ORNL/TM-2004/137
REPORT ON TOYOTA/PRIUS MOTOR DESIGN AND MANUFACTURING ASSESSMENT
J. S. Hsu C. W. Ayers C. L. Coomer Oak Ridge National Laboratory
This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.
Engineering Science & Technology Division REPORT ON TOYOTA/PRIUS MOTOR DESIGN AND MANUFACTURING ASSESSMENT
J. S. Hsu, Ph.D. C. W. Ayers C. L. Coomer
Publication Date: July 2004
Prepared by the OAK RIDGE NATIONAL LABORATORY Oak Ridge, Tennessee 37831 managed by UT-BATTELLE, LLC for the U.S. DEPARTMENT OF ENERGY Under contract DE-AC05-00OR22725
In todays hybrid vehicle market the Toyota Prius drive system is currently considered the leader in electrical, mechanical, and manufacturing innovations. It is significant that in todays marketplace Toyota is able to manufacture and sell the vehicle for a profit. This projects objective is to analyze and study the Prius drive system to understand the design and manufacturing mechanisms Toyota utilized to achieved their performance and cost goals. During the course of this research effort ORNL has dissected both the 2003 and 2004 Toyota/Prius drive motors. This study is focused primarily on motor design considerations and an assessment of manufacturing issues. _____________________________________________________________________________ Figure 1 shows the assembly of the Toyota/Prius hybrid THS II drive train system. The motor occupies a large portion of the drive train. Studying and analyzing the motor in detail can accomplish a through understanding of the design and manufacturing considerations of the drive system. Motor Sun Gear Damper Ring Gear Oil Pump Engine Generator
Planetary Gear Final Gear

Carrier Counter Gear

To wheel
Fig. 1. Motor, generator, and engine of Toyota/Prius hybrid THS II System. Source: Development of Electric Motors for the TOYOTA Hybrid Vehicle PRIUS, Kazuaki Shingo, Kaoru Kubo, Toshiaki Katsu, Yuji Hata, Toyota Motor Corporation.
Figure 2 compares the output power and torque versus speed of the 2003 (THS) and 2004 (THS II) model Prius motors. Both the power and the torque of the motor are significantly increased in the 2004 model.
Fig. 2. Comparison of output power and torque versus speed between 2003 (THS) and 2004 (THS II) Prius motor.
The power at base speed of the 2004 model, as tabulated in Table 1, is 50 kW, which is significantly higher than the 33 kW of the older model. The 400 Nm torque of the 2004 model is also higher than the 350/305 Nm of the 2003 model.
Table 1. Power and torque of PM synchronous motor of Toyota/Prius hybrid THS II system New Model (2004) Power: Torque: 50 kW at base speed 400 kW up to base speed Previous Model (2003) 33 kW 10405600 rpm 350 Nm305 Nm 0400 rpm 4001000 rpm
Source: Development of Hybrid Electric Drive System Using a Boost Converter, Masaki Okamura, Eiji Sato, Shoichi Sasaki, Toyota Motor Corporation.
How did Toyota improve the motor performance without increasing the motor size? In fact, the motor core length is even slightly shorter in the 2004 model (3.3 in. versus 3.5 in.). In dissecting the motor, ORNL researchers assessed that the windings of the 2004 and 2003 models have the same gauge wires, same number of turns per coil, same winding distribution, and the same stator punching. The only difference between the stators is that the windings are connected in series instead of in parallel in the 2004 model. The series winding would certainly boost the torque, because for a given current, doubling the turns of the winding that interact with the fixed flux of the permanent magnets (PMs) would double the torque. On the other hand, a series winding requires twice the voltage of a parallel winding. In the low-speed region of operation, the back electromotive force (emf) is low and the 200-V bus voltage is quite sufficient to drive the motor. For high-speed operation, a boost 4

converter to raise the bus voltage from 200 V to 500 V is required. The following table outlines the winding comparison between the old and new Prius technologies.
Table 2. The winding connections of the 2003 and 2004 Toyota Prius 2003 Parallel 3.5 in. core length (200 Vdc bus) 2004 Series 3.3 in. core length (200500 Vdc bus)
Figure 3 illustrates that because of the series winding, the motor leads of the 2004 model are substantially thinner than those of the 2003 model.
Fig. 3. Thinner motor leads after changing from two-parallel windings (2003) to series winding (2004).
The performance improvement is also obtained by increasing the quadrature-axis reactance Xq. This approach is well described in the literature. Figure 4 shows the 2003 and 2004 Prius motor rotor punchings. The new 2004 model punching has a wider quadrature-axis iron width than the older 2003 punching. The V-shape PM grooves in the new punching also help to increase the Xq value.
Fig. 4. Prius rotor punchings.
The width of bridges that contain the mechanical stresses holding the PMs against the centrifugal force has been optimized. A narrow bridge can reduce the leakage flux across the bridges and consequently can improve the motor performance. Both the synchronous torque produced by the PM and the reluctance torque affect the final shape of the total torque. Figure 5 shows these torques components along with the resultant total torque of the 2004 motor.
Fig. 5. Additional reluctance torque of Toyota Prius hybrid THS II motor. Source: Development of Electric Motors for the TOYOTA Hybrid Vehicle PRIUS, Kazuaki Shingo, Kaoru Kubo, Toshiaki Katsu, Yuji Hata, Toyota Motor Corporation.
The rotor punchings are made of 0.014 in. thick processed laminations. The burr of the punching is low, less than half a mil, as shown in the side view in Fig. 6. This helps to eliminate the machining after stacking the core. Consequently, the manufacturing costs can be reduced.
Fig. 6. Burr observation.
The Prius 2004 wound stator and its stator core are shown in Fig. 7. A very narrow stator slot is used to keep the flux density low to obtain the maximum air-gap flux produced by the PMs. The slot fill factor, calculated from the ratio of the total squares of insulated wires and the net available slot area, is 0.84. A master winder with Southern Armature, a well respected motor winding company in Knoxville, Tennessee can only produce a fill factor of 0.81 by hand winding. Normally the hand winding gets a higher fill factor than the machine winding. The end winding extensions per side is approximately 1.5 in. The slots and the stator core do not show any sign of machining or filing after the core is stacked, pressed, and welded. The slot insulation in the new model has been changed from a Nomex to a Mylar-type material. It is speculated that the Mylar-type material has superior mechanical holding property inside the stator housing filled with oil droplets. Figure 8 shows the shapes of the slot and phase insulations. Because the voltage rating of the 2004 model has been increased, two additional ties that hold the two ends of the phase insulation together have been added.

