The 2014 International Power Electronics Conference

Excitation System by Contactless Power Transfer
System with the Primary Series Capacitor Method
Ryosuke Nozawa, Ryota Kobayashi, Hikaru Tanifuji, Yasuyoshi Kaneko, Shigeru Abe
Department of Electrical and Electronic Systems
Saitama University
Saitama, Japan
high-frequency magnetic field generated by a contactless
power transfer system causes iron loss of the magnetic
materials surrounding the transformers and decreases thetransmission efficiency.
In this paper, we investigate the resonant capacitor
topology for leakage reactance compensation and the omissionof the smoothing capacitor, considering miniaturization andweight reduction of the secondary side. In addition, we also
investigate the magnetic shielding of the shaft.
For the resonant capacitor topology, we examine thecharacteristics of the contactless power transfer system with a
primary series resonant capacitor (S topology) and comparewith the series and parallel capacitor topology (SP topology).
In a contactless power transfer system with a large air gap for
an electric vehicle, the SP topology is often used for leakage
reactance compensation [6], [7]. On the other hand, the S
topology is able to omit the secondary capacitor and achieve
miniaturization and weight reduction of the secondary circuit.
Using the S topology, we derive equations to calculate thevalue of the primary series capacitor COS, the maximum
transformer efficiency ηTRmax, and its load resistance RLmax.
For omission of the smoothing capacitor, the inductance ofthe excitation windings around the rotor of the synchronous
motor is used. The effectiveness of an aluminum sheet has been
described to magnetically shield the motor shaft [8]. In order todetermine the most effective shape for the aluminum shield for
the S topology, we performed simulations and power transfer
experiments.

Abstract—For power transfer to the rotor circuit ofexcitation-type synchronous motors, we propose a contactless
power transfer system with a primary series capacitor (Stopology) to compensate for the leakage reactance. This systemdoes not contain secondary resonant and smoothing capacitorsand can reduce the number of components in the rotor circuit.
The value of the primary resonant capacitor is determined suchthat the input power factor is set to one for the operating
frequency. The rotating shaft of the rotor is covered with analuminum sheet to reduce the loss due to the leakage flux. Powertransfer tests were performed. A high efficiency of 91.8% wasachieved when power was transferred to the rotor circuit of thesynchronous motor.
Keywords—contactless power transfer system, electromagnetic
shielding, rotary transformer, synchronous motor

I.

INTRODUCTION

We study an excitation-type synchronous motor using acontactless power transfer system for the partial developmentof a variable magnetic flux motor. Conventionally, slip rings
and a brushless excitation system have been used as theexcitation method in excitation-type synchronous motors andgenerators. The slip ring is limited by the rotational speed andcreates dust and wear during sliding because it has contact
points. On the other hand, a brushless excitation system does
not have contact points, but it is not able to excite the rotor at
the start of operation. To overcome these problems, excitation
using a contactless power transfer system has been proposed[1]–[5]. A contactless power transfer system has no limit to
the rotational speed and does not create dust, wear, sparks, or
contact failure. Further, the contactless power transfer system
has the advantages of being clean and maintenance free and isable to excite the rotor of motor at the start of operation.
A contactless power transfer system using a rotarytransformer has been used in a video deck, resolver, etc.;
however, these systems are for dealing with small power suchas signal transmission. When developing a contactless powertransfer system for an excitation-type synchronous motor, it isnecessary to consider miniaturizing and reducing the weight ofthe secondary circuit and loss due to the shaft of magnetic
material. It is necessary for the secondary circuit to be small,lightweight, and maintenance free for mechanical strength andmaintainability if it is placed in the interior of the rotor. A©2014 IEEJII.

CONTACTLESS POWER TRANSFER SYSTEM

A. Series and Parallel Resonant Capacitor Topology (SPTopology)
Fig. 1(a) shows a schematic diagram of the contactless
power transfer system for the SP topology. A full-bridge
inverter is used as a high-frequency (f0 = 50 kHz) power
supply. Fig. 1(b) shows a detailed equivalent circuit. The
primary values are converted into secondary equivalent values
using the turns ratio a = N1/N2. Because the winding
resistances and ferrite-core loss are considerably lower thanthe mutual and leakage reactances at the resonant frequency,
the winding resistances (r'1 and r2) and the ferrite-core loss r'01115

The 2014 International Power Electronics Conference
VDC IDC

VIN IINI2

ID V2

VDC IDCVL IL

VIN IIN

CS
Regulated
DC Power
SupplyR

Regulated
DC Power
Supply

L

L

ηSP

ηS

P1

P2

P1

(a) Schematic diagram.

