3rd International Conference on

Innovative Academic Studies

September 26-28, 2023 : Konya, Turkey

All Sciences Proceedings © 2023 Published by All Sciences Proceedings https://www.icias.net/

http://as-proceeding.com/

Contactless Field Excitation System of Wound-Rotor Synchronous Motor

Ali AĞÇAL*
1
Electrical and Electronics Engineering Department/ Engineering Faculty, Suleyman Demirel University, Turkey
*
(aliagcal@sdu.edu.tr) Email of the corresponding author

Abstract – Transferring energy to the rotating parts of electrical machines through brush or slip ring has
always been a problem due to maintenance, friction loss and arcing. All of these problems are eliminated
with contactless energy transfer systems. In this study, a magnetic resonance coupling (MRC) based
wireless power transfer (WPT) system was used in the excitation of a synchronous motor. Single-layer
circular planar spiral coils were used in the transmitting and receiving coil design, thus achieving high
coupling factor and low thickness. The WPT system successfully transfers energy in low air gaps between
the rotating and stationary parts of the electrical machine. Simulations of the WPT model were made with
MATLAB and Maxwell 3D. According to the simulation results, the system can operate with over 98%
efficiency in a wide frequency band. The system was operated at 100 khz resonance frequency and 50 W
power was transferred to the output with 98.22% efficiency. This design is not affected by the rotation of
the rotor and does not create an imbalance in the rotor, since the receiver and transmitter coils are
positioned parallel and symmetrically. This WPT system offers a reliable, low-cost and low-complexity
solution to excite a synchronous motor.

Keywords –Wireless Power Transfer, Contactless Fıeld Excıtatıon, Contactless Energy Transfer, Electric Machinery, Wound-
Rotor Synchronous Motor

I. INTRODUCTION hand, inductive-coupled energy transmission is

Rotary electrical machines have been essential in preferred more than capacitive-coupled
our lives for over 100 years. Conventional rotating transmission due to less loss, lower operating
electrical machines such as synchronous and DC frequency range and higher power density.
machines use brush or ring structures to transfer In applying the inductively coupled WPT system
energy to the armature or excitation windings. to the rotating parts of the electrical machine, many
Friction loss, maintenance and repair cost, inability different coil designs, circuit topologies and hybrid
to be used in explosive areas, and arc risk are the circuit setups have been studied. U-type transmitter
most important disadvantages of ring or brush and spiral circular receiver coils are designed for
rotary electric machines. WPT provides efficient, the excitation of rotor windings of electrical
reliable and stable solutions for transferring energy machines [3]. However, in this design, it was
to the excitation windings of rotating electrical observed that severe eddy losses occurred in the
machines. It eliminates many of the disadvantages shaft, passing through the middle of the circular
of the ring and brush. coil in the receiver. For this reason, the efficiency
Various methods are used in the literature for did not exceed 51.19%. WPT system is designed
contactless energy transfer to rotating parts of for an in-wheel outer rotor switched reluctance
electrical machines. Since the most suitable motor drive using receivers with different
techniques for WPT from the near field are resonance frequencies [4]. A brushless excitation
capacitive [1] and inductive coupling [2], these two mechanism is designed with WPT based on MRC
methods are generally emphasised. On the other [5]. A WPT system was designed that determines
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the rotor's position and excitates the rotor In this study, the MRC-based WPT system is
wirelessly. The varying inductances of the WPT designed for contactless excitation of synchronous
system and the resulting variable frequencies in the machines. 50 watts of power was transferred from
power electronics circuit were used to evaluate the 2 mm air gap with 98.22% efficiency. It was
rotor position [6]. A primary side controller is operated at a frequency of 100 kHz. Eddy losses on
designed for brushless excitation of the excitation the shaft were investigated. The subsequent
windings of the synchronous motor [7]. The WPT sections of the paper is outlined below. In Section
system based on resonant inductive power II, the contactless field excitation system for the
transmission was designed to supply field power to excitation windings of the synchronous motor is
brushless alternating current (BLAC) or brushless explained. The features of the WPT design and the
direct current (BLDC) motors without permanent simulation results are given in Section III. Results
magnets in the rotor. A single-layer helix coil is and discussion are presented in Section IV.
used in the receiver, and a multilayer coil structure
is used in the transmitter [8]. A coaxial nested II. ROTARY WPT DESIGN
rotary WPT system was proposed for energising Inductive coupling is the most suitable method
the excitation windings of rotating electrical for brushless or ringless energy transfer to the
machines. The effect of the offsets formed due to excitation windings of rotating electrical machines.
the vibration of the rotor on the WPT system was In the inductively coupled WPT system, coils
investigated. The case was protected and losses transfer energy between the transmitter and
were reduced with a ferrite core design [9]. receiver. For WPT, a magnetic coupling must be
Various coil designs were investigated for between the receiver and transmitter coils. In the
contactless excitation of synchronous motor WPT system, coil designs are about increasing the
excitation windings. A prototype for high-speed coupling between the receiver and the transmitter
synchronous machines with a WPT system instead or obtaining the optimum coupling with a smaller
of rings was made [10]. A rotary WPT system with size. Although increasing the connection between
serial parallel topology was implemented for the the receiver and the transmitter increases the
excitation of rotating electrical machines [11]. An efficiency, more is needed. A compensation system
inductive power transmission system with a that balances the inductive reactive power of the
rotating transformer was proposed as an alternative coils and increases the power transmission capacity
solution to slip-ring systems. A prototype system and efficiency is required. The basic four
was built with a sensor mounted on the ball- compensation topologies are series-series, series-
bearing shaft. It was also examined in terms of parallel, parallel-serial, and parallel-parallel. In the
coupling factors and losses [2]. With the radial-flux literature, there are combined topologies such as
rotational WPT system, the rotor's position and LCC-S, S-LCC, and LCC-LCC, which are a
speed were determined and the excitation windings combination of these basic topologies. An
were energized [12]. Stability analysis and control overview of a rotary WPT system for the excitation
of the synchronous motor field excitation system of a synchronous motor is shown in Figure 1.
with rotary transformer was made. A dynamic R
S
model of the system based on variables was T
investigated using the harmonic balance technique
[13]. A low-cost and less complex contactless field Synchronous
excitation system was implemented that uses machine
Armature
existing motor drivers and does not require extra windings