Fig. 7. Prius 2004 wound stator and its stator core.

Phase insulation

Fig. 8. Slot insulation and phase insulation of 2004 Prius motor.
Figure 9 shows the Prius 2004 rotor. The periphery of the end piece is not perfectly round. It has eight semi-round grooves that are used to sling the oil to produce oil droplets inside the motor housing for cooling the motor windings and cores.
Fig. 9. Prius 2004 rotor.
Figure 10 shows the PMs for the 2004 and 2003 models. Figure 11 shows that the magnets are molded with a polymer that enables the final sizing of the PMs to be inexpensively honed to the correct dimensions and shapes for the final assembly.

Fig. 10. Prius PMs.

Fig. 11. Molded polymer is used to ensure the final dimensions of the PM.
The left side of Figure 12 shows the rotor core that has been assembled onto the rotor hub. The PMs with the polymer coating are inserted into the slots of the rotor core. The right side of Figure 12 illustrates the four keys on the rotor inner surface, which will be engaged with the key ways (not shown) on the rotor hub. After inserting the PMs, the top-clamping piece is then inserted on the rotor core. This is followed by compressing the core and securing the location of the clamping piece.
Fig. 12. Magnets were inserted into the slots of the rotor core.
The rotor assembly is now ready for magnetization of the permanent magnets.
Figure 13 shows the comparison of the measured air-gap flux densities in Gausses of the 2003 and 2004 motors. The 2004 model has a narrower but higher flux density distribution. This is accounted for by the use of the different rotor punchings between the two models shown in Fig. 4.
0 -2000 -4000 -6000 -02-ave.Gauss 030404-ave.Gauss

Slot Number

Fig. 13. Prius 2003 and 2004 no-load air-gap flux density comparison.
The PMs used were tested and compared with other types of magnets. This comparison is shown in Fig. 14. The Toyota Prius PMs are high-strength magnets. It appears that they are sintered.
Fig. 14. Comparison of Prius PM with others.
The die cast of the Prius aluminum housing is precise, which minimizes machining operations. No shrink-fit is needed between the stator wound core and the frame. This approach helps to cut the manufacturing costs. The oil level line when the rotor is at standstill is shown in Fig. 15.

oil level

Fig.15. Oil level in Prius motor housing.

Conclusions 1. Changing the winding connections from the parallel winding of the older (2003) model to a series connection in the 2004 model increases the torque and power in the low-speed region. 2. The rotor punching design has been changed to increase the Xq value for a higherreluctance torque component. 3. The punching quality is good. The burr is insignificant. No additional machining, grinding, and filing of the core assembly are required. This results in a reduction in manufacturing costs. 4. No shrink-fit is needed between the stator wound core and the frame. 5. Molded polymer is used to shape the PMs into their final dimensions. This results in a reduction in manufacturing costs. 6. The slot fill factor of the stator winding is very high. This increases the power density of the motor. 7. The phase insulation has been modified to add ties for meeting the higher voltage requirement. 8. The insulation has been changed from Nomex-type to Mylar-type to better withstand the oil-droplet environment inside the motor housing.
9. High-strength PMs are used. 10. The bridges that hold the PMs against the centrifugal force have been optimized for mechanial stress considerations. A narrow bridge reduces the leakage flux of the PMs and improves the motor performance. 11. Precision die casting is used to cut down machining requirements. 12. Utilizing cooling with oil droplets as well as a small water-to-water heat exchanger cast in the motor housing helps to cut the cost of motor cooling.
Distribution Internal 1. 2. 3. 4. External 10. S. A. Rogers, U.S. Department of Energy, EE-2G/Forrestal Building, 1000 Independence Avenue, S.W., Washington, D.C. 20585. 11. E. J. Wall, U.S. Department of Energy, EE-2G/Forrestal Building, 1000 Independence Avenue, S.W., Washington, D.C. 20585. D. J. Adams C. W. Ayers C. L. Coomer E. C. Fox 5. 6. 7. 89. J. S. Hsu L. D. Marlino J. W. McKeever Laboratory Records

