Suppression Method of Rising DC Voltage for the Halt
Sequence of an Inverter in the Motor Regeneration
Jun-ichi Itoh

Wataru Aoki

Nagaoka University of Technology
Nagaoka Niigata, 940-2188 Japan
itoh@vos.nagaokaut.ac.jp w_aoki@stn.nagaokaut.ac.jp

Abstract— In the power conversion system of electricvehicles, the inverter is shut down when the system fails in
the cause of a drastic load change or other protection
reasons. However, in the case if the inverter is shut down inregeneration mode, the DC link capacitor voltage isincreased dramatically, which will potentially break theswitching devices. In this paper, the authors propose a halt
method to overcome the over voltage and over current
problems in the case of system failure during theregeneration. The proposed method consists of two phases,in the phase I, the DC link capacitor voltage is controlled bycharge and discharge switching patterns based on space
vector of the inverter. Then, the phase II performs the shortcircuit operation in order to avoid the regenerating current
from the motor flows into the DC link capacitor. The
experimental results demonstrate that the DC link capacitor
voltage is 80% lesser comparing to the conventional method.
Furthermore, the circulating current can be suppressed by
46% from the proposed method.

I.

INTRODUCTION

Electric vehicles (EVs) have been remarkably researchand studied, as the vehicle offers zero CO2 commissions
and high driving performance.
The power conversion system of the EVs has adifferent structure from a standard industrial motor drive
system. In particular, the power conversion system
connects a relay between the inverter and batteries for the
purpose of system protection when the over voltage orover current occurs in the inverter due to system failure. In
this case, the power between the inverter and the batteries
is cutoff by the relay after cessation of normal operation.
Then, the regeneration current is forced to flow into theDC link capacitor if the inverter is shut down in a way that
all the switching devices are turned off at the same time.
Consequently, the DC link capacitor voltage rises sharply
in a short period because EV uses a small capacity such asfilm capacitor as the DC link capacitor. Electrolytic

Goh Teck Chiang
tcgoh@vos.nagaokaut.ac.jp

Akio Toba
Fuji Electric Co. Ltd
Tokyo, Japan
toba-akio@fujielectric.co.jp

capacitor is often not applied due to the size minimizationand high temperature operation condition. As a result, theswitching device in the inverter will be broken when theDC link capacitor voltage becomes higher than the voltage
rating of the switching devices.
One of conventional methods in order to solve thisproblem is connecting a brake chopper circuit in parallel to
the DC link capacitor [1]. This method is known as the
dynamic brake system which is typically applying in
general purpose motor drive systems. When the DC link
capacitor voltage exceeds the threshold voltage of thebrake chopper, the regeneration power is consumed by theresistance in the chopper circuit. However, the dynamic
brake system requires a switching device, a large
electrolytic capacitor and a large-capacity resistance even
it is only for the halt sequence during the system failures.
The cost and volume of the dynamic brake system is notpreferable in the power conversion system of EVs.
Several methods to reduce the volume of the dynamic
brake system have been reported [2-3]. One of themethods is to use the dynamic brake system with thedamping resistance. The proposed method utilizes theresistor in the dynamic brake system as the damping
resistance element in the EV system. As a result,additional damping resistors are not required. Therefore, it
can reduce the number of components. However, thismethod requires a large power capacity resistance.
A study also proposed that numbers of dynamic brakesystems can be combined with just a switching device on
the DC power supply wiring [3]. However, this method
cannot be applied to the power conversion system of theEVs since only one inverter is used in a typical system.
In this paper, the halt sequence without the dynamic
brake system is studied for EV system. Reference [4]shows a method to prevent the over voltage at the DC
capacitor. This method proposes to short the terminal ofmotor when the relay is cutoff during the regeneration
mode. However, the inverter current increases
dramatically during the operation results the over current
occurs. Here, the authors proposed a method to utilize therelationship between the output voltage vector of theinverter and the electromotive force voltage of the motor
during the regeneration in order to avoid the over voltage
at the DC link capacitor, and also prevent the over current
happening in the system. In addition, downsizing ispossible because dynamic brake system is not required in
the proposed method.
Firstly, the principle of the halt sequence method is
discussed, and the problem of the conventional halt
sequence that utilizes the vector control is described in thispaper. Next, the proposed method using the space vector
modulation is introduced. Finally, the effectiveness of the
proposed method is confirmed in simulations andexperimental results.
II.

