IEEJ Journal of Industry Applications
Vol.8 No.6 pp.953–959
DOI: 10.1541/ieejjia.8.953

Paper

Fault-Tolerant Function of DC-Bus Power Source
in A Dual Inverter Drive System and Its Operation Characteristics
Yoshiaki Oto∗a)Student Member,

Toshihiko Noguchi∗

Member(Manuscript received March 4, 2019, revised June 13, 2019)

The dual inverter drive system feeding an open-end winding permanent magnet (PM) motor has been studied fordeveloping autopilot technologies of hybrid vehicles. Autopilot systems require fault-tolerant functions, which enable
it to continue to drive the motor even if some failure occurs in the motor drive system. The fault-tolerant function of adual inverter drive system, which assumes that the DC-bus battery power source of the dual inverter drive system has
failed, is discussed in this paper. In the dual inverter drive system that is considered, both the inverters have a capacitor
in parallel with a battery across each DC-bus. The capacitor drives the motor continuously even if the DC-bus battery
has failed. The inverter, in which the DC-bus battery has failed, is operated with the capacitor instead of the failed
battery. It is required to both control the capacitor voltage at a constant value and simultaneously generate multilevel
voltage waveforms across the motor windings with the space vector modulation (SVM). In this paper, the fault-tolerant
function of the DC-bus battery in a dual inverter drive system is proposed, and its operation characteristics are examinedthrough several experiments and compared with those of a normal system.
Keywords: fault-tolerant function, dual inverter drive system, open-end winding motor, capacitor voltage control, space vector
modulation

1.

been studied (6)–(16) . The dual inverter drive system can achieve
several fault-tolerant functions and also can output multilevel
voltage waveforms across the motor windings. The faulttolerant function means that the system can continue to drive
the motor even if some failure occurs in the system. So far,the fault-tolerant techniques of inverter switching device havebeen studied, however, the fault-tolerant technique of the DCbus battery has not been studied in dual inverter drive system (17)–(19) .
The dual inverter drive system where both the inverters
have a battery and a capacitor in parallel across each DC-bus
is studied, and its fault-tolerant function of the DC-bus battery is proposed in this paper. The proposed fault-tolerant
function can continue to drive the motor even if the DCbus battery of the dual inverter drive system has failed, because the failure-side inverter is operated with the capacitor
instead of the failed battery. In the case, it is necessary tocontrol the capacitor voltage constantly, generating the multilevel voltage waveforms across the motor windings by thephase diﬀerence of the fundamental voltage components between the two inverters (10) (11) . In the proposed method, thespace vector modulation (SVM) is employed and the redundancy of the switching states of the dual inverter drive system
is focused on. In this paper, the dual inverter drive system
which has been studied by authors is adopted for the faulttolerant function of the DC-bus battery (20) , and the operation
characteristics are examined through additional experimental
tests to compare the operation characteristics of the normalcondition and the failure-condition.

Introduction

In recent years, autopilot technologies and mileage improvement techniques of hybrid vehicles are focused on.
Many of the current hybrid vehicles drive a high-voltage
permanent magnet (PM) motor with the combination of abidirectional chopper and a two-level inverter. Therefore,
it is diﬃcult to improve the eﬃciency of the current system
due to the bidirectional chopper where large current flow for
the voltage boost. The current system also cannot continue
to drive the motor if either the chopper or the inverter has
failed. And, the line-to-line voltage of the motor is a threelevel waveform, which may cause deterioration of the total
harmonic distortion (THD), conduction noise, and radiationnoise. Therefore, the dual inverter drive system, where two
2-level inverters feed an open-end winding PM motor, hasbeen focused on (1)–(5) . The dual inverter drive system is expected to improve the eﬃciency of the system, because thesystem can generate the multilevel voltage waveforms acrossthe motor windings by adding the voltages of the two inverters, and it is not required any voltage boosting chopper. In
particular, the dual inverter drive system, where the DC-bus
battery of one of the inverters is replaced with a capacitor, hasa) Correspondence to: Yoshiaki Oto. E-mail: ohto.yoshiaki.15@
shizuoka.ac.jp∗
Department of Electrical and Electronics Engineering, Facultyof Engineering, Shizuoka University
3-5-1, Johoku, Naka-ku, Hamamatsu, Shizuoka 432-8561, Japan
This work was supported by the New Energy and Industrial
Technology Development Organization (NEDO) Grant-in-Aid
for Scientific Research from the National Government and Ministry of Economy, Trade and Industry, Japan.
c 2019 The Institute of Electrical Engineers of Japan.
2.