V'IN I'IN V'1 r'1

jx'1 V'0 jx2

-jx'S

I'0

r2

V2 IL
I2

r'0
-jxP

V'IN I'IN V'1 r'1

jx'1 V'0 jx2-jx'OS

I'0

V'0

V2 IL
I2

r'0
RL
jx'0

(b) Detailed equivalent circuit.

jx2

V'IN I'IN V'1

V2 IL
I2

I'0

-jx'S

r2

RL

(b) Detailed equivalent circuit.

jx'1

P2

(a) Schematic diagram.

jx'0

V'IN I'IN V'1

VL IL

COS

R

CP

I2 V2

jx'0

-jxP

jx'1

V'0

jx2

V2 IL
I2

I'0

-jx'OS

jx'0

RL

RL

(c) Simplified equivalent circuit.

(c) Simplified equivalent circuit.

Fig. 1. Contactless power transfer system for the SP topology.

Fig. 2. Contactless power transfer system for the S topology.

can be ignored. Fig. 1(c) shows the simplified equivalentcircuit that ignores r'1, r'2, and r'0 from the detailed equivalentcircuit.
To achieve resonance of the input frequency f0 (=ω0/2π)
with the self-inductance of the secondary winding L2, which isequivalent to adding a mutual reactance x'0 and a leakage
reactance x2, the secondary parallel capacitor CP is given by
1= xP = x0′ + x 2
ω 0CP

η SP =

1= xS′ =

RLmaxSP = xP

(1)x0′ x2+ x1′x0′ + x2

x0′
x0′ + x2

1 r1′
+1
b 2 r2

(4)

(5)

RL

ηmaxSP =1+(6)

2r2 1 r1′
+1
xP b2 r2

The coupling factor k, the quality factor of the primarywinding Q1, and the quality factor of the secondary winding
Q2 are expressed as
ωLωLM, Q1 = 1 , Q2 = 2
k=
(7)
r
r

(2)

L1 L2

The input voltage V'IN and the input current I'IN can be
expressed as′ = I L / b, b =VIN′ = bV2 , I IN

1⎧⎪ ⎛ R ⎞ 2 ⎫⎪r1′
R L + 2 + r2 ⎨1 + ⎜⎜ L ⎟⎟ ⎬b⎪⎩ ⎝ x P ⎠ ⎪⎭The maximum transformer efficiency ηmaxSP is obtained
when RL = RLmaxSP.

The value of the primary series capacitor CS (C'S denotes its
secondary equivalent) is determined when the imaginary partof the impedance is zero, and the inverter output power factorof the fundamental wave is to be one. CS is given byω 0CS′RL I L2=′ 2 + r2 I L2
RL I L2 + r1′I IN

1

2Then, the equations for RLmaxSP and ηmaxSP can be expressed interms of k and Q.

(3)

rQ
RLmax = 2 2kThese equations show that the equivalent circuit of atransformer with these capacitors is the same as an idealtransformer at the resonant frequency.
Ignoring r'0, the efficiency of the transformer for the SP
topology in Fig. 1(b) is defined by

ηmaxSP =

1116

Q2
+ k2
Q1
1

2 1
k2
1+
+ 2
k Q1Q2 Q1(8)(9)

The 2014 International Power Electronics Conference

20mm
20mm

30mm

(b) Transformer dimensions.

(a) External form.
Fig. 3. Rotary contactless power transformer.
TABLE I. TRANSFORMER SPECIFICATIONS
Rated power
Mechanical gap
Primary
Windings wires
SecondaryPrimary
WeightSecondaryLitz wire
Ferrite core

TABLE II. TRANSFORMER PARAMETERS

1.0 kW
2 mm
24T1p
10T1p
890 g
610 g
0.1 mmφ × 800
TDK PE90

Situation
f0 [kHz]gap [mm]
r1 [mΩ]
r2 [mΩ]
l0 [μH]l1 [μH]l2 [μH]k
Q1
Q2
CS [μF]CP [μF]
RLmaxSP [Ω]ηmaxSP [%]SP

B. Primary Series Capacitor Topology (S Topology)
Fig. 2(a) shows a schematic diagram of the contactless
power transfer system for the S topology. A full-bridge inverter
is used as a high-frequency (f0 = 50 kHz) power supply. Fig.
2(b) shows a detailed equivalent circuit, and Fig. 2(c) shows
the simplified equivalent circuit ignoring r'1, r'2, and r'0 fromthe detailed equivalent circuit.
For the simplified equivalent circuit in Fig. 2(c), theimpedance seen from the primary input is given by
Z=