PRINCIPLES OF THE CONTROL METHOD

A. Over voltage problems in the halt sequenceFig. 1 shows the system configuration of the power
conversion system in EVs [6-5]. This system uses a twolevel inverter in order to control an Interior Permanent
Magnet Synchronous Machine (IPMSM). A small
capacitance DC link capacitor Cdc is used to absorb theswitching ripples. In addition, a relay is connected
between the batteries and the inverter for the protection of
the system.
Reference [4] shows the basic method to prevent theover voltage at the DC link capacitor, which intends toshort the terminal of motor when the relay is cutoff. Boththe upper and lower arms of the switches are turned on at
the same time which can be known as the short-circuit
mode. Then, the shorted arm is changing sequentiallyaccording to the phase voltage when only the zerocrossing currents are detected. From the operation, thecurrent can be reduced to zero and the regenerative energy
can be suppressed in a short period. However, this method
has a problem that the large current (circulating current)
flows inside the motor within the period of short circuit.
Here, the phenomenon of the current is consideredduring the short-circuit mode. Fig. 2 shows d-axis and qaxis equivalent circuits of the IPMSM without equivalentcore-loss resistance [7]. Voltage equations are given by (1)and (2),

vd = Ra id − ωLq iq +

did
Ld
dt

v q = R a i q + ωLd i d +

di q
dt

Lq + 3Ψ a ω

(1)
(2)

where Ld is the d-axis inductance, Lq is the q-axis
inductance, ω is the angular velocity, Ψe is the linkage
magnetic flux of armature by permanent magnet, id is thed-axis current and iq is the q-axis current.

Fig.1. System configuration of adjustable speed drives withsmall capacitor in DC Link for EVs.

id

Ra

iq

Lqiq

vd

Ra

vq
Ld

Ldid
Lq

a

(a) d-axis equivalent circuit. (b) q-axis equivalent circuit.
Fig.2. d-axis and q-axis equivalent circuits of IPMSM.
Table1. Motor parameters of IPMSM

In addition, vd and vq are zero when the motor isshorted. Moreover, provided that the transient period is notconsidered then did/dt and diq/dt are equal to zero. Under
these conditions, voltage equation can be expressed by (3)⎧ − ωLq iq + Ra id = 0⎨⎩Ra iq + ωLd id = 3Ψ a ω(3)d-axis and q-axis current are given by (4) and (5) fromsolving (3).

id = −

iq = −

3Ψ a ω
2

Ra
+ ωLd
ωLq
3Ψ a ωRa2

R a + ω 2 Ld Lq

(4)

(5)

Finally, maximum oscillation current iamax can beillustrated as (6).
2

ia max = id + iq

2(6)

From the above equation, it is possible that the maximum
oscillation current during the short-circuit mode can becalculated from the motor parameters.
Table 1 shows the motor parameters of a IPMSM. The
maximum oscillation current iamax is 4.65 p.u. during theshort-circuit mode. In order to confirm the validity of theexpression, simulation is run under the same parameters.
Fig. 3 shows output current waveform when the inverter
turned into short-circuit mode during the halt operation.
From the result, it is confirmed that the maximum
oscillation current iamax of steady state is 4.65 p.u., which isalmost consistent to the calculated value.
EV Motor is typically designed in a matter that the
maximum current is allowed up to 2.5 to 3.7 p.u. of the
rated current [8-11]. Therefore, the motor can potentiallysuffer from damages during the short-circuit mode,considered that the circulating current is 4.65 p.u. of the
rated current. Therefore, a method that can overcome theover voltage at DC link capacitor and also possible to keep
low circulating currents are required.
B. Explanation of proposed method
The regeneration energy from the motor depends on
the rotating speed and braking torque of the motor as (7).

Δ Wθ = − ∫ T

dθ
dt
dt

(

Fig. 4. Relationships between voltage command vector andvector of the motor current in phase I.

(7)

where θ is the rotation angle of the motor and T is
torque of the IPMSM. In addition, the output torque of the
IPMSM is given by (8).

{Fig. 3. Output current waveform with the short circuit control
method.

