Application Note - Wireless Energy Transmission

APPLICATION NOTE
WIRELESS ENERGY TRANSMISSION
Stefan Fassbinder
September 2014
ECI Publication No Cu0209
Available from www.leonardo-energy.org

Publication No Cu0209
Issue Date: September 2014
Page i
Document Issue Control Sheet
Document Title: Application Note – Wireless energy transmission
Publication No: Cu0209
Issue: 01
Release: September 2014
Author(s): Stefan Fassbinder
Reviewer(s):
Document History
Issue Date Purpose
1 September
2014
First publication in the framework of the Good Practice Guide
2
3
Disclaimer
While this publication has been prepared with care, European Copper Institute and other contributors provide
no warranty with regards to the content and shall not be liable for any direct, incidental or consequential
damages that may result from the use of the information or the data contained.
Copyright© European Copper Institute.
Reproduction is authorised providing the material is unabridged and the source is acknowledged.

Page ii
CONTENTS
Introduction.................................................................................................................................................... 1
Basic principles: electric and magnetic fields .........................................................................................................1
Inductance and magnetic fields..............................................................................................................................2
Capacitance and electric fields ...............................................................................................................................3
Comparison of formulae used to characterise electric and magnetic fields ..........................................................5
Small-scale wireless energy transfer............................................................................................................. 10
Not so new: The electric toothbrush....................................................................................................................10
New: Contactless charging of mobile phones ......................................................................................................12
No need for standardization?.................................................................................................................12
EMC ........................................................................................................................................................12
Efficiency ................................................................................................................................................13
RFID ................................................................................................................................................................14
EnOcean................................................................................................................................................................15
Listening to rock through concrete ......................................................................................................................17
Wireless energy transfer in ‘medium-power’ applications............................................................................ 19
Electrodeless fluorescent lamps in public spaces.................................................................................................19
Operating principle.................................................................................................................................19
Efficiency ................................................................................................................................................20
Don’t try this at home… .........................................................................................................................20
Health.....................................................................................................................................................21
Induction hobs and cookers .................................................................................................................................21
Large-scale wireless energy transfer ............................................................................................................. 22
Stationary systems................................................................................................................................................22
Excitation power in electric motors .......................................................................................................23
Charging electric cars .............................................................................................................................23
Transrapid: A non-hybrid mobile system .............................................................................................................24
The concept............................................................................................................................................25
Performance...........................................................................................................................................26
Is Transrapid technology ready for commercial use? ............................................................................27
Costs.......................................................................................................................................................29
Safety......................................................................................................................................................29

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Environmental impact ............................................................................................................................30
Energy consumption...............................................................................................................................31
The end...................................................................................................................................................34
Transrapid is dead, long live Transrapid! ...............................................................................................34
Hybrid mobile systems: Tramways without overhead line equipment................................................................35
Realistic – but not wireless.....................................................................................................................35
Wireless – but unrealistic.......................................................................................................................36
The age of miracles has returned… ............................................................................................................... 38
Looking for clues...................................................................................................................................................38
Looking for an explanation...................................................................................................................................39
Home testing ........................................................................................................................................................41
Stray fields and EMC.............................................................................................................................................44
Characteristics of resonant circuits ......................................................................................................................45
No short-circuit power mysticism please!............................................................................................................46
What we’ve learned so far….................................................................................................................................48
‘WiTricity’: With and without resonance..............................................................................................................49
Conclusions................................................................................................................................................... 50
The facts ...............................................................................................................................................................50
The remaining riddles...........................................................................................................................................51
The trade press and the internet..........................................................................................................................52
Final remarks ........................................................................................................................................................53

Page 1
INTRODUCTION
Electric current is used for two very different purposes: the transmission of energy and the transmission of
information. Although the methods and equipment used differ significantly, the same underlying properties of
electric current are utilised.
Information can be transmitted using voltages and currents via an electric conductor or ‘wirelessly’ via
electromagnetic fields. In certain circumstances, purely magnetic fields will also do the job. The induction
loops used to detect vehicles and to control traffic lights and car park barriers are a case in point – though
anyone riding an aluminium bike obviously has to wait a long time for the signal to turn green – or gets fed up
waiting and jumps the light. There is also the possibility of transmitting information down an optical cable
using light rather than electrical signals. However, transmitting energy always requires a connection made
from an electrically conducting material.
Always? Or is it actually possible to do without the electrical connection? Well, that very much depends on
how much copper wire one wants to use to establish the ‘wireless connection’, because ‘wireless’ energy
transmission deserves the epithet ‘wireless’ about as much as a compact fluorescent lamp deserves to be
called ‘compact’.
BASIC PRINCIPLES: ELECTRIC AND MAGNETIC FIELDS
Some years ago, a professor at Dortmund University of Applied Sciences said the following: ‘There aren’t
actually any electric motors, only magnetic motors.’ The point he was making was that in order to transform
electrical energy into mechanical energy, the following two principles can be exploited:
• electric voltages generate electric fields
• electric currents generate magnetic fields
Some of the properties of these two types of field are identical, while others complement each other. Both
fields, for instance, generate forces. Electrically charged particles in an electric field are repelled by the pole of
the same charge and are attracted to the pole of opposite charge. Neutral particles are attracted equally to
both poles, as both poles ‘hope’ to reduce their charge density by distributing charge over a greater mass,
volume or surface. A positively charged body made from a conducting material would, for instance, be
attracted to the negative pole where it would be discharged and then negatively charged. This negatively
charged particle would then be repelled by the minus pole and would migrate to the plus pole where the
process would repeat itself. That really would be an ‘electric’ motor, at least from the point of view of pure
physics. However, the forces involved are very small. Voltages of several megavolts would be needed to be
applied to make such a system technically usable. Taking off a pullover can hardly be said to make it more
attractive from an aesthetic point of view, but it does become more attractive in terms of electrostatics.
Nevertheless, despite being charged to a voltage of around 5 kV, the forces acting are sufficient merely to
cause our hair to stand on end or to keep hold of a few bits of fluff. Despite the high voltage, the electrical
discharge is harmless and shows just how little energy is actually stored in the pullover.
This is the reason why the magnetic forces associated with electrical current rather than the electrical forces
associated with voltage are used to generate mechanical energy. It is only magnetic forces that offer the
potential for any meaningful energy transfer
1
. In analogy to the situation in electrostatics, a magnetic north
1
www.bticcs.com/pub/d+e2005.pdf, table on p. 1

