Development of Small Size,
Light Weight and High PowerIPM Motor for Electric Vehicle
AKIRA NISHIO* 1
MASAHIRO HIRANO*1
YOSHIKI KATO* 1
TAKAYUKI IRIE*2
TSUTOMU BABA*3

Mitsubishi Heavy Industries, Ltd (MHI) has recently developed small-size, light-weight and high-power IPM
motor and inverter for electric vehicles. This paper describes the system configuration of the electric vehicle andhybrid vehicle and the technologies used for the small-size, light-weight and high-power IPM motor and inverter
developed by MHI for electric vehicles. With the skillful arrangement of permanent magnets, optimized combinationof rotor poles/stator slots, adoption of concentrated winding, etc., the motor is designed to ensure high current density, high torque constant and high power factor in order to realize high power, approximately 2.5 times higher, interms of power/volume ratio, than the conventional motor. Realization of small size and light weight has also beenachieved in inverter through optimization of water-cooling structure and by integrating the power circuit and thecontrol circuit in one board.
been drawn to the IPM (Interior Permanent Magnet)
motor suitable for high-speed rotation and capable ofbeing made smaller in size, lighter in weight and higher
in efficiency than the induction motor.
MHI has recently developed small-size, light-weightand high- power IPM motor and inverter for electric vehicle by making an effective use of its long amassed
technologies for the development of the conventional
servo-motor and amplifier. This paper describes the system configuration of electric vehicle and hybrid electricvehicle, the technologies used for small-size, light-weightand high-power IPM motor and inverter developed by
MHI for electric vehicles, and the product specifications.

1. Introduction
Because of the environmental problems such as exhaustion of natural resources, increase in the emissionof carbon dioxide, etc., developments are being actively
carried out recently in electric vehicles (EV), hybrid electric vehicles (HEV) and fuel cell electric vehicles (FCEV).
Motors for driving these vehicles are demanded to besmaller in size, lighter in weight and higher in power inorder to improve fuel consumption and to ensure larger
space inside the vehicles. There is particularly a highdemand for high power and high efficiency from the lowspeed rotation zone (region) through the high-speed
rotation zone.
With the rare earth magnet having its performance
improved and cost reduced in recent years, attention has

Battery

2. Outline of System Configuration
Fig. 1 shows the system configuration of electric ve-Battery

Battery

Battery

Generator
Inverter

Motor

Generator

EngineInverterInverterInverter

Motor

EnginePowerdividingmechanism Motor

Motor/
Generator

EngineElectric vehicle

Hybrid electric vehicle(parallel type)Hybrid electric vehicle(series type)Hybrid electric vehicle(series & parallel type)Fig. 1 System configuration of electric vehicle and hybrid electric vehicle
The electric vehicle and hybrid electric vehicle are classified mainly in 4 types of system.

*1 Kobe Shipyard & Machinery Works*2 Takasago Research & Development Center, Technical Headquarters*3 Seiryo Engineering

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Mitsubishi Heavy Industries, Ltd.
Technical Review Vol.40 No.5 (Oct. 2003)

3. Technologies for Small Size, Light Weight and HighPowerFig. 3 shows an example of the newly developed motor and inverter for electric vehicle.
3-1 Motor
The motor for electric vehicle is required of the operation up to a high-speed zone (area). Hence, theconventional servo motor type surface permanent magnet motor (SPM motor) with the magnet pasted on thesurface gives rise to a problem of the fly-off of the magnet due to the centrifugal force at high speed zone anddemagnetization resistance because of field-weakeningcontrol. We have therefore adopted the IPM motor where
the magnet is embedded inside. The IPM motor has anadvantage that it can maintain high power up to the highspeed zone by making use of field-weakening control.
In order to make a motor smaller in size, lighter inweight and higher in power it is necessary to increase theenergy density, the volume ratio to the output power ofthe motor. And in order to increase the motor energy density, the following three points must be taken into account.
(1) High current density
In case of a conventional IPM motor, excessive flowof current brings about magnetic saturation, leading
to a drastic change in inductance. So, it was not possible to flow excessive current. Further, it has acharacteristic of the inductance getting sharply increased when the current lead angle gets large at theFig. 4time of field-weakening control (Fig.
4). As is obvious in equation (1), these characteristics cause theterminal voltage of the motor to increase at the timeof high-speed rotation. Since there is an upper limit
for the inverter output voltage in relation to the battery voltage, no more current can be flowed if themotor terminal voltage exceeds the said upper limit.