converters in the receiver and transmitter [14]. Excitation

windings
Planar circular spiral coil sizes with PCB for Rectifier Inverter Compensation WPT Coils Compensation Rectifier
contactless electrical excitation of synchronous Fig. 1 An overview of a rotary WPT system for the
motors were discussed. PCB was chosen because excitation of a synchronous motor
of its simple manufacturing process, compactness,
power density, low AC resistance to proximity and This study used SS topology due to its simplicity
skin effect [15]. and ease of control. The basic WPT circuit for the
SS topology is shown in Figure 2.
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 factor resonators. However, high quality factor
C1 Lm C2 creates voltage stress in capacitors and increases
I1 I2 capacitor sizes. In contactless excitation of a
synchronous motor, designs with lower quality
factors are preferred to ensure energy transfer from
the near field and keep the capacitor size in the
I1 L1 L2 I2 receiver to a minimum. Equivalent impedance and
V1 RL
efficiency equations for the WPT system with SS
R2 topology are given in equations (3) and (4),
R1
respectively.

Fig. 2 WPT system with SS topology 1 𝐿𝑚 2 𝜔 2

𝑍𝐸𝑞 = 𝑅1 + 𝑗𝜔𝐿1 + (𝑗𝜔𝐶 ) + ( 1 ) (3)
1 𝑅2 +𝑗𝜔𝐿2 +( )+𝑅𝐿
𝑗𝜔𝐶2
Where, V1 and I1 are the input voltage and 𝑗𝐿𝑚 𝜔 𝑅
ɳ=( 1 )2 ∗ 𝑍 𝐿 (4)
current, respectively. I2 is the output current. L1, 𝑅2 +𝑗𝜔𝐿2 +(
𝑗𝜔𝐶2
)+𝑅𝐿 𝐸𝑞
C1, and R1 are the transmitter’s inductor, capacitor
and internal resistance, respectively. RL is the load ZEq is the equivalent impedance, η is efficiency,
resistance and Lm is the mutual inductance. L2, C2, and ω is the angular frequency. The resonant
and R2 are the receiver’s inductor, capacitor and frequency is not always one and constant in
internal resistance, respectively. When determining magnetically coupled systems. The resonance
the SS topology's inductance, capacitance and frequency may change and bifurcate depending on
resonant frequency, the basic LC circuit is used the change in circuit parameters. The resonance
first. The resonance frequency is the frequency at frequencies are found by Equation (5).
which inductive and capacitive reactive power is
balanced in compensation topologies. The natural 𝜕ɳ(𝜔)
resonance frequency is calculated by Equation (1). =0 (5)
𝜕𝜔

