1. OPTIMISATION IN WIRELESS POWER TRANSFER
Abstract
The paper deals with a detail study on how
to make wireless power much more efficient
and productive. In addition, the project also
opens up new possibilities in wireless
systems. The efficiency of power transfer is
considered by the amount of power that is
been received at the receiver side, for that a
combination of both inductive and resonant
frequency techniques are been tried out.
Testing of coils have been done on Maxwell
simulator to support the studies. Simulation
results prove that resonant inductive
technique is far better than inductive power
transfer.
Introduction
The conventional wire system creates a mess
when it comes to charging several devices
simultaneously. It also takes up a lot of
electric sockets and not to mention the fact
that each device has its own design for the
charging port. The solution to all these
dilemma lies within inductive coupling, a
simple and effective way of transferring
power wirelessly.
Wireless Power Transmission (WPT) is the
efficient transmission of electric power from
one point to another through vacuum or an
atmosphere without the use of wire or any
other substance.
Theory
An electric current flowing through a
conductor carries electrical energy. When
an electric current passes through a circuit
there is an electric field in the dielectric
surrounding the conductor; magnetic field
lines around the conductor and lines of
electric force radially about the conductor.
In a direct current circuit, if the current is
continuous, the fields are constant; there is
a condition of stress in the space
surrounding the conductor, which
represents stored electric and magnetic
energy, just as a compressed spring or a
moving mass represents stored energy. In
an alternating current circuit, the fields
also alternate; that is, with every half
wave of current and of voltage, the
magnetic and the electric field start at the
conductor and run outwards into space
with the speed of light. Where these
alternating fields impinge on another
conductor a voltage and a current are
induced
Any change in the electrical conditions of
the circuit, whether internal or external
involves a readjustment of the stored
magnetic and electric field energy of the
circuit, that is, a so-called transient.
Electromagnetic induction is proportional
to the intensity of the current and voltage in
the conductor which produces the fields
and to the frequency. The higher the
frequency the more intense is the induction
effect. Energy is transferred from a
conductor that produces the fields (the
primary) to any conductor on which the
fields impinge (the secondary).
Inductance and Inductive Coupling
In electromagnetism, inductance is the
ability of an inductor to store energy in a
magnetic field. Inductors generate an
opposing voltage proportional to the rate of
change in current in a circuit. This property is
also called self-inductance to discriminate it

2. from mutual inductance, describing the
voltage induced in one electrical circuit by
the rate of change of the electric current in
another circuit.
The electric field of a circuit over which
energy flows has three main axes at right
angles with each other:
a. The magnetic field, concentric with
the conductor.
b. The lines of electric force, radial to
the conductor.
c. The power gradient, parallel to the
conductor
Inductive Coupling
Inductive or Magnetic coupling works on the
principle of electromagnetism. When a wire
is proximity to a magnetic field, it generates
a magnetic field in that wire. Transferring
energy between wires through magnetic
fields is inductive coupling.
If a portion of the magnetic flux established
by one circuit interlinks with the second
circuit, then two circuits are coupled
magnetically and the energy may be
transferred from one circuit to the another
circuit.
This energy transfer is performed by the
transfer of the magnetic field which is
common to the both circuits.
Fig1-general diagram of inductive coupling
Resonant Frequency
Resonance is a phenomenon that causes an
object to vibrate when energy of a certain
frequency is applied. In physics, resonance is
the tendency of a system to oscillate with
larger amplitude at some frequencies.
Fig2-Resonant Frequency
Resonant Inductive Coupling
Resonant inductive coupling is the near
field wireless transmission of electrical
energy between two coils that are tuned to
resonate at the same frequency. The
equipment to do this is sometimes called a
resonant or resonance transformer. While
many transformers employ resonance, this
type has a high Q and is often air cored to
avoid iron losses.
Using resonance can help efficiency
dramatically. If resonant coupling is used,
each coil is capacitively loaded so as to form
a tuned LC circuit. If the primary and
secondary coils are resonant at a common
frequency, it turns out that significant
power may be transmitted between the
coils over a range of a few times the coil
diameters at reasonable efficiency.
Air core coil is an inductor that does not
depend upon a ferromagnetic material to
achieve its specified inductance. Air core

