A High Efficiency Three-phase AC Motor Drive
Converter that Utilized the Neutral Point of a Motor
Goh Teck Chiang

Jun-ichi Itoh

Nagaoka University of Technology
Niigata, Japantcgoh@stn.nagaokaut.ac.jp

Nagaoka University of Technology
Niigata, Japan
itoh@vos.nagaokaut.ac.jp

Abstract— This paper exams a three-phase power converter thatutilized the neutral point of the motor and demonstrates the
motor performance of the proposed converter under various
operating conditions. The structure of the proposed circuit isbasically consisting of an AC/AC converter (Indirect Matrix
Converter (IMC)) and a boost converter that connected to theneutral point of an induction motor. Boost converter will connect
to batteries to serve as a secondary input power source of thesystem. Secondary input power will be injected directly into theDC link voltage of the IMC. In addition, this paper also
demonstrates that the DC link voltage of the indirect matrix
converter is stable and capable for an additional DC powersource connection. Several motor test data such as speed-torquecharacteristic, torque impact and acceleration-deceleration
characteristic are included to confirm the performance of theinduction motor. Furthermore, experimental analysis proved that
this converter is low loss when controlling with carriercomparison method of either the symmetrical method or theasymmetrical method.

motor are analyzed in an experimental. These analyses include
the speed-torque curve, the torque impact characteristic and theacceleration-deceleration characteristic. In addition, theseanalysis are conducted in a way that modeling to a hybrid
system.

Keywords-component; Indirect Matrix Converter, Boost
Converter, Induction Motor, Neutral point of the motor, Zerovector output)

I.

INTRODUCTION

Hybrid electric vehicles (HEVs) is one of the promisingfuel efficiency vehicles that target to occupy 20% of the total
vehicle in Japan by 2020 [1]. The research in HEVs has
proven that this vehicle able to reduce 48% of CO2 compare tonormal vehicles. Therefore, the known issues in HEVs areactively studying recently. These issues are such as costing,size, battery technical issues and high reliability electric motor.

II.

CIRCUIT TOPOLOGY

An indirect matrix converter has shown good varieties ofusages as a three-phase AC converter [3]-[4]. This converter isdivided into two sides; the primary side which is a current
source inverter switches at zero current and result thisconverter to achieve high efficiency. Furthermore, if RBIGBTs are used in the primary side, the reverse blocking abilityof the IGBT can further reduce the conduction loss in the
primary side and increase the overall efficiency [5].
Besides from that, comparing an indirect matrix converter
to a matrix converter, indirect matrix converter has a simple
control structure because it can divided into two circuit
structures but also the DC link voltage can be simply connected
to other DC source. In Ref [6], this paper shows that an extra
circuit is required for the connection of DC link voltage of amatrix converter. The size and cost would be increased due tothe extra used components.
Fig. 1 shows the proposed circuit where it is basicallyconstructed by an indirect matrix converter and the DC link
voltage of the indirect matrix converter is connected a DC
source power and a DC boost converter [2]. Furthermore, theboost converter connects to the neutral point of the motor andutilizes the leakage inductance of the motor as a boost reactor.

The authors have proposed an AC/DC/AC circuit thatapplicable in a HEV system [2]. This proposed circuit greatly
reduced the size of the converter by removing two passive
components; electrolytic capacitors and a boost reactor, by
utilizing the neutral point of the motor. In the view of theconverter, the proposed circuit has successfully demonstrated
the fundamental operation.
However, in the view of the motor, a study needs toanalysis to confirm the stability and performance of the motor.
This paper studies the performance of a 750 W induction motorthat the neutral point connection is utilized by the proposed
circuit. The stability and the performance of the induction

Fig.1. Proposed circuit topology.

This paper exams and analysis the performance of a threephase induction motor that applied in the proposed circuit.
Table I shows the parameters of the induction motor and the
ES servo motor. Fig. 3 demonstrates the set up condition
between the induction motor and the ES servo motor. The ES
servo motor is performed as a motor load. Torque sensor andspeed sensor are located at the ES servo motor. The induction
motor is powered by the indirect matrix converter, and the ES
servo motor is directly connected to a grid power source. A
controller is attached to the ES servo motor to control thetorque directly. Speed and torque data can be read from thecontroller for the monitoring purpose.

Fig. 2(a). HEV mode energy flow diagram.

IMC

G

PrimarySide

Pbat

Secondary
Side

M

Boost
Converter
Fig. 2(b). EV mode energy flow diagram.
Fig.2. Energy flow diagram.

