Development of Inductive Power Transfer
System for Excavator under Large Load
Fluctuation-Consideration of relationship between load voltage and resonance parameter-

Jun-ichi Itoh1, Kent Inoue2*and Keisuke Kusaka2
1 Department of Science of Technology Innovation Engineering
2 Department of Electrical, Electronics and Information Engineering
Nagaoka University of Technology
Nagaoka, Niigata, Japan*E-mail: k_inoue@stn.nagaokaut.ac.jp

Abstract— An inductive power transfer system for an
excavator, which is operated under an air pressure
environment, is designed and developed by considering theload voltage fluctuation. In the conventional excavatorsystems, the power is supplied via the contact wires, which
may cause fire when a spark occurs because a working
chamber environment is under high air pressure. In the
proposed system, the series-parallel compensation is applied
to cancel out the leakage inductance. By using the seriesparallel compensation, the load voltage is ideally constantregardless of load fluctuation. However, the constant-voltage
characteristic degrades due to winding resistance and anerror of the resonance parameter. Thus, the resonanceparameters have to be designed considering the error. In thispaper, the design method of the resonance parameter is
proposed with the voltage ratio maps considering the errorof the parameter including the winding resistance. In theexperiments with a developed 15-kW IPT system, the voltage
fluctuation is smaller than 4.3%. Furthermore, the constantvoltage characteristic is maintained even when the output
power of an induction motor changes from 5 kW to 15 kW
and vice versa.

and the proposed charging system, respectively. An
excavator hangs onto the ceiling of the working chamberand moves along a traveling rail. Meanwhile, an electric
hydraulic pump is used in order to move and operate the
excavator. The power for an electric motor which drives
hydraulic pump is supplied via an insulated trolley wire
which is placed into the traveling rail. However, due to the
movement of the excavator along the traveling rail, there
is a threat of a spark which occurs at connection points.
Even a small spark may cause a large-scale fire because
the working chamber is under high air pressure
environment. Therefore, in order to reduce the risk of thefire which is caused by the spark, the application of an
inductive power transfer for the excavator has been
proposed.

GroundKeywords— Inductive power transfer, Excavator, Loadfluctuation, Constant voltage characteristic

Groundwater

I. INTRODUCTION
A pneumatic caisson method is used in many structures:
foundations of bridges and buildings, shafts for insertionof shield tunneling machines, tunnel and railways, e.g., theChuo Shinkansen [1]. Figure 1 shows the schematic of thepneumatic caisson method. First, a reinforced concrete
caisson is constructed on the ground. Second, an airtightworking chamber is formed at the bottom of the caisson.
Finally, the caisson is immersed at a predetermined depth
and the pressurized air is supplied into the working
chamber in order to prevent the underground water fromcoming into the chamber. Therefore, the working chamber
environment is under high air pressure.
Figure 2 shows the schematic of the charging system.
Fig. 2 (a) and (b) show the conventional charging system

Traveling rail

CaissonBucketExcavator
Dirt

Fig. 1. Schematic of pneumatic caisson method which is immersed at
predetermined depth and pressurized air is supplied into working
chamber in order to prevent underground water from coming into
chamber.

In the conventional method which is connected thecapacitor in series to the primary side and the secondaryside (SS compensation method), the secondary sidefeatures a constant-current characteristic when the primaryside is driven at a constant voltage [2]–[4]. Therefore, theadditional circuits, e.g., the buck converter and the boost
converter, on the secondary side are required for the load
voltage regulation. In addition, in the SS compensation
method, the high-speed communication is needed between
the primary side and the secondary side. On the other hand,
the method which is connected the capacitor in series tothe primary side and in parallel to the secondary side (SPcompensation method) is regulated the load voltage
without the additional circuits [5] [6]. Therefore, in the SP
compensation method, the high-speed communication is
unnecessary.
In this paper, the inductive power transfer system [7]–[14] for the excavator, which ensure the large load
fluctuation, is developed. In the proposed system, the SP
compensation is employed in order to cancel out theleakage inductance and a constant output voltage is ideallysupplied even under the large load fluctuation without theadditional circuit. However, the constant-voltage
characteristic degrades due to the winding resistance andthe error of the resonance parameter.
The new contribution in this paper is providing ananalysis how to develop the robust system against thevoltage fluctuation. Analysis which is indicated this paper
takes the relationship among the load fluctuation, theresonance parameter and the winding resistance takes into
account in order to optimize parameters which is
influenced the load voltage.
Based on the analysis on the voltage characteristic, a15-kW inductive power transfer system is developed andtested. The constant load voltage characteristic isevaluated using a resistance and an induction motor load.
II. INDUCTIVE POWER TRANSFER SYSTEM FOR
EXCAVATOR
A. System Configuration
Figure 3 shows the system configuration of theinductive power transfer system for the excavator. The
proposed system consists of a converter with pulse-width