3. coils have lower inductance than
ferromagnetic core coils.
Air core coil could be of
two types; (a) Single
Layer Coil (b) Multi-
Layer Coil.
Single Layer
A single layer coil, as shown in figure 3.1,
has two advantages. Firstly, like all air core
coils, it is free from iron losses and the
non-linearity mentioned above. Secondly,
single layer coils have the additional
advantage of low self-capacitance and thus
high self-resonant frequency.
In the simple case of a single layer
solenoidal coil the inductance may
be estimated as follows:
L = 0.001 N2 (a/2)2 / (114a + 254l)……..(1.0)
Where L is the inductance of the coil.
a=coil diameter in meters
l=coil length in meters
N=number of turns
Q factor
The Q factor of an inductor is the ratio of its
inductive reactance XL to its series
resonance RS. The larger the ratio, the
better the inductor is.
Q = XL/RS……………………………………………(1.1)
XL = 2πfL…………………………………………….(1.2)
Where f is the frequency in Hertz (Hz) and L
is the inductance in henries (H)
RS is determined by multiplying the length
of the wire, used to wind the coil, with the
D.C. resistance per unit length for the wire
gage used.
Q changes dramatically as a function of
frequency. At lower frequencies, Q is very
good because only the D.C. resistance of the
windings (which is very low) has an effect.
As frequency goes up, Q will increase up to
about the point where the skin effect and
the combined distributed capacitance begin
to dominate.
Multi Layer Coil
The ratio of the winding depth to length,
which is (b-a)/l, needs to be close to unity;
so the winding should have a square cross
section. This makes sense because only
with the square is the average distance
between turns at a minimum (a circular
cross section would be even better, but
that is hard to construct). Keeping the
turns close together maintains a high level
of magnetic coupling between them, and
so the general rule that the inductance of
a coil increases with the square of the
number of turns is maintained.
Where a is the inner radius and b is the
outer radius.
In the simple case of a multi-layer coil the
inductance may be estimated as follows:
L=0.008×D2×N2/(3D+9h+10g)……………. (1.3)
Where D is the average diameter of the coil;
h is the height of the coil;
g is the depth of the coil—
all in millimeters.

4. Fig3-multilayer coil
Oscillator
An electronic oscillator is an electronic
circuit that produces a repetitive electronic
signal, often a sine wave or a square wave.
They are widely used in many electronic
devices.
The harmonic, or linear, oscillator produces
a sinusoidal output. The basic form of a
harmonic oscillator is an electronic amplifier
connected in a feedback loop with its output
fed back into its input through a frequency
selective electronic filter to provide positive
feedback. When the power supply to the
amplifier is first switched on, the amplifier's
output consists only of noise. The noise
travels around the loop and is filtered and
re-amplified until it increasingly resembles
a sine wave at a single frequency.
Harmonic oscillator circuits can be
classified according to the type of
frequency selective filter they use in the
feedback loop:
I. RC oscillator: In an RC oscillator
circuit, the filter is a network of
resistors and capacitors. RC
oscillators are mostly used to
generate lower frequencies, for
example in the audio range.
Common types of RC oscillator
circuits are the phase shift
oscillator and the Wien bridge
oscillator.
II. LC oscillator: In an LC oscillator
circuit, the filter consistes of an
inductor (L) and capacitor (C)
connected together. Charge flows
back and forth between the
capacitor's plates through the
inductor, so the tuned circuit can
store electrical energy oscillating at
its resonant frequency. There are
small losses in the tank circuit, but
the amplifier compensates for
those losses and supplies the
power for the output signal. LC
oscillators are often used at radio
frequencies, when a tunable
frequency source is necessary,
such as in signal generators,
tunable radio transmitters and
the local oscillators in radio
receivers. Typical LC oscillator
circuits are the Hartley, Colpitts
and Clapp circuits.
III. A crystal oscillator is a circuit that uses
a piezoelectric crystal as a frequency
selective element. The crystal mechanically
vibrates as a resonator, and its frequency of
vibration determines the oscillation
frequency. Crystals have very high Q-factor
and also better temperature stability than
tuned circuits, so crystal oscillators have
much better frequency stability than LC or
RC oscillators.
WORKING PRINCIPLE

