28-10-2010, 11:54 PM
ABSTRACT
Because of high requirements in power, efficiency, installation space and weight, the design of electrical machines for hybrid electrical vehicles is a particular challenge. This seminar describes a comparative study allowing the selection of the most appropriate electric-propulsion system for a parallel hybrid electric vehicle (HEV). This seminar is based on an exhaustive review of the state of the art and on an effective comparison of the performances of the four main electric propulsion systems, namely the dc motor, the induction motor (IM) and the permanent magnet synchronous motor. With regard to a parallel hybrid system with a very restricted installation space, a further study is performed on the permanent magnet excited synchronous machine, preferred for its highest power density and overall efficiency.
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CONTENTS Page No.
1. INTRODUCTION 3
2. CONFIGURATION OFHYBRID ELECTRIC VEHICLE 4
3. HYBRIDIZATION FACTOR 6
4. MAJOR REQUIREMENTS OF HEVS ELECTRIC PROPULSION 7
5. TYPICAL HEV CHARACTERISTICS 8
6. MOTOR UNDER CONSIDERATION 9
7. DC MOTOR 10
8. INDUCTION MOTOR 12
9. PMSM 14
10. EVALUATION OF THE ELECTRICAL MACHINES 16
11. EFFICIENCY MAP 17
12. CONCLUSION 18
13. REFERENCES 19
LIST OF FIGURES Page No.
1. HEV PARALLEL CONFIGURATION 4
2. TYPICAL PARALLEL HYBRID POWER TRAIN 5
3. TYPICAL SERIES HYBRID POWER TRAIN 5
4. TYPICAL HEV CHARACTERISTIC CURVE 8
5. DIFFERENT MOTOR TYPES 9
6. CHARACTERISTIC CURVES OF DC MACHINES 11
7. CHARACTERISTIC CURVE OF IM 13
8. CHARACTERISTIC CURVE OF PMSM 15
9. EFFICIENCY MAP 17
1. INTRODUCTION
An increasing ecological awareness and the shortage of fossil-fuel resources are strong incentives to develop more efficient vehicles, with lower fuel consumption but without reducing driving comfort. The hybrid electrical vehicle (HEV), combining the drive power of an internal combustion engine (ICE) and of one or several electrical machines (EM), is a promising concept.
According to the pursued hybrid concept, the electrical machine has to be as efficient as possible at various operating points. Besides the fast start/stop function, it can operate as a generator, as support traction in the so called boost operation, as drive during electrical traction, as well as electro dynamic brake for recuperation. In addition, high demands are made upon these machines. Besides the specifications on torque and speed, the main demands are: a high overall efficiency within a large range of the torque-speed characteristic, a high overload capacity, small installation space and weight and a high reliability at low costs. With such requirements in power, efficiency, installation space and weight, the design of these machines is a particular challenge
2. CONFIGURATION OF HYBRID VEHICLES
The proposed comparative study has been done on the parallel HEV configuration (Fig). In fact, by being different from the series hybrid, the parallel HEV allows both the internal combustion engine (ICE) and the electric motor to deliver power in parallel to drive the wheels. Since both the ICE and the electric motor are generally coupled to the drive shaft of the wheels via two clutches, the propulsion power may be supplied by the ICE alone, by the electric motor, or by both. Conceptually, it is inherently using an electric-assisted ICE for achieving lower emissions and lower fuel consumption.
Fig.1 HEV parallel configuration. 1. Electric motor, 2. ICE, 6.Inverter, 7. Controller, 10. Battery, 11. Differential gear.
Better than the series HEV, the parallel hybrid needs only two propulsion devices; they are the ICE and the electric motor. Another advantage over the series case is that a smaller ICE and a smaller electric motor can be used to get the same performance until the battery is depleted. Even for a long-trip operation, only the ICE needs to be rated for the maximum sustained power, while the electric motor may still be about a half.