Traveling rail

Traveling rail
Receiver coil

Excavator

Contact wires

Excavator

Transmitter coil(a) Conventional charging system.

(b) Proposed charging system.

Fig. 2. Schematic of charging system which hangs onto ceiling of working chamber.

Lfilter1 Vdc2 Brake unit

Vdc1
Transmission
coil

REC

C1
Vin

Cdc1
+

vuv1

k

M

i1
v1

L1

GDU
Auxiliary
supply_1

iu1
INV
（VFD）

Controller

OV, OC
Detection

L2

C2 v2

Cfilter1

INV
（VFD）

iu2
vuv2

Excavator

M

Auxiliary Controller
supply_2 & GDU

Fig. 3. Inductive power transfer system for excavator.

Lfilter3
INV（CVCF）
Cfilter3

Auxiliary
supply(For Excavator)

modulation (PWM) and a single-phase inverter in the
primary side, a rectifier, a brake unit, a three-phasevariable frequency drive (VFD) inverter and a singlephase constant voltage constant frequency (CVCF)
inverter in the secondary side. The three-phase VFD
inverters are used for adjustable speed driving an
induction motors for an electric hydraulic pumps. The
single-phase CVCF inverter is used as an auxiliary supply
which is needed for an excavator control. The power istransferred from the primary side to the secondary sidethrough the transmission coils. The SP compensation is
employed in the system. Therefore, the secondary DC
voltage is constant in the ideal conditions because thesecondary side output features a constant-voltage
characteristic regardless of a load fluctuation even if the
primary side is driven at constant voltage. However, thewinding resistance and an error of the resonance parameter
practically degrade the constant-voltage characteristic.
B. Compensation Method of Reactive Power by Leakage
Inductance of Transmission CoilFigure 4 shows the typical compensation methods
which is connected a resonance capacitor in series orparallel to the primary side coil and the secondary side coilin order to cancel out the reactive power [15]. Reactancecomponents by leakage inductance are canceled out
because of the resonance with the transmission coil.
Consequently, the power factor seen from the primary sideis.
Fig. 4 (a) shows the SS compensation method, whichfeatures a constant current characteristic at the secondaryside when the primary side is driven at a constant voltage.
Therefore, the SS compensation is unsuitable for the
existing system because the additional circuit is needed inorder to convert the constant current characteristic into theconstant-voltage characteristic. In addition, the SS
compensation is undesirable for the existing systembecause the high-speed communication is needed between
the primary side and the secondary side. On the other hand,Fig. 4 (b) shows the SP compensation method, whichfeatures a constant voltage characteristic at the secondaryside when the primary side is driven at a constant voltage.
Therefore, the SP compensation is suitable to be applied
to the proposed system because the inductive powertransfer is possible to be applied into the existing
excavator system without any modifications. However,
the error of the resonance parameter needs to beconsidered.
III. DESIGN OF TRANSMISSION COIL
A. Specifications
Figure 5 shows the transmission coil. An excavatorsystem hangs onto the ceiling of the working chamber andmoves along a traveling rail. Therefore, the upper side is
the transmitter coil, the lower side is the receiver coil. In
the proposed system, the solenoid coil is employed toobtain higher magnetic coupling in comparison with acircular coil. The cores are employed with PC40
manufactured by TDK. The size of the core is W237 ×
H210 × D20 mm. Meanwhile, the transmission distance is

C2

C1
L1

L2

(a) SS compensation.

