IEEJ Journal of Industry Applications
Vol.8 No.3 pp.413–420
DOI: 10.1541/ieejjia.8.413

Paper

Inductive Power Transfer System for an Excavatorby considering Large Load Fluctuation
Keisuke Kusaka∗ Member, Kent Inoue∗
Jun-ichi Itoh∗a) Senior Member

Non-member(Manuscript received June 13, 2018, revised Nov. 7, 2018)

In this study, an inductive power transfer system (IPT) was designed and developed for an excavator, which will be
operated under high air pressure. In the conventional excavator systems, power is supplied through trolley wires. The
contact between the trolley wires and a pantagraph may be a reason of fire due to a spark because the excavator works
under high air pressure. In the proposed system, a series resonance capacitor on a primary side and a parallel resonancecapacitor in a secondary side is used to cancel out the leakage inductance. By using the series–parallel compensation,
the load voltage is ideally constant regardless of load fluctuation. However, the constant-voltage characteristic may
degrades when a winding resistance and an error of the resonant parameter cannot be ignored. Thus, the resonant
parameters must be designed considering the error. This paper proposes the design method of the resonant parameterwith the voltage ratio maps considering the error of the parameter including the winding resistance. The experimental
results confirmed that the voltage fluctuation is smaller than 4.3% with a developed 15-kW IPT system. Furthermore,
the constant-voltage characteristic is maintained even when the output power of an induction motor changes from 5 to
15 kW and vice versa.
Keywords: inductive power transfer, excavator, series-parallel compensation, pneumatic caisson method

1.

Introduction

Pressurized air is supplied into the working chamber for
excavation in order to prevent underground water from coming into the chamber.
A pneumatic caisson method is used for many structures:
foundations of bridges and buildings, shafts for insertion ofshield tunneling machines, tunnel and railways (1) . Figure 1
outlines a schematic of the pneumatic caisson method. In thefirst process of the pneumatic caisson method, a reinforced
caisson made of concrete is constructed on the ground. Then,an airtight working chamber is formed at the bottom of thecaisson. Finally, the caisson is immersed in a water to a predetermined depth. At same time, the pressurized air is supplied into the working chamber from the ground by a compressor in order to prevent the underground water from coming into the chamber. Owing to the pressurized air, the working chamber environment is always under high air pressure.
Figure 2 illustrates the schematic of a charging system
for the excavator, which is operated for the pneumatic caisson method. Figures 2(a) and (b) show the conventional
power supply system and the proposed system, respectively.
An excavator hangs onto the ceiling of the working chamber and moves along a traveling rail. Meanwhile, an electric hydraulic pump is used for the operation of the excavator. The power for an electric motor which drives hydraulicpump is supplied via trolley wires which are placed belong

Fig. 1. Outline of pneumatic caisson method
Pressurized air is supplied into the working chamber for excavation inorder to prevent underground water from coming into the chamber.

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 the contacts. Even a small spark may cause alarge-scale fire because the working chamber is under highair pressure environment. Therefore, in order to reduce therisk of the fire which is caused by the spark, an IPT system
for the excavator has been proposed. The IPT system transmits power without electrical contact. Thus, there is a norisk to have a spark as long as electric breakdown does nota) Correspondence to: Jun-ichi Itoh. E-mail: itoh@vos.nagaokaut.
ac.jp∗
Nagaoka University of Technology
1603-1, Kamitomioka, Nagaoka, Niigata 940-2188, Japanc 2019 The Institute of Electrical Engineers of Japan.
413

Development of IPT System for Excavator（Keisuke Kusaka et al.）(a) Conventional charging system(b) Proposed charging system