The boost converter operation depends on the zero vector
output of the inverter side. The battery current will be charged
through the motor leakage inductance at every zero vector
output period. Then, the accumulated battery energy will be
released into the indirect matrix converter at every non zero
vector period.
In the view of a hybrid system, the proposed circuit iscapable of operating in three modes. Fig. 2 illustrates theenergy flow diagram of the proposed circuit. Fig. 2(a) shows
the typical hybrid mode, where the battery power is used as analternate energy to drive the motor and the main power is fromthe generator. Fig. 2(b) shows the EV (electric vehicle) mode
operation, where all the power is drain from battery power only.
The third is the normal mode, where all the input power is fromgenerator only.
III.

MOTOR CONTROL DESCRIPTION

The selection of motor for the proposed circuit is important.
The motor needs to have two functions; one is the normal
motoring function and regenerating function; and the second isconnection point of neutral point for boost converter operation.

Fig. 4 shows the control block diagram for the motor andthe boost converter. This control is based on carrier comparison
method [7]. First of all, V/f control could easily control a wide
range of motor parameters [8]-[9]. The motor voltage isadjusted proportionally to the output frequency. Then, themagnetizing current of the motor is kept practically constant ina wide adjustment range. As a result, a constant torque can becontrolled across the entire speed adjustment range.
Then the output voltage command needs to compensate
with the DC link ripple which contains 6 times frequency of
the input frequency. A carrier generator which will produce avariable carrier that based on the duty of the primary side, will
be used in the secondary side and the chopper side. There are
two types of variable carrier which is known as thesymmetrical carrier and the asymmetrical carrier [2]. These
two methods are formed based on the primary duty, but thesymmetrical generates a 20 kHz switching frequency and theasymmetrical generates a 10 kHz switching frequency.
Boost converter controller contains of a PI control for thecontrol of battery current and then is multiplied with the DC
link frequency compensator to obtain the desired switchingpattern.
TABLE I.

MOTOR PARAMETERS

Induction motor (Fuji: MLH6085M)
Motor Power

750 W

4/ 50 Hz

Rated Voltage

200 V

Poles
Rated
RPM
Leakage
Rated Current
3.6 A
inductance
ES Servo motor (Fuji: GRK1151A)
Rated
Motor Power
1500 W
RPM
Rated
Rated Votlage
200 V
TorqueMax
Rated Current
8.6 A
Torque1420

Controller

R

ES Servo Motor
T Power Converter

SU

ES Servo
Motor

Induction Induction V
Motor
Motor

Power grid
200V /50 Hz

W

Indirect
MatrixConverter

R
S
T

Power grid
200V /50 Hz

Fig. 3. Motor experimental set-up condition.

4.42 mH

2000
7.16 N.m
10.8 N.m

ir*
is*
it*
0

ω*

V/f conv.

Carrier
1/X generator

DC linkripple cal

vu*

vd*

dq uvw vv*
conv.

vw*

vq *

PI
1
s

θ*

Ib* +

X
X
X
X
vc*

Sup

V2

U2

+
+ Svp
+ Swp
+
-

StatorWinding
W2

N
W3

U1

iu =0

V1

iv

Rator

Sbp

Flux

V3

- Boost converter
Ib controller

U3

W1

Fig. 4. Control block diagram.

In ref [2], this paper has been explained the boost converter
is operating at every zero vector output of the inverter side. Fig.
5 illustrates about the current flow direction and the magnetic
flux behaviour of a motor structure between the zero vector
output and the non zero vector output.

iw

S

W4

V4

U4

StatorFig. 5 (a). Positive-phase current flow diagram.

The motor stucture in Fig. 5 is based on a standard threephase 4-pole induction motor. The stator and the rotor are theelectrical circuits that perform as electromagnets in an
induction motor. For the stator winding, each group of coils
surrounds the steel core to form an electromagnet. The rotor
consists of a stack of steel laminations with evenly spaced
conductor bars around the circumference. Phase windings areplaced 300 apart in this example.
In Fig. 5(a) shows the positive-phase operating behaviourof an induction motor. Assuming the current is flowing at 600
intervals time, the output current iu is zero current, iv is anegative current and iw is a positive current. As iw flows into
the stator, a magnetic flux will be formed from W1 to W3.
Eventually, the rotor will be induced by the magnetic flux
direction and rotates accordingly.
In Fig. 5(b) shows the zero-phase motor behaviour during
the zero vector output, since the upper arm or the lower arm of
the secondary side are all turned on together, no voltage flows
across the motor. Battery current will take this period and flow
into the leakage inductance (stator winding) thru the inverter
switches and release out at the neutral point connection. Notethat battery current flows into one direction only, and therefore
magnetic flux cannot be produced.
In
Vn

R1

L2

L1
Lm

Rg

In
R2
s

Stator circuit Rotor circuit(a)R1=Stator resistor
L1=Stator leakage inductance
R2=Rotor resistor
L2=Rotor leakage inductance

R1

L1

Vn

Lm
Stator circuit(b)s = Slip
Lm=Magnetizing inductance
Rg= Magnetizing reactanceFig. 6. Induction motor equivalent circuit. .