Page 2
pole is repelled by another north pole and attracted to a magnetic south pole and vice versa. Unmagnified but
‘magnetically conducting’ (ferromagnetic) part is attracted equally to both poles.
Both fields, electric and magnetic, contain energy – though not a great deal – and if an electric field is
discharged along an electrically conducting path, an electric current will be generated, and this electric
current, which is nothing more than moving electrical charges, produces an associated magnetic field. As the
strength of this magnetic field grows, it will – in accordance with the principle of electromagnetic induction
discovered and described by Michael Faraday – generate a voltage (emf) in the conductor such that the
induced voltage opposes the applied voltage (from the applied electric field) and therefore inhibits the rate at
which the electric current grows (so-called self-inductance). If the magnetic field weakens, the voltage (emf)
that is now induced has the inverse polarity and now acts to retard the decreasing current.
INDUCTANCE AND MAGNETIC FIELDS
A magnetic field can be characterized in terms of its field strength H (Figure 1) or in terms of the magnetic flux
density B. The formula for the magnetic flux density contains the dimensionless factor µr that expresses the
ratio of the permeability of a specific material to magnetic fields relative to the magnetic permeability of free
space (or air, which for the present purposes is a good approximation of free space). The magnetic materials
typically used in inductors and transformers exhibit µr values of around 300. Certain special materials (‘mu-
metals’) have relative permeabilities of 30,000. An electric current flowing in a coil will generate a magnetic
field of strength H and with a magnetic flux Φ. If one were to connect the ends of the coil (i.e. short-circuit the
coil) the current would continue to flow if the wire was a perfect conductor (i.e. if the wire had no ohmic
resistance). Superconductors are almost perfect conductors and this phenomenon is indeed observed: a
current in a superconductor will essentially flow forever, as the magnetic field that it generates represents the
‘inertia’ of this electric current. If one were to now introduce a medium into the (short-circuited,
superconducting) coil and if this medium was significantly more magnetically permeable than air, the coil’s
magnetic flux would remain unchanged, though the magnetic field strength required to maintain this flux
would be substantially smaller. The current in the coil would decrease so that the energy stored in the
magnetic field is unchanged. This would be equivalent to introducing an iron core into a current-carrying coil –
something that is easier to imagine than to achieve in practice. A mechanical analogy would be to imagine
loading iron into a moving vehicle, whereby the velocity of the iron when it is being loaded is lower than that
of the vehicle. Theoretically, loading the iron onto the vehicle and the associated acceleration of this additional
mass would result in a reduction in the vehicle’s speed by such an amount that the total kinetic energy of the
moving system (vehicle plus load) remains unchanged. In the case of our coil, the current in the coil and
therefore the magnetic field strength would decrease by the factor µr. The magnetic flux density, in contrast,
remains unaffected by the ‘load’ (i.e. by the introduction of an iron core into the coil). The inductance of the
coil has increased by factor of µr, because inductance can be thought of as the time-integrated voltage divided
by the current and the current is now smaller. To re-establish the former current in the coil, the time-
integrated voltage must therefore increase by a factor of µr. In the conventional interpretation of this
phenomenon, the magnetic field associated with current uniformly aligns the normally unordered ‘elementary
magnets’ (or ‘magnetic domains’) within the magnetic material causing a significant increase in the magnetic
flux.
The inductance of a wire coil whose length is much greater than its diameter is proportional to the square of
the number of turns and to the enclosed area of the coil and is inversely proportional to the length of the coil.
The energy contained within the coil is proportional to the inductance of the coil and to the square of the
current flowing. Halving the cross-sectional area of the wire would allow twice as many turns of wire on the
coil without changing the size of the inductor, but it would also increase the ohmic resistance of the wire by a
factor of four. The current would then have to be halved in order to keep power loss due to Joule heating in
the coil unchanged. So for an inductor of fixed size and shape, doubling the number of turns will increase the

Page 3
inductance fourfold, but the energy stored in the coil will remain unchanged if Joule heating losses in the wire
conductor are not allowed to increase. So it is not just inductance, but also the coil’s operating current that has
to be taken in to account when designing and dimensioning an inductor. Another factor that has to be taken
into account when designing inductors is the operating frequency. If the inductor has a core, the magnetic
losses in the core increase with increasing frequency and therefore contribute to heating the coil.
Compensating for these losses would require a larger coil. In practice, however, the size of an inductor
typically decreases with increasing operating frequency, as inductance increases linearly with frequency and a
smaller inductor operating at a higher frequency can produce the same effect (i.e. store the same amount of
energy) as a larger inductor operating at a lower frequency.
Figure 1 – If a current of 1A flows through a straight wire conductor, a magnetic field with a field line of 1m
(and therefore a field strength of 1 A/m) forms at a distance of about 159 mm from the wire.
Figure 2 – An electric field strength of 1V/m is rather weak.
CAPACITANCE AND ELECTRIC FIELDS
An electric field can be described in terms of its field strength E or the electric displacement field D. The
formula for the electric displacement field contains the dimensionless factor that expresses the ratio of the
permittivity of a specific (dielectric) material to electric fields relative to the permittivity of free space (or air,
which for the present purposes is a good approximation of free space). The dielectric materials typically used
in capacitors exhibit relative permittivity values of around 2 to 10. If a voltage source is applied across a
capacitor it will become charged with a charge Q, as free electrons on the capacitor plate connected to the
positive terminal of the voltage source will move towards the positive pole, leaving a net positive charge on

Page 4
that plate, while free electrons in the conductor connecting the negative pole of the source to the other plate
of the capacitor will flow towards the plate creating a net negative charge on that plate.
However, the magnitude of the charge created is not particularly large. If a voltage of 1000 V is applied across
a capacitor with a capacitance of 1 µF, the charge on the capacitor is 10
-3
As or 1 mC, which is equivalent to
precisely 6.24∙10
15
electrons. For the purposes of comparison, a capacitor similar to that shown in Figure 3 will
contain somewhere in the region of 10
26
electrons (of which 10
24
to 10
25
are free electrons). Doubling the
electron density in this material would therefore require the application of around 10
10
∙1000 V = 10
13
V!
Figure 3 – The two components (L=870 mH; C = µF) combine to form a resonant circuit with a resonant
frequency of 50 Hz.
If the charged capacitor is then disconnected from the voltage source, the voltage across the capacitor plates
will remain, as there is no way for charge equalisation to occur. If one were to now introduce into the region
between the plates a medium with a greater electrical permittivity than air, the charge on the capacitor would
remain unchanged, but the voltage and therefore the electric field strength would decrease by the factor of εr.
However, introducing this higher-permittivity dielectric material does not affect the electric displacement field.
Introducing the higher-permittivity dielectric therefore increases the capacitance of the capacitor by a factor of
εr because it enables a greater charge to be stored for a given applied voltage. If the capacitor were to be
reconnected to the voltage source, it would charge up until the voltage across the capacitor is equal to the
applied voltage and the charge on the capacitor would be correspondingly greater than before. One way of
interpreting this is to consider the electrons in the dielectric material. The dielectric is an insulator and thus by
definition has no free electrons. However, dielectrics are frequently organic materials that are made of
comparatively large complex molecules. While not able to move freely within the dielectric material, the
electrons in any one molecule are able to be displaced within the relatively large amount of space provided by
the molecule. The electrons are therefore shifted to one side, creating in many cases a greater degree of
charge displacement, and therefore a greater electric displacement field than when a vacuum or only air was
present between the plates.
In a capacitor used in low-frequency applications, such as one that might be used for power-factor correction
(Figure 3), the distance between the plates is only a few micrometres and the dielectric must therefore be able
to withstand field strengths of several tens of kilovolts per millimetre (Figure 2). The surface area of the
capacitor plates is of the order of square centimetres. As far as is technically feasible, the thickness of the
insulating layer (dielectric) is constant. Under these conditions, it can be assumed that the electric field is
homogeneous. As capacitance is inversely proportional to the distance between the plates, capacitance can be
doubled by ‘simply’ halving the thickness of the dielectric. If this makes it necessary to halve the operating
voltage, then the energy stored in the capacitor is also halved. The energy stored in a capacitor actually varies

Page 5
with the square of the voltage, so halving the voltage would reduce the energy by a factor of four, but halving
the distance between the plates would compensate for half of this loss. So not only the capacitance, but also
the operating voltage has to be taken in to account when designing and dimensioning a capacitor. Another
factor that has to be taken into account when designing capacitors is the operating frequency as it can affect
the mass and volume of material used. The dielectric losses in the dielectric increase with increasing frequency
and may well be the main cause of heating losses in the capacitor. Generally, however, heating losses are very
small. According to one manufacturer, for a capacitor operating at 50 Hz, the main cause of heat loss is the
stipulated discharge resistance.
COMPARISON OF FORMULAE USED TO CHARACTERISE ELECTRIC AND MAGNETIC FIELDS
The following table compares and contrasts the most important quantities used to characterise inductors and
capacitors and illustrates the complementary nature of these two devices.
Figure 4 – The energy in an ideal resonant circuit is constant over time.
Magnetic fields:
l
I
H 
HB r0
l
A
IB
Electric fields:
l
U
E 
ED r0
l
A
UDQ 