)}T = Pniq 3Ψe + Ld − Lq id(8)where Pn is the number of the pairs of poles.
From (8), the negative torque will cause the increase of
the DC voltage if the q-axis current is not controlled tozero. Thus, in order to prevent the over voltage at the DC
link voltage, the q-axis current should be zero in the halt
sequence immediately.
One of the strategies for the halt sequence, the current
commands in the vector control should be zero before theinverter shuts down. However, this operation is dependingon the current response of the regeneration current that is
flowing into the DC link capacitor voltage.
Here, the proposed control method consists of twophases; in the phase I, the control ensures that the q-axis
current equals to zero without a current regulator. The DC
link voltage is controlled by selecting the switchingpatterns between the charge mode and discharge mode in
the DC link capacitor (Switching patterns are illustrated inTable 2.). After the q-axis current becomes zero, the phase
II is implemented in order to conduct the short-circuitmode until the d-axis current becomes zero.
In a matter of fact, the phase I alone can achieve thehalt sequence and preventing the DC link capacitor fromover voltage, since the motor currents are circulating in the
inverter during the halt sequence. However, the motor
current will be increased drastically in high speed region
because of the electromotive force in the motor. As a resultof the short circuit condition, the large motor current
causes the irreversible flux loss in the magnet of the
IPMSM. In addition, the inverter is required to implementwith high current rating switching devices. Therefore, thephase I is introduced to prevent the occurrence of large
motor current. By implementing the phase I, the maximum
current in the halt sequence is suppressed to less than threetimes of the rating current of the motor. Note that theirreversible flux loss of the magnet does not occur in the
IPMSM when the motor current is less than three times ofthe motor rating current generally.
Fig. 4 shows the relationship between the voltage
command vector and the motor current vector in the phase
I. The voltage command is 90 degrees ahead of the motor
current vector. Therefore, the phase of the voltage
command vector is controlled to a lead of 60 to 120
degrees with respects to the current vector of the motor. As
a result, the q-axis current achieves zero at the very shorttime because the active current is changed into a reactive
current.
However, the remaining regeneration energy ischarged into the DC link capacitor because all of the active
current cannot change quickly into the reactive current.
Therefore, the proposed method includes a switching table
to control the DC link capacitor voltage by charging anddischarging the DC link capacitor during the phase I.

Table 2. Switching table at phase I(a) Discharge mode(b) Charge mode
Fig.5. Operational modes in phase I

charge mode, respectively. These two operations are thefundamental switching patterns which are used to suppress
the fluctuation of the DC link capacitor voltage. (Thecorresponding switching devices are changed according to
the direction of motor current.)
Fig. 6 shows the operation flow chart in phase I. Oncethe relay is opened, the direction of the current is recorded
then the q-axis current signal is comparing in the system todecide the operation between the phase I and phase . Then,the q-axis current is changed into reactive current bychanging the voltage vectors in the inverter.
Table 2 illustrated the switching table that is
implemented in the inverter that is depending on thecapacitor voltage in subjects to the charge and discharge
modes. The voltage command vector becomes a lead of
30-90 degrees with respects to the motor current vector
during the discharge mode. Similarly, the voltage
command vector becomes the lead of the phase of 90-150
degrees with respects to the motor current vector during
the charge mode. As a result, the voltage command vector
becomes the leading phase of 60-120 degree with respects
to the motor current vector. The phase I operation ends
when the q-axis current becomes zero.

Fig. 6. Operation flow chart in phase I

Fig. 5 shows the equivalent circuit of the inverter in thephase I condition with the proposed method. Fig. 5(a)illustrates the discharge mode and Fig. 5(b) illustrates thepattern that against the current vector as set forth in Fig 5

Fig. 7 shows the short circuit operation mode that
intends to prevent the DC link capacitor voltage fromrising by circulating the regenerating current inside theinverter. In phase II, the switching states in the inverter
create a short condition to prevent the DC link capacitor
voltage from increasing. The following shows theswitching states of the inverter, first, all of upper side arms
(or lower side arms) of the inverter are opened as shown in
Fig. 7(a). The switching state of Fig. 7(a) can avoid the
motor current flows into the DC link capacitor because it
is a short-circuit condition. Then, when the zero-crossing
current is detected, the corresponded switch arm is opened
sequentially as shown in Fig. 7(b), (Assuming that u-phasedetects the zero-crossing). Thus, the inverter becomes a
condition of a single phase operation. In addition, thetorque is not produced in this state because the magnetic
field of the stator becomes an alternating magnetic field.

