Loss Evaluation of a Two-stage Boost Converter
Using the Neutral Point of a Motor
Jun-ichi Itoh

Daisuke Ikarashi

Member, IEEE
Nagaoka University of Technology
1603-1 Kamitomioka-cho Nagaoka City
Niigata, Japan
itoh@vos.nagaokaut.ac.jp

Student Member, IEEE
Nagaoka University of Technology
1603-1 Kamitomioka-cho Nagaoka City
Niigata, Japandai1446@stn.nagaokaut.ac.jp

Abstract -- This paper evaluates the loss of the proposed twostage boost converter using the neutral point of a motor, which
consists of a small boost chopper and a three-phase inverter.
The proposed converter uses the leakage inductance of a motor
instead of an ordinary boost-up reactor as a passive boost up
component. However, the DC current is imposed to the phasecurrent of the motor in the proposed circuit. Thus the proposed
circuit is difficult to separate the power loss between the motor
part and the inverter part. The basic operation of the proposed
circuit is confirmed by experimental results. In additionally, ameasurement method of a power loss in each part of the
proposed circuit is established and demonstrated in the test
circuit. The efficiency of the boost converter stage of the
proposed circuit is increased by 2.3% in compared to theconventional circuit.
Index Terms—DC-DC power conversion, DC-AC power
conversion, Pulse width modulated inverters, Permanent
magnet motors, Motor drives, Inductance, Loss measurementI.

INTRODUCTION

Recently, motor drive technologies that drive with abattery are highly demanding in a lot of applications such as
electric vehicles and rail trains [1]. In order to obtain highefficiency, high terminal voltage at the motor is needed.
However, a higher voltage needs to increase the volume of
the battery bank and results an increase in term of cost.
Therefore, the battery is usually connected to a boost
converter before an inverter.
Fig. 1 shows the conventional DC-AC power converter,
which is composed of batteries, a boost converter and a threephase inverter. Battery voltage is increased by the boost
converter when the inverter outputs high voltage to the motor.
However, the boost converter requires a large boost-up
reactor, which is high cost and bulky. In addition, higher
switching frequency is applied in order to reduce the volumeof the boost-up reactor; however this strategy will increase
the switching losses. One of solutions to reduce the switchinglosses under high frequency, resonance converters are used[2-4]; however this method increased the number ofcomponents in the boost-up converter.
In order to solve this problem, the authors have been
proposed a reactor free boost-up converter that uses theleakage inductance of a motor instead of a boost-up reactor[5-6].
Fig. 2 shows the DC/AC conversion reactor free boost-up
converter [6-7]. In the proposed converter, the neutral pointof a motor is connected to a battery directly. So, the proposed
converters can increase the inverter output voltage without
increasing the number of components.
However, the proposed converter is having two problems.
First, the maximum peak value of the inverter output phasevoltage is limited by the battery voltage. Therefore, when the
boost-up ratio is two times higher and above, the modulation
factor of the inverter will be decreased.
Fig. 3 shows the controllable range of fig. 2. Second, thesix-step modulation can not be applied to the proposed
converter. In the six-step modulation, the neutral point
voltage of a motor will fluctuate at a frequency which is threetimes of the output frequency. Since the battery is connectedto the neutral point of the motor, the input current willcontain large ripple due to the fluctuation of the neutral point
voltage of the motor. Therefore, another small boost-up stage
is proposed to connect to the neutral point of a motor [8].
This paper evaluates the loss of the proposed two-stage
converter in order to optimize the motor design and power
converter design. Especially, the proposed circuit generates
the motor and inverter loss by zero-phase current where theDC current of the battery is added to the phase current of the
motors in the proposed circuit. Thus the proposed circuit isdifficult to separate the motor part and the inverter part into
two items for power loss measuring. This paper describes ameasurement method of a power loss in each part of the
proposed circuit. The validity of the measurement method is
confirmed by the simulation and the experiment. Furthermore,
the loss distribution of the proposed system is demonstrated
in the test circuit. A power loss in each part of the proposed
circuit is measured and compared with the conventional
circuit. In additionally, the basic operation of the proposed
circuit is confirmed by experimental results.
II.
A.