Page 6
Inductance:
i
t
uL



dt
di
LuL 
L
udt
iL

fLXL 2
2
2
i
L
WL 
Capacitance:
u
t
iC



C
idt
uC

dt
du
CiC 
fC
XC
2
1

2
2
u
C
WC 
In which:
H magnetic field strength
Φ Magnetic flux
B Magnetic flux density
Am
Vs6
0 102566.1 
 magnetic constant (also: vacuum permeability; permeability of free space)
µr Relative permeability (material constant)
L Inductance
E Electric field strength
Q Charge on capacitor
D Electric displacement field
Vm
As12
0 10854.8 
 Electric constant (also: vacuum permittivity; permittivity of free space)
εr Relative permittivity (material constant)
C Capacitance
l Length of field line (see Figure 1, Figure 2)
A Area of the capacitor plate / Cross-sectional area of a single winding (‘turn’), so current in coil
has to be multiplied by the number of turns as the current ‘acts’ in each turn.
t Time
i Current (instantaneous value)
u Voltage (instantaneous value)
X Reactance.
In an ideal (i.e. lossless) resonant circuit, the energy oscillating back and forth between the inductance L and
the capacitance C is constant.
constti
L
tu
C
tW  )(
2
)(
2
)( 22
In our idealised system, energy is neither added to nor taken away from the circuit. The current that flows
through both components is the same. The voltage across the inductance leads the current by 90°, while the

Page 7
voltage across the capacitance lags the current by 90°. Both the voltage and current waveforms are sinusoidal
– and as we all remember from school…
1cossin 22
 
THE INTERACTION BETWEEN CAPACITORS AND INDUCTORS
The complementary nature of the behaviour of inductors and capacitors is demonstrated when the two
components are connected to form a resonant circuit (or ‘LC circuit’). Once the electric field has discharged,
the current has reached its maximum but does not cease to flow instantaneously. As the current decays, it
charges up the capacitor – though now with reversed polarity – and once charged, the electric field decays
again and the cycle begins again. As the one field declines, the other increases in magnitude and vice versa.
Whenever one field (magnetic or electric) passes through zero, thus changing its polarity, the strength of the
other field (electric or magnetic) reaches its maximum. The resonant frequency f0 of the circuit is given by the
formula:
LC
f
2
1
0 
The resonant frequency is such that the energy in the inductor when the current is at its maximum is equal to
the energy in the capacitor when the voltage is at its peak. If the inductance L in the above formula is made ten
times greater while the capacitance C is reduced by the same factor, then the resonant frequency of the circuit
will not change – though practically every other aspect of the circuit will be altered. The relative magnitudes of
L and C determines whether the current in the resonant circuit is larger while the voltage is lower, or whether
the reverse is true.
The inductance of an electrical circuit can be considered to represent the ‘inertia’ of the electric current,
whereas the capacitance can be thought of representing the tension in a spring or some other elastically
tensioned mechanical parts in the system (‘parasitic capacitances’ of wires and cables).
SPECIAL FEATURES AND DIFFERENCES
Although, as mentioned above, it is better to harness magnetic forces rather than electric forces in technical
applications, the amount of energy stored in an inductor is significantly lower than that stored in a capacitor of
comparable size. The inductor shown in Figure 3 has a current rating of 0.67 A. To get 0.67 A to flow, a voltage
of 195 V would have to be applied if the supply frequency is 50 Hz. The capacitor, on the other hand, is
designed to operate at 220 V at 50 Hz. The energy capacity of the capacitor, and thus its reactive power rating,
is 27 % greater than that of the inductor despite the fact that the two components are of about the same size.
What's more, the induction coil is built of iron and copper and has a mass more than twice that of the
capacitor, which is made primarily of aluminium.
Strangely, when electric and magnetic fields are discussed, for instance, when considering their effects on
organisms, it has become commonplace to always talk about magnetic fields in terms of the magnetic flux
density, but to use the electric field strength to characterise electric fields.
Another factor that has to be taken into account is saturation, which limits the magnetisability of magnetic
materials. When an external magnetic field is applied to a ferromagnetic material, the elementary magnets
(‘magnetic domains’) will all align themselves with the external field. Depending on the material this process of
magnetic saturation occurs gradually or abruptly, but is typically complete once the flux density has reached
about 1.8 T (or 0.2 – 0.4 T in the case of HF ferrite cores). Once the material is saturated the magnetisation of

Page 8
the material cannot be increased further. Any further increase in the magnetic field strength will more or less
act as if the iron core was not present (µr ≈ 1).
This type of saturation effect has not been observed in dielectric materials exposed to an electric field. For
dielectrics, the critical factor is the insulating power of the material. As is well-known, all electrically non-
conducting materials suddenly begin to conduct electricity when exposed to an electric field strength above a
certain threshold. As the electric field strength increases, so too does the associated electric force and at some
point this force is great enough to tear electrons out of the atoms that comprise the insulating material. This
sudden ionisation results in catastrophic failure of the insulating material. As most of the dielectric insulators
are made from organic materials, ionisation is usually accompanied by the formation of soot (i.e. particulate
carbon), which being electrically conductive, initiates the avalanche-like electrical breakdown of the dielectric.
FROM THE ELECTRIC AND MAGNETIC FIELD TO THE ELECTROMAGNETIC FIELD
At low frequencies, an alternating magnetic field and an alternating electric field can (like constant magnetic
and electric fields) be considered to behave as two quite distinct fields. In terms of practical applications, only
the magnetic field is considered useful when generating mechanical energy or when transmitting electrical
energy. At high frequencies, however, the two fields gradually merge to form a single alternating
electromagnetic field. A visual illustration of this process is provided by Figure 5.
Figure 5 – Illustration of an electric field and a magnetic field (1) gradually merging at high frequencies to form
a single electromagnetic field (7).
The resonant frequency of the circuit will increase if L and C are made smaller by reducing the number of turns
and the diameter of the inductor coil or by decreasing the plate area and increasing the plate separation in the
capacitor (see Figures 5-1 and 5-2). If the separation of the plates becomes appreciable relative to the plate
area, which can be roughly said to occur when plate separation is greater than plate diameter, the associated
electric field becomes inhomogeneous and the equipotential field lines begin to bulge outwards emerging
beyond the confines of the ‘actual’ capacitor (see Figures 5-3 and 5-4). As this process continues, the ‘inductor’
eventually becomes nothing more than a single open conductor loop while the capacitor is simply the two
open ends (Figure 5-5). As the inductances of the wires and the capacitance between the wire ends begin to
enhance the actual inductance and capacitance, the magnetic field (blue) begins to shift to the right, entering

Page 9
into the space occupied by the electric field, while the electric field (red) expands leftward where it begins to
enter the space occupied by the magnetic field (Figure 5-6). The final result is a straight length of wire –
essentially a sending antenna – that exhibits a tiny bit of longitudinal inductance and a tiny bit of capacitance
between its ends. The resonant frequency is now in the gigahertz region and the once separate magnetic and
electric fields have merged to a single electromagnetic field.
So when does this occur? Well the transition is a continuous one and dependent on the spatial dimensions
being considered. As is well known, the wavelength λ of an electromagnetic wave is defined as its speed of
propagation (i.e. the speed of light c = 299,792.5 km/s) divided by the frequency f:
f
c

The wavelength is therefore the distance from the crest of one wave to the crest of the preceding wave. If the
spatial region being considered at the source of the waves is substantially larger than one wavelength, then
there will be a great many ‘wave crests’ from both electric and magnetic travelling waves arranged
concentrically around the source and the region will be saturated with ‘electromagnetic’ waves. If, in contrast,
the spatial dimension being considered is less than a wavelength, then this region will be alternately occupied
by an electric and a magnetic field. The components then appear as clearly separate phenomena. When the
size of the region being considered is of the order a wavelength, the nature of the situation that then prevails
is open to debate.