Fig. 2. Power supply systems for excavator hanging onto ceilingFig. 3. Inductive power transfer system for excavator

occur. Note that, the breakdown voltage in high-pressurized
air environment is higher than the atmosphere according toPaschen’s law.
Besides, as the topology of the IPT system, a seriesseries compensation method and a series-parallel compensation methods are well known (2)–(4) . In the series-series compensation method, the resonant capacitors are connected into
the primary coil and the secondary coil in series. The secondary side behaves a constant-current characteristic by theseries-series compensation method when the constant voltage
source is used on the primary side (2)–(4) .
Therefore, the additional 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 seriesseries compensation method, the high-speed communicationis needed between the primary side and the secondary side.
On the other hand, the series-parallel compensation method,which connects the capacitors to the primary side in series
and to the secondary side in parallel provide the constant output voltage characteristic without the additional circuits (5) (6)as long as the winding resistance and parameter error forthe resonance can be ignored. However, the parameter error should be considered because inductor and capacitor typically have an error. Moreover, the winding resistance cannot
be ignored in such high-power IPT system.
In this paper, the IPT system (7)–(14) , which accepts large load
fluctuation and resonant parameter error, is developed for the
excavator. In the proposed system, the series-parallel compensation is employed in order to cancel out the leakage inductance. Moreover, an output voltage is ideally maintained

at constant even under the large load fluctuation without theadditional circuit. However, the constant-voltage characteristic degrades due to the winding resistance and the error of
the resonant parameter. The output voltage change has to be
discussed because the load power dynamically fluctuates in
the IPT system for the excavator. Thus, this paper analyzes
the output voltage characteristic of the series-parallel compensation method by considering the circuit parameter error
and the winding resistances.
The new contribution is providing an analysis how to develop the robust system against the voltage fluctuation in thispaper. The analysis takes into account the relationship among
the load fluctuation, the resonant parameter and the winding
resistance. Based on the analysis on the voltage characteristic, a 15-kW IPT system is developed and tested. The constant load voltage characteristic is evaluated using a resistance and an induction motor load.
2.

Inductive Power Transfer System for Excavator

2.1 System Configuration
Figure 3 shows the system configuration of the IPT system for the excavator. The
PWM converter as a grid-tied converter and a single-phase
inverter are in the primary side. A single-phase rectifier,a brake unit, three-phase variable frequency drive (VFD)
inverters and a single-phase constant voltage constant frequency (CVCF) inverter are in the secondary side.
The three-phase VFD inverters, which are operated withthe constant voltage per frequency control, are used for adjustable speed drive of induction motors for electric hydraulic
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IEEJ Journal IA, Vol.8, No.3, 2019

Development of IPT System for Excavator（Keisuke Kusaka et al.）pumps. The rated synchronous rotational speed of the induction motors are 1800 rpm (at 60 Hz). The brake unit is connected at DC side on the secondary side. The IGBT on thebrake unit turns on when the secondary side voltage reaches
a threshold of the overvoltage detection to decrease the secondary DC voltage for a protection. The brake unit is normally oﬀ, however, it will work when the regeneration current comes to the secondary DC side to avoid the breakdownof the secondary circuit. The single-phase CVCF inverter isused as an auxiliary supply, which is needed for the controlof the excavator because the existing control circuit of the excavator, which has been operated by AC 400 V, is used. The
output voltage of the single-phase CVCF inverter is 400 V at
60 Hz. The rated power of the CVCF inverter is 1.5 kW.
The power is wirelessly supplied from the primary side andthe secondary side. The series-parallel compensation is employed in the system. Therefore, the secondary DC voltage
is constant in the ideal conditions because the secondary side
output features a constant-voltage characteristic regardless ofa load fluctuation when the primary side is driven at constantvoltage. However, an error of the resonant parameter and thewinding resistance practically degrade the constant-voltage
characteristic.
2.2 Compensation Method
Figure 4 shows the typical compensation methods, which connects resonance capacitors to the primary side coil and the secondary side coil inseries and parallel, respectively, in order to cancel out thereactive power (15) . Reactance by leakage inductance are canceled out because of the resonance with the transmission coil.
Consequently, the fundamental power factor from the primary side is unity.
Figure 4(a) shows the series-series compensation method,which features a constant current characteristic at the secondary side when the primary side is driven by a constantvoltage. Therefore, the series-series compensation is unsuitable for the existing system because the additional circuit isneeded in order to convert the constant current characteristic into the constant-voltage characteristic. In addition, theseries-series compensation is unsuitable for the existing system because a high-speed communication is needed between
the primary side and the secondary side.
On the other hand, Fig. 4(b) shows the series-parallel compensation method, which features a constant voltage characteristic at the secondary side, when the primary side is driven
by a constant voltage. When the winding resistance can beignored, the voltage gain from the input and the output is expressed by(a) series-series compensation.