ORNL/TM-2004/247

Evaluation of 2004 Toyota Prius Hybrid Electric Drive System Interim Report
C. W. Ayers J. S. Hsu L. D. Marlino C. W. Miller G. W. Ott, Jr. C. B. Oland
DOCUMENT AVAILABILITY Reports produced after January 1, 1996, are generally available free via the U.S. Department of Energy (DOE) Information Bridge. Web site http://www.osti.gov/bridge Reports produced before January 1, 1996, may be purchased by members of the public from the following source. National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 Telephone 703-605-6000 (1-800-553-6847) TDD 703-487-4639 Fax 703-605-6900 E-mail info@ntis.fedworld.gov Web site http://www.ntis.gov/support/ordernowabout.htm Reports are available to DOE employees, DOE contractors, Energy Technology Data Exchange (ETDE) representatives, and International Nuclear Information System (INIS) representatives from the following source. Office of Scientific and Technical Information P.O. Box 62 Oak Ridge, TN 37831 Telephone 865-576-8401 Fax 865-576-5728 E-mail reports@adonis.osti.gov Web site http://www.osti.gov/contact.html
This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.
ENGINEERING SCIENCE & TECHNOLOGY DIVISION
EVALUATION OF 2004 TOYOTA PRIUS HYBRID ELECTRIC DRIVE SYSTEM INTERIM REPORT

November 2004

Prepared for the U.S. Department of Energy FreedomCar and Vehicle Technologies Program
Prepared by the OAK RIDGE NATIONAL LABORATORY Oak Ridge, Tennessee 37831 managed by UT-BATTELLE, LLC for the U.S. DEPARTMENT OF ENERGY under contract DE-AC05-00OR22725

TABLE OF CONTENTS

Page LIST OF FIGURES.... LIST OF TABLES..... ACRONYMS..... ACKNOWLEDGEMENTS.... ABSTRACT..... 1. INTRODUCTION.... 1.1 PROGRAM OVERVIEW.... 1.2 TESTING FACILITIES.... 1.3 SCOPE AND OBJECTIVE.... 1.4 APPROACH..... 2. HYBRID ELECTRIC DRIVE SYSTEM DESCRIPTION.. 2.1 DESIGN REQUIREMENTS.... 2.2 SYSTEM COMPONENTS.... 2.2.1 Engine.... 2.2.2 Power Split Device.... 2.2.3 Generator.... 2.2.4 Motor.... 2.2.5 Inverter.... 2.2.6 Battery.... 3. LABORATORY TESTING.... 3.1 ANL VEHICLE LEVEL PERFORMANCE TESTS... 3.1.1 Recent Work and Tests.... 3.1.2 Future Test Plan.... 3.2 ORNL COMPONENT-LEVEL PERFORMANCE AND VALIDATION TESTS.. 3.2.1 Locked Rotor Tests.... 3.2.2 Back-emf Tests.... 3.2.2.1 Motor Tests.... 3.2.2.2 Generator Tests.... 3.2.3 Hybrid Drive System Losses Tests.... 4. INVERTER AND CONVERTER EVALUATION... 4.1 FUNCTIONAL AND ARCHITECTURE STUDY... 4.2 CONTROL DEVELOPMENT.... 4.3 INVERTER MODIFICATIONS AT ANL... 5. SUMMARY AND CONCLUSIONS... 5.1 FINDINGS AND OBSERVATIONS... 5.2 NEEDED RESEARCH AND DEVELOPMENT... REFERENCES..... iii iv v vi 34

LIST OF FIGURES

Figure 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 Component arrangement for the THS II.... 2004 Prius engine and hybrid drive assembly... Heat removal and lubrication scheme for the 2004 Prius hybrid electric drive system.... Diagram of the 2004 Prius power split device... Diagram of the 2004 Prius gear train between the motor and wheels.. 2004 Prius gears with number of gear teeth shown... 2004 Prius generator rotor.... 2004 Prius motor rotor and stator... 2004 Prius inverter and converter unit... Instrumentation locations for ANL testing... Sample power measurements under varying speed conditions.. Sample power measurements under controlled speed conditions.. Motor shaft angle versus torque (rotor locked).. Locked rotor peak torque as a function of current... Motor back-emf voltage versus motor shaft speed.. Generator back-emf voltage versus generator shaft speed.. Hybrid electric drive system and component losses at 25C.. Configuration B losses as a function of oil temperature.. Page 25 28
LIST OF TABLES Table 2.1 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 3.11 3.12 3.13 5.1 5.2 5.3 Specification for THS II components... Measurement variables for ANL testing program... Motor torque versus motor shaft angle... Test conditions for back-emf voltage measurements.. Back-emf voltage measurements for the 2004 Prius motor.. Back-emf voltage measurements for the 2004 Prius generator... Component configurations for loss determinations... Summary of hybrid drive system losses... Configuration B losses at a nominal oil temperature of 28C.. Configuration B losses at a nominal oil temperature of 40C.. Configuration B losses at a nominal oil temperature of 50C.. Configuration B losses at a nominal oil temperature of 60C.. Configuration B losses at a nominal oil temperature of 70C.. Configuration B losses at a nominal oil temperature of 80C.. Summary of gear train losses at 25C... Summary of back-emf test results... Summary of motor current and torque test results... Page 33