Spv

Spw

iu
iv
IPMSM
iw

Cdc

Snu

Snv

Snw

(a) State 1(short mode)

Spu

Spv

Spw
iv
IPMSM
iw

Cdc

Snv

Snw

(b) State 2(single-phase mode)(c) State 3 (all gate-off mode)
Fig. 7. Operational modes in phase II of the proposed halt
sequence control method

Finally, the remaining two switching arms are opened
when the zero-crossing current are detected, respectively
as shown in Fig. 7(c). Therefore, the suppression of thecirculating current and the rising of the DC link capacitor
voltage can be achieved.
C. The proposed method in a d-q diagram
Fig. 8 shows the relationships between the voltage
vectors and current vectors in a d-q axis when theregenerative current is controlled by the vector control. In
Fig.8, as soon the current vector is start moving, it can benoticed that the current vector is moving along the q-axis
only. On the other hand, the voltage vector moves into thefirst quadrant as soon the control is started. Normally, thevoltage vector is move to second quadrant in the motoring
operation [12]. However, during the regenerating, themotor is working similarly as a generator, the voltage
vector can be expressed by (9)•••V out = V g − jωLx I out

Fig. 8. Output voltage vector and output current vector in dq
diagram. (Vector control is applied).
q-axis Current and Voltage [p.u.]Snu

q-axis Current and Voltage [p.u.]

Spu

(9)

where Vg is the induced EMF of the generator, Lx is
synchronous reactance of the generator.
Fig. 9 shows the relationship between the voltage
vector and current vector when the proposed method isFig. 9. Output voltage ve ctor and output current vector in dq
diagram. (Proposed method is applied).

applied. As shown in Fig. 9, the current vector is started at
q-axis and ended at the d-axis when the phase I iscompleted. Then, the current vector moves back to theorigin point from the fourth quadrant during the phase II.
For the case of voltage vector, subjecting to the capacitor
voltage condition, the voltage vector is moving towards
the q-axis during the phase I, which is 90-120 degreesubjecting to the current vector. As the phase 2 is started,the voltage vector is moving back to the origin pointaccordingly.
III.

SIMULATION RESULTS

A. Verification of the proposed method
The IPMSM parameter that is shown in Table 1 is usedin the simulation. The DC link capacitor voltage Edc is 400
V and the capacity of the DC capacitor Cdc is 100μF. Fig.
10 shows the vector control with the propose method
diagram [10].
Fig. 11 illustrates the waveform of the output current
and the DC link capacitor voltage which are obtained byvector control only. Once the relay is opened at t1, the qaxis command current is controlled to zero immediately. It
can be note that the rising of DC link capacitor voltage is76.4 V.

id *

ACR

iq *

ACR

Cdc

Vbat

vector control digtam
vd*

relay

dq
vq*
abc

decoupling
control

iu

iq

M

iv

iw

PG

re Speed signal and
re

iq
id

INV

phase signal
processor

vdc
Switching
table

dq
abc
the propose method diagram

Fig. 10. vector control with the propose method diagram

From the result, the output current can be suppressed toless than 2.80 p.u of the rated current by applying the
proposed method. In addition, the fluctuation of the DC
link capacitor voltage is suppressed to less than 0.022 p.u.
of the rated voltage, which is sufficiently small.

Fig. 11. Output current and DC capacitor voltage waveform
without the proposed method

Outputcurrent[p.u.]

Phase 1
iw
2
0
-2 iv
iu

Outputcurrent[p.u.]-2

Phase 22.8[p.u.]

0DC
capacitor 450
voltage
400
VC[V]

iq

id
5.42[ms]
0.022[p.u.]
10

12

14
Time [ms]16

Fig. 12. Output current and DC capacitor voltage waveformwith the halt sequence control method and countermeasure to
current raised the recondition.