(b) series-parallel compensation.

Fig. 4. Typical compensation methods to compensate aleakage inductance due to weak magnetic coupling.

Note that, the derivation of these equations will be shownin the next chapter.
When the winding resistances r1 and r2 can be ignored, andthe resonant conditions are satisfied, Eq. (1) can be simplified
into (16)
v2 1 L2=· · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · ·(3)v1 k L1
It means that the secondary voltage will be constant regardless of the load when the winding resistances can be ignored,
and the resonant capacitors C1 and C2 are in resonant condition. Therefore, the series-parallel compensation is suitable
to be applied to the proposed system because the IPT is possible to be applied into the existing excavator system with asmaller number of codifications.
However, the error of the resonant parameter needs to beconsidered because the resonant capacitors and transmission
coils typically have an error as against the ideal value for the
resonance. In addition, the winding resistances cannot beignored in the high-power IPT system because the winding
resistance causes a large voltage drop regarding to the output power, which is determined by the operation of hydraulic
pump.
3.

3.1 Specifications
Figure 5 shows the transmission
coils. The primary coil is placed in the upper side. On theother hand, the secondary coil is on the excavator. Besides,in the proposed system, the solenoid coil is employed to obtain higher magnetic coupling in comparison with a circularcoil. The cores are PC40 manufactured by TDK. The sizeof the core is W237 × H210 × D20 mm. Meanwhile, thetransmission distance is 50 mm. The transmission distanceis determined so that the secondary coil does not contact the
primary coil when the excavator moves along the rail.
3.2 Parameter Design
Figure 6 shows the equivalentcircuit for designing the IPT system. The circuit equations of
the equivalent circuit, which is shown Fig. 6 are calculated as1V1 = r1 + jω L1 − 2
I1 − jωLm I2 · · · · · · · (4)
ω C1



1
1
I3
I2 + j0 = − jωLm I1 + r2 + jω L2 − 2
ωC2ω C2· · · · · · · · · · · · · · · · · · · · (5)v2 Req ω2 Lm (L1 − Lm )· · · · · · · · · · · · · · · · · · · · · · · · ·(1)=v1Δ
where Req is the equivalent AC resistance, r1 and r2 are thewinding resistances, Lm is the mutual inductance, and Δ is





r + jω L − 1
0
−
jωL
1
m

 1
2C
ω
1




1
1

Δ = 
−
jωL
r
+
jω
L
−
j
m
2
2

ωC2 
ω2 C 2

1
1 

0
j
Req − jωCωC 
2Design of Transmission Coils

2· · · · · · · · · · · · · · · · · · · · · · · · · (2)415

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Development of IPT System for Excavator（Keisuke Kusaka et al.）

where P2 is the output power and V dc,2 is the DC voltage on
the secondary side.
The secondary inductance should be designed according to
L2 =

Req
k
· · · · · · · · · · · · · · · · · · · · · · · · · · · · · · (12)√
ω 1 + k2

where V dc,1 is the DC voltage on the primary side and k is thecoupling coeﬃcientEq. (13) is calculated with (12).
2
8 Vdc,1L1 = L2 2· · · · · · · · · · · · · · · · · · · · · · · · · · · (13)π k Vdc,2Fig. 5. Transmission coils which is used core with PC40
manufactured by TDK. Upper side is primary coil, lower
side is secondary coil. Coil size is W237 × H210 ×D20 mm. Transmission distance is 50 mm

