Development of Low-loss
Inverters for Electric Vehicle(EV) Motors
KOICHI OKUBO*1

MAKIO MASUDA*1

YOSHIKI KATO*1YUTAKA NAKATANI*1

The reduction of emissions of carbon dioxide (CO2 ) is presently an urgent task given that such emissions are playing a large partin global warming. Improved fuel consumption technologies have been developed and stringent emissions rules have been applied inrelation to automobiles as environmental awareness has increased. Ever since it started to produce a series of motors and inverters
for hybrid compact trucks in May 2006, Mitsubishi Heavy Industries, Ltd. (MHI) has been contributing to the production ofenvironmentally friendly automobiles that comply with automobile emissions control. This paper reports MHI’s new development of
an inverter with a control system that changes the switching frequencies to reduce power module loss which in turn drives the motor
more efﬁciently.

1. Introduction

2. Basic inverter control system

Reducing CO 2 emissions is mandatory, as they have asignificant impact on global warming. Since automobiles
account for a high percentage of total CO 2 emissions, theautomobile industry has been keen to promote the spread ofhybrid cars and electric vehicles.
MHI began producing a series of motors and inverters
for hybrid compact trucks in May 2006 by way of developing
environmentally friendly vehicles that comply with emissions
control. With more hybrid trucks expected to be marketed,
MHI must produce efﬁcient vehicle motors that will improvef uel consumpt ion whi le cont r ibut i ng to env i ron ment
preservation.
This paper describes a newly developed control systemthat reduces inverter loss as a measure to drive motors more
efﬁciently.

Electric vehicles (EVs) generally use interior permanent
magnetic synchronous (IPM) motors, which control theelectric current in response to the torque commands.
Figure 1 shows a schematic block diagram of the control
system.
The current detected by the three-phase current sensor
is coordinate-transformed from triple-phase to d- and q-axes
so that the current feedback control takes place in the d-q
axes coordinate system. Since the d-q axes form a rotating
coordinate system, the cur rent vector cont rol occurs
while dividing the current into torque and field-weakening
components.
Since the induction voltage is higher than the power line
voltage in the high-speed region, the current would normallybe unable to flow. To avoid this, field-weakening control

Torque command
Current
command block

Switching frequencies
f1 f2 f3
Du,Vu*,Dv,Vv*,Id*, Iq*
Vd*, Vq*
Dw
Vw*
Current
d-q axes/
Duty
PowerPI compensator
3-phase conversion
calculator
converter
Id, Iqd-q axes/
3-phase conversion

Iu, Iv, Iw

MotorRotor angle
detector

Speedcalculator
Fig. 1 Schematic block diagram of the motor and inverter control
The current is controlled by controlling the torque commands by switching frequency.
*1 Kobe Shipyard & Machinery Works

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 3 (Sep. 2008)

16

IGBT steady-state loss

Inverter

IGBT module

Total loss ofthe power module

IGBT loss

IGBT switching loss
FWD steady-state loss

Battery power

FWD loss+FWD reverse recovery loss

MotorFig. 3 Total power module losses
The causes of power module loss are classiﬁed into four categories.

FWD–Changing the switching frequencies helps to minimize theheat generation and loss in the low-speed region, as shownin Fig. 4. The resulting significant reduction in heat andloss enables the construction of a smaller power module andcooling system, which in turn allows a compact inverter.
Nevertheless, this method has a problem: torque shock
and abnormal noise are generated when the switchingf requencies change. T he di f ference in the sw itchingfrequencies causes a delay in the current control system.

IGBT
IGBT : Insulated Gate Bipolar Transistor
FWD : Free Wheeling DiodeFig. 2 The basic hardware conﬁguration
The IGBT is PWM-controlled to drive the motor.

generally occurs where a negative d-axis current flows andweakens the magnetic ﬂux in the direction of the d-axis, so
as to suppress the induction voltage.
Figure 2 shows the basic hardware conﬁguration for theIPM motor system. The inverter must be compact and highly
efﬁcient, as it has to be mounted on a car. Since driving an
EV motor requires a large current, the heat generated by theinsulated gate bipolar transistor (IGBT) module, which is thepower module in the inverter, has been a stumbling block toachieving both compactness and high efﬁciency. Figure 3classiﬁes the causes of IGBT module heat generation (totalloss).
The IGBT switching loss and F W D reverse recover yloss increase with the switching frequency while the total
loss increases as the current increases. In other words,
a higher switching frequency and current both induce
more power module loss and heat generation. Copingwith heat generation requires a larger power module and acooling system. Attempting to suppress the current would
compromise the performance. To solve this problem, MHI
have adopted a control system that changes the switchingfrequencies.

