Switching over technique to fault tolerant functionof DC-bus battery in dual inverter motor drive
Yoshiaki Oto, Melinda Badriatul Fauziah, and Toshihiko Noguchi
Department of Electrical and Electronics Engineering, Faculty of Engineering
Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu, Shizuoka 432-8561, Japanohto.yoshiaki.15@shizuoka.ac.jp
Abstract. The dual inverter motor drive feeding an open-end winding PM motor is focused onfor mileage improvement and autopilot technologies of hybrid and electric vehicles. Autopilot
operation requires the fault tolerant functions which achieve to keep controlling the motor, even
if some failure occurs in the system. In this paper, the fault tolerant function of the DC-bus
battery in the dual inverter motor drive system is discussed. The system has a battery and acapacitor across the DC-bus of each inverter. The capacitor is utilized to keep operating themotor, instead of the failed battery in the fault tolerant function. In this time, however, it isnecessarily to control the capacitor voltage of the failed side inverter at constantly with the spacevector modulation (SVM). In particular, it is required to regulate the capacitor voltage of thefailed inverter at a half of the battery voltage of the normal condition inverter. Therefore, thecapacitor voltage must be discharged immediately after some failure is occurred in the batteryof one of the inverters. In this paper, the switching over techniques from the normal condition to
the fault tolerant function is proposed, and the proposed technique is examined through several
computer simulations.

1. Introduction
Recently, the mileage improvement and autopilot technologies of hybrid and electric vehicles arefocused on. However, the current system operates the motor by boosting the battery voltage andoperating the 2-level inverter as shown in Fig. 1. It is difficult to improve the efficiency of the systemand to achieve a fault tolerant function because the bidirectional chopper and the 2-level inverter areconnected in series, and the system cannot drive the motor even if either the copper or the inverter isfailed. Therefore, the dual inverter motor drive feeding an open-end winding PM motor with two
inverters has been studied as shown in Fig. 2. The dual inverter system can generate the voltage acrossthe motor windings by adding the output voltages of two inverters. Therefore, it does not require any
voltage-boosting chopper, which is expected to improve the efficiency of the system. The dual inverter
Capacitor

Three-Phase
Three-Wire Motor

Chopper

Open-EndWinding Motor

Capacitor

Battery

Battery

300 V

300 V

InverterFigure 1. Conventional single inverter motor drive.

INV1 (u1, v1, w1)(u2, v2, w2)’INV2

Figure 2. Studied dual inverter motor drive.

system also can generate the multilevel voltage waveforms and also can achieve some fault tolerantfunctions, e.g., the DC-bus battery and the switching device. However, the fault tolerant function of theDC-bus battery in the dual inverter system has not been studied, although the fault tolerant functions of
the switching devices have been studied[1]-[3]. Therefore, in this paper, the fault tolerant function of theDC-bus battery in the dual inverter motor drive is discussed. In the fault tolerant function, the inverter
whose DC-bus battery is failed is operated with the capacitor, instead of the failed battery. The dual
inverter motor drive with the capacitor has been already introduced[4]-[5]. On the other hand, it is noveltyof this paper that the space vector modulation (SVM) is employed to control the capacitor voltage in thedual inverter motor drive. In particular, it is required to regulate the capacitor voltage of the failed
inverter at a half of the battery voltage of the normal condition inverter. Therefore, the capacitor voltage
must be discharged immediately after some failure is occurred in the battery of one of the inverters. In
this paper, the switching over techniques from the normal condition to the fault tolerant function isproposed, and the proposed technique is examined through several computer simulations.
2. Dual inverter motor drive
In the dual inverter motor drive system studied in this paper, the left-hand side inverter is called theINV1 and the right-hand side is called the INV2, and both inverters have a battery and a capacitor acrosseach DC-bus. The combination of the switching states of the INV1 and the INV2 is described as(u1,v1,w1) (u2,v2,w2)’, and the switching devices in each leg are complementarily operated. In thispaper, it is assumed that the battery of the INV2 is failed, and the failed battery is separated as shown inFig. 3. In this case, however, it is required that the capacitor voltage is regulated at constant value tooperate the INV2 with the capacitor in stead of the failed battery, which must be achieved at the sametime of generating the multilevel voltage waveforms to the motor.
The dual inverter motor drive can generate the voltage vector by adding the output voltages of thetwo inverters. The output voltage vectors with the studied dual inverter system which are enlarged from0 to 60 degree are shown in Fig. 4. The dual inverter system has a redundancy in the switching states,
which achieves to generate particular voltage vectors with the several redundant states. The redundancy
also achieves to control the capacitor voltage across the DC-bus of the failure side inverter, because thedirections of the current flowing into the capacitor are different from each switching state. However,
either capacitor charging or discharging mode can be used in particular redundant states in case of theDC-bus voltage ratio 1:1 as shown in Fig. 4 (a). On the other hand, both capacitor charging andCapacitor