1. INTRODUCTION

The 2004 Toyota Prius is a hybrid automobile equipped with a gasoline engine and a battery-powered electric motor. Both of these motive power sources are capable of providing mechanical drive power for the vehicle. The engine can deliver a peak power output of 57 kilowatts (kW) at 5000 revolutions per minute (rpm) while the motor can deliver a peak power output of 50 kW at 1300 rpm. Together, this engine-motor combination has a specified peak power output of 82 kW at a vehicle speed of 85 kilometers per hour (km/h). In operation, the 2004 Prius exhibits superior fuel economy compared to conventionally powered automobiles. To acquire knowledge and thereby improve understanding of the propulsion technology used in the 2004 Prius, laboratory tests were conducted to evaluate the electrical and mechanical characteristics of the 2004 Prius and its hybrid electric drive system. This testing was undertaken by the Oak Ridge National Laboratory (ORNL) as part of the U.S. Department of Energy (DOE) Energy Efficiency and Renewable Energy (EERE) FreedomCar and Vehicle Technologies (FCVT) Program through its vehicle systems technologies subprogram. 1.1 PROGRAM OVERVIEW The Hybrid Electric Vehicle (HEV) program officially began in 1993 as a five-year cost-shared partnership between DOE and American auto manufacturers: General Motors, Ford, and DaimlerChrysler. They committed to produce production-feasible HEV propulsion systems by 1998, first-generation prototypes by 2000, and market-ready HEVs by 2003. Currently, HEV research and development is conducted by DOE through its FCVT Program. The mission of the FCVT program is to develop more energy efficient and environmentally friendly highway transportation technologies. Program activities include research, development, demonstration, testing, technology validation, and technology transfer. These activities are aimed at developing technologies that can be domestically produced in a clean and cost-competitive manner. The vehicle systems technologies subprogram, which is one of four subprograms under the FCVT program, supports the efforts of the FreedomCAR through a three-phase approach intended to:
Identify overall propulsion and vehicle-related needs by analyzing programmatic goals and reviewing industrys recommendations and requirements, then develop the appropriate technical targets for systems, subsystems, and component research and development activities; Develop and validate individual subsystems and components, including electric motors, emission control devices, battery systems, power electronics, accessories, and devices to reduce parasitic losses; and Determine how well the components and subsystems work together in a vehicle environment or as a complete propulsion system and whether the efficiency and performance targets at the vehicle level have been achieved.

Source: http://www.eere.energy.gov/vehiclesandfuels/technologies/systems/index.shtml The research performed under the Vehicle Systems subprogram will help remove technical and cost barriers to enable technology for use in such advanced vehicles as hybrid and fuel-cell-powered vehicles. 1.2 TESTING FACILITIES Evaluation of the 2004 Prius and its hybrid electric drive system involved both vehicle-level and component-level performance testing. Vehicle-level testing is being conducted at the Advanced Powertrain Research Facility (APRF) located at the Argonne National Laboratory (ANL), 9700 S. Cass Avenue, Argonne, Illinois. The APRF is a multi-dynamometer vehicle test facility capable of testing conventional and hybrid vehicle propulsion systems and vehicles. Component-level testing is being conducted by ORNL at its Power Electronics and Electric Machinery Research Center (PEEMRC). The PEEMRC is a broad-based research center for power electronic inverters and electric machinery (motor) development. Located in the recently constructed National User Facility known as the National Transportation Research Center (NTRC), the PEEMRC has more than 9000 square feet of space for developing and building the next generation prototypes of inverters, rectifiers, and electric machine technology. 1.3 SCOPE AND OBJECTIVE This interim report summarizes vehicle-level and component-level test results obtained to date for the 2004 Prius and various electrical and mechanical components of its hybrid electric drive system. The primary objective of these tests was to characterize the electrical and mechanical performance of the 2004 Prius. Information about the 2004 Prius and its technical design features are presented in Chapter 2 to serve as a foundation for subsequent discussions about the various components of the hybrid electric drive system that were tested. Laboratory test results are summarized in Chapter 3. They include electrical and mechanical data that have been acquired to date at ANL and ORNL. The objectives of these tests were to characterize the functional performance of the hybrid electric drive system and to understand the design methodology used in the construction of the various system components specifically the generator, traction motor, and inverter. Information about the inverter and converter is presented in Chapter 4. This information includes discussions about the functional characteristics and control development for the inverter and converter and a description of inverter modifications that will provide a way to measure current and voltage values at specific locations. Preliminary conclusions and findings based on the acquired test data along with areas of needed research and development are presented in Chapter 5.