Rg

Fig. 5(b). Zero-phase current flow diagram.
Fig. 5. Induction motor interior structure.

Fig. 6(a) shows the induction motor equivalent and Fig.
6(b) shows the induction motor zero phase equivalent circuit.
For the zero-phase, since there is no flux produced, the rotor
circuit does not occur. Output current is derived as following,*i
I n = I n + bat

3

.

(1)

where In = Positive phase motor current; n= u, v & w.
ibat = battery current;
The copper loss can be derived as following,
1 2
2
Pn = ( I n + ibat ) R1 .
9
Where Pn= Per-phase motor power loss;(2)

EXPERIMENTAL RESULTS

Table II shows the experiment parameters and the system
connection is as described in Fig. 3. The conducted analyses
are classified into three sections, the speed and torque curve,
the torque impact characteristic and the acceleration anddeceleration characteristic.
TABLE II.

EXPERIMENT PARAMETERSLC Filter Cut-off
frequency

2.4 kHz

50 Hz

DC source

100 V

Carrier frequency

10 kHzMeasure leakage
inductance

4.42 mH

Variable carrier
frequency

20 kHzOutput frequency

10 Hz –
45 Hz

Input voltage

200 V

Input frequency

A. Speed and torque Curve
Fig. 7 shows the relations between the speed and torquecharacteristic of an induction motor. Fig. 7(a) shows the
motoring data where only the indirect matrix converter isconnected. Note that the V/f control is controlled at constant at
all time. This graph shows a good control over an induction
motor. The reason the speed is not constant to the torque isbecause induction motor consists of slip. The slip creates adifference in speed between the rotor and rotating magnetic
field because no relative motion would exist if the two were
turning at the same speed.
Fig. 7(b) shows the relations between speed and torquecharacteristic of the proposed circuit connected to an induction
motor. The battery current was pre-set at 0 A when the torqueis below than 20%. When the torque is larger than 20%, battery
current will increase to 2 A. At 35 Hz output frequency, when
the torque is 100%, the motor speed is 879 rpm. Likewise forthe indirect matrix converter connection, under the same output
frequency, when the torque is 100%, the motor speed is 877
rpm. The data in these two graphs show no difference, even theneutral point of the motor is connected to a boost converter.
That is, the motor is running under a good operating condition.

input power eventually increases to keep the motor speed. The
output current iu demonstrates a good sinusoidal waveform at
all the time which is the same as Fig. 8(a).
C. Acceleration and deaccleration characteristic
This section will demonstrate the speed characteristic of the
proposed circuit under two different set-up conditions, normal
mode and HEV mode. This experiment was conducted under ano-load condition. For the acceleration, the output frequency is
increased from 10 Hz to 30 Hz. As for the deceleration, theoutput frequency is decreased from 30 Hz to 10 Hz.
Fig. 9(a) shows the indirect matrix converter only (normalmode) analysis data. During accelerating, large input power isrequired until the speed is at constant. Output current iu showsa good sinusoidal waveform and change of frequency due tothe accelerate and decelerate control.
Fig. 9(b) shows the proposed circuit speed characteristic
data. When the motor is at low speed (fout= 10Hz), the battery
current is set at 0 A. Once the acceleration starts, the battery
power increases to 200 W immediately. Therefore, during
accelerating ir shows a low current magnitude because battery
power is used to drive the motor. When the speed is constant,battery power continues supplying and the input power drops.
Comparing between Fig. 9(a) and Fig. 9(b), it can be seenthat the output current iu is performing a good sinusoidal
waveform. That is, the connection of neutral point of the motor
for the used of a boost converter does not affect theperformance of a motor. Furthermore, the results demonstrate agood control of batter power management is possible.
120

100

80

Torque (%)

IV.

60

40

B. Torque Impact
This section will demonstrate the torque impact
characteristic under an EV mode and normal mode which isshown in Fig. 8. Fig. 8(a) shows the normal mode operation
where the boost converter did not connect to the motor andonly the indirect matrix converter is operating. The output
frequency is 30 Hz, and the step increase of torque is 100%.

The performance of a motor under the EV mode is shownin Fig. 8(b). The experimental condition is the same as in Fig.
8(a). The battery power is at 200 W at all the time, and the
input power is nearly to 50W, therefore the ir shows a bad
quality of waveform due to low input power. When the torqueincreases to 100%, battery power maintains at 200 W and the

0
0

200

400

600

800

1000

1200

Fig. 7(a). Indirect matrix converter.