CIRCUIT TOPOLOGY

Circuit configuration
Fig. 4 shows the proposed circuit configuration. The

proposed circuit is composed of batteries, a three-phase
inverter and a first stage boost converter that connects to theneutral point of a motor. The inverter in the proposed circuit
comes with a boost-up function when the motor leakage
inductance is utilized. However, the DC current is imposed tothe phase current of the motor in the proposed circuit.
The proposed circuit can reduce the volume of the boostup reactor and switching losses in the first stage boost
converter. When a sinusoidal wave triangle carriercomparison method is used to generate PWM pulses, theneutral point voltage of the motor is the same as the neutral
point voltage of the DC link part, i.e. half of the DC link
voltage Edc1. Therefore, the rating voltage of the boost-up
reactor and switching device in the first stage boost converter
is reduced to the half of DC link voltage. It should be notedthat the capacitor C2 that is connected to the neutral point of
the motor is 100 times smaller than the DC link capacitor C1of the inverter.
The applied voltage of the boost-up reactor assumes to beconstant during a switching cycle. So, the relations between
the inductance of boost-up reactor L and the input current
ripple ΔIin can be expressed as
1 d / f sw
vd
vL dt = L .............................................. (1)
L ∫0
Lf sw

where the d is duty ratio, fsw is the switching frequency, vLis the applied voltage of reactor.
When the input current ripple ΔIin is designed, the required
inductance of the boost-up reactor L is derived from equation(1).
Fig. 5 shows the relationship between the boost-up ratio
and the reduction ratio of inductance in the proposed circuit.
When the boost-up ratio is 3, the proposed circuit can reducethe inductance of boost-up reactor by half as compared to theconventional circuit. Therefore, the proposed circuit can
reduce the size of boost-up reactor.

Conventional circuit diagram
S3
S5

Edc

C

u

S2
Fig. 2.

v
S4

w

Motor

S6

Iin
Ebattery

DC/AC conversion reactor free boost-up converter

Vun/Edc

Δiin =

Fig. 1.
S1

Controllable range of fig. 2

Fig. 4.

Proposed circuit diagram

B.

Vout =

3
2 2

L1/L2

Positive phase sequence equivalent circuit
Fig. 6 shows the positive-phase sequence equivalent circuit.
The proposed circuit can be divided between the positive andnegative sequence components. The boost converter does notappear in the positive-phase sequence equivalent circuit due
to the connection of the neutral point of the motor.
Consequently the positive-phase sequence equivalent circuit
is similar to a conventional three-phase inverter. During thePWM control, when the sinusoidal modulation is based on
the triangle carrier comparison to generate PWM pulses, thefundamental voltage of the output is given by (2)Fig. 3.

Edc ⋅ a .......................................................... (2)where Vout is the output line voltage, Edc is the DC link
voltage and a is the modulation index.
In the proposed circuit, a six-step operation is applied tothe three-phase inverter in order to reduce the switching lossof the inverter in comparison to a PWM inverter. During the

Fig. 5.

The relationship between the boost-up ratio and the reduction ratio
of inductance in the proposed circuitsix-step operation, the switching frequency agrees with theoutput frequency. The output line voltage becomes a 120
degree square waveform, then, the fundamental voltage ofthe output is given by (3)Vout =

6uEdc ................................................................. (3)πwhere Vout is output line voltage, Edc is the DC link
voltage.

S2

Fig. 6.

C.