C2

C1
L1

L2

(b) SP compensation.
Fig. 4. Typical compensation method which is connected capacitor inseries or parallel in order to cancel out leakage inductanceFig. 5. Transmission coils which is used core with PC40
manufactured by TDK. Upper side is transmitter coil, lower side isreceiver coil. Coil size is W237 × H210 × D20 mm. Transmission
distance is 50 mm.

i1 C
1

r1

i2 L2-Lm

L1-Lm

f

i3

r2
C2

v1

Lm

Req
v2

Fig. 6. Equivalent circuit of IPT system with SP compensation.

50 mm.
B. Parameters Design
Figure 6 shows the equivalent circuit for designing the
IPT system. The circuit equations of the equivalent circuit
which is shown Figure 6 are calculated as
1  
V1   r1  j  L1  2  I1  j Lm I 2 
 C1  





1  
10   j Lm I1   r2  j  L2  2  I 2  j
I3
 C2
 C 2  where Vdc,2 is the DC voltage on the secondary side and P2
is the output power.
The inductances of the primary and the secondary coils
are designed according to the following equations
L2 
0 j

 2 V dc , 2
8 P2
2

R eq 

Req

k



1 k2


1
1 
I 2   Req  j I 3 
 C2
 C2 


2where V1 is the primary voltage, Req is the equivalent load
resistance, r1 is the equivalent series resistance of the
primary winding, r2 is the equivalent series resistance ofthe secondary winding, L1 is the primary inductance, L2 isthe secondary inductance, C1 is the primary compensation
capacitor, C2 is the secondary compensation capacitor, Lm
is the mutual inductance, and  is the angular frequencyof the power supply.
The currents I1, I2 and I3 are calculated by (4), (5) and(6) when an input voltage V1 is applied into the primary
side.
I1 

I2 

I3 





V1 
r2  j x2  x0  x p


V1


V1




where Vdc,1 is the DC voltage on the primary side and k is
the coupling coefficient.
The compensation capacitors are selected in order tocancel out the reactive power at the input frequency. Thus,
the value of the compensation capacitors is calculated as
1

C1 

 2 L1 1  k 2  

C2 

1
 L2

  R  jx   x  
eq

p

2
p



2

jx0 Req  jx p 

x0 x p 
1 
r1  j  L1  2 
 C1 


 8 Vdc ,1 

L1  L2  2
  k V  
dc
,
2



 j Lm0 j Lm

C. Influence of Parameter Error and Winding Resistance
Figure 7 shows the voltage ratio v2/v1 against the errorof the resonance parameter including the winding
resistance. Fig. 7 (a) and (b) show the v2/v1 ratio against
the error of L1 and C1 and the error of L2 and C2 from 80%
to 120%, respectively. Figure 7 represents the v2/v1 ratio
which is calculated by (13) and (7).

0

v2

1 
r2  j  L2  2 

C2 

1
j
C2

j

v1

1
C2

Req  j1

C2
Note that the voltage V1 is the fundamental componentof the output voltage of the inverter.
The parameters of the transmission coil are designedwith the equivalent circuit. The resistance Req indicatesthat equivalent load resistance considering the full-bridge
rectifier. Then the equivalent load resistance is given by[16]

Req  2 Lm  L1  Lm 

As a result of Fig. 7 (a), it is confirmed that v2/v1 ratio
is high in a large area of the primary inductance L1 and asmall area of the primary capacitor C1. As a result of Fig.
7 (b), it is confirmed that v2/v1 ratio is high in a large areaof the secondary inductance L2. Thus, in the resonanceparameter of the secondary side, v2/v1 ratio depends on thesecondary inductance L2. From the results, it is confirmedthat the error of the resonance parameter affects v2/v1 ratio,i.e., the secondary DC voltage.
D. Coil size design
The size of the coil is decided based on the desired
coupling coefficient and the transmission distance, which
is obtained from the coupling coefficient maps [17]. The

core length is decided larger than the core depth because
the coupling coefficient may be smaller than the design
value.