Page 10
SMALL-SCALE WIRELESS ENERGY TRANSFER
As mentioned earlier, magnetic electric fields contain energy. However, the amount of energy or power that
can be transmitted by such a field depends not only on the strength of the field but also on how often the
fields are ‘filled’ and then ‘emptied’ again, i.e. on from the frequency of the alternating field.
NOT SO NEW: THE ELECTRIC TOOTHBRUSH
In principle, every normal transformer represents an example of the ‘wireless’ transmission of electrical
energy, as the energy in the primary coil is transferred via a magnetic field to the – galvanically isolated –
secondary coil. However, the terms ‘wireless’ or ‘contactless’ are only really used in connection with a
transformer that does not have a core, thus enabling the coils to be arranged so that – as one of the relevant
technical standards states – when ‘used in the intended way’ the coils can be separated ‘without the use of
tools’. Electrical tooth brushes are a well-known example of this type of arrangement. The primary winding is
located in the charger stand, while the secondary winding is situated at most a few millimetres above in the
base of the brush unit (Figure 6, Figure 7). Placing the two coils in such close spatial proximity ensures that
most of the primary magnetic field is able to permeate the secondary winding and thus generate a voltage in
the secondary coil that is proportional to the rate of change of magnetic flux, as expressed by the law of
induction. The rate of change of the magnetic flux increases the higher the peak value and the higher the
frequency. The missing core is often approximated by a spike or pin-like protrusion on the charger unit (see
Figure 8). This enables a sufficiently large magnetic flux to be generated from a relatively small electrical
current, thus reducing resistive losses in both coils. A flux frequency higher than that of the mains supply is
required in order that the small-sized coils used can generate sufficient (though still modest) power to charge
the battery in the brush unit.
While the advantages of this design are self-evident (users do not have to worry about dangling cables while
brushing their teeth), the question arises as to just how efficient this form of energy transfer really is. But
that’s not an easy question to answer, as the power consumed by the charger stand shown here is always the
same whether it’s got something to charge or not. The battery has a capacity of about 3 Wh and a full charge
cycle takes about 16 hours to complete, so the net charging power is around 0.2 W and the efficiency during
the charging cycle is just short of 15 %, which is hardly something to write home about. If we assume that the
toothbrush is actually in use for about 10 minutes a day, which is probably a pretty generous estimate, and
that the charger stand is permanently connected to the power supply, which is also probably a realistic
assumption, the annual efficiency is less than 0.5 %. Unfortunately that sort of figure is typical and cannot be
attributed to the contactless inductive charging technology used. The efficiency would not be greatly improved
if a wire connection were to be used to charge the battery. The focus here needs to be on the power being
delivered by the charger relative to its permanent power consumption level of 1.3 W, and whether this latter
figure could be reduced by, say, a factor of 10 by implementing some relatively simple technical
improvements. As things stand, it would take around 25 years before the cumulative energy costs caught up
with the purchase price. And there are – or there were – far worse examples in this market sector. Much has
already been achieved thanks not least to effective EU regulations. As far as electric toothbrushes are
concerned, however, the biggest driver of operating costs is the expensive replacement brush heads.

Page 11
Figure 6 – Whether this electric toothbrush is currently being charged…
Figure 7— …whether the battery is fully charged, or…
Figure 8 – …whether the brush unit is not even on the charger – the power consumed by the device is always
the same!

Page 12
Nevertheless, using wireless energy transfer in this particular application offers real benefits in terms of
comfort and usability. There are no major drawbacks apart from the fact that these sorts of consumer
appliances are relatively expensive. But relative to what? Any alternative design using a wire connection to the
hand-held electric brush is not available on the market and the energy savings that would result would be
pretty small. This type of wireless technology has been around for decades and so can hardly be thought of as
particularly innovative. And having been around for so long, there is less pressure to market the technology as
particularly ‘energy efficient’.
NEW: CONTACTLESS CHARGING OF MOBILE PHONES
At present there is some heated debate about using contactless induction charging to charge mobile phones
and other small devices that are powered exclusively by a battery and not directly from the outlet socket.
Given today’s ‘gadget chaos’, in which we have an ever-increasing number of devices all needing to be directly
attached to an outlet socket, the idea is an attractive one and could be of major interest if it ever became
possible to charge a phone with anything other than the charger unit supplied with it.
NO NEED FOR STANDARDIZATION?
This latest development has stalled because, once again, the EU wishes to put in place regulations that ensure
that uniform standardized connections are used for charging small devices. Equally, those modernists, for
whom anything new is always synonymous with progress, fail to see that certain standards need to be
established for inductive charging. For example, how high should the frequency be? Should the frequency be
continuously available or clocked in some way to facilitate communication with the energy consumer? Perhaps
a variable or alternating frequency should be used for the purpose of communication or for adjusting the
power? Or should the power be controlled by varying the intensity of the field, that is, the excitation current in
the charger unit? Sinusoidal or rectangular? And does the device being charged also need to be able to
communicate with the charger? Does the charger unit need to be able to be switched off manually, or can it
switch itself off automatically when the device is fully charged or when no device has been placed on the
charger unit? Oh, the questions that life throws at us!
EMC
Electromagnetic compatibility (EMC) is another issue that has to be addressed. The radiation from
electromagnetic fields is subject to legal limits and in an inductive charging system, it is not only the secondary
winding that experiences the relatively strong alternating field, but also all the other electronics housed in that
part of the device. Can the phone function while being charged inductively or is it unable to receive signals
during this time? How do the sensitive electronic components respond to the strong stray fields? And what
should a charger unit do when it’s got nothing to do is another question being currently debated. In this type
of application we are a long way from concentrically arranged coils neatly placed one above the other and we
have nothing like the layout found in the electric toothbrush, where we were dealing with coils of the same or
similar diameter and a centrally located pin containing a little ferromagnetic material that forces the coils into
a concentric arrangement while also strengthening the coupling field. With the coils arranged in a less than
optimal fashion transferring even a small amount of energy from one coil to the other on the basis of Faraday’s
principle of electromagnetic induction is a difficult task no matter how high the frequency. Increasing the
frequency would cause a linearly proportional increase in the inductive reactance making it harder for the
electronics to ‘press’ the alternating electric current into the primary coil, as only the smaller (coupled) portion
of the flux will be opposed by the magnetic field generated by the induced current in the secondary coil. This
technology is already mature and market-ready. Well it is if you believe the marketing. There are lots of
diagrams showing a panel on which one or more devices can be laid at random to be charged. The reality,
however, is that these are still just scenarios. There are no marketable products – just lots of wishful thinking.
There are numerous websites claiming that wireless charging technology of this type is or will be commercially

Page 13
available, but they always shy clear of ever saying when (now, soon, the distant future?). Readers are
therefore left to draw their own (rather negative) conclusions.
If the receiving coil in the device to be charged, i.e. the ‘secondary winding’, has a diameter one tenth of that
of the transmitting coil (the ‘primary winding’), then the ratio of the cross-sectional areas of the two coils will
be 1:100. This means that about 99% of the alternating magnetic field generated in the primary coil goes
unused. This situation is acceptable if the powers to be transmitted are correspondingly small, such as the
power required by a wireless PC mouse, a TV remote control, or similar devices. Occasionally one finds
wireless products that, being available commercially, really deserve the name – but they always turn out to be
applications, like the electric toothbrush, in which there is close spatial proximity between the primary and
secondary coils or the energy demands of the specific application are very modest.
EFFICIENCY
For products that aren’t actually available for purchase, there is a remarkable amount of marketing talk
surrounding them. One of the issues that rarely gets touched upon is their efficiency – and that perhaps is the
core of the problem. The somewhat arbitrary arrangement of the coils does not, however, mean that 99 % of
the energy inevitably disappears into nirvana and is lost forever. After all, an ‘unused’ alternating magnetic
field will feed its energy back to the source during the negative half of the cycle. We are effectively dealing
with an air-core inductor, i.e. with reactive power. However, the power factor for the energy transfer is only
about 0.01. Therefore, to achieve 5 W of charging power (see Figure 9), an apparent power of 500 VA would
need to be applied to the transmitting coil. The electronic circuitry would thus need to generate this 500 VA of
apparent power – technically not a problem, but expensive to realize. Or we accept longer charging times.
Using a 100 VA primary coil would result in charging times five times as long. If the frequency is high, this
primary coil can be kept thin, i.e. with few windings, which in addition to its correspondingly large area would
also require using a wire of large cross-section if energy consumption is not to be excessive.
Figure 9 – Charging a mobile telephone.
In contrast to the electric toothbrush, a conventional charger unit sets the bar pretty high (Figure 10). The
battery has a charge of 0.9 Ah at a nominal voltage of 3.7 V, which corresponds to about 3 Wh of stored
energy. After charging for two hours, the battery is fully charged and the charger unit has taken 8 Wh from the
mains network, which corresponds to an efficiency of 37.5 %. After charging – and irrespective of whether the
telephone is removed from the charger or remains connected to it – the charger unit has a standby power
consumption of only 0.16 W (Figure 9).