Fig.2 Typical Parallel Hybrid Power Fig.3 Typical Series Hybrid Power
Train Train
3. HYBRIDIZATION FACTOR
Sizing the electric motor is a key point in a HEV to improve fuel economy and for dynamic performances. The ratio between the maximum power of the electric motor (PEM) and the ICE (PICE) is characterized by the hybridization factor (HF) that is defined as
HF = PEM/(PEM + PICE)
= PEM/PHEV
where PHEV is the maximum total traction power to propel the HEV. It has been demonstrated that hybridization improves HEV fuel economy and dynamic performances up to a critical optimum point (HF = 0.3 to 0.5). After this point, increasing the electric-propulsion system capacity will not improve the HEV performances.
4. MAJOR REQUIREMENTS OF HEV’s ELECTRIC PROPULSION
Major requirements of HEV’s electric propulsion are summarized as follows;
1) a high instant power and a high power density;
2) a high torque at low speeds for starting and climbing, a well as a high power at
High speed cruising;
3) a very wide speed range, including constant-torque and constant-power regions;
4) a fast torque response;
5) a high efficiency over the wide speed and torque ranges
6) a high efficiency for regenerative braking;
7) a high reliability and robustness for various vehicle operating conditions; and
8) a reasonable cost.
5. TYPICAL HEV CHARACTERISTICS
Figure illustrates the standard characteristics of an electric motor used in EV’s or HEV’s. Indeed, in the constant-torque region, the electric motor exerts a constant torque (rated torque) over the entire speed range until the rated speed is reached.
Once it is past the rated speed of the motor, the torque will decrease proportionally with speed, resulting in a constant power (rated power) output. The constant-power region eventually degrades at high speeds, in which the torque decreases proportionally with the square of the speed.
Fig. 4 Typical HEV Characteristic Curve
6. MOTORS UNDER CONSIDERATION
In an industrial point of view, the major types of electric motors adopted or under serious consideration for HEV’s include the dc motor, the induction motor (IM) and the permanent magnet (PM) synchronous motor. Cross sections of each of these three motor types are provided in figure below.
DC MACHINE IM PMSM
Fig. 5 Different Motor Types
7. DC MACHINE
The DC machine allows the simplest regulation and, due to the possibility to connect this machine directly to the vehicle‘s battery, no complex power electronic is required.
However, for powers higher than 20 kW, DC machines require commutating poles and compensation windings, so they are larger and more expensive. Due to the missing possibility of field weakening, the permanent magnet excitation, which would increase the machine‘s power density, is not feasible.
Another disadvantage is the commutator and its brushes, which decreases the reliability and increases the maintenance costs. Most losses of the DC machine occur in the rotor, which makes it necessary to add a complex cooling system at high power and restricts the overload capacity.
In summary, the DC machine has a moderate power density, a small efficiency and reliability but has the advantage of low costs and simple controllability, especially for small rated powers.
Figure shows torque speed relation of a dc motor. Slightly torque region is part of shunt characteristics and highly varying torque region is a part series characteristics. Using a switching mechanism motors changes from shunt to series motor
Fig. 6 Characteristic Curves Of DC Machines
8. INDUCTION MACHINE(IM)
Induction machines with squirrel-cage rotor belong, as well as the DC machine, to the most technically mature machines, but they offer a higher power density and a better efficiency when compared to the DC machine. The dominant losses in IM machines are the copper losses. Due to the lower magnetization current in the range of field weakening, the copper losses are copper losses are reduced and accordingly the IM provides a wide speed range in combination with a comparatively good efficiency at high speeds. The required magnetization current and the copper losses in the rotor decrease the efficiency in the range of nominal speed compared to PMSM’s. A disadvantage is the heat in the rotor as a result of the losses, which requires cooling and restricts overload capacity. Furthermore, an air gap as small as possible is necessary to decrease the magnetization current, but this requires tighten tolerances during fabrication and thus increases production costs.
Figure shows the typical characteristics of an IM drive. Vector control of IM’s can decouple it torque control from field control. Extended speed range operation with constant power beyond the base speed is accomplished by flux weakening.