where;
V a: Motor terminal voltage [Vrms]
R a: Motor winding resistance [ Ω ]i d, i q: Current in d, q axes [A]
L d, L q: Inductance in d, q axes [H]ω : Electrical angular velocity [rad/sec]
ψa: Armature flux linkage due to permanent
magnet [Wb]80
Fuel consumption improvement rate (%)

hicle and hybrid electric vehicle (series type, parallel typeand series & parallel type)The electric vehicle uses a motor as power source andoperates only by the power from the battery. At the timeof speed reduction (slowdown) or at downgrade hill drive,the motor works as a generator, regenerating power tothe battery. The battery of this system is replaced witha fuel cell in the case of a fuel all electric vehicle.
The hybrid electric vehicle, on the other hand, uses
two power sources: engine and motor. There are mainly
three types of hybrid electric vehicle.
The series type uses the engine to operate the generator, and the generated power is used for driving themotor for traveling. In the case of parallel type, the engine plays main role for traveling. In some cases, theengine is used as a power source to charge the battery,
therefore the motor gets activated and works as an auxiliary at the time of start or acceleration. The series &parallel type is a combination of series type and parallel
type and uses both motor and generator, with motor being used for low-speed drive. At a speed above a certain
level the engine starts operation, and keeps on traveling while generating power when the drive load getslight.
In the aforesaid system types the motor and inverter
work as a substitute or an auxiliary for the engine, sothat they are demanded to be smaller in size, lighter inweight and higher in efficiency in order to improve fuelconsumption and to ensure low emission of exhaust gas.
The effect of improvement in fuel consumption of a
hybrid electric vehicle is shown in Fig. 2 (1), indicatingthat the higher the motor power, the larger the effect ofimprovement in fuel consumption. Since a hybrid electric vehicle needs both motor and engine, the installation
place for motor and inverter is limited, so that smallsize, light-weight and high-power motor and inverter areindispensable.

: With 33 kW motor (mounted): With 8 kW motor (mounted)

706050
4030
20
100
11.5

22.5

33.5

4

Engine displacement ( x 1 000 cc)
Fig. 2 Mileage/Liter(1)The higher the motor power, the greater the effect of improvement in fuel cost.

MotorInverter

Fig. 3 Motor and inverter for electric vehicle
Shows the example of the motor and inverter developed for electric vehicle.

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol.40 No.5 (Oct. 2003)

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Inductance (relative value)

3.5

With the rotor diameter, winding system and theother electrical characteristics being equivalent, it isnecessary to increase the number of poles in order toincrease the torque constant.
Further, with the stator diameter, thickness andnumber of poles being equivalent, the torque constantis proportional to the number of winding turns. Inorder to increase the number of winding turns, it isnecessary to increase the cross-section of the slot, thewinding groove,; and in case the number of slots isthe same, the preliminary condition for the above willbe the small cross-section of the tooth of 1 piece ofslot. Taking the aforesaid factors into consideration,MHI carried out optimization of the combination ofpole number and slot number.
In the case of a motor with optimized numbers ofpoles and slots, the winding factor against the fundamental magnetic flux can be brought closer to 1 byadjusting the opening width of the slot, and the winding factor against the high-frequency magnetic flux
can also be reduced to ensure high torque constant.
MHI takes account of this fact, and has optimized
the width of slot opening to ensure high torque.
(3) High power factor
In the case of a conventional IPM motor, the inductance in q-axis is large and the effect of armature
reaction is also large, causing the power factor to get
decreased when a large torque is generated. The MHI
IPM motor, however, has the distance between thepermanent magnet embedded in the rotor and therotor surface optimized to reduce the q-axis inductance to ensure high power factor even when thegenerated torque is large.
Besides, MHI IPM motor has slot-saving winding
system and adopts the concentrated winding that canbe carried out using an automatic machine. Further,
the motor has been made smaller in size by adopting
water cooling system instead of the forced air coolingused in the conventional servo motor.
A comparison between the conventional servo motor and the newly developed IPM motor is shown inTable 1
1. By using the aforesaid technologies, the: Conventional IPM motor: IPM motor newly developed by MHI