Configuration of Dual Inverter Drive System

Figure 1 shows a circuit diagram of the conventional motor
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Fault-Tolerant Function of A Dual Inverter Drive System（Yoshiaki Oto et al.）Fig. 1. Conventional motor drive system of hybrid vehicles with single inverter drive system

Fig. 4. Redundant switching states of normal-condition
dual inverter drive system enlarged from 0 to 60 degree
Fig. 2. Studied motor drive system of hybrid vehicles
with dual inverter drive system(a) Open-circuit failure of switching device of upper arm in W-phase of INV2.

Fig. 3. Principle of voltage vector generation of dual inverter drive system(b) Short-circuit failure of switching device of upper arm in W-phase of INV2.

drive system of hybrid vehicles which consists of a bidirectional chopper, a single two-level inverter, and a three-phase
PM motor with a neutral-point inside of the motor. Figure 2 shows the dual inverter drive system studied in thispaper, where the left-hand side and right-hand side inverters
are called INV1 and INV2, respectively. Both the inverters
have a capacitor in parallel with a battery across each DCbus. Each leg of both the inverters is complementary operated, and the combination of the switching states is expressedas (u1, v1, w1) (u2, v2, w2)’, where the switching state “1”means that the upper arm is turned on and the “0” means the
opposite. The studied dual inverter drive system has the advantages of generating the multilevel voltage waveforms andutilizing the redundancy of the switching states. And also, thestudied system does not require any voltage boost power converter such as the bidirectional chopper, however, the inverter
counts and the DC-bus power source counts are increased,compared with the conventional single inverter motor drive
system.
The dual inverter drive system can output the voltage vector by adding the voltage vectors generated with the INV1
and the INV2 as shown in Fig. 3, therefore, the system cangenerate the multilevel voltage waveforms across the motor
windings. The multilevel voltage waveform generation can
reduce the dv/dt and the harmonic component rate of the output voltage waveforms, which results in the improvement of
the THD. It is expected to reduce the copper and the ironlosses as a result of the improvement of the THD of the output multilevel voltage waveforms and is expected to reducethe conduction and the radiation noises as a result of theFig. 5. Conventional fault-tolerant functions of one ofinverter switching devices

reduction of the dv/dt. The dual inverter drive system also
has the redundancy in the switching states, which means thatparticular voltage vectors can be generated to the motor withthe several diﬀerent redundant switching states as shown in
Fig. 4. The studied dual inverter drive system can achieve theproposed fault-tolerant functions with the redundancy of theswitching states with the SVM.
The conventional fault-tolerant function of one of the inverter switching devices has been studied (17)–(19) . The failureside inverter where one of the inverter switching devices has
failed makes the neutral-point of the motor by shorting thenormal-condition arms as shown in Fig. 5, that is, the failed
inverter keeps the switching state either (000) or (111). The
fault-tolerant functions of the switching device of the upperarm in the U-phase of the INV2 is in the open-circuit failure
and the short-circuit failure are shown in Figs. 5(a) and (b),respectively. The conventional fault-tolerant function makes
it possible that the studied dual inverter drive system can continue to operate the motor as a single inverter motor drive system, even if one of the inverter switching devices has failed.
The conventional function also may achieve the fault tolerance of the DC-bus battery as the single inverter drive system
by making the short circuit with the inverter whose DC-bus
battery has failed. On the other hand, the proposed faulttolerant function is operated as the dual inverter system byutilizing the capacitor voltage control with the space vector
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IEEJ Journal IA, Vol.8, No.6, 2019

Fault-Tolerant Function of A Dual Inverter Drive System（Yoshiaki Oto et al.）Fig. 6. Proposed fault-tolerant function of DC-bus battery of INV2

Fig. 7. Simultaneous control of multilevel voltage
waveform generation and capacitor voltage across DCbus of failure-side inverter with SVM

modulation. Therefore, the proposed function is expected to
improve the THD of the output voltage waveforms, that is,
to reduce harmonic losses because the proposed function can
generate the multilevel voltage waveforms across the motor
windings.
3.