5. Oscillator
Fig4-Royer oscillator
When power is applied, DC current flows
through the two sides of the coil and to the
transistors’ drain. At the same time the
voltage appears on both gates and starts to
turn the transistors on. One transistor is
invariably a little faster than the other and
will turn on more. The added current flowing
in that side of the coil does two things.
One, it takes away drive from the other
transistor. Two, the auto- transformer
action impresses a positive voltage on the
conducting transistor, turning it hard on. The
current would continue to increase until the
coil (transformer) saturates. The resonating
capacitor C causes the voltage across the
primary to first rise and then fall in a
standard sine wave pattern.
Assuming that Q1 turned on first, the voltage
at the drain of Q1’s will be clamped to near
ground while the voltage at Q2’s drain rises
to a peak and then falls as the tank formed
by the capacitor and the coil primary
oscillator through one half cycle.
The oscillator runs at the frequency
determined by the inductance of the coil, the
capacitor value and to a lesser extent, the
load applied to the secondary (Source coil).
The operating frequency is the familiar
formula for resonance,
F=1/2×π ×√(LC)………………………………(1.4)
Design
Oscillator Design
in voutV V ………………………………………….(1.5)
vout inV AV ………………………………………….(1.6)
By equating both the equations we get
(1 ) 0inV A  …………………………………….(1.7)
Hence we get
) 1A  …………………………………………………(1.8)
The total phase angle should be either 0 or 360
and the loop gain is 1 hence the barkhausen
criteria for sustainable oscillation is satisfied.
out
v m L
gs
V
A g R
V
   …………………………………..(1.9)
2
2
1
1 LC




……………………………………….(2.0)
Ct is calculated by substituting the capacitor
branch with 2

Hence we get 1
2
C
C
  ………………………………(2.1)
The Transmitter Coil
For this project the transmitter coil is
designed with 6mm copper tube with a
diameter of 16.5cm
(6.5 inches) and a length of 8.5cm.
From the equation of inductance of a single
layer air core coil we get,
L = 0.001 N (a/2)2 / (114a + 254l) H………(2.2)

6. L = 0.001×22× (0.165/2)2 / ((114×0.165) +
(254×0.085)) H L = 0.674 µH
Reciever Coil design
For this project the receiver coil is designed
with 18 awg (American Wire Gauge) copper
wire with a diameter of 8cm.
From the equation of inductance of a single
layer air core coil we get,
L = 0.001 N2 (a/2)2 / (114a + 254l) H……..(2.3)
L = 0.001×32× (0.08/2)2 / ((114×0.08) +
(254×0.01)) H L = 1.235 µH
Reciever Circuit
Fig5-Reciever circuit
Components
Transmitter
Table1-Transmitter
Reciever Components
Component’s Name Component’s Value or
code
Diode, D1 D4007
Diode, D2 D4007
Diode, D3 D4007
Diode, D4 D4007
Capacitor, C1 6.8 nF
Capacitor, C2 220 µF
Resistor, R 1k ohm
Voltage Regulator IC IC LM 7805
Receiver coil, L 1.235 µH
Table2-reciever
Observations
Inductive
Power transfer efficiency of inductive
coupling can be increased by
1) Increasing the number of turns in the
coil
2) Increasing the strength of the current
3) Increasing the area of cross-section of
the coil
4) Increasing the strength of the radial
magnetic field.
Component’s Name Component’s Value or code
Voltage Source, Vdc 30V
Capacitor, C 6.8nF
Resistor, R1 1k ohm
Resistor, R2 10k ohm
Resistor, R3 94 ohm
Resistor, R4 94 ohm
Resistor, R5 10k ohm
Diode, D1 D4148
Diode, D2 D4148
MOSFET,Q1 IRF540
MOSFET, Q2 IRF540
Radio Frequency Choke,L1 8.6 µH
Radio Frequency Choke, L2 8.6 µH
Transmitter coil, L 0.674 µH