32.5
2Sharp increase of inductance

1.5
10.50No increase of inductance
102030

4050

60

70

8090

Lead angle (deg)Fig. 4 Armature inductance vs. current lead angle
The MHI IPM motor has the armature inductance hardly increased evenwhen the current lead angle gets increased.

In the case of MHI IPM motor, however, the magnet arrangement is carefully devised in order to reducethe inductance as well as to raise the limit of magnetic saturation. Further, the rise in motor terminal
voltage is controlled by minimizing the change in inductance even when the current lead angle gets large
at the time of field-weakening control in high-speed
rotation zone, ensuring high current density.
(2) High torque constant
As is clear from equations (2) and (3), increasing
the motor torque constant (armature flux linkage due
to permanent magnet) to increase the energy density has conventionally been avoided since it resultsin high electro-motive force (e.m.f.). However, the active utilization of field-weakening control canincrease the torque constant for realization of a highpower motor.

where;
T : Torque [Nm]P n: The number of motor pole pairs
K e: Electro-motive force constant [Vrms/rpm]
N : Rotational speed [rpm]Table 1 Comparison of motor main specification

1Motor for electric vehicle
Conventional servo motor(example)

Item

No.

Rated power(kW)

40

222

Rated input current(Arms)

133

120

3

Maximum output power (kW)

7547

4

Maximum input current (Arms)

340

310

5Volume(m3)0.01410.0242

6Weight(kg)5876

7Cooling system

Water cooling

Forced air cooling

8Structure

Waterproof specification-Mitsubishi Heavy Industries, Ltd.
Technical Review Vol.40 No.5 (Oct. 2003)

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Table 2 Comparison of inverter main specification
ItemNo.

Inverter for electric vehicle Conventional servo motor

1

Rated output current(Arms)

133

138

2

Maximum output current (Arms)

340

345

3Current control system

Sinusoidal PWM control

Sinusoidal PWM control

4Input power

DC 288AC 200 - 220

Totally enclosed/Watercooling system

Forced air cooling(V)5Structure

6Weight(kg)

1840

7

Volume(m3)0.01734

0.05337

InverterInverter board
CAN

External interface

Control circuit section

Power circuit section

Smoothingcondenser
Discharge
resistance
N

R1

Upper
controller

Battery+PPower module
Current detector
UVW

MotorFig. 5 Inverter system configuration
Adopts the international standard CAN for communication with upper controller.

power/volume ratio has been increased to 2.5 times
larger than that of the conventional servo motor.
3-2 Inverter
The main specifications of the inverter for electric vehicle and the conventional servo amplifier are given in
Table 2 and the internal configuration of inverter inFig
Fig.. 5 . The inverter largely differs from the conventional
servo amplifier in (I) the adoption of CAN, the international standard for communication with upper controllerand (II) the field-weakening control by means of current
control.
The conventional servo amplifier uses forced air cooling system for cooling the power module, while the newly
developed inverter uses water cooling in order to reduce
the size. In addition, the water cooling system is designed
to have optimized cooling fan to ensure small size andhigh efficiency.
Furthermore, the inverter is designed to totally enclosed and waterproof structure so as to be installedinside the engine room. In addition to this, the powerand control circuits are integrated into one board for lesswiring and small size to upgrade work efficiency at thetime of assembly.
By using the aforesaid technologies, the volume couldbe reduced to one-third and the weight to half as compared with the conventional servo amplifier with thesame power (output).