Torque (%)

In Fig. 8(a), input current ir shows an unbalanced sinusoidal
waveform due to the low input power, where the input power isnearly 170 W. As the torque increases to 100%, input power
immediately produces a larger power to maintain the speed.
The output current iu shows a good sinusoidal waveform at all
the time.

20

Fig. 7(b). Proposed circuit.
Fig. 7. Speed-torque characteristic analysis.

1400

D.

DC Link Voltage
This section will confirm the DC link voltage stability of
the indirect matrix converter for another power source
connection. In the proposed circuit, the DC link voltage isutilized to connect to an addition DC source. The batteryenergy will be released to the DC link part during the non zero
vector output. The experimental results in Fig. 10 show that thebattery voltage does not affect the DC link voltage of theindirect matrix converter.

Besides that, the asymmetrical method average produces
switching frequency of 10 kHz per carrier time which will
reduce the switching loss in the secondary side. On the other
hand, the symmetrical method produces switching frequency of
20 kHz averagely. Therefore, most of the time, the loss of thesymmetrical method is rather higher.

Two conditions were verified in the experimental, mid
speed region and high speed region. Figs. 10(a) and 10(b) show
the experimental results where the motor speed is 475 rpm andthe output power is 700W. In Fig. 10(a), the battery current is
0 A, and the DC link voltage is 270 V constantly. In Fig. 10(b),the battery current increases to 2 A, note that the DC link
voltage can keep at constant even the input current magnitude
has reduced.
The second condition is the high speed region where themotor speed is 900 rpm and the output power is 700W. Fig.
10(c) shows the result when the battery power is 0 W, and theDC link voltage is constantly at 270 V. Note that when thebattery power increases to 200 W as shown in Fig. 10(d), thebattery current contains of fluctuation but the DC link voltage
still can maintain at constant. One of the reasons of thefluctuation is due to the tiny zero vector output cannot be
formed due to the dead time error under high switchingfrequency. These results confirmed that the DC link voltage ofthe indirect matrix converter can be utilized and maintained byanother DC power source.
V.

Fig. 8(a). Indirect matrix converter (normal mode).

POWER LOSS EXPERIMENTAL MEASUREMENT

Fig. 11 shows the proposed circuit experimental loss
analysis data. The data (input power, battery power and output
power) were directly taken from a power meter where theoutput power is controlled proportional to the load torque.
The symmetrical carrier and the asymmetrical carrier wereapplied one at a time to compare the losses of each method.
The output frequency is 40 Hz, the output power is 750 W, andthe battery power is 200W. Initially, the battery power does not
supply, the battery power supplies when the output power is
300 W and above.
When the output power is low, the loss of the asymmetricalcarrier is higher than the symmetrical carrier. One of thereasons is because when applying the asymmetrical method the
input current contains of high harmonic component [10], thiswill cause the loss in LC filter increase. Furthermore, as theasymmetrical method has lesser zero output periods than thesymmetrical method, eventually the conduction loss in primaryside will increase.
When the output power is 300 W and above, the loss inchopper for the symmetrical method is 22 W and for theasymmetrical method is 15 W, respectively. This is because thesequence of zero vector output for the symmetrical method ismore than the asymmetrical method. That is, the switching lossof the chopper is increased when symmetrical method applied.

Fig. 8(b). Proposed circuit (EV mode).
Fig. 8. Torque Impact characteristic.
N
150
r/min/div
ir
50mA/div
iu
100mA/div
2000ms/div

Fig. 9(a). Indirect matrix converter (normal mode).

Fig. 9(b). Proposed circuit (HEV mode).
Fig. 9. Acceleration and Deceleration characteristic.

Power Loss (Watt)

Fig. 11. Power loss experimental data

Fig. 10 (a). Mid speed region (Output frequency =20 Hz, ibat=0 A).

CONCLUSION
This paper discusses about the performance of an induction
motor where the neutral point of the motor is connected to a
DC boost converter. Several experimental test results haveshown that the motor is under a good working condition eventhe stator leakage inductance is utilized to use in a boost
converter operation. Besides, the DC link part of the indirect
matrix converter is shown a stable connection point for an
additional DC power source. Lastly, the total loss is shown to
be 60 W+10% for a 750 W induction motor system.
Fig. 10 (b). Mid speed region (Output frequency =20 Hz, ibat =2 A).

REFERENCES[1]

Fig. 10(c). High speed region (Output frequency =40 Hz, ibat =0 A).

Fig. 10(d). High speed region (Output frequency =40 Hz, ibat =2 A).

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