1.4 APPROACH Complementary electrical and mechanical data from vehicle-level and component-level tests were acquired to gain a full understanding of the 2004 Prius performance. These data were then used to determine back-emf voltage and energy loss values over the specified operating range of the vehicle. Vehicle-level tests are being performed at the APRF with the electrical and mechanical systems installed in the original factory configuration. The inverter, motor, generator, axles, and related components are instrumented to acquire power flow data needed to characterize vehicle performance. Component-level tests are being performed at the NTRC by removing the hybrid electric drive system and inverter from the vehicle and mechanically connecting the shaft to a dynamometer. This arrangement also requires a reconfiguration of the inverter so that it will operate outside the vehicle. Using this approach makes it possible to separately evaluate the performance of each power-related component of the hybrid electric drive system. Component-level test results provide data needed to characterize the overall performance of the 2004 Prius.
2. HYBRID ELECTRIC DRIVE SYSTEM DESCRIPTION
The 2004 Prius is a new-generation hybrid automobile that was introduced into the market in September 2003 by the Toyota Motor Corporation. As a hybrid vehicle, the 2004 Prius uses both a gasoline-powered internal combustion engine capable of delivering a peak power output of 57 kW and a battery-powered electric motor capable of delivering a peak power output of 50 kW as motive power sources. Combining these two-motive power sources results in improved fuel efficiency and reduced emissions compared to traditional automobiles and provides the 2004 Prius with the following energysaving characteristics. Energy-loss reduction is achieved by automatically stopping the engine when idling. Energy is recovered and reused by capturing kinetic energy that is normally wasted as heat during deceleration and braking. The starter and electric motor then convert this energy to electricity for use. Engine is able to operate at peak efficiency speed a high percentage of the time. Supplementary power is provided by the electric motor during acceleration when engine efficiency is low. Optimal vehicle efficiency is realized by using the electric motor to run the vehicle under operating conditions when engine efficiency is low and by generating electricity when engine efficiency is high. Enhanced performance of the 2004 Prius is attributed to the new-generation Toyota Hybrid System (THS II). This system is a power train consisting of a high-power motor, generator, and a battery of relatively low power. Major components of the THS II are shown in Fig. 2.1. As this figure indicates, a mechanical component referred to as a power split device (planetary gear set) separates power supplied by the gasoline engine into two paths. In the mechanical path, engine power is transmitted to the vehicles wheels directly through the transmission. In the electrical path, a generator converts mechanical energy from the engine into electrical energy. Electricity produced by the generator is then available for either supplementing the battery power to the electric motor or charging the battery, or both. By using energy in this manner, the 2004 Prius requires no external power source for battery charging. The power split device allows the engine to function at or near its optimal operating speed, regardless of vehicle speed, while still being able to efficiently add power to the wheels and simultaneously drive the generator. A photograph of the engine and hybrid drive assembly after removal from the car is shown in Fig. 2.2. In operation, the 2004 Prius is capable of functioning in the following modes: When engine efficiency is low, such as during start-up and mid-range speeds, motive power is provided by the motor alone using energy stored in the battery. Under normal driving conditions, overall efficiency is optimized by controlling the power allocation so that some of the engine power is used for turning the generator to supply electricity for the motor while the remaining power is used for turning the wheels. During periods of acceleration when extra power is needed, the generator supplements the electricity being drawn from the battery so the motor is supplied with the required level of electrical energy.

Hybrid Drive Coolant System Pressure Differential: 4.1 psi Flow Rate: 2.8 liters per min.
Hybrid Drive Coolant System Overflow Reservoir
Hybrid Drive Coolant System Pump Generator Power Split Device

Inverter

Hybrid Drive Housing Engine Lubricating Oil Drip Cup
Hybrid Drive Lubricating and Cooling Oil
Hybrid Drive Coolant System Radiator
Engine Coolant System Pump Engine Coolant System Radiator Engine Coolant System Overflow Reservoir Heat Storage Tank and Water Pump 3-Position Water Valve
The 2004 Prius uses two separate liquid coolant systems to remove excess heat from the hybrid electric drive system. Oil inside the hybrid drive housing splash lubricates the bearings, gears, and other moving parts while an oil pump supplies lubricating oil to the power split device. Heat produced by the motor, generator, and gears is transferred to the hybrid drive housing by the lubricating oil. The hybrid drive housing contains a flow path for the hybrid drive coolant. Excess heat from the hybrid drive housing and the inverter is removed by the hybrid drive coolant and discharged to the atmosphere by the hybrid drive coolant system radiat or. Excess engine heat is removed by the engine coolant and discharged to the atmosphere by the engine coolant system radiator. During normal engine operation, the three-position water valve allows coolant to flow from the engine to the radiator and to the heat storage tank. When the coolant in the heat storage tank is at operating temperature, the water valve redirects all flow to the radiator. After the engine stops operating, the water valve redirects flow from the engine to the heat storage tank where hot coolant is stored. To reduce emissions during cold-engine startup, hot coolant is pumped from the heat storage tank into the engine.
Fig. 2.3. Heat removal and lubrication scheme for the 2004 Prius hybrid electric drive system.
The hybrid drive coolant system is separate from the engine coolant system because the two systems operate at different temperatures. Powered by an electric pump, the liquid coolant in this closed-loop system flows continuously through the motor, generator, inverter, and radiator. Heat removed from these electrical components is transferred to the surrounding atmosphere by the radiator. Like the engine coolant system, the hybrid coolant system also includes a tank that functions as an overflow reservoir. Detailed information about the 2004 Prius and its operation and maintenance is provided in the threevolume repair manual published by the Toyota Motor Corportaion.2,3,4
Although the reason for using two separate coolant systems cannot be confirmed, it is believed that this decision was made to allow the motor, generator, and inverter to operate well below 100C, the boiling point of water.

*planet carrier locked

Planetary Carrier

Pinion Gear 23 Teeth

Planetary Gear Details
Engine (connected to planetary carrier) Pinion Gear 23 Teeth
Motor (connected to ring gear)
Sun Gear 30 Teeth Planetary Carrier Generator (connected to sun gear) Ring Gear 78 Teeth
Fig. 2.4. Diagram of the 2004 Prius power split device.
Gear Designation G1 G2 G3 G4 G5 G6 Number of Gear Teeth 26 75
wheel = motor (G1/G2)(G3/G4)(G5/G6) wheel = motor (36/35)(30/44)(26/75) wheel = motor (0.2431) motor = 4.113 wheel

Differential Gearbox

Fig. 2.5. Diagram of the 2004 Prius gear train between the motor and wheels.
Fig. 2.6. 2004 Prius gears with number of gear teeth shown.