Zero phase sequence equivalent circuit
Fig. 7 shows the zero-phase sequence equivalent circuit.
The back electromotive force (EMF) does not appear in thezero-phase sequence equivalent circuit but only the leakage
inductance exists. In zero-phase sequence, the inverter legs
could consider to a single leg. Then the output voltage of thesingle leg is the same as the neutral point voltage of the
motor. A two-stage boost up operation is achieved by two
choppers, which are constructed by switch S1, S2 and switch
S3 and S4. However, the leakage inductance of the motor
decreases to 1/3 since the leakage inductance of the motor isconnected in parallel in the zero-phase sequence equivalentcircuit.
On the other hand, the neutral point voltage of the motor
is given by (4)E dc 2 =III. CONTROL STRATEGY

Control method of output voltage
Fig. 8 shows the control block diagram of the proposed
circuit. The control of motor current in the proposed circuit is
the same as a conventional three-phase inverter because thepositive-phase sequence equivalent circuit is similar to aconventional three-phase inverter. The stabilization control isapplied to achieve a V/f control for the permanent magnetic

S5

Edc1

v

Motor

w

S4

S6

Positive phase sequence equivalent circuit

S1

Leakageinductance

C1

S3

S2
Edc2
Fig. 7.

C2
S4

L
Iin
EbatteryZero phase sequence equivalent circuit

vu∗
vv∗

1
1
(vu + vv + v w ) + E dc1 .................................... (4)32

where Edc2 is the neutral point voltage of the motor, vu, vv
and vw are the motor phase voltage and Edc1 is the DC link
voltage.
Therefore, the neutral point voltage of the motor can becontrolled by the zero-phase voltage command. In particular,
the average of the neutral point voltage of the motor for one
switching period is zero when the symmetrical three-phasesinusoidal waveform is used as modulation signal. Therefore
the neutral point voltage of the motor equals to double of theDC link voltage. Furthermore, sum of the maximum inverter
output voltage command and the zero-phase voltage
command is limited by the neutral point voltage of the motor.
If the battery connects the neutral point of the motor directly,
the maximum phase voltage of the motor is constrained by
the battery voltage. However in the proposed circuit, themaximum phase voltage is not constrained by the battery
voltage because the neutral point voltage of the motor iscontrolled by the boost converter.

A.

S3

S1
Edc

vw∗
iu

ωl∗ 1
s

iδ

iv
iw

Fig. 8.

Control block diagrams

motor [9]. Additionally, the PWM control is slowly changed
into a six-step operation via the trapezoidal pulse modulation.
The transition control in Fig.8 is a proportional compensation
to the amplitude of the output voltage command because theoutput voltage is nonlinearly increasing at the trapezoidal
pulse area [10].

B.

Battery current compensation control
In the proposed circuit, when the six-step operation isapplied to the inverter control, the input current is distortingat three times of the output frequency. This is because theneutral point voltage of the motor has fluctuation of ±1/6Edc1 with three times of the inverter output frequency inthe six-step operation. In order to compensate the voltage
fluctuation, a feed forward control is applied to a current

regulator in the first stage boost converter. The voltage
fluctuation is estimated by the DC link voltage and the pulse
pattern of the inverter.
IV. LOSS MEASUREMENT METHOD
Fig. 9 shows the power flow of the proposed circuit. In the
proposed circuit, the motor losses by zero-phase current
occur between the boost converter losses and the inverter
losses because the neutral point of a motor is connected to aboost converter with a battery voltage as the input voltage. In
consequence the inverter losses can not be calculated as thedifference between the boost converter output power and theinverter output power merely. Therefore, it is difficult toseparate the power loss between the motor part and theinverter part. However, the proposed circuit can beconsidered as one of the three-phase four wires system. Thenthe total power of the converter is measured by the sum of
the instantaneous power of each phase.
Fig. 10 shows the method for measurement of the inverter
losses. The inverter losses in the proposed circuit are givenby (5) based on a four wires system.

Fig. 9.

Fig. 10.

Power flow of the proposed circuit

Method for measurement of the inverter losses
in the proposed circuit
TABLE I
IPM MOTOR PARAMETERS

Pinvloss = v1i1 + v2i2 + v3i3i
= edc 2idc − (vu iu + vv iv + vwiw ) − dc (vR1 + vR 2 + vR 3 ) .............. (5)3where vu, vv and vw are positive-negative phaseinstantaneous voltage, iu, iv and iw are positive-negative phaseinstantaneous current.
V.