The compensation capacitors C1 and C2 on the primaryside and the secondary side are respectively designed in orderto cancel out the reactive power at the input frequency. The
value of the compensation capacitors is calculated by (14),(15)
1· · · · · · · · · · · · · · · · · · · · · · · · · · · · (14)
ω2 L1 1 − k2
1
C2 = 2 · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · (15)ω L2
3.3 Influence of Parameter Error and Winding Resistance
Figure 7 shows the voltage ratio v2 /v1 against theerror of the resonant parameter including the winding resistance. Figure 7 represents the v2 /v1 ratio which is calculatedby (13) and (10). The vertical axis and the horizontal axis arenormalized by the designed value calculated by Eqs. (12)–(15). Figures 7(a) and (b) show the v2 /v1 ratio against theerror of L1 and C1 and the error of L2 and C2 from 80% to120%, respectively.
C1 =Fig. 6. Equivalent circuit of IPT system with seriesparallel compensation

0= j



1
1
I2 + Req − j
I3 · · · · · · · · · · · · · · · · · · (6)ωC2
ωC2

where L1 is the primary inductance, L2 is the secondary inductance, C1 is the primary compensation capacitor, C2 is thesecondary compensation capacitor, Lm is the mutual inductance, V 1 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 of the secondary winding, and ω is the angular frequency of the power
supply.
The currents I 1 , I 2 and I 3 are calculated by (7), (8) and (9)when an input voltage V 1 is applied into the primary side.

v2 Req ω2 Lm (L1 − Lm )· · · · · · · · · · · · · · · · · · · · · · · (16)=v1
Δ
As a result of Fig. 7(a), the v2 /v1 is sensitive to the secondary inductance L2 even when the secondary capacitance
C2 is maintained at 100%. Meanwhile, the sensitivity of the
primary inductance L1 on v2 /v1 is mitigated when the primarycapacitance is maintained at 100%. By considering the parameter change of the inductance, the precise primary capacitance and secondary inductance are required for the constantvoltage characteristic on the secondary side.
3.4 Influence of Parameter Error and Winding Resistance
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.

V1[{r2 + j(x2 + x0 − x p )}(Req − jx p )+ x2p ] · · · · · · · (7)
Δ
V1
I2 =
jx0 (Req − jx p ) · · · · · · · · · · · · · · · · · · · · · · · · · · (8)ΔV1x0 x p · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · (9)
I3 =
Δ




r + jω L − 1

0
−
jωL
1
m
 1

2C
ω
1




1
1

Δ = 
−
jωL
r
+
jω
L
−
j
m
2
2

ωC2 
ω2 C 2

1
1 

0
j
Req − jωC2
ωC2 
· · · · · · · · · · · · · · · · · · · (10)
I1 =

4.

Experimental Results

4.1 Experimental Conditions
Table 1 shows the parameters of the prototype. The inductances in Table 1 are themeasured value including an error. In this test, a resistance oran induction motor is used as the load.
4.2 Resistance Load
Figure 8 shows the operation
waveforms of the developed system with the resistance load.
Figures 8(a) and (b) show the input and output waveforms of
the IPT system at an output power of 5 kW and 10 kW, respectively. In the figure, the secondary voltage is constant

In (10), V 1 is the fundamental component of the output
voltage of the inverter.
The parameters of the transmission coil are designed withthe equivalent circuit. The resistance Req indicates that equivalent load resistance of the full-bridge rectifier. Then theequivalent load resistance is given by Ref. (16).
2π2 Vdc,2
Req =· · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · (11)
8 P2
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Development of IPT System for Excavator（Keisuke Kusaka et al.）(a) Error of primary inductance L1 and primary capacitor C1 .

(b) Error of secondary inductance L2 and secondary capacitor C2 .

Fig. 7. v2 /v1 ratio against error of resonance parameter including winding resistance(a) Load of 5 kW.

(b) Load of 10 kW.

Fig. 8. Operation waveforms with resistance load(a) Voltage gain.