Page 14
Figure 10 – The charging time profile of a mobile phone.
The practical implementation of ‘wireless’ technology clearly requires a lot of wire. Comparing the inductive
charging and the conventional charging scenarios discussed above, we can see that wireless charging requires
replacing a short thin wire with a long thick one. But the efficiency is still not great. Losing 10 % of the
apparent power in the copper and the electronics would generally be considered a good value, but it would
still be nine times greater than the active power transferred. This would, at worst, correspond to an efficiency
of 10 % and, at best, a power factor of 0.1. Low energy efficiency is the other major cost driver in wireless
technology – making it hard to sell as a ‘green technology’.
RFID
Radio-frequency identification is another modern electronic technology that has been in use for some time
and that – although perhaps not obvious to most people – also operates on the principle of wireless energy
transmission. RFID tags are used in retail outlets to prevent theft, in person identification devices for
controlled access areas, in time and attendance recording (Figure 11 and Figure 12) and in many other
applications. RFID technology is also likely to play a future role in the trade and logistics sectors. There are
even RFID implants for pets. A cat fitted with an RFID implant could, for example, automatically open its own
‘cat flap’, while the neighbour’s cat could not – a nice example of automated controlled access. The more usual
approach is to fix the ID tag, the so-called transponder, to the animal’s collar, but collars can, of course, get
lost.
Figure 11 – Contactless time and attendance recording for employees on flextime.

Page 15
Figure 12 – Time to knock off – having spent a full four minutes at work!
The transponder, which is typically integrated into a plastic card or a key fob, contains an induction coil that is
not normally visible (Figure 13). The principle is simple: A high-frequency alternating magnetic field is
generated at the point to be monitored. If the coil that is mounted on the tag comes close enough to this field,
a high-frequency alternating voltage will be induced that is sufficient to power a miniature transmitter that
then transmits a radio signal.
Figure 13 – RFID chip – one of many possible deigns, but the smaller an RFID chip is, the smaller the amount of
energy it can receive.
But the problem is the same as always: only when the transmitting and receiving coil are in close proximity to
one another is it possible to transfer enough energy or power. In principle, transponders can be designed to be
extremely small, but size is also a crucial factor in determining the operating range.
If one wants to be able to monitor an entire person and so save that person the effort of taking the
transponder out of their pocket, then the whole head and body must pass through an induction coil, which
would have to be dimensioned accordingly. This is one of the reasons why the theft prevention scanners used
in large clothing stores are so big and so clearly visible, the other no doubt being that they are also designed to
act as deterrents to put off potential perpetrators.
ENOCEAN
Refitting or converting electrical equipment in older residential buildings is often problematic, particularly
when the property is not undergoing general renovation at the same time. Installing new cable runs is usually

Page 16
unfeasible, making it practically impossible to reconfigure the switching on existing lighting installations or
individual lamps in an effort to reduce energy consumption. Installing sensors and actuators, each of which has
to be powered from the available power supply, often nullifies the theoretically achievable energy savings as
the sensors and actuators run on small DC voltages that have to be generated by transforming the line voltage
– a process associated with high conversion losses. As the saying goes: good advice doesn’t come cheap, nor
does good technology. But such technology, though expensive, is actually available.
Figure 14 – Viewed from the outside, it’s just another standard flush-mounted light switch…
Figure 15 – …but in fact it’s a surface-mounted switch containing a miniature radio transmitter. RF remote
switches of this type are useful whenever cable-based installations are not an option.

Page 17
‘EnOcean’
2
is a consortium of some 107 companies offering products that can communicate wirelessly with
one another without requiring an external energy source, not even a battery. The energy to power these
devices comes from harvesting ambient energy, such as harvesting ambient light energy by means of battery-
buffered photovoltaic cells or harvesting the mechanical energy used to operate a light switch (Figure 14). In
the latter case, the mechanical energy of pressing the switch is converted by a magnet and wire coil into
electrical energy. That short burst of electrical power is enough to transmit a tiny radio signal. Originally, these
self-powered switches made use of piezoelectric generators, but the wound copper coil has proved to be the
better option (Figure 15).
Once again, we have an example of ‘wireless’ technology that, strictly speaking, still relies on wire. But the
‘wireless’ descriptor is fair enough as the technology does indeed represent a means of transferring energy
without the need to lay cables or wires.
LISTENING TO ROCK THROUGH CONCRETE
It is a long time ago that the author of this article used to spend his time tinkering about in his parent’s
basement workshop, no doubt taking the first steps that would lead on to a career in electrical engineering.
The problem was that the cellar was not my bedroom, so listening to Deep Purple or Uriah Heep on my
headphones was not an option. At that time, portable music players had either not been invented or, if they
were available, were at a price that would have left me with no spare cash for buying the resistors, electrolytic
capacitors, rectifiers, thyristors and all the other components that my teenage heart desired. But if portable
players weren’t available, magnet wire from old vacuum cleaners or from the scrap box of a local transformer
manufacturer was. And, luckily, my father had recently moved into the world of solid-state hi-fi, leaving me
with the old stereo system – an interesting combination of a transistor-based pre-amp and a valve power amp
(2x7 W; input power: 70 W) that I was allowed to set up in my bedroom, which, as it happened, was
conveniently located directly above the cellar workshop. But instead of plugging in some headphones or a pair
of speakers, I chose to take the magnet wire and loop it six times around the walls of my bedroom, running
along or behind the skirting board, behind the cupboard and bed and carefully fastening it in the corners. And
then I repeated the process for the other channel to amplify the signal. By placing a large wire-coil block (of
unknown origin but containing around half a kilogram of copper) vertically on the workbench in the cellar and
attaching a pair of headphones to this secondary coil, it was now possible to listen to music at an acceptable
volume and in really quite good quality down in the cellar, provided the stereo system in the bedroom had
been turned up to full volume and the output from both power amps was fed into the windings of the ‘primary
coil’ that ran around the walls of the bedroom. The output was only in mono as getting stereo reception would
have meant installing the second of the primary coils in a perpendicular orientation on the wall and that would
have been a little less simple to conceal. Bass reproduction was significantly improved by inserting a strip of
electrical steel sheet from an old transformer into the secondary coil in the basement. Strangely, this had no
effect on the treble volume. The reason is probably that at high frequencies the leakage reactance of the
‘secondary winding’ of the ‘transformer’ (i.e. the coil in the basement) was greater than the impedance of the
load (i.e. the loudspeakers). The presence of the steel insert ensured that both low and high frequencies were
‘captured’ more efficiently, but that the higher frequencies were lost due to the greater leakage flux at these
frequencies. Things would probably have sounded different if the headphones would have had greater
impedance.
2
www.enocean-alliance.org

Page 18
Nevertheless, the set up was a form of inductive energy transfer, albeit a highly inefficient one with an
estimated efficiency of 0.01%. Of the 2x7 W upstairs, only 1 mW was left to drive the headphones in the cellar.
But despite the low power they still managed to deliver a full 95 dB(A) without the need for an external
amplifier, i.e. without a separate power source. The 95 dB were of course more than was allowed by law, or by
my parents, who luckily for me remained blissfully unaware of the situation.

