Development of Motors andInverters for Light-Duty HybridTrucks
KENJI FUJIWARA* 1

YOSHIKI KATO* 1

SHINJI OKADA* 1

SHINJI TAKAHASHI* 1

KOJI MINAMI* 1

NORIFUMI YAMAMOTO* 2

From the perspectives of preventing global warming and maintaining resources, the issues of fuel efficiency
and vehicle emission regulations have intensified in recent years, and the demand for green cars is increasing
every year. Mitsubishi Heavy Industries, Ltd. (MHI) has been supplying motors and inverters for prototype hybrid vehicles to various car manufacturers and also started to supply them for light-duty hybrid trucks in May
2006. This paper gives an outline of our reliability assurance in the processes from designing to manufacturing of
the motors and inverters.

were completed in 2006 and commercial production of
motors and inverters for light-duty hybrid trucks began
in May of that year.
This paper describes the development of these motors and inverters in processes from design tomanufacturing by focusing on reliability assurance.

1. Introduction
In recent years, there have been increasing discussions on the effects of vehicle exhaust gas on the global
environment, for example the California's Low-Emission
Vehicle Regulations in the U.S, the New Long-Term
Emission Regulations in Japan, and emission gas controls in Europe. Meanwhile Brazil produces alternative
fuels such as ethanol which is known as clean fuel. The
world's nations decided on CO 2 reduction targets under
the Kyoto Protocol and have launched efforts to prevent
global warming. When viewed by the industrial sector,
the ratio of CO2 emissions from vehicles is considerable.
In addition to the global environmental problems,
there is also a problem of reserve-production ratios (RPR)
of fossil fuels, and the development of technology to reduce petroleum consumption is becoming an importantmission for manufacturers. Petroleum consumption in
Japan is about 300 million liters per year, 35% of which
is used as vehicle fuel. Vehicle exhaust gases include
CO 2, NOx and PM. Carbon dioxide causes global warming, while NOx and PM pollute the environment.
Therefore, improving fuel consumption and controllingexhaust gas are strongly required worldwide. Further,
responding to the hikes in gasoline prices in the U.S.
over the past few years, car manufacturers are accelerating their efforts to expand the supply of hybrid vehicles
and develop new products.
MHI started to develop servo motor technology as
early as the 1970s and has introduced it to special vehicles and industrial machinery. The compact andlightweight features of MHI motors are highly regarded,
and motors and inverters for prototype hybrid vehicles
have been supplied to various car manufacturers since
2000. Long term verification tests with actual vehicles

2. Motors and inverters for light-duty hybrid trucks
The features of MHI's motors are compactness, lightweight, and high power, and so can be mounted between
the automatic clutch and the automated mechanicaltransmission in a compact manner. Because of this, thevehicle is started by the motor alone and energy is efficiently regenerated when the engine is detached during
deceleration. Further, this compactness also allows parts
sharing between the base and hybrid vehicles. Figure 1
shows a light-duty hybrid truck equipped with an MHI
motor, while Fig. 2 shows a light-duty hybrid truck inverter developed by MHI. (The title photo shows themotor itself.)

Fig. 1 Light-duty hybrid truck (photo provided by Mitsubishi
Fuso Truck & Bus Corporation)

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 44 No. 2 (Jul. 2007)*1 Kobe Shipyard & Machinery Works*2 Seiryo Engineering Co., Ltd.

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Steep slope
travelHig
h speed
uphill travelHigh speed travel

Inner-city
travelPowerTorque: Max. Torque: Max. PowerCruising using engine
Speed
Fig. 3 Performance curve of hybrid vehicle motor: Max. Torque: Max. Power

Fig. 2 Inverter for light-duty hybrid truck
Table 1 Required specifications input to design tools(1) Max. TorqueTmax

Nm(2) Max. Speed

Nmaxrpm(3) Max. Voltage

VBrVDC(4) Max. Current

ImaxArms(5) Outer diameter of statorDomm(6) Max. PowerPoutkW

Input value

Example of data list outputs of design tools

PowerUnit

Speed

Motor specifications ----- Armature resistance, inductance, torque constant
Performance characteristics ----- N-T characteristics, output characteristics,
temperature characteristics, efficiency map
Losses ------- Copper loss, iron loss, wind loss
Driving characteristics ----- Temperature trend during continuous driving,temperature trend during 10-15 mode drive,
temperature trend during any given drive patternFig. 4 N-T characteristics and efficiency map (example of output)
The contour line map shows the efficiency distribution,
where efficiency increases toward the center.