Generator

The THS II includes a synchronous-type alternating current (ac) generator that rotates at high speeds up to 10,000 rpm. By rotating at high speeds, the generator, which is an 8-pole PM device, provides highpower density for charging the battery and supplementing motor power requirements. In addition, the generator also functions as the engine starter. At start up, the generator rotates the sun gear in the power split device and thereby provides cranking power for the engine. The configuration of the generator rotor is shown in Fig. 2.7.
Fig. 2.7. 2004 Prius generator rotor.
The 8-pole, PM synchronous motor features high low-speed torque and high power output. It is designed as a high-efficiency, direct current (dc) brushless motor that uses ac. The motor rotor is constructed with interior PMs and laminated stacked electromagnetic steel plates. The PMs are arranged in a V-shape as opposed to conventional radial alignment. In addition, with a high supply voltage up to 500V, the peak power output of the motor is 50 kW. The configuration of the motor and stator are shown in Fig. 2.8. Additional details about the design and manufacture of the motor are contained in a report that was recently published by ORNL.5 Supplementary information about locked rotor torque and current performance, which is addressed in Sect. 3.2.1, is contained in another ORNL report.1

For motor testing, the engine input spline was allowed to float with either the motor or the dynamometer providing the driving power. During the generator tests, the engine spline was fixed from rotating which effectively locks the planetary carrier arm. In this configuration, the planetary gear train transmitted torque to the generator shaft. Details of the power split device and the gear train that connects the motor to the wheels are presented in Figs. 2.4 and 2.5. To provide a better understanding of the thermal management system, gearbox lubricating oil temperature and hybrid drive coolant system flow data were collected as part of the overall testing effort. A diagram showing the lubricating and cooling oil inside the three compartments of hybrid drive housing is presented in Fig. 2.3. This figure also presents the hybrid drive coolant system flow rate and pressure that were determined as part of the testing effort. Besides lubricating the bearings and gears, this oil also removes excess heat from the gears, motor, and generator and transfers it to the hybrid drive system coolant. 3.2.1 Locked Rotor Tests A series of locked rotor tests1 was performed to determine general operating capabilities of the traction motor. To perform the motor starting torque evaluation, a lever arm was devised and calibrated to a zero cogging torque position that corresponded to zero degrees. The lever arm allowed the motor shaft position to be incrementally moved in degree segments while otherwise remaining locked. Various torque values were produced by supplying current to the motor windings at varying degrees of shaft angle. The resulting data were used to produce a plot, which is shown in Fig. 3.4, that represents torque versus shaft angle at various current levels. Locked rotor torque and current were also studied to characterize the startup torque capability of the motor. Current and corresponding torque values are listed in Table 3.2 and plotted in Fig. 3.5. This series of tests was effective in characterizing the starting torque capability of the 2004 Prius traction motor.

3.2.2.1 Motor Tests Measured back-emf voltage values from the motor are shown in Table 3.4 and plotted in Fig. 3.6. The lubricating oil temperature during this motor test was a nominal 25C. It should be noted that the Vpeak to Vrms ratio is greater than the square root of 2 because of the harmonics content in the back emf.
Table 3.4. Back-emf voltage measurements for the 2004 Prius motor Axle speed, rpm 1337 1458
Motor shaft speed, rpm 5500 5998
Axle torque, Nm 8.0 8.4 9.3 10.2 10.8 11.3 12.0 12.6 13.1 13.6 14.6 15.6
Frequency, Hz 33.8 66.5 99.8 134.4 168.1 200.2 233.9 265.4 295.7 333.0 366.3 401.3
Scaled back emf (Vrms) 42.0 85.7 132.3 181.6 221.8 269.3 315.7 354.6 405.5 440.4 503.4 539.8
Scaled back emf (Vpeak) 775 850
Note: Testing was conducted with the differential gears blocked from rotating and oil near room temperature.

Root mean square (rms).

6500 Motor Shaft Speed, rpm
Fig. 3.6. Motor back-emf voltage versus motor shaft speed.
3.2.2.2 Generator Tests Measured back-emf voltage values from the generator are shown in Table 3.5 and plotted in Fig. 3.7. The lubricating oil temperature during this generator test was a nominal 80C.
Table 3.5. Back-emf voltage measurements for the 2004 Prius generator Axle speed, rpm 600 Generator shaft speed, rpm 6420
Axle torque, Nm 8.2 9.4 9.6 9.0 9.1 9.5 10.2 10.8 11.3 11.6 12.2
Frequency, Hz 70.0 109.4 141.3 180.6 213.3 247.9 287.0 320.6 357.9 392.2 430.5
Scaled back emf (Vrms) 31.6 49.4 67.0 83.5 96.5 113.5 134.5 144.5 167.0 182.0 195.0
Scaled back emf (Vpeak) 52.5 80.0 110.0 135.0 160.0 190.0 210.0 240.0 260.0 290.0 320.0
Note: Testing was conducted with the differential gears blocked from rotating and a nominal oil temperature of 80C.

6000 7000

Vrms Vpeak
Generator Shaft Speed, rpm
Fig. 3.7. Generator back-emf voltage versus generator shaft speed.
In order to mechanically link the generator into the system, the engine shaft was not allowed to rotate during the tests (i.e., the planetary carrier was fixed from rotating). Using this arrangement allowed the generator to either drive or be driven by the hybrid drive gear train. The location of the planetary carrier relative to the other hybrid electric drive system components is shown in Fig. 2.4. 3.2.3 Hybrid Drive System Losses Tests
Three types of power losses that affect the overall efficiency of the hybrid electric drive system were studied. These losses, which are reported in watts (W), include: (1) gear losses; (2) motor rotor losses; and (3) planetary gears, sun gear, and generator rotor losses. Determining the magnitude of each of these types of losses was achieved by separately testing three hybrid drive system configurations at different motor shaft speeds and lubricating oil temperatures. Components installed as part of each configuration are identified in Table 3.6.