Page 19
WIRELESS ENERGY TRANSFER IN ‘MEDIUM-POWER’ APPLICATIONS
In low-power applications, practical use rather than efficiency is usually the key concern. However, in
applications in which more than just a watt or two of power is being transferred, the question of efficiency
begins to grow in importance.
ELECTRODELESS FLUORESCENT LAMPS IN PUBLIC SPACES
Another wireless energy transfer application that has been much talked about is the electrodeless fluorescent
lamp, or induction lamp. Like many other applications in the field, these products are rarely encountered in
practice despite being officially available on the market. These lamps consist of an annular closed glass tube
that is enclosed at one or two locations by a core (Figure 16). The core is wrapped with wire to form a coil that
acts as the primary winding of a toroidal transformer. The glass tube functions as the secondary winding.
OPERATING PRINCIPLE
A high-frequency current flowing through the coil induces an electric current in the glass tube. From there on
the technology corresponds to that of a conventional fluorescent lamp with the exception that in this case the
current in the gas starts to flow ‘on its own’, i.e. it does not require a starter or any kind of electrodes. As the
lamp manufacturer Osram correctly states: ‘It has long been known that a fluorescent lamp could be
illuminated by inducing a current in the tube’
3
. The company continues quite correctly by saying that, ‘this fact
can be demonstrated quite easily by simply rubbing a fluorescent lamp with a piece of nylon fabric or fur. The
rubbing action generates static electricity on the surface of the glass tube so that small electric fields are
created within the tube.’
Figure 16 – Principle of an external inductor lamp.
(http://en.wikipedia.org/wiki/File:External_Inductor_Type_Induction_Lamp_Dwg.jpg)
3
www.osram.de/osram_de/Tools_%26_Services/Training_%26_Wissen/Lichtlexikon/popups/pop_Geschichte_I
nduktionslampe.jsp

Page 20
The company then states: ‘These small fields induce small electric currents that cause the tube to light, though
only in those regions in which the tube was rubbed.’ What the experts at Osram fail to mention is that the
induction referred to here is electrostatic induction and not electromagnetic induction! In this case the
electrostatic field generated by rubbing the glass in the manner described causes some localized ionization of
the mercury atoms in the tube. The excited mercury atoms emit UV light that itself excites the phosphor
coating on the inside of the glass leading to the emission of visible light. In an induction lamp, however, the
mercury atoms are excited purely by the high-frequency magnetic field generated by the coil.
EFFICIENCY
The crucial advantage claimed for induction lamps is their extremely long lifetimes of around 60,000 operating
hours, which is achievable as these lamps do not have an electrode inside them that can age and eventually
fail. This is a major benefit whenever fluorescent lamps are installed in inaccessible locations such as in high
halls. However, conventional fluorescent lamps have also improved and now offer a typical service life of
45,000 operating hours. The efficiency of inductive energy transfer via high-frequency fields is, once again,
nothing like one would get were a wire connection to be used. And, last but not least, the inductor inevitably
blocks some of the light from the lamp. To reduce such losses, the ‘toroidal transformer’ would need to be
built to be as small as possible, which in turn would reduce its electrical efficiency. In those regions where
there is still a market for induction lamps, such as in Brazil
4
, the performance efficiency is typically 75 lm/W;
today’s conventional fluorescent lamps are expected to deliver an efficiency of 100 lm/W.
DON’T TRY THIS AT HOME…
It is sometimes stated that putting a compact fluorescent lamp, even a dead one, into a microwave oven and
switching the oven on not only causes the lamp to light up (at least initially), but also nicely illustrates the
lamp’s underlying operating principle. While such an experiment is no doubt interesting and impressive, we
definitely do not recommend trying it. After all, a lamp with a rated power of say 13 W would be exposed to
radiative power of around 800 W and it is certainly not clear how the lamp would react. Whereas the
commonly assumed health hazard in which 3 mg of mercury vapour would be released is a myth
5
, the
presence of splintered glass from a burst lamp inside the oven is hardly compatible with the appliances true
purpose, namely the preparation of food. The release of other contaminants from the electronic components
is not easy to calculate as that depends on their conductivity and how they interact with the radiation field
whose intensity varies considerably depending on how the microwaves are transmitted, reflected or absorbed
by the respective materials, giving rise to extremely high, localized temperatures within the oven cavity. The
fact that microwaving a plate with a gold rim emits a shower of sparks, gives one a feel for just how high the
induced voltages can be. On the one hand, the gold rim is effectively a short circuit winding, making it difficult
to see where the voltage drop needed to ionize the surrounding air could develop. However, microwave ovens
operate at a frequency of 2.45 GHz and this single large-diameter winding has a very significant reactance at
this frequency, which explains the flurry of sparks typically observed.
Unlike some of the other applications discussed above, most of the power within the oven does in fact interact
with the designated target. At a frequency of 2.45 GHz, the wavelength is around 122 mm. That is the distance
between the crests of successive microwaves. Clearly several wavelengths are able to fit within the dimensions
of the oven. The oven housing is made of metal and as metals reflect electromagnetic waves, the microwaves
4
See, for example, www.everlastlight.com
5
Stefan Fassbinder: “Tragische Sparlampen?”, Schweizer Zeitschrift für angewandte Elektrotechnik 10/2011, p.
27

Page 21
remain within the oven. This also applies to metal mesh as long as the mesh size is significantly smaller than
half a wavelength. The inside of the glass oven door is therefore fitted with a thin metal mesh, which is
transparent to visible light but which prevents microwaves from escaping.
The efficiency of these appliances is typically quoted as about 70 %, i.e. about 70 % of the input power is
actually transferred in the form of electromagnetic radiation to the food being processed. The microwave
radiation is absorbed by the food and converted into heat energy. As an oven is by definition designed to
generate heat, it is perhaps not particularly surprising that in this case the technology has a much greater
efficiency than when used to charge a battery.
By the way, where does the power go when a microwave oven is operating without anything in it? Something
manufacturers do not recommend! With nothing in the oven to absorb the microwaves, the radiation intensity
rises significantly eventually making its way back to the magnetron, the component that generates the high-
frequency microwaves, potentially causing it to overheat and become damaged.
HEALTH
Occasionally reports surface that people living in the immediate proximity of powerful radio transmitter masts
(100 kW) don’t need to connect their fluorescent lights to the mains supply – they just light up on their own.
Apparently, these lights can be quite literally ‘turned off’ by simply rotating the lamp through 90° until it is
perpendicular to the polarization of the field. Another myth? Another urban legend? The reports are probably
just that. Reliable witnesses are still being sought!
What has been reliably documented, on the other hand, is that the very sight of a mobile radio mast (20 W) in
the local vicinity is enough to cause entire neighbourhoods to fall ill – even before the transmitter mast has
been connected to the grid! A number of authenticated cases have been recorded.
INDUCTION HOBS AND COOKERS
Here, too, the application is ‘simply’ the generation of heat. However induction heating is a circuitous process.
Initially, electrical energy creates a high-frequency alternating magnetic field that then generates electrical
eddy currents in the base of a cooking pot, which itself heats up as a result of resistive heating and this heat is
then used to heat the contents of the pot. That is hardly a straightforward procedure. And the investment
required goes beyond the price of the hob itself, as special pots and pans are also required. Induction heating
can be thought of as analogous to the heating effect generated in friction brakes. The ‘brake pad’ and the
‘braking force’ have to be selected to match the frequency. The brake has to function effectively, but not so
effectively that the ‘wheel’ locks, which would generate heat in the tire and not in the brake pad. Specifically,
the base of the pot has to be thick enough and its conductivity high enough so that the ‘eddy voltage’ induced
in the material generates a sufficiently large eddy current. However, the electrical conductivity of the base
material must not be so large – or equivalently its resistance so low – that the ‘brakes’ appear to have been
oiled. The eddy currents in the base of the pot have to big enough to produce the required Joule heating
effect. The situation is not unlike that in a conventional transformer, where the short-circuit current is strongly
inductive in nature, because the conductivity of the secondary coil is very high and the short-circuit current is
therefore predominantly limited by the size of the stray inductance, i.e. the stray magnetic field. If the
resistivity in the base of the pot is too high or too low, too little heat will be generated – at least, there where
it is needed. If the pot is not present on the cooking zone and there is therefore no opposing field, the primary
field would be able to spread further afield and could potentially cause unwanted heating effects in other
electrically conducting parts in the vicinity. Eddy currents are eddy currents, no matter where. This potential
problem has to be monitored by sensors that detect whether a pot has actually been placed on a cooking zone
that has been switched on.