Proposed Fault-Tolerant Function of DC-Bus
Battery with SVM

The fault-tolerant function of DC-bus battery is proposedin this paper, which is achieved with the capacitor voltage
control across the DC-bus of the failed inverter. Figure 6
is assumed that the proposed fault-tolerant function of theDC-bus battery source of INV2. The fault detection of theDC-bus battery can be achieved with the current and voltage sensors in the system. The over current of the battery
can be detected with the current sensor of the DC-bus. And,the separation of the battery can be detected with the voltage
sensors which is in the battery management system of thebattery. Then, the separation of the DC-bus battery can beachieved with the electromagnetic contactor of the DC-bus.
In the case, the failure-side inverter is operated with the capacitor, which is connected in parallel to the battery acrossthe DC-bus. In the proposed fault-tolerant function, it is necessary both to generate the multilevel voltage waveforms to
the motor and simultaneously to control the capacitor voltage across the DC-bus of the failure-side inverter constantly.
As described before, the dual inverter drive system has theredundancy in the switching states, which makes it possible
to generate particular voltage vectors with the several diﬀerent redundant states. The capacitor charging or discharging
modes of the each switching state can be calculated by the
directions of the motor phase currents. Both of the capacitor voltage control of the failed inverter and the multilevel
voltage waveform generation can be achieved with the SVM
by selecting the appropriate switching state in the redundantstates, which charges or discharges the capacitor voltage appropriately as shown in Fig. 7. The SVM is employed to operate the proposed fault-tolerant function, because the switching sequence can be designed flexibly in the SVM.
The 1:0.5 voltage ratio is employed to utilize the redundancy of the switching states in the proposed fault-tolerant
function, which means that the capacitor voltage across theDC-bus of the failed inverter is regulated at a half of the battery voltage of the normal-condition inverter. The relationship between the output voltage vectors from 0 to 60 degree
and the capacitor charging/discharging modes in the steadystate is shown in Fig. 8. As shown in the figure, both thecapacitor charging and discharging modes can be used redundantly in particular redundant switching state in case of
the 1:0.5 DC-bus voltage ratio, on the other hand either the(a) Dual inverter with 1:1 DC-bus
voltage ratio.

(b) Dual inverter with 1:0.5 DCbus voltage ratio.

Fig. 8. Relationship between output voltage vectors
from 0 to 60 degree and capacitor charging/discharging
modes in steady state

capacitor charging or discharging mode can be used in particular redundant states in case of the 1:1 DC-bus voltage ratio. That is why the 1:0.5 DC-bus voltage ratio is employed,
and the capacitor voltage of the failed inverter should be controlled at a half of the DC-bus battery voltage of the normalcondition inverter in the proposed fault-tolerant function.
The modulation-index m is also indicated in Fig. 8. The
dual inverter system can generate the voltage vectors byadding the voltages of the two inverters. However, the proposed failure-condition system cannot generate the voltage
vectors in m > 0.5, because the system is required to achieveboth the capacitor voltage control and the voltage waveformgeneration to the motor at the same time. Therefore, the maximum motor rotating speed of the proposed failure-condition
system is limited to a half of the normal-condition system.
On the other hand, the proposed failure-condition system is
expected to improve the THD of the output voltage waveforms because the system can generate the multilevel voltage
waveforms across the motor windings. The proposed failurecondition system can generate the voltage vectors with a half
amplitude of the normal-condition system in m < 0.25, thatis, with the closer amplitude to the voltage reference than
the normal-condition. Therefore, the harmonic content rateof the output voltage waveforms and the harmonic losses in
the failure-condition system are reduced in m < 0.25, which
result in the improvement of the THD of the output voltage waveforms. On the other hand, the harmonic losses andthe THD of the proposed failure-condition system is also expected to be improved in 0.25 ≤ m < 0.5, compared with thenormal-condition system, because the proposed system cangenerate the 9-level voltage waveforms although the normalcondition system generates the 5-level voltage waveforms.
That is why, the eﬃciency of the INV1 and the motor in the
proposed system is expected to be improved.
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IEEJ Journal IA, Vol.8, No.6, 2019

Fault-Tolerant Function of A Dual Inverter Drive System（Yoshiaki Oto et al.）Fig. 11. Control block diagram
Table 1. Experimental test conditions

Fig. 9. Experimental setup of dual inverter drive system

Fig. 10. Block diagram of controller

4.

Experimental Setup and Test Results

4.1 Experimental Setup
As shown in Fig. 9, the experimental setup of the dual inverter drive system has been
conducted to examine the steady operation characteristics ofthe studied dual inverter drive system in the normal-condition
and the failure-condition of the DC-bus battery. The SVM is
implemented in the digital signal processor (DSP) and thefield programmable gate array (FPGA) of the controller asshown in Fig. 10, where the DSP calculates the time durationsof the output voltage vectors and communicates the results
to the FPGA, and the FPGA outputs the switching signals
to the gate drivers of the INV1 and the INV2 with the highresolution of 100 MHz. The open-end winding PM motor iscontrolled by a field-oriented control (vector control) algorithm with the maximum torque per ampere (MTPA) control
method as shown in Fig. 11, and its speed is regulated constantly by a load servo motor directly connected to the test
motor. The rated output power of the test motor is 1 kW
which is fed by the two 5-kVA inverters, and other experimental test conditions are shown in Table 1.
As described before, the proposed fault-tolerant function
achieves both to control the capacitor voltage across the DCbus of failed-side inverter and to generate the multilevel voltage waveforms to the motor at the same time with the SVM.
In this case, it is required to select the appropriate switching state among the redundant states considering the instantaneous motor power factor, because the capacitor charging/discharging modes of each switching state can be determined by the directions of the motor phase currents (20) . That
is why, the phases of the commanded voltage vector and the
motor current vector are calculated with the feedback valueof the capacitor voltage, and are utilized to control the capacitor voltage constantly in the SVM as shown in Fig. 11.