1 At the resonant frequency, the derivative of the

𝑓𝑟 = 2𝜋√𝐿𝐶 (1)
efficiency with respect to the frequency is zero.
The extreme points found with the derivative are
Where, fr is the natural resonant frequency, L is the resonance frequencies. While one or three
the inductance and C is the capacitor. The optimum resonant frequencies occur in basic topologies, the
working frequency in WPT differs according to the number of resonant frequencies can be increased
application areas. High frequencies are used in with combined topologies.
low-power application areas, while low frequencies Coil design is one of the main issues of the WPT
are used in high-power applications. It is also system. In coil design, it is necessary to keep the
available in frequency bands recommended for coupling between the receiver and transmitter high,
some application areas or determined by standards. reduce eddy losses, reach higher air gaps, or pay
In contactless excitation of rotating electrical attention to special restrictions depending on its
machines, WPT applications are generally made in application area. Square, rectangular or polygonal
the low-frequency band. After the frequency is coils are not preferred in rotating electrical
determined, the inductance and capacitance values machines as they create an imbalance in the shaft.
are selected. While determining the inductance and Circular coil designs are generally used. Spiral,
capacitance values, the resonator's quality factor is helical or multi-layer coils are often used in the
desired to be high. The quality factor is given in rotating WPT literature. The coupling factor
Equation (2). between the receiver and transmitter of helical coil
designs is lower than that of spiral coil designs.
𝐿 Therefore, a single-layer planar spiral coil is
𝑄 = √𝐶 𝑅 (2)
generally preferred. In low-frequency applications
where the inductance of the single-layer planar
Where, Q is the quality factor. WPT can be spiral coil is insufficient, the multilayer coil is
achieved from higher air gaps with high quality
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preferred. In this study, a single-layer circular Table 1. Parameters of the coils
planar spiral coil was used due to its high coupling Number of turns 9
factor, low thickness and suitability for rotating Distance between wires 0.2 mm
WPT. The single-layer circular planar spiral coil is Wire width 1 mm
shown in Figure 3. Outer diameter of the coil 90 mm
Inner diameter of the coil 68.8mm
Self-inductance 11.3 uH
Mutual inductance (2mm air gap) 8.96 uH
Length of wire 2.25 m
Coupling factor (2mm air gap) 0.79
Inner resistance of wire 49 m

Rotary WPT design in Maxwell 3D is shown in

Figure 4.

Fig. 3 Single-layer circular planar spiral coil [16]

A Single-layer circular planar spiral coil was

designed by using (6) and (7).

N2 A2
L = 30A−11D (6)
i
Di+ N(W+S) Fig. 4 Rotary WPT design in Maxwell 3D
A= (7)
2
Resonance capacitance was determined 224 nF
Where, W is diameter of the wire, N is the for 100 kHz resonance frequency. the load
number of turns, and S is the space between wires. resistance was selected 5 . Efficiency and
D0 and Di are the coil's outer and internal equivalent impedance according to frequency for 5
diameters, respectively.  load was shown in Figure 5.

III. WPT DESIGN FEATURES AND SIMULATION

RESULTS

The dimensions of the rotary coil were

determined so that the coil would be placed
between the stator and the shaft. According to a
synchronous machine with a stator inner diameter
of 95 mm, the transmitter and receiver coils were
designed in Maxwell 3D. The parameters of the
coils are given in Table 1.
Fig. 5 Efficiency and equivalent impedance according to
frequency

As can seen in Figure 5, WPT system efficiency

is higher than 98 % at the wide frequency band.

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The magnetic field distribution of the coils is design, since the receiver and transmitter are
shown in Figure 6. parallel and symmetrical to each other, there is no
change in the mutual inductance due to the rotation
of the rotor.
IV. CONCLUSION
Excitation of synchronous machines with the
help of brushes and rings has always been a
problem due to maintenance, friction loss and
arcing. It has limited the application areas of
synchronous machines. This paper provides an
effective alternative for the field excitation of
synchronous motors with an MRC-based WPT
system design. The designed WPT system
Fig. 6 Magnetic field distributions of the rotary WPT
achieved operation in low air gaps, high frequency
design
and high efficiency. Using single-layer planar
The circuit of the rotary WPT system for spiral coils in contactless excitation of rotating
excitation of synchronous motors is set up in electric machines provided a high coupling factor
Simplorer and shown in Figure 7. and low thickness. 50 W power was transferred to
the excitation windings on the rotor of the
PULSE2 MOS2 D3 D1 D6
synchronous machine with 98.22% efficiency.
C1 C2
R1
The simulation results show that the designed
Rxout
Txout

E1
C3
WPT system is a suitable solution for the excitation
C4 MOS4 D2 D4
of synchronous motors. The system offers a low-
PULSE1 D5
cost, reliable, maintenance-free and low-
Rxin
Txin

complexity alternative for the field excitation of

gnd_term
synchronous motors. It eliminates the problems
Fig. 7 The circuit of the rotary WPT system for excitation associated with contact energy transfer to the
of synchronous motors excitation windings of rotating electrical machines.
This design can be developed and further
The transmitter circuit was fed with a half-bridge optimised for use in industrial applications. This
inverter. Since the excitation windings of paper highlights the potential of MRC-based WPT
synchronous motors work with DC, the output of systems to provide excitation to synchronous
the WPT system is rectified with a full bridge motors and may inspire future research in this area.
rectifier. The voltages, currents and powers of the
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