Motor Shaft Speed, rpm

Fig. 3.8. Hybrid electric drive system and component losses at 25C.
Losses that were determined for Configuration B at various elevated lubricating oil temperatures are listed in Tables 3.8 to 3.13. As Fig. 3.9 indicates, losses tend to decrease as the lubricating oil temperatures increases.
Table 3.8. Configuration B losses at a nominal oil temperature of 28C Motor shaft speed, rpm 5500 6006
Axle speed, rpm 1337 1458
Axle torque, Nm 5.8 6.7 7.6 7.9 8.4 8.9 9.4 9.8 10.3 10.7 11.2 12.1
Losses, W 72.8 170.4 291.1 400.2 534.6 681.0 837.3 997.0 1180.5 1360.7 1565.0 1849.0
Oil temperature, C 27.0 27.0 27.0 28.0 28.5 29.0 29.0 29.5 30.0 30.5 31.5 32.0
Table 3.9. Configuration B losses at a nominal oil temperature of 40C Motor shaft speed, rpm 5500 6006
Axle torque, Nm 5.3 6.0 6.7 7.2 7.7 8.2 8.5 9.1 9.5 10.2 10.6 11.3
Losses, W 66.6 152.6 256.7 364.7 490.0 627.4 757.1 925.8 1088.8 1297.1 1481.1 1726.8
Oil temperature, C 40.5 40.5 40.5 41.0 41.0 41.5 42.0 42.5 43.0 44.0 44.5 45.0
Table 3.10. Configuration B losses at a nominal oil temperature of 50C Motor shaft speed, rpm 5500 6006
Axle torque, Nm 5.2 6.0 6.5 7.0 7.4 7.9 8.3 8.6 9.1 9.7 10.2 10.7
Losses, W 65.3 152.6 249.0 354.6 470.9 604.4 739.3 874.9 1043.0 1233.5 1425.2 1635.1
Oil temperature, C 50.0 50.5 51.0 51.5 51.0 51.5 52.0 52.0 52.5 53.0 53.5 54.5
Table 3.11. Configuration B losses at a nominal oil temperature of 60C Motor shaft speed, rpm 5500 6006
Axle torque, Nm 5.0 5.5 6.2 6.8 7.2 7.9 8.3 8.6 8.9 9.4 10.0 10.6
Losses, W 62.8 139.9 237.5 344.5 458.2 604.4 739.3 874.9 1020.0 1195.4 1397.3 1619.8
Oil temperature, C 59.5 59.5 60.0 60.0 60.0 60.0 60.0 60.0 60.5 61.0 61.5 62.0
Table 3.12. Configuration B losses at a nominal oil temperature of 70C Motor shaft speed, rpm 5500 6006
Axle torque, Nm 4.7 5.5 6.2 6.8 7.1 7.7 8.2 8.3 8.8 9.1 9.6 10.1
Losses, W 59.0 139.9 237.5 344.5 451.8 589.1 730.4 844.4 1008.6 1157.2 1341.4 1543.4
Oil temperature, C 70.0 70.0 70.0 70.0 70.0 69.5 69.5 69.5 70.0 70.5 71.0 71.5
Table 3.13. Configuration B losses at a nominal oil temperature of 80C Motor shaft speed, rpm 5500 6006
Axle torque, Nm 4.0 4.4 5.5 6.1 6.6 7.2 7.8 8.2 8.5 8.8 9.2 9.7
Losses, W 50.2 111.9 210.7 309.0 420.0 550.9 694.8 834.2 974.2 1119.1 1285.5 1482.3
Oil temperature, C 82.5 82.0 80.5 80.5 80.5 81.0 81.0 81.0 81.0 81.5 82.0 82.0
Run at 28 C (Oil Temp.) Run at 40 C (Oil Temp.) Run at 50 C (Oil Temp.) Run at 60 C (Oil Temp.)
Run at 70 C (Oil Temp.) Run at 80 C (Oil Temp.)

Increasing Temperature

Motor Shaft Speed, rpm 7000
Fig. 3.9. Configuration B losses as a function of oil temperature.

4. INVERTER AND CONVERTER EVALUATION
The inverter that is part of the hybrid electric drive system was partially disassembled to reveal its architecture, to identify its method of cooling, and to understand the manufacturing techniques used in its construction. A description of the inverter and its components is provided in Sect. 2.2.5. 4.1 FUNCTIONAL AND ARCHITECTURE STUDY After careful study of a 2003 and 2004 Prius inverter, differences were noted between the two models. The 2004 inverter is packaged in roughly the same volume as the 2003 unit; however, the 2004 inverter contains the new buck/boost converter in addition to the motor, generator, air conditioning compressor inverter, and dc-to-dc inverter and converter. The 2004 Prius inverter is cooled using a cold plate located in the center of the package. This cold plate serves as a separator between the generator-motor-boost sections that are located above the cold plate and the air conditioning compressor inverter and dc-to-dc converter located below the cold plate. The cold plate transfers excess heat from the inverter to the hybrid drive system coolant as it circulates through internal passages in the cold plate. A flow diagram for the hybrid drive system coolant is presented in Fig. 2.3. The main inverter sections (motor and generator) are packaged in one module referred to as the 12 pack. The boost converter is a separate module from the 12 pack. The main dc link capacitors in the 2004 Prius are slightly smaller in volume than those in the 2003 Prius and are packaged in a plastic module making them different from the commercially available can-type electrolytic capacitors used in the 2003 model. Most of the integrated circuits in the 2004 Prius are identified with a Toyota label, as compared to those in the 2003 unit that used commercially available electronic components. 4.2 CONTROL DEVELOPMENT In preparing for future component-level tests, the inverter is being modified to operate and properly control the Prius traction motor and generator while outside the vehicle. Accomplishing this objective requires an understanding of the interface between the inverter and the Prius control system. Speed and position feedback signals need to be understood, and algorithms and interfaces need to be developed to provide control signals to the inverter and converter power devices. Currently, the RT-LAB real-time computing platform from OPAL-RT Technologies is being used to model and replace the Prius onboard control system. The RT-LAB system interfaces with the MATLAB SIMULINK software for quick controller development without tedious assembler programming. The system consists of a host PC running a user-selected operating system and two target PCs running the QNX Neutrino operating system. One of the PCs is a dual-processor computer with additional counter, encoder, and analog/digital I/O PCI boards. The model of the Prius controller and a user interface is built in MATLAB SIMULINK using both builtin SIMULINK blocks and RT-LAB blocks. Using the Real-Time Workshop Toolbox of SIMULINK, the model is converted to C-source code and the executable is uploaded to the target PCs. The controller software runs on two target PCs that communicate with each other through a firewire connection while the host computer is used to command the controller through an ethernet connection. The software allows the control development to be flexible and versatile, with the capability of quickly making required development changes. This approach bypasses some of the more difficult hardware development efforts