4.2 Test Results of Dual Inverter Drive System inNormal-Condition
The test results of the studied dual
inverter drive system in the normal-condition are shown in
Fig. 12. The figures show the three-phase motor currents andthe voltages across the U-phase. On the condition of m < 0.5,
the 5-level voltage waveforms are generated across the motor
windings. In the case, the INV2 outputs the switching state(000) or (111) throughout the SVM, in other words, the INV2
makes the neutral-point of the motor. Therefore, it is diﬃcult
to measure the eﬃciency of the INV2 in m < 0.5. And also,
the output voltage waveforms in Figs. 12(a) and (b) are similar to the voltage waveforms which is measured between the
U-phase terminal and the neutral-point of the motor in a single inverter system. On the other hand, the 9-level voltage
waveform is generated by adding the voltages of the INV1
and the INV2 in m > 0.5.
4.3 Test Results of Proposed Fault-Tolerant Functionof DC-Bus Battery
The experimental test results of the
proposed fault-tolerant function are shown in Fig. 13 and Table 2, where the steady operation characteristics of the dual
inverter drive system in the failure-condition of the DC-bus
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Fault-Tolerant Function of A Dual Inverter Drive System（Yoshiaki Oto et al.）(a) m = 0.2 (400 r/min).

(a) m = 0.2 (400 r/min).

(b) m = 0.4 (800 r/min).

(b) m = 0.4 (800 r/min).

Fig. 13. Test results of proposed fault-tolerant functionof DC-bus battery of INV2(c) m = 0.8 (1600 r/min).

Fig. 12. Test results of dual inverter drive system innormal-condition

motor on the condition of each modulation-index. The efficiency measured results are calculated with the following
equation. In the equation, ηmotor is the motor eﬃciency, PINV1and PINV2 are the output of the INV1 and INV2, ωm is the
motor rotating speed, and T is the motor torque.

Table 2. Measured eﬃciency resultsηmotor =ωm T
PINV1 + PINV2

However, PINV2 is only a conduction loss in the normalcondition in m < 0.5, which is a negative value, because theINV2 makes the neutral-point of the motor and there are noswitching transitions. On the other hand, PINV2 is the sum of
a conduction loss and a switching loss in the failure-conditionin m < 0.5, which is also a negative value, because the INV2
is operated with the capacitor and the capacitor utilizes onlya reactive power. That is why, the eﬃciency of the INV2
cannot be obtained at m = 0.2 and m = 0.4 in the normalcondition and in the failure-condition. In the experimental

battery of INV2 are examined. In the figures, the motor line
current, the capacitor voltage, and the U-phase voltage areshown, which indicate that the SVM achieves both to control the capacitor voltage of the INV2 at a half of the battery
voltage of INV1 and simultaneously to generate the multilevel voltage waveforms to the motor. The table shows themeasured eﬃciency results of the INV1, the INV2, and the957