waveforms are misaligned from the resonance point.
Figure 9 shows the frequency characteristics of thevoltage gain. Fig. 9 (a) and (b) show the input/output

IV. EXPERIMENTAL RESULTSTABLE 1

A. Experimental Conditions
Table I shows the experimental conditions. The rated
power is 15 kW. The self-inductance of the primary sideand the secondary side in Table I is measured value. In thisexperiment, a resistance or an induction motor is used as
the load.

SymbolSwitching frequency
Rated power
Coupling coefficient
Primary inductance
Secondary inductancePrimary capacitance
Secondary capacitancePrimary winding resistance
Secondary winding resistance
MOSFETs
DiodesB. Resistance Load
Figure 8 shows the operation waveforms with theresistance load. Fig. 8 (a) and (b) show the waveforms
obtained at an output of 5 kW and 10 kW, respectively. In
the figure, the secondary voltage is constant against theload power. Though the resonance condition is correct, thelow-order harmonics affect the primary current in the lightload. Therefore, the primary voltage and current

110

1.1

100

1

90
0.9

80
0.8

1.2

0.9

80
0.8

90
110
100
0.9
1
1.1
0.8
Inductance
of
L
1
1
1.1
1.2 [%]

Valuef
20 kHz
P
15 kW
k
0.41
L1
393 HL2
113 HC1
198 nFC2
582 nFr1
107 m
r2
70 m
BSM120D12P2C005
DH 2X61-18A

1.2 v2 / v1 ratio 1.35 -1.40
120
1.2
1.35-1.40
1.35
-1.40
1.30 -1.35
1.30-1.35
1.30
-1.35
1.25 -1.30
110
1.1 1.25
1.25-1.30-1.30
1.1
1.20 -1.25
1.20-1.25
1.20
-1.25
1.15 -1.20
1.15-1.20
1.15-1.20 1.10 -1.15
100
1
1
1.10-1.15
1.10-1.15 1.05 -1.10
1.05-1.10
1.05-1.10 1.00 -1.05
90
0.9 1.00
1.00-1.05-1.05 0.95 -1.00
0.9
0.95-1.00
0.95-1.00 0.90 -0.95
0.90-0.95-0.95 0.85 -0.90
80
0.8 0.90
120
90
110
100
0.85-0.90
0.85-0.90 0.80 -0.85 80
0.8
0.9
1
1.1
1.2
0.8
Inductance
of
L
2
0.8
0.9
1
1.1
1.2 [%]
0.80-0.85
0.80
-0.85

Capacitance of C2 [%]Capacitance of C1 [%]120

EXPERIMENTAL CONDITIONS.

1.2v2 / v1 ratio 1.35 -1.40
1.35-1.40
1.35
-1.40
1.30 -1.35
1.30-1.35
1.30
-1.35
1.25 -1.30
1.1 1.25
1.25-1.30-1.30
1.20 -1.25
1.20-1.25
1.20-1.25 1.15 -1.20
1.15-1.20
1.15-1.20 1.10 -1.15
1
1.10-1.15
1.10-1.15 1.05 -1.10
1.05-1.10
1.05-1.10 1.00 -1.05
0.9 1.00
1.00-1.05-1.05 0.95 -1.00
0.95-1.00
0.95-1.00 0.90 -0.95
0.90-0.95
0.90-0.95 0.85 -0.90
0.8
120
0.85-0.90
0.85-0.90 0.80 -0.85
1.2
0.80-0.85
0.80
-0.85

(a) Error of primary inductance L1 and primary capacitor C1 from 80% to 120%. (b) Error of secondary inductance L2 and secondary capacitor
C2 from 80% to 120%.
Fig. 7. v2 / v1 ratio against error of resonance parameter including winding resistance. Graph legends shows v2 / v1 ratio.