7. Magnetic fields decay quickly, making
inductive coupling effective at a very short
range.
Advantages/disadvantages
Inductive charging carries a far lower risk
of electrical shock when compared with
conductive charging, because there are no
exposed conductors.
The main disadvantages of inductive
charging are its lower efficiency and
increased resistive heating in comparison to
direct contact. Implementations using lower
frequencies or older drive technologies
charge more slowly and generate heat for
most portable electronics.
Losses in an Air Core
At high frequencies, particularly radio
frequencies (RF), inductors have higher
resistance and other losses. In addition to
causing power loss, in resonant circuits this
can reduce the Q factor of the circuit,
broadening the bandwidth. In RF inductors,
which are mostly air core types, specialized
construction techniques are used to
minimize these losses. The losses are due to
these effects:
I.Skin effect: The resistance of a wire to high
frequency current is higher than its
resistance to direct current because of skin
effect. Radio frequency alternating current
does not penetrate far into the body of a
conductor but travels along its surface.
Therefore, in a solid wire, most of the cross
sectional area of the wire is not used to
conduct the current. This effect increases
the resistance of the wire in the coil, which
may already have a relatively high resistance
due to its length and small diameter.
II.Proximity effect: This occurs in parallel
wires that lie close to each other. The
individual magnetic field of adjacent turns
induces eddy currents in the wire of the coil,
which causes the current in the conductor to
be concentrated in a thin strip on the side
near the adjacent wire. Like skin effect, this
reduces the effective cross-sectional area of
the wire conducting current, increasing its
resistance.
Results
Single coil output
When we apply excitation to a single metal
coil then the effect of magnetic field on a
copper piece is shown in Maxwell.
Fig5-single coil design
Fig6-Multi Layer Coil

8. 1)Now we will consider a normal oscillator
,lets consider colppits oscillator designed
for 9mghz
Fig6-colppit oscillator
Output
Fig7-Output of colpits oscillator
Here we can see that the current it
produces is almost zero so its almost not at
all efficient in wireless power transfer.
2)Now we consider Royer Oscillator
Fig8-Royer Oscillator
Output from Royer oscillator
Fig8-Simulink output
Initially when you compare with the collpit
oscillator here we get a current output
about 1.242Amps. So its very much
beneficial for wireless power transfer.
3)Now we will consider the royer oscillator
using a power mosfet
Fig9-Royer oscillator using power mosfet

9. Fig10-simulink results
Initial simulink results prove that the above
circuit is best for inductive power transfer.
References
[1] Russell M Kerchner and George F
Corcoran, ―Alternating-Current Circuits‖,
pp. 273-324, 1960.
[2] G. Grandi, M.K. Kazimierczuk, A.
Massarini, ―Optimal Design of Single-Layer
Solenoid Air-Core
Inductors for High Frequency Applications‖,
Circuit Systems, Vol. 1, pp. 358-361, 1997.
[3] A. Kurs, A. Karalis, R. Moffatt, J. D.
Joannopoulos, P. Fisher, M. Soijacic,
―Wireless Power
Transfer via Strongly Coupled Magnetic
Resonances‖, Massachusetts Institute of
Technology, 2007
Science, Vol. 317. no. 5834, pp. 83— 86,
2007
[4] Jacob Millman and Christos C. Halkias,
―Integrated Electronics: Analog and Digital
Circuits and
Systems‖, pp. 103-107, 2007
[5] Muhammad H. Rashid, ―Power
Electronics: Circuits, Devices, and
Applications‖, pp.37-63, 2nd
Edition, 2000
[6] Robert L. Boylestad and Louis
Nashelsky,‖Electronic Devices and
Circuit Theory‖,9th
Edition,2006, pp. 79-82
[7] William H.Hayt,Jr. and John
A.Buck,‖Engineering
Electromagnetics‖,7th
Edition,2006,pp.292-299 [8]
http://info.ee.surrey.ac.uk/Workshop/ad
vice/coils/air_coils.html
[9]http://en.wikipedia.com
[10]
http://www.smeter.net/electronics/solnoi
d3.php
Submitted By
Adharsh John Chundammanal
L20352216