[

]

⎧⎪ x0′ RL2 + x2 (x0′ + x2 )⎫⎪RL x0′2j++ x1′ ⎬⎨
2
2
2
RL + (x0′ + x2 )⎪⎩ RL + (x0′ + x2 )⎪⎭ηSP [%]*(10)

The value of the primary series capacitor COS is determinedwhen the imaginary part of the impedance is zero, and theinverter output power factor of the fundamental wave is to beone. COS is given by[]

1
x′ R 2 + x2 (x0′ + x2 )′ = 0 2L
= xOS
+ x1′
2
′
ω0COS
RL + ( x0′ + x2 )

S

(11)

Therefore, the relationship between the input and theoutput is expressed as
′ =−j
I IN

x′ + x2
1
VL + 0IL
x0′
x0′

Ignoring r'0, the efficiency of the transformer for the S
topology in Fig. 2(b) is defined as[]ηS [%]*

pf1*
pf2** experimental value( ) Calculated value of efficiency
pf: power factor

Without
shaft
2
88.6
19.7
185.8
48.8
8.97
0.79
832
657
0.114
0.246
19.53
99.54
―
―
―
0.071
17.3
99.48
―
―
―

The maximum transformer efficiency ηmaxS is obtained when
RL = RLmaxS.

(12)

RL I L2
RL
ηS =
=
2
r1′ 2
′ 2 + r2 I L2
2
RL I L + r1′I IN
RL + (x0′ + x2 ) + RL + r2
x0′ 2

pf1*
pf2*
COS [μF]
RLmaxS [Ω]ηmaxS [%]

With shaft andaluminum sheet (1.5 mm)
50
1
2
3
188.1
103.3
85.6
6.87
23.3
28.6
123.8
110.1
101.1
60.8
63.2
68.7
8.46
8.80
8.80
0.70
0.67
0.64
308
527
623
1370
376
289
0.106
0.103
0.099
0.338
0.363
0.384
30.1
14.26
12.2
99.54
99.14
98.99
97.16
96.95
97.2
(99.42)(99.14)(98.97)
0.88
0.91
0.93
0.63
0.62
0.62
0.071
0.072
0.071
9.91
11.13
11.28
98.60
98.91
98.91
97.67
97.16
96.35
(98.60)(98.90)(98.90)
0.82
0.81
0.77
0.87
0.86
0.86

RLmaxS =1

ηmaxS =

(13)

′2
r1′

(x0′ + x2 )2 + x0 r2

1+

2r1′
x0′2

′2
r1′

(x0′ + x2 )2 + x0 r2

(14)
(15)

The equations for RLmaxS and ηmaxS expressed in terms of k and
Q are as follows:

1117

The 2014 International Power Electronics Conference

Bush
Shaft
Aluminum sheet

Ferrite 2(b) Without an aluminum sheet.

Coil 1
Coil 2Ferrite 1(a) Revolved section of the transformer.

(c) With an aluminum sheet (thickness: 0.5 mm).
Fig. 4. Calculated results for the magnetic field analysis.

Joule loss density

Joule loss density(b) Aluminum sheet with a thickness of 1.5 mm.

(a) Aluminum sheet with a thickness of 0.5 mm.

Fig. 5. Calculated results for the Joule loss density.

2

RLmaxS = kr2 Q1Q2 +ηmaxS =

III.

Q2
k2

1
2 1
1
+
1+
k Q1Q2 k 2Q12

(16)
(17)

MAGNETIC SHIELDING OF THE SHAFT

A. Rotary Contactless Power Transformer
Fig. 3(a) shows the exterior of the rotary contactless powertransformer, and Fig. 3(b) shows the dimensions of thetransformer. Table I summarizes the specifications of thetransformer. The cores are made of ferrite, and Litz wires (0.1mmφ × 800) are used for the windings. The gap of between
the primary side and the secondary side of the transformer is 2

Fig. 6. 8 kW synchronous motor.

1118

The 2014 International Power Electronics Conference

-10

200

40

100

20

0

0

-100

-20

-200
0

-40
0.05 0.06

0.01 0.02

0.03 0.04
Time[ms]

-10

-100

VIN
IIN

IIN[A]

VIN[A]

0

VIN
IIN

-20
100

20
10

0

0

ID[A]

-100

10

0

V2[V]

0

IIN[A]

0

100

ID[A]

VIN[V]

100

V2[V]

20

10

-10

V2
ID

-20

-100
0

0.01

0.02 0.03 0.04
Time[ms]

0.05

V2
ID

0.06

(b) S topology (RL = 10 Ω).