Open-EndWinding MotorCapacitor
150±5 V

BatteryBattery
300 V300 V

INV1 (u1, v1, w1)(u2, v2, w2)’INV2

Figure 3. Fault tolerant function of DC-bus battery in dual inverter motor drive.
(000)(001)’(-) (110)(001)’(-)(111)(001)’(-)(010)(011)’(-)(100)(101)’(-)(110)(000)’(0)(110)(111)’(0)(000)(000)’(0)(000)(111)’(0)(111)(000)’(0)(111)(111)’(0)(001)(001)’(0)(010)(010)’(0)(100)(100)’(0)(011)(011)’(0)(101)(101)’(0)(110)(110)’(0)m = 0.5(000)(011)’(-)(111)(011)’(-)(101)(001)’(-)(110)(010)’(-)(100)(000)’(0)(100)(111)’(0)(100)(001)’(-)(110)(011)’(-)

m = 1.0(100)(011)’(-)(110)(000)’(0) (110)(001)’(-)(110)(111)’(0)(110)(011)’(-)(000)(001)’(-)(100)(101)’(0)(111)(001)’(-)(110)(010)’(0)(110)(110)’(+)(000)(000)’(0)(100)(001)’(-)(000)(111)’(0)(111)(000)’(0) m = 0.25 m = 0.5 m = 0.75(111)(111)’(0)(100)(011)’(-)(000)(011)’(-) (100)(000)’(0)(111)(011)’(-) (100)(111)’(0)(100)(100)’(+)(+): Charging Mode(‐): Discharging Mode(0): Holding ModeFigure 4. Redundancy of switching states and capacitor charging/discharging modes.

discharging modes can be used redundantly in case of the DC-bus voltage ratio 1:0.5 as shown in Fig.
4 (b). Therefore, the DC-bus voltage ratio 1:0.5 is employed in this paper, that is, the capacitor voltageof the failure side inverter is regulated at a half of the battery voltage of the other inverter. Fig. 4 also
shows the definition of a modulation index m, where the maximum amplitude of the voltage vector
generated with the dual inverter motor drive in the normal condition is defined as m = 1.0.
3. Capacitor voltage control technique with SVM
The fault tolerant function of the DC-bus battery in the studied dual inverter motor drive requires tocontrol the capacitor voltage at constantly across the DC-bus of the failure side inverter, generating themultilevel voltage waveforms across the motor windings at the same time. The simultaneous control of
the capacitor voltage and the multilevel voltage waveform generation is achieved with the redundancyof the switching state in the dual inverter system. As shown in Fig. 5, the simultaneous control can beachieved by selecting the switching state which charges/discharges the capacitor of the failure side
inverter appropriately among several redundant states when particular voltages are generated. That iswhy, the SVM which can configure the switching sequence flexibly is employed in this paper.
However, the relationship between the capacitor charging/discharging modes and switching states isaffected by the instantaneous motor power factor. The relationship between the phase of the motor line
current vector and the line current polarities enlarged from 300 to 60 degree is indicated as Fig. 6 (a).
Although the voltage vector V is generated as shown in Fig. 6 (a), the current polarity may be A or F
because the motor has a inductive road, where the motor line current is delayed behind the output voltage
vector, and the phase difference between the current vector and the voltage vector is up to 90 degree.
And also, the current polarity may be B in case that the maximum torque-per-ampere (MTPA) control
SVM 1 Period
V0V0in(100)(000)’(0)(100)(111)’(0)Hold(000)(011)’(-)(111)(011)’(-)

V30

V0in(000)(011)’(-)(110)(010)’(-)(100)(100)’(+)(100)(100)’(+)

Discharge

ChargeCapacitor Voltage Command・・・(111)(011)’(-)(100)(101)’(+)

Discharge(+): Charging Mode(-): Discharging Mode(0): Holding Mode

Capacitor Voltage

Figure 5. Capacitor voltage control technique with SVM.