SIMULATION RESULTS

In order to demonstrate the loss distribution of the
proposed system, the loss is analyzed by the simulation. The
converter loss analysis was implemented by a circuit
simulator (PSIM, Powersim Technologies Inc.) and DLL files(Dynamic Link Library) [11]. The converter losses are
estimated by the applied voltage for the switching device, thecollector current and the characteristics of the data sheet. The
circuit uses IGBT (2MBI50N-060), the rated collectoremitter voltage is 600 V and the rated collector current is
50A. It should be noted that in the converter loss analysis, theloss of a boost-up reactor is neglected. The motor loss isanalyzed in a 2D finite element method (FEM) by simulating
a 1.5-kW interior permanent-magnet motor.
Table 1 shows the internal permanent magnet (IPM) motor
parameters. The motor phase current of the conventional
system is assumed as sinusoidal current without the DC
current. By contrast, the proposed system condition is with
the DC current, where the DC current is －4.0A per single
phase.
Fig. 11 shows a total efficiency comparison between the
proposed system and the conventional system during thePWM control. The efficiency of the proposed system is lower
than the conventional system approximately 3% because thecopper loss of the motor is increased 30% by the zero-phaseFig. 11.

Total efficiency comparison of the proposed system with theconventional systemcurrent. In contrast, the switching loss of the boost converterin the proposed circuit is decreased by half. So, the proposed
circuit can increase the converter efficiency.
Fig.12 shows a total loss analysis comparison between the
proposed system and the conventional system during theLoss[W]Fig. 12.

Total loss analysis in simulation

PWM control by applying 5 difference load conditions. The
total loss of the motor drive system that drive with a battery
is consist of a boost converter loss, a three-phase inverter loss,
a copper loss of motor and an iron loss of motor. The loss
simulation estimates that the proposed circuit can reduce theboost converter loss by 20% as compared to the conventional
circuit because the proposed circuit can reduce the terminal
voltage of the boost converter. However, the conduction loss
in the inverter is increased by the zero-phase current.
Furthermore, the copper loss of the motor is increased by
the DC current components because the copper loss is
proportional to the square of the motor current. On the other
hand, the iron loss caused by zero-phase current is very low.
Fig. 13 shows the magnetic field analysis results insimulation. Fig. 13(a) shows the line of magnetic flux where
the motor model contains sinusoidal three-phase current only.
Fig 13(b) shows the line of magnetic flux where the motor
model contains only DC current. In the Fig 13(b), the main
flux of the DC current is denied with each other. Therefore,
the DC current excites only a leakage flux. So, the zero-phasecurrent in the motor does not generate motor torque [12].

(a) The magnetic flux for sinusoidal three-phase current

VI. EXPERIMENTAL RESULTS
The proposed system was tasted in the prototype circuit.
Table 2 shows the experimental parameters, the IPM motor is
750 W, 175 V and 90 Hz, and the battery voltage is 70 V. In
this experiment, the output torque is controlled to 100%.
Fig.14 shows the operation waveforms during a PWM
control of the inverter. In Fig. 14, the DC link voltage Edc1 isincreased to twice of the neutral point voltage Edc2 by the
inverter side boost-up function. Furthermore the DC link
voltage Edc1 is controlled to 286 V by a voltage regulator. On
the other hand, the motor current contains the DC
components because the motor current is added with thezero-phase current. However, the sinusoidal motor current
waveform and sinusoidal line voltage of the motor are
obtained. It should be noted that the output line voltage Vuv isobserved by using a low-pass filter of 1.5 kHz cut-off
frequency to observe the low frequency component distortion.

(b) The magnetic flux for zero-phase current
Fig. 13 Magnetic field analysis for the magnetic flux

Fig. 15 shows the experimental result during a six-step
operation. When the six-step operation is applied to theinverter control, the input current is distorting at a frequencyof three times of the output frequency. However, Fig. 15
shows that the ripple in the input current has been suppressedby the proposed control. Furthermore, the DC link voltage isEfficiency[%]

TABLE II
EXPERIMENTAL PARAMETERS00

DC link voltage Edc1[V]

250[V/div]

Neutrl point voltage Edc2[V]

250[V/div]

Line voltage Vuv[V]（LPF）

250[V/div]

Fig.16.