: Coil (thermistor): Stator: Rotor: Housing

3. Design of motors and inverters

Temperature

As hybrid vehicle motors require increased compactness and light weight, a wider driving range, and higher
efficiency compared with motors for industrial machinery, they are designed using the procedures below.
3.1 Requirements for hybrid vehicle motors
Figure 3 shows the motor performance of various travelmodes. Hybrid vehicle motors are expected to have enoughperformance to meet the specific objectives of each travel
type. Through optimizing the number of poles and slotsof the motor, MHI has realized (1) increased torque stability (reduction of the high harmonic components ofmagnetic flux), (2) maximum utilization of magnets andmagnet wires, and (3) reduced magnetic resistance. As aresult, downsizing and weight saving of about 60% in comparison with MHI's conventional model has been achieved.
3.2 Automatic design by use of tools
When the requirements specified in Table 1 are input
to the design tools, they output a data list (mentioned below) to complete the basic design of the motor and inverter.
The design tools have been improved based on prototype
vehicle test data to ensure higher accuracy (+/-5% or: Radiator: Copper loss
: Iron loss

Copper loss, iron loss

SymbolTorqueItemTime
Fig. 5 Temperature trend during continuous driving(example of output)

below). The design tools can quickly provide the motor
design specifications needed to attain the target performance and have contributed to a sharp reduction of design
man-hours. As output examples, Fig. 4 shows the N-T characteristics and efficiency map, while Fig. 5 shows thetemperature trend during continuous driving.
Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 44 No. 2 (Jul. 2007)

2RotorMotor shaft

Housing

StatorFig. 6 Results of magnetic field analysis (example of output)

Fig. 7 Example of digital mock-up for vibration and heatstress analysis

Table 2 Core tools
APQP

Advanced product quality planning

PPAP

Production part approval process

FMEA

Failure mode and effects analysis

MSA

Measurement system analysis

SPC

Statistical process control

4. Manufacture of Motors and Inverters
By using the quality system for automobile parts(TTable 2 shows the core tools of the quality system) MHI
has established a production line and developed an automatic winding machine suitable for the new motors.
The quality of the products currently being manufactured
shows less disparity and the process capability is good.
We plan to keep working on pursuing the fundamentalsof the quality system to improve and maintain our quality level, thus responding to customers' needs.

3.3 Design verification by analysis
When there is a large change in the motor structure
or motor mounting conditions, we carry out two dimensional and three dimensional analyses of the magnetic
field, heat and vibration in the initial stage of the design process, verifying the feasibility of the targetedperformance. As an example, the results of the magnetic field analysis of a hybrid vehicle motor are shownin Fig. 66, and an example of a digital mock-up for thevibration and heat strength analysis is shown in Fig. 7
7.
3.4 Selection of component parts
Unlike the motor, as the inverter is equipped with anumber of electronic devices, we consider that not only
thermal analysis by the design tools but also reducing
the failure rate of electronic parts based on Expression(1) is an important issue.

5. Conclusion
Regarding the motors and inverters whose commercial production for light-duty hybrid trucks has alreadybeen started, we have outlined our efforts in improving
reliability through the design to manufacturing processes.
Various kinds of system such as hybrid cars and natural gas vehicles are under development as earth- conscious
cars all over the world. Energy regeneration is the characteristic which is inherent in a vehicle using a motor,
where the degree of regenerated energy effect of the motor is always an important issue. Further, when low-cost
fuel cells are developed in future, the motor will become
their core driving power source. We will continue to reinforce our technological development and promote thedevelopment of low-cost, high-performance motors.

Note: Fit (Failure In Time) = 10 -9 /hour

As the power device generates the largest loss of theinverter component parts, it is a particularly importantcomponent which influences both the performance andthe cost. MHI has achieved the target power by raising
the motor power factor and by using a highly reliable
power device with a small capacity. Further, in the design of inverters, especially in the design of electronic
circuits, the aim is to reduce the initial failure rate bymeans of screening and aging, and to reduce the failure
rate through selecting parts with a proven long service
life. As MHI is also working on improving the reliabilityof electronic parts supplied to the aviation, space andatomic industries, we will address this important issue
by integrating these technologies.

Kenji Fujiwara

Yoshiki Kato

Shinji Okada

Shinji Takahashi

Koji Minami

Norifumi Yamamoto

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 44 No. 2 (Jul. 2007)3