Page 22
The advantage of induction cooking is that it is only the pot gets hot. The ceramic surface of the cooker can
and should, in principle, stay cold; in practice it becomes only moderately warm. Other benefits of this
technology are lower energy consumption, despite the rather circuitous route by which electrical energy is
converted into heat energy, and the speed at which the pot heats up. The risk of burning one’s fingers is also
considerably lower as the residual heat in the cooking surface is minimized once the cooking vessel has been
removed. How many of these advantages are of real practical value is perhaps best illustrated by quoting a
user: ‘The induction hob is really a lot quicker than a standard electric cooker. It’s like cooking with gas, when
you turn it off, the heat disappears immediately. If you have a saucepan of boiling milk you only have to turn
off the cooking zone rather than remove the pan from the hob as you used to have to do with a conventional
electric cooker. Of course the surface does get hot when the bottom of the pot is heated inductively, but the
warning lights that indicate the presence of residual heat extinguish very quickly because the cooking zones
never get really hot.’
Nevertheless, induction hobs are controlled electronically and when manufacturers start to introduce
electronic control systems they usually don’t know when to stop and end up automating so many functions
that the user often gets confused by the sheer number of possibilities on offer. Unconfirmed reports state that
standby power levels are about 8 W, which is at a level that could easily negate the energy savings that an
induction cooker offers over a conventional electric cooker.
The story is always the same: any new technology must be marketed as an energy-saving development and
somehow the manufacturers always manage to cobble together some argument to convince customers that
this is the case. Usually, however, the new technology does a lot more than its predecessor, offering greater
ease-of-use and possibly greater safety, it will also no doubt be more efficient despite its greater capabilities or
higher performance levels, but, almost inevitably, absolute power consumption will rise.
Perhaps the answer is to make more and varied use of existing technology. Maybe in future we will be able to
charge our smart phones on our cookers.
LARGE-SCALE WIRELESS ENERGY TRANSFER
The transport sector is currently considering a number of stationary and mobile applications of wireless energy
transfer, some of which have reached the planning stage, while others have already been implemented. In the
case of mobile applications, it is also important to distinguish between conventional (single power source)
systems and hybrid systems. A system is referred to as hybrid when it consists of two or more subsystems that
can work alternately or in parallel, but each of which is also able to accomplish the required task individually.
While superficially similar, these systems differ quite significantly in terms of their design concepts.
STATIONARY SYSTEMS
As mentioned previously, a conventional transformer can also be thought of as transferring electrical energy
wirelessly. Indeed some transformers – so-called isolating transformers – are designed not to switch one
voltage level to another, but to galvanically isolate the voltage on the input side from the same voltage level
on the output side. As is well known, restricting oneself to the existing line frequency will limit the distance
over which the energy can be transferred. Generally speaking, the electrical energy will need to be converted
to a higher frequency in order to achieve any meaningful transmission distance.
There are also cases of wireless energy transfer in which the appropriate frequency can be said to be
generated automatically and, interestingly, these applications have been around for some time. While the

Page 23
distance to be covered in these cases is often less than 1 mm, the essential point is that electrical energy is
transferred in a contactless manner from a stationary component (the ‘stator’
6
) and a rotating component (the
‘rotor’).
EXCITATION POWER IN ELECTRIC MOTORS
Figure 17 – Sliding contacts are avoided whenever possible. In this case they are made from a sintered copper-
graphite material.
As is well known, the Achilles’ heel of commutator motors and three-phase slipring motors is the commutators
or sliprings themselves. As the shaft is typically equipped with rolling-contact bearings, the commutator or
slipring is often the only sliding element in the system and therefore the only part to be subject to substantial
wear. Nevertheless, even in a synchronous machine, excitation power has to be transferred to the rotor
winding. Sliprings offer an obvious, but often unfavourable solution. A more complex but considerably better
answer is to make use of inductive energy transfer. Even if the excitation power of, say, a large power station
generator is only 1 ‰ of nominal power, it still represents about 1 MW of power. Even if permanent-magnet
excitation was technically feasible, it would not be the method of choice from a power system management
perspective as excitation has to be variable in order to control the generator voltage. While auxiliary
permanent-magnet excitation is conceivable, it would still not be possible without the exciter winding.
Providing 1 MW of excitation power anyway requires a separate generator and this would have its own exciter
winding (so-called brushless excitation systems). In a brushless exciter, the configuration is reversed to that in
the main machine, the exciter winding is now configured as a stator, while the main winding (the armature
circuit) is mounted together with a rectifier on the rotor shaft of the main machine. By controlling the DC field
current of the exciter generator, the exciting current (i.e. field current) in the main machine can be adjusted
and thus a significant amount of electrical energy can be transferred without any contact between rotor and
stator.
CHARGING ELECTRIC CARS
6
lat. stare = stand

Page 24
While a plug-in connector for an electric car would undoubtedly represent a major improvement on the
unwieldy fuel hose that we currently use on our petrol- or diesel-powered cars, for those who want to keep
their hands clean at all costs, a fully hands-free, contactless means of charging an electric vehicle would be a
dream come true. Systems of this type were exhibited by a number of companies at the 2011 Hanover Trade
Fair. The systems on show were designed for charging e-bikes, scooters, wheelchairs and similar vehicles, but
the stated goal was clearly to use this technology to charge electric cars. Efficiencies of around 90 % to 95 %
were being claimed. Recent tests of wireless charging of electric vehicles yielded similar results: ‘Although the
efficiency of the system has not reached that of cable-based charging, a figure of 90 % is already looking very
promising and only slightly worse than that of pluggable, cable-connected charging solutions – and that
includes all system components from the outlet socket to the battery’
7
. That certainly sounds impressive,
especially to the layperson, until one realizes that the losses over those final few metres of the energy
transmission path are greater than the losses incurred within the entire power network, from the power
station to the socket. Those losses continue to be incurred, as do the charging/discharging losses in the battery
and the losses in the rectifier / charge regulator. As is often the case, however, one can choose to present the
facts of a situation in a positive or negative light depending on the argument one wants to win. Efficiencies of
more than 90 % sound good, but the losses here are 20 times those in a conventional cable connection. This
compounds the question of whether the technical effort involved and the benefits accrued are proportionate
and whether the electric car, should it ever become widely available, will actually end up saving any primary
energy, which was, after all, the very reason for designing it in the first place. There seems to be a potential
conflict of objectives here. Which car owner would accept stay-in-the-car, hands-free refueling if that involved
spilling five litres of fuel every time? Probably the only ones prepared to pay that price for the comfort are
those who can afford a chauffeur and who would therefore not be getting out to do the refueling anyway.
What has already been said about low- and medium-power wireless energy transfer systems is even more
applicable in the present case. The charging system will only really function efficiently when the coils are
located almost exactly above one another and when the distance between them is only a fraction of the
diameter. Any other configuration results in a huge increase in both system size and reactive power and a
corresponding reduction in energy transfer efficiency. The smaller the portion of the magnetic flux in the
primary winding that actually permeates the secondary winding, the less able the system is to function as
intended as a transformer, and the more it resembles two mutually independent inductors.
TRANSRAPID: A NON-HYBRID MOBILE SYSTEM
For decades the Transrapid maglev train ran round its test track in Emsland in northern Germany and waited
for potential customers to show an interest. In Europe, not a single client, public or private, was convinced of
the commercial viability of the Transrapid train. And the reasons are not hard to find. The magnetic levitation
(maglev) transport system suffered essentially the same fate as the induction lamp. When the idea was first
conceived, the new system was miles ahead of the competition. However, during the years it took to develop
maglev technology, conventional rail transport systems were able to make up surprisingly large amounts of
lost ground. Suddenly, ‘high-speed trains’ were running on the national rail network with scheduled operating
speeds of 200 km/h, increasing to 300 km/h a relatively short time later. New speed records were set, and
with top speeds increasing from 525 km/h to 575 km/h, the advantages of the Transrapid system became
increasingly marginalized. Almost overnight, the technology gap that Transrapid had enjoyed, had shrunk such
7
Press release issued by Conductix-Wampfler on 5 Dec. 2011: ‘Praxistest zum kabellosen Laden von Elektro-
fahrzeugen von Daimler und Conductix-Wampfler’ [Practical testing of inductive charging of e-vehicles from
Daimler and Conductix-Wampfler].See www.conductix.de/index.asp?vid=12&id=14&news_id=346&lang=D