The converter efficiency comparison of the analysis resultswith the experimental results0Motor current Iu[A]

5[A/div]

0Fig. 14.

0

10[ms/div]
Experimental results with PWM control

DC link voltage Edc1[V]

250[V/div]

Line voltage Vuv[V](LPF)

250[V/div]00Motor current Iu[A]0

5[A/div]

Input current Iin[A]

10[A/div]
10[ms/div]
Fig. 15. Experimental results during six-step operation

controlled constant by the voltage regulator in the first stage
boost converter.
Fig. 16 shows a converter loss comparison between theanalysis results and the experimental results during a PWM
control. The inverter loss in the proposed circuit is measured
by the three-phase four-wire system. From fig. 16, notice thatthe analysis result is different from the experimental result at
low power region. This is due to the no-load loss. So, when
the converter outputs at the rated power, the error of loss
analysis is about 15%. Meanwhile, it is confirmed that, theproposed circuit can reduce converter loss by 10% ascompared to the conventional circuit at the rated operation
because the proposed circuit can decrease the first stage boost

Fig. 17

Loss separation of the proposed system

converter loss.
Fig. 17 shows a loss separation result of the proposedsystem. The result shows that major loss is dominant by the
motor loss in the proposed system. Especially in the
proposed system, the copper loss of motor is large due to thezero-phase current. On the other hand, the loss in the boostup reactor is dominant in the converter. Therefore, a
reduction on the loss of the boost reactor loss is needed for
obtaining high efficiency in the converter.
Fig. 18 shows the total loss comparison of the proposedsystem with the conventional system. The efficiency of theboost converter in the proposed circuit is increased by 2.3%.
The efficiency is better because the applied voltage for theswitching device in the boost converter decreases to less than
1/2 times in compared to the conventional circuit. Therefore,
the switching loss in the boost converter is decreased by half.
Similarly, the low voltage resistance switching device can beused in the first stage boost converter of proposed circuit.

Furthermore, the copper loss of the boost-up reactor can be
decreased and the proposed circuit can reduce the size of theboost-up reactor to a half due to the low voltage terminal.
However, the inverter losses and the motor losses are
increased because the DC current is imposed into the phasecurrent of the motor in the proposed circuit. In future, an
optimization method on the motor is required in order toreduce the motor losses caused by zero-phase current.
VII.

REFERENCES

[2]

[3]
[4]

[5]
[6]
[7]
[8]

Loss [W]250

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200

Output power 730[W]
Efficiency 72.7%
Efficiency 75.8%

Motor loss

150
100

CONCLUSION

A novel two-stage DC-AC power converter connected to
the neutral point of the motor has been proposed. The
proposed circuit can reduce the volume of the boost-up
reactor and switching losses in the boost converter. This
paper discussed a measurement method of a power loss ineach part of the proposed circuit. The measurement methodof a power loss in each part of the proposed circuit isestablished and demonstrated in the test circuit. Then, theconverter loss of the experimental result is compared to theanalysis result. Furthermore, a power loss in each part of the
proposed circuit is measured and compared with theconventional circuit. From the loss separation result, theefficiency of the boost converter of the proposed circuit isincreased by 2.3% in compared to the conventional circuit. In
additionally, the basic operation of the proposed circuit isconfirmed by experimental results. A six-step operation isapplied to the proposed converter with a feed forward control
to compensate the fluctuation at the motor neutral point
voltage. In future work, the motor will be optimized toreduce the loss that is caused by zero-phase current.

[1]

300

50
0

Inverter loss
Boost converter
loss

Conventional
Proposed circuit
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method for PM Motor,” Trans. IEEJ, Vol.122-D,No.3 No.2 pp.253259, 2002.
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