Primary voltage v1 1000 V/div
00Primary voltage v1 1000 V/div

00Primary current i1 10 A/div
Secondary voltage v2 2000 V/div

Primary current i1 10 A/div00
Output voltage vo 1000 V/div
40 s/div0(a) Load of 5 kW.

0Secondary voltage v2 2000 V/div
Output voltage vo 1000 V/div(b) Load of 10 kW.
Fig. 8. Operation waveforms with resistance load.

40 s/div

020log10(i / v) [dB]20log10(vo / vi) [dB]

5 kW
8 kW
10 kW

-20
-40
-60

Resonant freq.
(20 kHz)
10

100

5 kW
8 kW
10 kW

-20

-40
Resonant freq.
(20 kHz)

-60

10

100

frequency [kHz]frequency [kHz](a) Input-output voltage.

(b) Input admittance.

Fig. 9. Frequency characteristics of input-output voltage and input admittance gain

20000
st
20log軸ラベル
10 Yi [dB]00

20 (1 )

Frequency
60000[kHz]軸ラベル
60 (3rd)

100000th
100 (5 )-20-20-40-40-60-60

kW
55ｋW
kW
88kW
10 kW
10
kW

Fig. 10. Frequency characteristics of primary admittance. 1st:
fundamental harmonic component (20 kHz), 3rd: triple harmonics
component (60 kHz), 5th: fifth-order harmonics component (100 kHz)

11.10
vdc,2 / vdc,1 ratio [.]voltage ratio characteristic and the input admittancecharacteristic, respectively. The frequency characteristics
in Fig. 9 are obtained in simulation in order to evaluate theeffect of harmonics on the load. Around the fundamental
frequency of the voltage gain frequency responses, thegain is same value regardless of the output power. On theother hand, it is concluded from the frequency
characteristic of the input admittance; the gain around thefundamental frequency decreases at light load because
low-order harmonics component of the input current isrelatively large.
Figure 10 shows the frequency characteristics of the
primary admittance. The gain of the fundamental
component becomes smaller at light load, which has been
expected from the frequency characteristics which isshown in Fig. 9 (b). Therefore, the distortion of the
primary current at 5 kW is larger than the waveforms at 10
kW because low-order harmonics component of the
primary current are relatively large at the light load.
Figure 11 shows the secondary/primary DC voltage
ratio characteristic against the output power. The blue line
in Fig. 11 represents the calculation value which is
calculated without consideration of the resistancecomponents in the rectifier. In the proposed system, thesecondary/primary voltage ratio characteristic is expected
to be constant because the SP compensation is employed
in order to cancel out the leakage inductance. However,the secondary/primary DC voltage ratio decreases by 1.9%
when the output power increases because the effect ofvoltage drop which is caused by the winding resistance islarge at the high output power.
Figure 12 shows the secondary/primary voltage ratio
characteristic against the output power. The blue line in
Fig. 12 represents the calculation value by (13) and (7)using actual parameters considering the error from anominal value. The green line in Fig. 12 represents
calculation value which is calculated using (13) and (7)with design parameters (theoretical value). The
experimental results agree with the calculation value of the
prototype model with a small error (less than 0.5%). The
error is caused by the difference between the nominal
values and the actual values in the process of the resonanceparameter design. Nevertheless, the secondary/primaryvoltage ratio characteristic is constant regardless of theoutput power. Therefore, it is confirmed that the decrease

11.08
11.06

Calculation value

11.04
Experimental result

11.02
11.00 551.9% drop
in vdc,2 / vdc,1 ratio
6

6

7

8

7 power8[kW]Output

9

9

10

10

Fig. 11. Secondary/Primary DC voltage ratio. Red point isexperimental results. Blue line is calculation value without
consideration of resistance components in rectifier.

in the output/input voltage ratio at the high output poweris mainly caused by the post-stage conversion, i.e., therectifier.
Figure 13 shows the operation waveforms of thetransient characteristic with the resistance load. Fig. 13 (a)and (b) show the step load response from 10 kW to 5 kW,
and vice versa, respectively. The secondary DC voltage ismaintained at constant even when the load step occurs.
Consequently, it is confirmed that the secondary DC

voltage is constant regardless of the output power by using
the SP compensation as a leakage inductance cancelingmethod. In particular, the secondary DC voltage isconstant even when a large load fluctuation occurs.