(b) Input admittance.

Fig. 9. Frequency characteristics of input-output voltage and input admittance gain
Table 1. Experimental conditionsin the light load. Thus, the primary voltage and current waveforms are misaligned from the resonance point.
Figure 9 shows the frequency characteristics of the voltage
gain. Figure 9(a) is the voltage ratio of the input voltage tothe output voltage. Figure 9(b) is the frequency characteristic of the input admittance. The frequency characteristics
are obtained by a simulation to evaluate the eﬀect of harmonics components. Around the fundamental frequency of thein Fig. 9(a), the gain is same value regardless of the output
power. Meanwhile, the input admittance in Fig. 9(b) at thefundamental frequency depends on the output power in spiteof the constant admittance on the low order harmonics frequency. This frequency characteristic cause the large current
distortion at light-load region.
Figure 10 shows the frequency characteristics of the input
admittance. The admittance of the fundamental component
becomes smaller at light load, which has been expected fromthe frequency characteristics which is shown in Fig. 9(b).

against the load power. Although the resonance condition isadequate, the low-order harmonics aﬀect the primary current
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Development of IPT System for Excavator（Keisuke Kusaka et al.）Fig. 12 represents the calculation value by (13) and (7) using
actual parameters considering the error from a nominal value.
The green line in Fig. 12 represents calculation value whichis 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 (lessthan 0.5%). The error is caused by the diﬀerence between thenominal values and the actual values in the process of the resonant parameter design. Nevertheless, the secondary/primaryvoltage ratio characteristic is constant regardless of the output power. Therefore, it is confirmed that the decrease in the
output/input voltage ratio at the high output power is mainly
caused by the post-stage conversion, i.e., the rectifier.
Figure 13 shows the operation waveforms of the transientcharacteristic with the resistance load. Figures 13(a) and (b)show the step load response from 10 kW to 5 kW, and viceversa, respectively. The secondary DC voltage is maintained
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 series-parallel compensation as a leakage inductance canceling method. In particular, the secondary DC voltage is constant even when alarge load fluctuation occurs.
4.3 Excavator (Induction Motors)
Figure 14 shows
the operation waveforms with the induction motor loads,
which are connected to the pump. The output power is 15 kW.
Figure 14(a) shows the primary voltage and current waveforms whereas Fig. 14(b) shows the secondary DC voltage, output voltage, and current waveforms. As shown in
Fig. 14(a), the fundamental power factor of the inverter output is almost unity. The power is transmitted at 20 kHz.
Moreover, the primary current is confirmed that the waveform distortion is small because the fundamental components
is relatively large, i.e., the influence of low-harmonics components is relatively small. Figure 14(b) shows the output
voltage and current of the three-phase inverter. The output
frequency of the three-phase inverter is 60 Hz because theinduction motor is drive at a rated speed 1800 rpm with thenumber of poles of four.
Figure 15 shows the operation waveforms of the transientcharacteristic in the induction motor load. Figures 15(a) and(b) show the step response from 5 kW to 15 kW and viceversa, respectively. Due to the load change, the output currentof the proposed IPT system is increased. It means that theoutput three-phase currents increase because the three-phase
inverter is operated with the constant voltage per frequency
control at rated speed. The secondary DC voltage is maintained at the constant value even when the load step occurs.
The voltage fluctuation owing to the load change is less than
4.3%. Consequently, it is confirmed that the secondary DC
voltage is constant regardless of the output power by using
the series-parallel compensation. In particular, the secondary
DC voltage is constant even when a large load fluctuation occurs.