V60
V60in

V0in

V300inV300IfV

Redundant switching states generating V30（1，1，0） （0，1，0）’

V＋

B

V30Ib
IaVz

Current
Polarity

Current
Polarity B
U＋V＋W−
Current
Polarity A
U＋
V0 V −W−

A

V330

Charging

V－U＋
W － Discharging
V－Current
Polarity F
U＋
V−
W＋

F

＋: Current direction from INV1 to INV2－: Current direction from INV2 to INV1(a). Relationship between phase ofcurrent vector and current polarities.

U＋

Discharging

W－U＋W－（1，0，0） （1，0，1）’

U＋

U＋

V＋Charging

W－U＋

V－Charging

W＋W ＋ Discharging

V－(b). Relationship between current polarity current
polarity and capacitor charging/discharging modes.

Figure 6. Impact of instantaneous motor power factor.

is adopted and the d-axis current flows, because the phase of the current vector may be advanced rather
than the output voltage vector. On the other hand, the relationship between the capacitor
charging/discharging modes and the current polarity of the redundant switching states which generate
the voltage vector V30 is indicated as Fig. 6 (b). The switching state (1,1,0) (0,1,0)’ is the capacitor
charging mode in case of the current polarity B, however, it is the capacitor discharging mode in caseof the current polarity A or F. Therefore, it is required to select the appropriate switching state among
the redundant states, considering the instantaneous motor power factor to control the capacitor voltageof the fault tolerant function in the studied dual inverter motor drive.
4. Switching over technique to fault tolerant function of DC-bus battery
The studied dual inverter motor drive in the normal condition is operated with the DC-bus voltage ratio
1:1 to enhance the operating area by adding the voltages of the two inverters. On the other hand, thestudied system in the failure condition of the DC-bus battery is operated with the DC-bus voltage ratio
1:0.5 to utilize the redundancy of the switching state for the capacitor voltage control. In case that thefailure of the DC-bus battery is occurred, therefore, it is required to smoothly discharge the capacitor
voltage of the failure side inverter up to a half of the battery voltage, generating the multilevel voltage
waveforms across the motor windings with the SVM.
In this paper, two switching over techniques to the fault tolerant function of the DC-bus battery areproposed as shown in Fig. 7. In case of 0 < m ≦ 0.25, the zero voltage vector Vz whose amplitude isequal to 0, and the voltage vectors Va and Vb whose amplitude depend on the capacitor voltage are used
for the SVM. In case of 0.25 < m ≦ 0.5, on the other hand, the voltage vectors Va, Vb, Va’, and Vb’
whose amplitude depend on the capacitor voltage, and the voltage vectors Vc whose amplitude andphase depend on the capacitor voltage are used for the SVM. In this way, the capacitor voltage of failed
inverter is discharged up to a half of the battery voltage of the other inverter, generating the multilevel
voltage waveforms to the motor at the same time with the SVM, and it is achieved to switch over to thefault tolerant function smoothly. However, it is required to select the switching state which discharges
the capacitor voltage among the redundant states, considering the instantaneous motor power factor.
5. Computer simulation
Several computer simulations were conducted to examine the proposed switching over techniques to thefault tolerant function. The control block diagram and simulation conditions are shown in Fig. 8 andTable 1, respectively. In the simulation, the open-end winding PM motor is controlled with the speed

Vb

Vb

|v*|

Vb

Vz

Vz

Vb

Vz

Va

Vz
Va

Va

Va

Vdc2 = Vdc1

Vdc2 = Vdc1/2(a). 0 < m ≦ 0.25.
Vb
Vb’

Vb

Vb

Vb

Vb’

|v*|

Vb’

Vc

Vz
Va
Vdc2 = Vdc1

Va’ Va

Vc
Va’