5kW

v2 / v1 ratio [.]C. Induction Motor Load
Figure 14 shows the operation waveforms with theinduction motor load which is connected to the pump. The
output power is 15 kW. Fig. 14 (a) shows the primaryvoltage and current waveforms whereas Fig. 14 (b) shows
the secondary DC voltage, output voltage and current
waveforms. The primary current is confirmed that the
waveform distortion is small because the fundamental
components is relatively large, i.e. the influence of lowharmonics components is relatively small.
Figure 15 shows the operation waveforms of transientcharacteristic in the induction motor load. Fig. 15 (a) and(b) show the step response from 5 kW to 15 kW and viceversa, respectively. The secondary DC voltage ismaintained at the constant value even when the load step
occurs. Consequently, it is confirmed that the secondary

11.20
11.18

Calculation value (Prototype model)

11.16
Experimental result

11.14
11.12
11.10
5
5

Calculation value (Design model)
6

6

7

8

7 power8[kW]Output

9

9

10

10

Fig. 12. Secondary/primary voltage ratio of transmission coil. Red
point is experimental results. Blue line is calculation value by (13)and (7) using actual parameters considering error from nominal value.
Green line is calculation value by (13) and (7) with design parameters.

10 kW

10 kW

5 kW

0

0

Primary voltage v1 1000 V/div

Primary voltage v1 1000 V/div

00Primary current i1 10 A/div
Primary current i1 10 A/div
Output voltage vo 500 V/div

100 ms/div0

Output voltage vo 500 V/div
0(a) Step load response from 5 kW to 10kW.

100 ms/div(b) Step load response from 10 kW to 5 kW.

Fig. 13. Operation waveforms of transient characteristic with resistance load.

Primary voltage v1 1000 V/div

Primary voltage v1 1000 V/div

00

0 Secondary DC voltage v

dc2 1000 V/div
Output voltage vuv1 1000 V/div

00
Output current iuv1 10 A/div

00

Primary current i1 10 A/div
Secondary DC voltage vdc2 1000 V/div

40 s/div(a) Primary voltage and current waveform.

20 ms/div(b) Secondary DC voltage, output voltage and current waveform.

Fig. 14. Operation waveforms with induction motor load.

5 kW

15 kW

15 kW

Secondary DC voltage vdc2 250 V/div

Secondary DC voltage vdc2 250 V/div

Primary current i1 10 A/div

Primary current i1 10 A/div

0

0

0

0

400 ms/div(a) Step load response from 5 kW to 15 kW.

5 kW

400 ms/div(b) Step load response from 15 kW to 5 kW.

Fig. 15. Operation waveforms of transient characteristic with induction motor load.

DC voltage is constant regardless of the output power byusing the SP compensation. In particular, the secondary
DC voltage is constant even when a large load fluctuation
occurs.
V. CONCLUSIONSIn this paper, the inductive power transfer system wasapplied in the existing excavator in order to reduce the firerisk which was caused by the spark of contact charging. In
the proposed system, the load voltage against the load
fluctuation should be stabilized without the additional
circuit. In order to stabilize the load voltage against theload fluctuation, the SP compensation method was applied
as the method in order to cancel out the leakage inductance.
Moreover, the constant-voltage characteristic dependingon the error of the resonance parameter and the winding
resistance is theoretically analyzed in order to obtain theconstant-voltage characteristic. In the experiments with an
output power of 15 kW, the voltage fluctuation was
smaller than 4.3%. The constant voltage is maintainedeven when the load step of the induction motor occurs, i.e.
from 5 kW to 15 kW operation and vice versa. From theexperimental results, the constant secondary DC voltage
characteristic was confirmed, i.e. the constant output
voltage.
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