4. Product Specifications
Specifications of motors so far developed for electric
e 3,vehicle and hybrid electric vehicle are given in Tabl
able
indicating that the small size, light-weight and highpower motor and inverter have been realized by using
the technologies for small size, light weight and highpower mentioned in Section 3.
The rotational frequency (speed) vs. torque and power
characteristics of Type 3 motor applied to parallel hybrid system is shown in Fig. 66, while the system
configuration of the hybrid electric vehicle using thismotor in Fig. 77. In this system, the battery voltage is 42
V and the motor has three functions working as a starter,
generator and auxiliary driving force at the time of startand acceleration. The use of MHI small-size, light-weightand high-power motor and inverter has been verified toimprove the fuel consumption by 40%.

5. Conclusion
We have so far described the technologies for smallMitsubishi Heavy Industries, Ltd.
Technical Review Vol.40 No.5 (Oct. 2003)

4Table 3 Specification of Motor for conventional (already developed) electric vehicle and hybrid electric vehicle
Item
Maximum output power(kW)

Type-1

Type-2Type-3Type-4

100

33

1335

Maximum torque(Nm)

250

82

90

197

Maximum rotational speed(rpm)

5 750

6 000

6 000

3 750

Case diameter/stator diameter*1(mm)

320

280

238

330

Case length/stator length*1(mm)

290

189

3560

Water coolingsystemWater coolingsystemWater coolingsystemWater coolingsystem

380

240

42288

Electric vehicle

HybridParallel hybrid

Parallel hybrid

Motor type

Motor type

Build-in type

Build-in type

Cooling system
Power voltage(V)Applicable vehicle type
Type*2*1: Indicates case diameter and case length in the case of motor type and stator diameter and stator length
in the case of build-in type.
*2: Motor type indicates normal motor case type, while build-in type indicates the type installed to the engine case.
14140

12: Power: Torque100

1080

860

640

420

20

1 000

2 000

3 000

4 000

5 000

InverterPower (kW)Torque (Nm)120

BatteryEngine
Motor0Rotational speed (rpm)Fig. 6 Speed vs. torque and power characteristics (Type-3)
Indicates the high power maintained over the high-speed zone.

Fig. 7 System of hybrid electric vehicle using Type-3 motor
Battery voltage is 42V, and the motor carries out functions of starter,
generator and auxiliary to driving force at start and acceleration.

size, light-weight and high-power motor and inverter
developed by MHI for electric vehicles. The newly developed motor has realized a power about 2.5 times as large
as that of a conventional servo motor of equivalent volume through skillful arrangement of permanent magnet,
optimization of rotor pole number/stator slot number,adoption of concentrated winding, etc. Further, the inverter has realized a volume as large as one-third of aconventional servo amplifier of equivalent power throughwater-cooling structure and integration of control andpower circuits in one board.
Furthermore, these technologies were used in themotor and inverter for hybrid electric vehicle system,and have been found to be substantially effective forimproving the fuel consumption.

We are determined to carry on our development mission to produce the motor and inverter smaller in size,
lighter in weight and higher in power, to improve reliability of our products and to widen the applicable vehicle
types.
Reference(1) The Relation between Motor Generator Output Power andFuel Consumption Improvement, and Manufacture’s Catalog Data, D & M Nikkei Mechanical, Nikkei BP No.573 (2002)
p.81

Akira Nishio

Masahiro Hirano

Yoshiki Kato

Takayuki Irie

Tsutomu Baba

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol.40 No.5 (Oct. 2003)

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