(a) SP topology (RL = 15 Ω).
Fig. 7. Input and output waveforms.

mm. Table II lists the parameters of the transformers measured
by an LCR meter. The value of resonant capacitors CP, CS, and
COS are calculated from (1), (2), and (11), respectively.
Because the efficiency of the contactless power transfer
system is altered by changing the gap, we also measured thetransformer parameters of for gaps of 1 mm and 3 mm.
B. Magnetic Shielding of the Shaft Using an Aluminum Sheet
For a large synchronous motor, it is necessary to support
both ends of the shaft of the rotor. Therefore, by placing thetransformer inside of the synchronous motor, the shaft isnecessary to penetrate the center of transformer. The 8 kW
synchronous motor in Fig. 6 uses a carbon steel (S45C) shaft
with a diameter of 25 mm. The primary transformer(transmitter) has a gap of 2.5 mm between the outer diametersof the shaft. The secondary transformer (receiver) is fastened
to the shaft by bushing made of carbon steel (S45C).
A high-frequency magnetic field generated by thecontactless power transfer system causes iron loss of themagnetic materials surrounding the transformers, and thetransformer efficiency decreases [5], [8]. Therefore, weinvestigated the effectiveness of an aluminum sheet formagnetically shielding the shaft in this system. Fig. 5 shows
the results of magnetic field analyses of the shaft with andwithout an aluminum sheet, as analyzed by the magnetic field
analysis software “JMAG.” The thickness of the aluminum
sheet is 0.5 mm, which is thicker than the skin depth at 50 kHz.
The leakage flux into the shaft from the transformers is
reduced by the aluminum sheet. Thus an improvement intransformer efficiency can be realized when reducing the loss
due to the leakage flux.
Furthermore, we carried out a magnetic field analysis of the1119

shaft with a 1.5-mm-thick aluminum sheet to reduce theleakage flux into the pointed end of the bushing. Fig. 6 shows
the calculated results for the Joule loss density of the bushing.
We confirmed that the loss of the pointed end of the bushing is
reduced by increasing the thickness of the aluminum sheet.
We performed 1 kW power transfer experiments with the S
topology to confirm the effectiveness of the aluminum sheet.
The inverter output frequency f0 was 50 kHz, and thethicknesses of the aluminum sheets are 1.5 mm and 0.5 mm.
The smoothing capacitor and load resistance (10 Ω) areconnected to the rectifier. From the results of the experiments,
the transformer efficiencies are 97.0%, 95.3%, and 92.3% witha 1.5-mm-thick aluminum sheet, with a 0.5-mm-thickaluminum sheet, and without an aluminum sheet, respectively.
These results mean that the transformer efficiency is improvedby the presence of the aluminum sheet for magneticallyshielding the shaft, and the loss of the pointed end of thebushing is reduced by using a thicker aluminum sheet. Thisshielding method is effective for reducing the loss and leakage
flux into the shaft in this system.
IV.

EXPERIMENTAL RESULTS

A. S Topology Versus SP Topology
In the S topology, the value of Cos is altered by load
fluctuations, and the power factor of the primary side (inverter
output) is lower. However, the load of the contactless powertransfer system is constant for power transfer to the excitation
windings of the rotor. Therefore, the S topology is useful inthis case. In addition, the S topology has advantages such ascompactness and no maintenance on the secondary side.
We performed 1 kW power transfer experiments for the S

The 2014 International Power Electronics Conference

98

98

96

96

experimental(S)experimental(SP)calcutated(S)calculated(SP)

94
92
90
0

η[%]

100

η[%]

100

5

10
15
RL[Ω]

20

94
92
90
0

25

Fig. 8. Efficiency as a function of the load resistance.

1000
rotational speed[rpm]2000

Fig. 9. Characteristics for changes in the rotational speed.

and SP topologies. The inverter output frequency f0 was 50
kHz, and the thicknesses of the aluminum sheets were 1.5 mm.
The smoothing capacitor and load resistance were connected
to rectifier. Fig. 7 shows the results of experiments. In the SP
topology, V2 is a sine wave, and ID flows into the rectifier
when VL is larger than the voltage of the smoothing capacitor.
Therefore, the power factor of the secondary side is low
because the time that ID flows into the rectifier becomes short.
On the other hand, I2 flows into the rectifier at all times
because V2 of the S topology is a rectangular wave. When thesystem is connected to the smoothing capacitor and load
resistance, the power factor of the secondary side of the S
topology is higher than that of the SP topology.