required to allow the inverter to be controlled outside the vehicle. It also enhances the ability to make changes during testing, if required. When this work is completed, inverter performance will be verified by driving simple resistive or inductive loads. After verification is completed, the inverter will then be used to operate the traction motor and generator during the component-level performance tests in a motor test cell. 4.3 INVERTER MODIFICATIONS AT ANL A 2004 Prius inverter is being modified at ANL with installation of instrumentation to allow control of the system and to provide an effective way to monitor power flow through the inverters and converters. Currently, a preliminary version of this instrumentation is being used to perform 2004 Prius testing on the ANL chassis dynamometer. Additional information about these tests is presented in Sect. 3.1. The inverter that will be used for the ORNL component-level tests is currently being modified with conventional sensors that are similar in design to those installed in the ANL inverter. Changes that are being made include installation of: 1. Shunt-style current sensors on dc link bus bars on both the 200V and 500V sides of the boost converter, 2. Voltage divider sensing points on the 200V and 500V bus bars, 3. A shunt-style current sensor on the 12V auxiliary charging system, and 4. A shunt-style current sensor on the 200V dc-to-dc converter for the air conditioning compressor inverter drive. Additional inverter instrumentation is required to achieve all the ORNL component-level testing goals. This additional instrumentation is non-standard and involves developmental sensors and installation techniques. Issues related to this development include: Installation of Giant Magneto Resistive (GMR) current sensors directly on the dc bus bars (embedded in the module) to sense current in the feed to the traction motor inverter section. Installation of GMRs on the dc bus bars that feed the generator inverter section.
The GMRs, which supplement the conventional sensors being installed on the inverter and converter sections, are much more difficult to install and are very intrusive into the inverter, thus requiring very careful design and installation. These special sensors are important because they will allow power flow inside the inverter and converter package to be resolved (separate power flow to and from the motor and power flow from the generator are normally mixed on the internal dc link and packaged so tightly inside the housing that it is difficult to measure as separate power quantities). These sensors were specified because they are very small and the space in which they are being placed is very restricted, and because they provide a way to resolve all the separate power flows in the hybrid electric drive system. This instrumentation scheme, which is depicted in Fig. 3.1, will allow ORNL to best evaluate the operating methods and electrical and thermal performance of the hybrid electric drive system. Inverter modifications by ANL are nearing completion and the unit should be ready for operation in the near future.

REFERENCES

1. J. S. Hsu, C. W. Ayers, C. L. Coomer, R. H. Wiles, S. L. Campbell, K. T. Lowe, and R. T. Michelhaugh, Report on Toyota/Prius Motor Torque Capability, Torque Property, No-Load Back EMF, and Mechanical Losses, ORNL/TM-2004/185, UT-Battelle, LLC, Oak Ridge National Laboratory, Oak Ridge, Tennessee, October 2004. 2. Prius Repair Manual, 1, Pub. No. RM1075U1, Toyota Motor Corporation, 2003. 3. Prius Repair Manual, 2, Pub. No. RM1075U2, Toyota Motor Corporation, 2003. 4. Prius Repair Manual, 3, Pub. No. RM1075U3, Toyota Motor Corporation, 2003. 5. J. S. Hsu, C. W. Ayers, and C. L. Coomer, Report on Toyota/Prius Motor Design and Manufacturing Assessment, ORNL/TM-2004/137, UT-Battelle, LLC, Oak Ridge National Laboratory, Oak Ridge, Tennessee, August 2004.

INTERNAL DISTRIBUTION

1. 24. 5. 68. 911. 12. D. J. Adams C. W. Ayers E. C. Fox J. S. Hsu L. D. Marlino S. C. Nelson, Jr. 13. 14. 15. 16. 1718. 19. C. B. Oland G. W. Ott, Jr. R. H. Staunton Central Research Library ORNL Laboratory Records (OSTI) ORNL Laboratory Records (RC)

EXTERNAL DISTRIBUTION

20. S. A. Rogers, U.S. Department of Energy, EE-2G/Forrestal Building, 1000 Independence Avenue, S.W., Washington, D.C. 208585. 21. E. J. Wall, U.S. Department of Energy, EE-2G/Forrestal Building, 1000 Independence Avenue, S.W., Washington, D.C. 208585.