Va

Vc
Va’

VaVdc2 = Vdc1/2(b). 0.25 < m ≦ 0.5.
Figure 7. Proposed switching over technique to fault tolerant function with SVM.

control and the vector control with the studied dual inverter system. The instantaneous motor power
factor is calculated from the difference between the phases of the output voltage vector and the motor
current vector on the dq reference frame. The SVM is performed with the output voltage command, themotor power factor, the capacitor voltage of the INV2, and the fault signal of the DC-bus battery of theINV2. At first, the two inverters are operated with the battery across each DC-bus. After the fault signalof the battery of the INV2 is detected, the battery of the INV2 is separated with the relay and the INV2is operated with the capacitor. The capacitor voltage is discharged with the proposed switching overtechniques up to a half of the battery voltage of the INV1, and the dual inverter system is switched overto the fault tolerant function smoothly with the SVM. Then, the capacitor voltage of the INV2 isregulated at a half of the battery voltage of the INV1, generating the multilevel voltage waveforms acrossthe motor windings with the SVM.
The computer simulation results on the condition of m = 0.2 and m = 0.4 are shown in Fig. 9. The
motor rotation speed, the fault signal of the battery of the INV2, the capacitor voltage, and the U-phasevoltage waveforms are indicated in the figure. In the normal condition, the dual inverter motor drive cangenerate the 5-level voltage waveforms across the motor windings. In the fault tolerant function at m=0.2, the 5-level voltage waveform is generated with a half amplitude because the DC-bus voltage ofthe INV2 is regulated at a half of the battery voltage of the INV1. In the fault tolerant function at m =0.4, on the other hand, the 9-level voltage waveform is generated. Furthermore, the dual inverter systemis switched over to the fault tolerant function smoothly with the proposed techniques, generating theid*ωm*＋－PIT*

1/Ψ

iq* ＋ －＋PIPI

vd*

dq
↓
αβvq*

－tan−1
tan−1

sw u1vα*

swv1vβ*sww 1θ v*
θi*SVM

Vca pFaultSignal

ωmidiqαβ↓
dqiαiβiu

INV1iviw

PM

Motorsw u2
sw v2
INV2sww 21suvw↓αβFigure 8. Control block diagram.
Table 1. Simulation conditions.

Motorparameters

Switching frequency

10 kHzVoltage of battery

300 V

Voltage command of capacitor (INV2)

150 ± 5 V

Capacitance of capacitor (INV2)

330 µF

Motor speed command

1150, 2300 r/min

Dead time

0 µs

Number of poles

8

Number of flux linkage

0.174 Wb

Moment of inertia

0.00173 kgm2

Damping coefficient

0.005 N/rad/s

Phase resistance

1.1 ΩPhase inductance

5 mH

Motor Rotation Speed(r/min)Fault Signal
U-Phase Voltage Capacitor Voltage
Vu (V)
Vcap (V)
Motor Rotation Speed(r/min)Fault Signal
U-Phase Voltage Capacitor Voltage
Vu (V)
Vcap (V)

2500
2000
1500
1000

wm_rpmwm_rpm_ref

500
0

10
320
300
280
260
240
220
200
180160
140
300
200
100
0-100-200-300

050

100

150

Time (ms)

200

250

300

(a). m = 0.2 (1150 r/min).

2500
2000
1500
1000

wm_rpmwm_rpm_ref

500
010320
300
280
260
240
220
200
180160140300200100
0-100-200-300

0255075Time (ms)100125150(b). m = 0.4 (2300 r/min).
Figure 9. Computer simulation results.
multilevel voltage waveforms to the motor, that is, the fault tolerant function of the DC-bus battery isachieved with the proposed techniques.
6. Conclusion
The fault tolerant function of the DC-bus battery in the dual inverter motor drive, which has a battery
and a capacitor across the DC-bus of each inverter, is focused on in this paper. And the smooth switchingover techniques to the fault tolerant function of the DC-bus battery are proposed. The proposedtechniques were examined m < 0.5 through several computer simulations and the validity is verified.
The consideration of the switching over technique to the fault tolerant function on the condition of m >0.5 is our future work.
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