VIN

50V

IIN

6A

V2

100V
3A

ID
VL

100V

IL

3A

B. Characteristics with a Change in the Gap Length (STopology Versus SP Topology)
The gap between the transformers might be changed byvibration during rotation. In order to confirm thecharacteristics with a change in the gap length, we performed
1 kW power transfer experiments for a change in the gap
length with the S and SP topologies. The inverter output
frequency f0 was 50 kHz, and the thickness of aluminum sheet
was 1.5 mm. The smoothing capacitor and load resistance (10Ω) were connected to rectifier. The gap is varied from 1 mm
to 3 mm.
The results of experiments are listed in Table II. The
experimental values of the transformer efficiency are lower
than the calculated values because the iron loss was ignored.
However, the transformer efficiencies of the two topologies
are approximately constant when the gap changes from 2 mm.

0

10

20
30
time [μs]4050Fig. 10. Waveforms connected to the inductive load.

Figure 8 shows the transformer efficiency as a function of
the load resistance. The experimental values for thetransformer efficiency of the S and SP topologies are similar
to the calculated curves. The experimental values of thetransformer efficiency for the two topologies are at an
approximately equal level. RLmax of the S topology is smaller
than that of the SP topology.
D. Characteristics with a Change in the Rotational Speed (STopology)
In order to confirm the characteristics for a change in therotational speed, we performed 1 kW power transfer
experiments for rotating secondary transformer and circuit
conditions. The rotational speed was varied from 0 rpm to2000 rpm, and the thickness of aluminum sheet was 1.5 mm.
The smoothing capacitor and load resistance (10 Ω) wereconnected to rectifier. Fig. 9 shows the relationship between
the rotational speed and the transformer efficiency. The results
show that the transformer efficiencies are greater than 96%
and are about same under rotating conditions. Therefore, theC. Characteristics with a Load-Resistance Change (STopology Versus SP Topology)
In order to confirm the characteristics with a loadresistance change, we performed 1 kW power transfer
experiments with a load-resistance change for the S and SP
topologies. The thickness of the aluminum sheet was 1.5 mm,
and the smoothing capacitor and load resistance (10 Ω) wereconnected to rectifier.

1120

The 2014 International Power Electronics Conference[2]transformer efficiency is not affected by the rotational speed.
E. Power Transfer to the Inductive Load (S Topology)
We performed experiments for excitation windings aroundthe rotor of the 8 kW synchronous motor in Fig. 6. The
parameters of excitation windings of the rotor are R = 20 Ω, L
= 600 mH, and an excitation current of 3 A. In order to adjust
the value of COS to the load resistance, COS is set to 0.063 μF.
When placing a contactless power transfer system into theinterior of the synchronous motor, a secondary transformer andcircuit need to be constructed on the rotor side of the motor.
On the secondary side, the smoothing capacitor of the rectifier
is omitted, and the secondary current is smoothed by the
inductance of the excitation windings. Fig. 10 shows theexperimental results under excitation-current conditions (3 A,
184 W) that excite the windings of the rotor. The voltage atthe excitation winding VL is not constant owing to theomission of the smoothing capacitor. Further, the field current
IL is smoothed by the inductance of excitation windings and isconstant. The input current is in phase with the input voltage,
and the input power factor is 0.83. The transformer efficiency
ηTR is 91.8%, which is lower than the case of power transfer tothe load resistance (10 Ω) with a smoothing capacitor, because
the resistance of the excitation windings is 20 Ω, which isshifted from RLmaxS. These results reveal that the S topology iseffective for power transfer to an inductive load.
V.

[3]
[4]
[5]

[6]

[7]
[8]

CONCLUSION

In this paper, we proposed a resonant circuit with a primaryseries capacitor (S topology) for miniaturization and weight
reduction of the secondary rotating part. The S topology uses
only the primary series capacitor; thus the secondary circuit iscompact and maintenance free. We derived ηTRmaxS and RLmaxS,
and we performed power-transfer experiments to investigate
the validity of the proposed system.
The experimental results demonstrated that the transformer
efficiency of the S topology is equal to that of the SP topology,
and the S topology has high power factor performance at thenormal gap length (2 mm). When the gap changes from 1 mm
to 3 mm, the transformer efficiency remains approximately
constant.
The results also showed that the transformer efficiency andpower factor of the inverter output are not affected by therotational speed or inductive load. Therefore, the S topology iseffective for power transfer to an inductive load. A highefficiency of 91.8% is achieved when power is transferred to
the rotor circuit of the synchronous motor.
In proposed system, an aluminum sheet for magneticallyshielding the shaft is effective for reducing the loss andleakage flux into shaft.
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