TorqueTorqueSwitching frequency

f3 Hz

f2 Hz

f1 Hz

Rotor speed

Power module loss

IGBT switching loss and FWD
reverse recovery loss reductions
in low speed region

3. Low-loss motor control system
When driving a motor at a high speed, the pulse widthmodulation (PWM) control system has to be set to a highswitching frequency to ensure controllability in responseto the increase in the sinusoidal frequency of the motor
current. By contrast, in a low-speed region where a highelectric current flows, the motor does not require a highswitching frequency. Despite a short drive time in thehigh-speed range, driving an EV motor requires that theswitching frequency always be compatible with the highspeed region. The resulting switching loss reduces theefﬁciency signiﬁcantly. To solve this problem, MHI divided
the switching frequencies into three steps for the low-,intermediate-, and high-speed regions, as shown in Fig. 4.

Without changing
switching frequencies

With changing
switching frequencies

Rotor speed
Steady-state loss
(IGBT steady-state loss andFWD steady-state loss)
Fig. 4 Changing regions of the switching frequencies andpower module loss
The top diagram is the N-T diagram and shows the changing
regions of the switching frequencies. The bottom diagram indicatesthe effect on reducing power module loss.

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 3 (Sep. 2008)

17

The resulting fluctuation in the motor voltage and current
leads to torque shock and abnormal noise.
MHI have developed a current-control software program
to cope with this delay, one that successfully suppresses thetorque shock and abnormal noise.

is necessary only when a customer requires anything otherthan the standard MHI I/O system, which reduces the timeneeded for software design, while improving the reliability ofthe software.

5. Conclusion

4. Improving reliability

T his paper has outlined the development of low-loss
inver ters for E V motors and ways to improve productreliability.
The development of various hybrid and electric vehicles,
as environmentally friendly vehicles, has been given highpriority worldwide. Electric motors will surely require
attention as the engines for such vehicles. MHI is involvedin the development of high-efficiency EV motors, with theaim of developing inexpensive, high-performance motor
systems while improving, for instance, the regeneration rateof regenerative energy.

4.1 Reliability evaluation
To evaluate the reliability of the product, each part andcomponent of the major constituent parts (power modules) of
the inverter are tested in terms of performance, temperature,and vibration to ensure that the vehicle-mount criteria aresatisﬁed. In addition to the performance tests, the inverters
must pass various car-mount tests concerning electricalcharacteristics, thermal environment, mechanical strength(vibration and impact), electromagnetic interference, andimmunity (electromagnetic compatibility (EMC)).
MHI obtained certification for the quality management
system ISO/T S16949, for the EV motors and inverters
products in March 2008. The product design and production
quality management system (QMS) comply with accredited
ISO/TS16949 of product quality.
4.2 Improving software reliability
I nver ter sof t wa re prog ra ms for cont rol have beendesigned for each vehicle. From the viewpoint of parts
standardization and reuse, the functions of these software
programs have been reorganized.
The inverter software programs are roughly classified
i nto core (t he core sof t wa re responsible for cont rol
calculation), hardware-dependent, and vehicle-compatible( I/O compat ible) sof t wa re. From t he core sof t wa reprograms, which account for almost 40 % of all of thesoftware, representative programs are selected based on thecontents and actual implementation as common software
programs.
The programs are also reorganized in line with the seriesmodels of the inverter hardware products. Additional design

Reference
1. Takeda, Y. et al., Design and Control of Interior Permanent
Magnet Synchronous Motor, 1st Edition, Ohmsha (2001)

Koichi Okubo

Makio Masuda

Yoshiki Kato

Yutaka Nakatani

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 3 (Sep. 2008)18