B. Evaluation of IGBT Junction temperatureThis section evaluates the junction temperature of theswitching devices, considering that the proposed method isapplied in an inverter with a 55 kW IPMSM. Table 3
shows the motor parameters. Table 4 shows the device
parameters that are used in this section. The rated current
for the selected device is approximately two times of the
rated current of the motor. Considering that the heat
capacity of the heatsink is large, therefore the heat
resistance is not taken into accounts.
Fig. 13 shows simulation result of junction temperaturefor devices TSnu TSnv and TSnw. In Fig. 13, the rise of
temperature for IGBT TSnv is approximately 9 degrees athighest and the rise of temperature for FWD TSnu is 8.1
degrees at highest. From the results, it can notice that thechange of temperature in IGBT is low for the
implementation of proposed method. From the result, it
can demonstrate that the implementation of the proposed
method is not required high rated current devices, that is,
the same rating devices that used in the conventional
method can be applied. Furthermore, a large heatsink is
not required since the change of temperature is relatively
low.
IV. EXPERIMENTAL RESULTS

Fig. 12 shows the waveform of the output current andthe DC link capacitor voltage which are obtained by the
proposed method. In Fig. 10, once the relay is opened at
10 ms. Then, the switching patterns in Table 1 is
implemented depending on the current polarity to ensurethat the q-axis current equals to zero. At approximately 11
ms, the q-axis current reaches zero, then the phase II
operation is implemented.

Fig. 14 shows the configuration of the experimental
system. The parameters of the IPMSM are identical to thesimulation condition as shown in Table 1. The DC link
capacitor voltage Edc is 200 V, the rated speed of IPMSM
is 0.5 p.u. and a 7.5kW induction motor is used as the load
machine. In addition, the halt sequence control is begun

Table 3. Motor parameters used in junction temperature
simulations

Table 4. inverter module parameters of inverter used in junction
temperature simulations
Fig. 13. Junction temperature waveform.

when DC link capacitor voltage Edc is detected after therelay is opened.
Fig. 15 shows the experimental waveform of the output
current and the DC link capacitor voltage which are
obtained by vector control only. In Fig. 13(a), the DC link
capacitor voltage is increased by approximately 110 V
right after the relay is opened at t1. After that, the dynamic
break system is operated to ensure that the DC link
capacitor voltage did not exceed the design value. As theenergy is being consumed in the dynamic brake system,
the DC link capacitor voltage drops immediately start fromt2.
Fig. 16 shows the experimental waveform of the output
current and the DC link capacitor voltage which are
obtained by the short circuit control method only. The
maximum value of the circulating current is 4.6 p.u, which
has been calculated from (6). However, for the safety
purpose, the level of over current is set to 3.0 p.u. of the
rated current. Therefore, in Fig. 16, the maximum value ofthe circulating current is shown as 3.0 p.u.. From the result,
it is confirmed that this method is not practical because theDC link capacitor voltage rises sharply at this time. The
reason is that increasing of the current causes theregenerative torque is increasing in the short circuit control
method. Consequently, a large output current flows
depending on the direction of the regeneration energy isincreasing.
Fig. 17 shows the experimental waveform of the output
current and the DC link capacitor voltage which are
obtained by the proposed method. In Fig. 17(a), phase I is
implemented between the time t1 and t2. During this period,

Fig. 14. Configuration of the experimental system.

(a) DC link capacitor voltage waveform.

(b) Output current waveforms.
Fig. 15. Experimental result without the halt sequence control
method.

it is confirmed that the DC link capacitor voltage
fluctuates during from t1 to t2 as the switching patterns inTable 1 is applied. In addition, it is confirmed that the
maximum capacitor voltage is 22 V during the phase I,which is approximately 80% lesser comparing to theresults in Fig. 15.
On the other hand, from Fig. 17(b), the maximum
output current is 2.5 p.u., which is a 46% of suppression
comparing to 4.6 p.u. as calculated early. From theexperimental results, it is confirmed that without the
dynamic brake system, the proposed method can preventthe over voltage happening at the DC link capacitor
voltage, and also suppressing the circulating current.
IV. CONCLUSION(a) DC link capacitor voltage waveform.

In this paper, the suppression method for the rising of
the DC link voltage during the halt operation of theinverter while in the motor regeneration is proposed. The
proposed method can stop the regeneration operation by
utilizing the switching states of the inverter without the
dynamic brake system. The simulation and experimental
results demonstrate the effectiveness of the proposed
method.
In the future works, the studies including the
optimization of control of phase 1 and numerical analysisof the circulating current.
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Fig. 16. Experimental result with the short circuit control
method.

[1](a) DC link capacitor voltage waveform.

(b) Output current waveforms.
Fig. 17. Experimental result with the halt sequence control
method.
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