it to the output shaft and adds it to the traction motor TM torque.
When the ACM2 shaft is turning then SGPM torque transfers mechanical power which increases with
output shaft speed. Traction motor power decreases because SGPM differential speed decreases too.
Power and losses in electric machines falls and power transmission efficiency increases. Electric
power splitting generator supplies the traction motor via EC3 converter. When stator SGPM speed is
near to the rotor SGPM speed the clutch ELM is switched on and power transmission from ACM1 to
ACM2 takes place on a pure mechanical way.


                EC1                              EC3                           EC2

                                            ELM CLUTCH

                ACM1            SGPM                            TM             ACM2


                                  Fig. 14 Experimental working stand

2.3. Flexibility of the experimental stand
Many experiments may be performed on the experimental working stand.
They are:

1) Starting ICE form the battery or super condenser by starter-generator (Fig. 15): this
   configuration unites the function of a conventional starter and alternator to one electric machine.
   Mass of starter-generator rotor can act as a flywheel and its more robust construction permits more
   frequent starting, for instance on queue in front of a crossing.

2) The battery or super condenser charging (Fig. 15): the super condenser is a new hopeful energy
   storage component, which allows faster charging in contrast to the battery and it can be used for
   ICE starting or recuperation breaking. Both energy storage components can be compared on the
   experimental working stand.

3) Running as the series hybrid drive (Fig. 16): this type of hybrid drive can be simulated by
   mechanical disconnection between SGPM stator and TM rotor and by fixation of SGPM stator.

4) Running as the parallel hybrid drive (Fig. 17): the power splitting system permits working as a
   parallel hybrid drive by switching on of the clutch ELM. Then the electric power splitting generator
   SGPM does not operate and traction motor TM is used for charging the battery by recuperation
   breaking or for driving the wheels when TM is supplied from the battery.
5) Control strategy to optimise the fuel consumption of the ICE: the experimental working stand
   allows to find optimal control strategy regarding to the fuel consumption.

6) Control strategy to reach the lowest gas emission: analogous to the previous point it can be
   found strategy to achieve the lowest gas emission.


            EC1                              EC3

                                                FIXED STATOR
            ACM1           SGPM


                   Fig. 15 Configuration for ICE starting or battery charging


       EC1                                  EC3                                 EC2

                                FIXED STATOR

       ACM1              SGPM                                TM                 ACM2


                       Fig. 16 Configuration for the series hybrid drive
7) Transient phenomena in the drive: transient phenomena in electric drives and in mechanical
   clutch can be tested on physical model.
8) Efficiency studies: efficiency studies under different control conditions can be proved for different
   traffic conditions (for instance city traffic, highway traffic, cross country traffic) and for different
   vehicle types (for instance passenger car, lorries, buses, light railcars, heavy lorries and so on).

9) Studies for energy production and consumption in the island car network: increasing number
   of electric power consumers in new car types requires to study this problem which can be also
   simulated on experimental working stand.


         EC1                                        EC3                                    EC2

         ACM1                                                           TM                 ACM2


                           Fig. 17 Configuration for the parallel hybrid drive

[1] Sousedik J.: Patent document Czechoslovakia Nr 53 735 from 25. February 1936
[2] Bilek J.: Elektricka vyzbroj motorovych vozu “Slovenska strela” (Electric drive of motor cars “Slovenska
    strela”. Elektrotechnicky obzor 1937, Nr. 16, Pg. 249-253, Nr. 21 Pg. 331-336
[3] Klima V: Elektromechanicky pohon DELKA a jeho srovnani s normalnim Diesel-elektrickym pohonem.
    (Electro-mechanic drive DELKA and its comparison with Diesel-electric drive.) Elektrotechnicky obzor
    1949, Nr. 19, Pg. 489-496
[4] Mierlo J.: Simulyation software for comparison and design of electric, hybrid electric and internal
    combustion vehicles with respect to energy, emission and performances. Vrije Universiteit Brussel
[5] Doh-Hyoung Kim, Youngjin Park: Modeling and Design of Hybrid Electric Vehicles Drivetrains. FISITA
    World Automotive Cogress 2000 Seoul, Korea

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