IEEJ Journal IA, Vol.8, No.6, 2019

Fault-Tolerant Function of A Dual Inverter Drive System（Yoshiaki Oto et al.）tests, however, it is not achieved to switch over the dual inverter drive system from the normal-condition to the failurecondition transiently.
The proposed fault-tolerant function can generate the voltage vectors with a half amplitude of the normal-condition
system in m < 0.25, that is, with the closer amplitude to thevoltage reference than the normal-condition system as shownin Fig. 8, which result in the reduction of the harmonic content rate of the output voltage waveforms. That is why, themeasured eﬃciency results of the INV1 and the motor in the
failure-condition are slightly improved, and the THD measured results of the output voltage waveforms in the failurecondition of the DC-bus battery are reduced by 67.8% at m =0.2, compared with the normal-condition system.
On the other hand, the proposed fault-tolerant function
can generate the 9-level voltage waveforms across the motor windings in 0.25 ≤ m < 0.5 as shown in Fig. 8. Becauseof the reduction of the harmonic component rate of the output
voltage waveforms ,therefore, the measured eﬃciency resultsof the INV1 and the motor are also slightly improved, and the
THD measured result of the output voltage waveforms in the
proposed system is reduced by 52.4% at m = 0.4, comparedwith the normal-condition system.
4.4 SVM Technique to Reduce Error Voltages in Output Voltage Waveforms of Proposed Fault-Tolerant Function
The proposed fault-tolerant function, where one of
the inverter is operated with the capacitor, has a problem thatan unexpected error voltages are generated during a deadtime (16) (20) . The error voltages are generated due to particular transients of the switching states, where the switching
devices in the same phase of both the inverters are synchronously turned on/oﬀ, or the several switching devices in
one inverter are synchronously turned on/oﬀ. Therefore, thesynchronous switching technique to reduce the error voltages
generated due to the synchronous switching transient is introduced (16) (20) . The introduced synchronous switching technique is performed according to the directions of the motor
line currents. Therefore, the introduced technique may be incomplete in particular when the line currents are near by thezero-cross points, because it is diﬃcult to detect the directions of the motor line currents. The proposed fault-tolerant
function employs the introduced technique.
The conventional switching sequence, for example a triangle wave comparison PWM, has the seven switching statesin one PWM sequence. However, the conventional PWM
sequence has a problem that the unexpected error voltages
are generated during the dead time in the dual inverter drive
system, even if the synchronous switching technique is employed. Therefore, the switching sequence to reduce the error voltages is introduced (20) . The introduced sequence has
five switching states in one PWM period. In the introduced
switching sequence, therefore, the number of switching transitions are less than the conventional sequence, which meansthat the switching loss is reduced in the introduced sequence.
In this paper, the proposed fault-tolerant system is operated
with the SVM which employs the introduced sequence. That
is why, the introduced sequence also contributes to improvethe measured eﬃciency results of the INV1 and the INV2
in the failure-condition. It is achieved that the measured efficiency results of the motor in the fault-tolerant function isequivalent to the normal-condition. The reason why is thatthe dv/dt of the output voltage waveforms in the fault-tolerant
function is a half of the normal-condition, although the error
voltage pulses are generated in the output voltage waveformsof the fault-tolerant function even if the introduced SVM
techniques to reduce the error voltages are employed.
5.

Conclusion

This paper proposed the fault-tolerant function of the DCbus battery in the dual inverter drive system. The studied dual
inverter drive system has a capacitor in parallel to a battery
across the DC-bus of each inverter. In the proposed faulttolerant function, the failure-side inverter can be achieved
the continuous operation with the capacitor instead of theDC-bus battery, even if the DC-bus battery has failed. The
proposed fault-tolerant function requires that the capacitor
voltage of the failure-side inverter is regulated at a half ofthe battery voltage of the other inverter and simultaneously
the multilevel voltage waveforms are generated to the motorwith the SVM. In this paper, the operation characteristics ofthe proposed fault-tolerant function are examined through theseveral experimental tests, and the experimental test results
are evaluated from the view point of the multilevel voltage
waveform generation, the THD measured results of the output voltages to the motor, and the eﬃciencies of the inverters
and the motor, compared with the normal-condition system.
Although the transient switching over techniques from thenormal-condition to the fault-tolerant functions in the studied
dual inverter drive system have not been considered, which
will be investigated in detail in future works.

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Yoshiaki Oto (Student Member) received the B.Eng. and M.Eng. degrees in Electrical and Electronic Engineering from
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Shizuoka University since 2017. His research interest
is a motor drive technology with a dual inverter system feeding an open-end-winding motor for hybrid
vehicles. Mr. Oto is a student member of the Instituteof Electrical Engineers of Japan.

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Toshihiko Noguchi (Member) received the B.Eng. degree in electrical engineering from Nagoya Institute of Technology, Nagoya, Japan in 1982, and the M.Eng. and
D.Eng. degrees in electrical and electronics systems
engineering from Nagaoka University of Technology,
Nagaoka, Japan, in 1986 and 1996, respectively. In
1982, he joined Toshiba Corporation, Tokyo, Japan.
He was a Lecturer at Gifu National College of Technology, Gifu, Japan, from 1991 to 1993. He was an
Assistant Professor and an Associate Professor in Department of Electrical, Electronics and Information Engineering, Nagaoka
University of Technology from 1994 to 1995 and from 1995 to 2009, respectively. Since 2009, he has been a Professor in Department of Electricaland Electronics Engineering, Faculty of Engineering and Graduate Schoolof Engineering, Shizuoka University. His research interests include new circuit topologies of static power converters and motor drives including electricmachine hardware. Dr. Noguchi is a member of the IEE-Japan and a senior
member of the IEEE.

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