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WIRELESS POWER TRANSFER VIA INDUCTIVE

COUPLING

Mirsad Hyder Shah

Ex-Fellow, Department of Electrical Engineering, DHA Sua University. Karachi, (Pakistan).

E-mail: itsmirsadhyder@yahoo.com ORCID: https://orcid.org/0000-0003-2476-5887

Nasser Hassan Abosaq

Assistant Professor, Computer Science and Engineering Department. Yanbu University College, Yanbu

Industrial City, (Saudi Arabia).

E-mail: abosaqn@rcyci.edu.sa ORCID: https://orcid.org/0000-0003-1354-3170

Recepción:

16/01/2020

Aceptación:

20/03/2020

Publicación:

30/04/2020

Citación sugerida Suggested citation

Shah, M. H., y Abosaq, N. H. (2020). Wireless power transfer via inductive coupling. 3C

Tecnología. Glosas de innovación aplicadas a la pyme. Edición Especial, Abril 2020, 107-117. http://doi.

org/10.17993/3ctecno.2020.specialissue5.107-117

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ABSTRACT

The concept of transferring electrical power to a load wirelessly is an intimidating and a

challenging idea. The genius of powering systems wirelessly has pulled the curtains to a

new world. In the 19

century, Nikola developed ‘Tesla Tower’ in hope to transfer power

wirelessly. Since then, the world is trying hard to say goodbye to wires. WPT using Inductive

Coupling which falls under the domain of NFWPT, uses a transmitter coil to transmit power

to the receiver coil via a magnetic eld. Inductive coupling is an ecient way to transmit

power through short distances and making its way in smartphones and the health industry.

Electric vehicle charging stations are also trending thanks to wireless power transfer. This

paper discusses the theoretical foundation of Inductive coupling and presents results of

an experimental work done on WPT via Inductive Coupling. In the process above, an

eciency of 72% was achieved.

KEYWORDS

Wireless Power Transfer, Inductive Coupling.

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1. INTRODUCTION

Many Engineers and Physicists credit Nikola Tesla for the concept of wireless power transfer,

ignoring Faraday being the pioneer of the concept of transferring energy wirelessly when

he demonstrated how Electromotive-Force and Current were induced in a conductor when

subjected to a changing magnetic eld, and hence the concept of Wireless Power Transfer

was derived. In 1892, Nikola Tesla believed wireless power transfer was possible and began

building what he called the ‘Tesla Tower’. This 200 feet high tower was energized with

300kW of power but couldn’t prove to succeed because of the long distance approach

(Johnson, 1990). The late 19

century saw an attempt to power electric vehicles through

electrodynamic induction, but combustion engines proved much more ecient. In 1978, the

United States powered an electrical vehicle successfully; while in 1987, Canada successfully

ew the rst fuel-free airplane model. Commercial use of powering smartphones wirelessly

came up on the scene after 2009, when Palm Inc. introduced wireless charging in their

smartphones. Samsung and Apple followed the lead and presented wireless charging in

2013 and 2014 respectively.

1.1. RESEARCH SIGNIFICANCE

Wirless power transfer is the dawn of a new age. At present, many companies have

introduced commercial use of wireless charging for smartphones, EV cars and other

electronic devices. Wireless Power Transfer has even made its mark in the healthcare,

especially in the implantable medical devices. This study presents a simple prototype which

can be employed for smartphones or cars with an impressive eciency of 72% when placed

in proximity.

1.2. OVERVIEW OF WPT

Wireless power transfer can be classied into two elds; NFWPT (Near Field Wireless power

technology) and FFWPT (Far Field Wireless power technology) (Hassan & Elzawawi, 2015).

NFWPT is further classied as electromagnetic induction, as it depends on the coupling

of the magnetic-eld between the two coils, which explains why it has a short range. The

eld (range) of NFWPT decreases exponentially. It includes Inductive Power Transfer

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(IPT), Resonant Inductive Power Transfer, Capacitive Power Transfer (CPT), Resonant

Capacitive Coupling and Magneto-dynamic coupling (Hassan & Elzawawi, 2015).

FFWPT is further classied as electromagnetic radiation. It is most convenient for long

range applications. But due to the power losses, it is comparatively less ecient. It includes

lasers (radiowaves) and microwaves to transmit power.

Table 1. Classication of WPT technologies.

WPT

Technolgies

Range Frequency Efciency Power transfer via

NFWPT Short-Medium Hz-kHz High Electric or Magnetic Fields.

FFWPT Long MHz Low

Infrared or Ultraviolet or

Microwaves

Source: (Hassan & Elzawawi, 2015).

2. WPT USING INDUCTIVE COUPLING

Any Wireless Power Technology must have two core components for it to work; a transmitter

and a receiver. In the case of Inductive Coupling, the transmitter and receiver are two

separate coils wound on materials with high permeability. This increases the eciency of

the circuit by increasing the inductance of the coils. The transmitter transfers AC power

to the receiver which can then be converted to DC for in use applications. As DC power

transfer has higher energy loss hence the model comprises of transferring AC power.

In Inductive power transfer, longitudinally arranged dipole elds are produced. These elds

decrease with the cube of distance between the transmitter and the receiver. Hence one of

the factors aecting the eciency is the distance between the two coils. Hence, the closer

the receiver and the transmitter, the better the eciency (Van Schuylenbergh & Puers,

2009).

Inductive coupling solely involves magnetic elds for transferring power and therefore can

be referred to as ‘Magnetic coupling’ as well. It works on the basic principle of faraday’s law

of induction which explains how a magnetic eld will interact with an electrical circuit to

produce an electromotive force (EMF) in the secondary coil. The power transfer in Inductive

coupling is directly proportional to the frequency as well as the mutual inductance between

the coils. The mutual inductance between the transmitter and receiver can be calculated by:

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(1)

Where, k is the coupling coecient. It is a dimensionless parameter.