Fig. 7 Characteristic Curve Of IM
9. PERMAMENT MAGNET SYNCHRONOUS MACHINE(PMSM)
The excitation of the PMSM is provided by permanent magnets in the rotor. This machine benefits from the high energy density of the magnets, because the permanent magnet excitation requires little space. Since no excitation current is required, the PMSM provides a high overall efficiency in the range of nominal speed. The dominant losses of the PMSM are the iron losses, which mostly occur in the stator, so they can be easily dissipated by a case cooling system. Hence, the PMSM exceeds the IM in power density and efficiency. Its major disadvantage is the high costs of rare-earth magnets such as NdFeB. Another disadvantage is the additional current component required for field weakening, whereby higher stator losses occur and the efficiency decreases at high speeds. Furthermore the overload capacity is restricted by the magnet characteristics. To prevent them from irreversible demagnetization, high magnet temperatures in combination with high stator currents must be avoided - a reliable temperature detection is essential.
PMSM motors inherently have a short constant-power region due to their rather limited field weakening capability, resulting from the presence of the PM field (the fixed PM limit their extended speed range). In order to increase the speed range and improve the efficiency of PM brushless motors, the conduction angle of the power converter can be controlled at above the base speed.
Fig.8 Characteristic Curve Of PMSM
10. EVALUATION OF THE ELECTRICAL MACHINES
The machine characteristics and their advantages and disadvantages are summarized in Table.
Table.1 Comparative Evaluation Of Electrical Machines
The direct current machine has a good technical maturity at low costs for machine and power electronics. But it offers the lowest power density and a bad efficiency. Furthermore it provides an insufficient reliability and requires a high amount of maintenance. The disadvantages exceed the advantages, so that the DC machine does not achieve the high requirements of an HEV.
The induction machine features the best reliability at low production costs. It has the best average overall efficiency over the whole speed range, but its maximum efficiency does not reach the values of a PMSM. So the IM is advantageous if a good efficiency over a wide speed range is required. But it only allows a moderate power density and a complicated and expensive field oriented control is required to reach high powers and dynamics.
The permanent magnet synchronous machine offers the best power density; this permits a high power machine with small weight and even in the restricted installation space of a vehicle‘s engine compartment. It offers the best maximum efficiency in a defined speed range. For these reasons the PMSM may be most suitable to achieve a fuel saving hybrid electrical vehicle. However, due to its rare-earth magnets, it is the most expensive machine type as well.
11. EFFICIENCY MAP
Fig.9 Efficiency Map
The choice of the machine type also depends on the control strategy of the hybrid electrical vehicle. It is to be determined in which operation points the electrical machine will be used. That means, the frequency distribution of the operation points during a drive cycle has to be considered. Most operation points are in the range of low speeds up to 2000 min−1, the maximum speed does not exceed6000 min−1 - so the operation points are distributed over a limited speed range. In Figure the exemplary efficiency map of different machine types is depicted. The lines are equipotential lines, that surround the range of an efficiency>85%. The PMSM has its best efficiency at low speed whereas the induction machine. In this case the PMSM would be the best choice. But if most of the operation points are at higher speeds or over a wide speed range, the IM should be preferred.
12. CONCLUSION
The characteristics of the machines like the power density or the efficiency and their advantages and disadvantages were compared regarding their applicability in HEV’s. With regard to a parallel hybrid system with very limited installation space, the permanent magnet excited synchronous machine (PMSM) was chosen for application.
13. REFERENCES
[1] L. Chang, “Comparison of AC Drives for Electric Vehicles - A Report on Experts’ Opinion Survey”, IEEE AES Systems Magazine, August 1994
[2]Thomas Finken, Matthias Felden and Kay Hameyer, “Comparison and design of different electrical machine types regarding their applicability in hybrid electrical vehicle, Institute of Electrical machine”, RWTH Aachen University
[3] Paper by Mr. Aubrey Corbett & Mr. Chris. Mellors. University of Warwick / Rover Group.
[4] M. Zeraoulia, M.E.H. Benbouzid, D. Diallo, “Electric Motor Drive Selection Issues for HEV Propulsion Systems: A Comparative Study” IEEE Transaction on Vehicular Technology, Vol.55, No.6, November 2006.
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