Fig. 10. Frequency characteristics of primary admittanceFig. 11. Voltage ratio between primary side and secondary side

Fig. 12. Voltage gain between the primary voltage andsecondary voltage

Therefore, the distortion of the primary current at 5 kW islarger than the waveforms at 10 kW because low-order harmonics component of the primary current are constant.
Figure 11 shows the voltage ratio of the primary DC voltage to the secondary DC voltage against the output power.
The blue line in Fig. 11 represents the calculation value,
which is calculated without a consideration of the resistancecomponents in the rectifier. In the proposed system, the voltage ratio of the secondary and the primary voltage is expected to be constant because the series-parallel compensation is employed in order to cancel out the leakage inductance. However, the voltage ratio of the primary DC voltage
to the secondary DC voltage ratio decreases by 1.9% when
the output power increases because the eﬀect of voltage drop
which is caused by the winding resistance is large at the highoutput power.
Figure 12 shows the voltage ratio of the primary to secondary voltage against the output power. The blue line in5.

Conclusion

In this paper, the inductive power transfer system was applied into the existing excavator in order to reduce the firerisk which is caused by the spark of contact charging system. In the proposed system, the load voltage against the418

IEEJ Journal IA, Vol.8, No.3, 2019

Development of IPT System for Excavator（Keisuke Kusaka et al.）(a) Step load response from 5 kW to 10 kW.

(b) Step load response from 10 kW to 5 kW.

Fig. 13. Operation waveforms of transient characteristic with resistance load(a) Primary voltage and current waveform.

(b) Secondary DC voltage, output voltage and current waveform.

Fig. 14. Operation waveforms with induction motor load(a) Step load response from 5 kW to 15 kW.

(b) Step load response from 15 kW to 5 kW.

Fig. 15. Operation waveforms of transient characteristic with induction motor loadconfirmed, i.e. the constant output voltage.

load fluctuation should be stabilized without the additional
circuit. In order to stabilize the load voltage against the load
fluctuation, the series-parallel compensation method has been
applied as the method in order to cancel out the leakage inductance. Moreover, the constant-voltage characteristic depending on the error of the resonant 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 maintained even when
the load step of the induction motor occurs, i.e. from 5 kW
to 15 kW operation and vice versa. From the experimental
results, the constant secondary DC voltage characteristic was

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Jun-ichi Itoh (Senior Member) was born in Tokyo, Japan, in 1972. Hereceived his M.S. and Ph.D. degree in electrical andelectronic systems engineering from Nagaoka University of Technology, Niigata, Japan in 1996, 2000,
respectively. From 1996 to 2004, he was with Fuji
Electric Corporate Research and Development Ltd.,Tokyo, Japan. He was with Nagaoka University ofTechnology, Niigata, Japan as an associate professor.
Since 2017, he has been a professor. His research interests are matrix converters, dc/dc converters, power
factor correction techniques, energy storage system and adjustable speed
drive systems. He received IEEJ Academic Promotion Award (IEEJ Technical Development Award) in 2007. In addition, he also received Isao Takahashi Power Electronics Award in IPEC-Sapporo 2010 from IEEJ, 58th
OHM Technology Award from The Foundation for Electrical Science andEngineering, November, 2011, Intelligent Cosmos Award from Intelligent
Cosmos Foundation for the Promotion of Science, May, 2012, and Third
prize award from Energy Conversion Congress and Exposition-Asia, June,
2013. Prizes for Science and Technology (Development Category) from theCommendation for Science and Technology by the Minister of Education,
Culture, Sports, Science and Technology, April 2017. Dr. Itoh is a senior
member of the Institute of Electrical Engineers of Japan, the Society of Automotive Engineers of Japan and the IEEE.

Keisuke Kusaka (Member) received his B.S. and M.S. degrees from
Nagaoka University of Technology, Niigata, Japan in
2011, 2013, respectively. From 2015 to 2016, he waswith Swiss Federal Institute of Technology in Lausanne (EPFL), Switzerland as a trainee. In 2016,
he received his Ph.D. degree in energy and environment science from Nagaoka University of Technology. From 2016 to 2018, he was with Nagaoka University of Technology as a researcher. He is currently
an assistant professor at Nagaoka University of Technology. His current research interests include an inductive power transfer
system and high-frequency converters. He received the second prize paper
award in IPEC-Niigata 2018.

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IEEJ Journal IA, Vol.8, No.3, 2019