Another factor which aects the eciency of WPT is misalignment tolerance. Misalignment

is the displacement of the receiver coil with respect to the transmitter coil that leads to a

decline in both the eciency and power transfer of the IPT system. Since,

(2)

For maximum ux, the dot product requires the angle between the ux density (

) and the

area enclosed to be 0 deg [cos 0 =1]. This can be ensured when both the transmitter and

receiver coils overlap each other.

Graphic 1 shows the basic model of a complex circuit which transfers power via inductive

coupling. The transmitter consists of an AC voltage source (V

) and a primary coil (L

) on

the left hand side, while the receiver consists of a secondary coil (L

) and R

load

. A bridge

rectier and further electronic circuitry are lumped as R

load

on the right hand side. The

transmitter is powered through an AC source (a coil driver) which produces a magnetic

ux in the primary coil. This induces a voltage in L

which in turn produces a ux in the

secondary coil L

. This ux produces a voltage in the secondary coil which can be rectied

for further use (Van Schuylenbergh & Puers, 2009).

Graphic 1. Basic circuit diagram of WPT using inductive coupling. Source: (Van Schuylenbergh & Puers,

2009).

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To calculate the link eciency of the coupling circuit, Graphic 2 is used. Since, Graphic

1 is a generalized block of Graphic 2; it neglects the coil resistances of the receiver and

transmitter. From transformer theory, we know that the two coils employ the coupling

coecient (k) and that both the inductances L

and L

are aected by k.

From transformer theory we know that, reducing the circuit with respect to primary side

will yield:

Graphic 2. Inductive circuit referred to the primary side and coil losses included. Source: (Van Schuylenbergh

& Puers, 2009).

Where,

(3)

Graphic 3. Equivalent circuit for the calculation of link efciency. Source: (Van Schuylenbergh & Puers, 2009).

And the equivalent resistance and inductance are given by:

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(4)

(5)

The link eciency is given by,

(6)

(7)

It can be observed that one of the factors the link eciency is dependent on is the square

of primary inductance of the coil. A higher value of mutual coupling ’k’ is also desirable

for better link eciency (Van Schuylenbergh & Puers, 2009).

3. THEORY OF WPT

According to amperes law, the loop integral of the eld equals the net current i enclosed

by the loop.

(8)

Where

is the magnetic ux density, i is the net current and is the permeability.

[

]

According to Biot-Savart Law,

(9)

Where:

dl: is the innitesimal length of conductor carrying the electric current i.

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r: is the distance from the length element dl to the eld point P.

The magnetic ux is given by equation (2):

(2)

Where, A is the area enclosed in a given loop.

Solving the dierential in (9) and then after equating in (2) we get,

(10)

Ampere’s law in term of reluctances is given by,

(11)

Where,

Rm is the reluctance of the magnetic loop.

According to faradays law,

(12)

Where,

is the emf or the electromotive force, is the magnetic ux.

The voltage in the coils is given by,

(13)

(14)

The coupling coecient k and inductance ratio are given by

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(15)

(16)

4. EXPERIMENTAL WORK AND RESULTS

For the construction of an inductive coupling circuit, the transmitter has an AC source (V

)

and a coil with inductance (L

). The input frequency can be adjusted as per the application

of usage. The greater the frequency the more the transmission eciency. The receiver

is comprised of a coil of inductance (L

) which produces an EMF for the power to be

delivered to the load. As the coil Ls delivers AC voltage, a bridge rectier is used to convert

AC voltage to DC voltage which is then delivered to the load.

AC Voltage source Primary Coil Secondary Coil Rectification Remote Electronics

Graphic 4. Block diagram of WPT using inductive coupling. Source: (Yahaya et al., 2018).

The proposed model in Graphic 4 was implemented for charging a mobile phone.

Transmitter and receiver coils were wound on plastic forms, both the coils had the same

number of turns. The core was made from steel plates and was arranged around each of

the coils. The steel plates were held together by a coating of shellac.

The radius of both the coils were 0.07m, while the input voltage provided was 240V.

Following results were obtained when a piece of paper was placed between the two coils

and consequently the distance between the two coils was increased. A similar approach to

the work done in Yahaya et al. (2018).

Table 2. Voltage, current and power when distance is changed.

Distance

(cm)

Input

Voltage

(V)

Input

Current

(A)

Input

Power

(W)

Output

Voltage

(V)

Output

Current

(A)

Output

Power

(W)

Efciency

(%)

0 208 0.12 24.96 22.087 0.881 19.468 78

1 208 0.12 24.96 17.12 0.816 13.977 56

2 208 0.12 24.96 10.935 0.707 7.737 31

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Distance

(cm)

Input

Voltage

(V)

Input

Current

(A)

Input

Power

(W)

Output

Voltage

(V)

Output

Current

(A)

Output

Power

(W)

Efciency

(%)

3 208 0.12 24.96 6.12 0.57 3.494 14

4 208 0.12 24.96 2.055 0.242 0.499 2

5 208 0.12 24.96 0.12 0.149 0.0179 0.072

As it was predicted by Akpeghagha et al. (2019), the eciency drops drastically with the

increase of distance between the two coils. Here, the input voltage and input current are the

voltage and currents of the primary coil (L

) , while the output voltage and output current

are the voltage and currents of the secondary coil (L

Graphic 5. Graphical representation of Transmission efciency vs Distance.

REFERENCES

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