Page 25
an extent that is was no longer worth investing all that time and money on – if indeed it ever was. The reasons
for its demise will be discussed in detail in the following sections.
THE CONCEPT
It is probably fair to say that reinventing the ‘wheel’ in such a way that the wheel’s very existence then
requires the consumption of energy is a questionable way to develop a new transport system. An anonymous
and rather old calculation obviously produced by Deutsche Bahn, the German national railway company,
states, though without providing any figures to back it up, that simply energizing the support magnets (also
referred to as lift or levitation magnets) and the guidance magnets to suspend the Transrapid requires as much
energy as a conventional ICE high-speed train needs to run at 120 km/h. The magnets are DC electromagnets
with solid-state controllers that are able to adjust the magnetic force so that the vehicle floats at a distance of
about 10 mm from the steel guide rails. The superconductors that were used in the 1972 version of the vehicle
(that’s how long these vehicles have been running on test tracks) are no longer part of the design. The energy
that would have been saved by the use of superconducting magnets would have been spent on permanently
cooling the magnets to -170 °C (prior to 1986, the magnets would have had to be have been cooled down to -
270 °C!). As is so often the case when superconductors are forced into the role of delivering energy savings
8
:
the hoped-for savings become worthless when weighed against the associated (and costly) operational issues
involved. In this particular case, using superconducting magnets on the Transrapid would have meant either
stabling the vehicle with its cooling system permanently running, or having a long wait for the magnets to cool
down sufficiently for the vehicle to be used.
The long stator of the linear drive motor can be thought of as a ‘cut open and straightened out’ version of the
stator of an air-core three-phase asynchronous motor. It is installed in the guideway rather than in the vehicle.
The guideway is therefore divided up into a sequence of such stators and energizing the stator section
currently below the train will cause the train to levitate above that portion of the track. According to a model
of an earlier version of the Transrapid system (originally on show at Hanover University of Applied Science in
Germany, now housed in Zhejiang province, China), the section length proposed at the time was 1.6 km.
However, the stators used at present
9
are only marginally longer than the train. Another source
10
states that
the sections used for the Transrapid of 1988 were of varying lengths ranging from 300 m to 2,080 m.
Information on the segment lengths for the subsequent Transrapid08 and Transrapid09 generations is very
hard to find – almost as if there was something to hide. As the length of a stator segment (i.e. the ‘magnetic’
length) is always longer than the physical length of the train, energy is lost as stray magnetic fields that extend
beyond the ends of the vehicle into the surroundings. And as the stators are essentially air-cored inductors, a
much greater conductor cross-section would be needed than in a coil with an iron core for the same level of
apparent power. In this case, it is not possible to make use of a higher frequency in order to reduce the size of
the stators, as the frequency has to be consistent with the speed of the vehicle – as well as the number of
poles and the length (formerly: diameter) of the long stator – and does not therefore exceed 230 Hz.
The earlier proposal of a combined lift and propulsion magnet was never realized in practice. These combined
lift-and-propulsion magnets were configured to be mechanically and electrically isolated from the long stator
of the linear drive motor. It would have proved too complicated to simultaneously control the variable
8
Stefan Fassbinder: ‘Elektrische Leiter – Alternativen zu Kupfer? ’ [Electrical conductors – Alternatives to
copper?]. Schweizer Zeitschrift für angewandte Elektrotechnik 4/2008, p. 27
9
www.transrapid.de
10
http://de.wikipedia.org/wiki/Transrapid

Page 26
horizontal propulsion and braking force and the more or less constant, precision-regulated vertical lifting force
using just a single coil – especially as the idea of using a synchronous drive with permanent magnets on the
vehicle had already been rejected at an earlier stage. Precisely controlling the interaction of the travelling
wave magnetic field with the vehicle’s support magnets in order to make optimal use of both attractive and
repulsive forces for propulsion requires careful control of the opposing fields in what would be the ‘squirrel-
cage rotor’ in an AC induction motor, but in a maglev system is simply referred to as the ‘secondary’ that is
installed underneath the vehicle. From a system control perspective, this is far harder to manage than the
constant opposing field from a permanent magnet ‘secondary’.
PERFORMANCE
In terms of what it can deliver, the Transrapid concept is way ahead of existing railway transport. Travel speeds
are now quoted to be around 500 km/h with an ability to climb gradients of ‘up to 10 %’. Although the exact
significance of the ubiquitous term ‘up to’ is, as so often, unclear, conventional railway services measure track
gradients in parts per thousand
11
. According to the manufacturer, the Transrapid will have reached a speed of
300 km/h after accelerating over a distance of only 5 km; a ‘modern high-speed train’ would require 20 km to
achieve the same speed. The latter figure corresponds to an average acceleration of 0.175 m/s² and is
compatible with calculations made elsewhere. An ICE3 train takes 42 s to reach a speed of 100 km/h from a
standing start, equivalent to an average acceleration of 0.67 m/s². The Transrapid08 achieves the same feat in
34 s. In addition, the acceleration of the ICE drops significantly with increasing speed, whereas, rather
curiously, the Transrapid08 takes less time to accelerate from 100 km/h to 200 km/h than it does to accelerate
from nought to 100 km/h. But the crucial factor for running at high speeds is the vehicle’s initial acceleration.
Unfortunately, no information on the running dynamics of the Transrapid09 is publicly available. The equation
of motion
asv 2
Which relates the acceleration a (assumed to be constant), the distance travelled s, and the velocity v, can be
rearranged to:
2
2
2
92.1
10000
139
2 s
m
m
s
m
s
v
a 







An acceleration of 1.92 m/s² is of course several times greater than that achievable with an ICE train, but it
comes at a price. The force F required can be calculated as follows:
kN
s
m
kgmaF 38592.1000,200 2

The power dissipated at the end of acceleration is thus
MWkN
s
m
vFP 6.53385139 
11
Stefan Fassbinder: ‘Energieeffizienz im Schienenverkehr’[Energy efficiency in railway transport].
Elektropraktiker 1 and 2 / 2011

Page 27
Which is equivalent to 10 locomotives or 7 ICE3 multiple units.
It would seem then that the Transrapid has access to unlimited amounts of electrical power. Acceleration is
unlikely to be constant over time. Towards the end of the acceleration phase, when the maglev vehicle is
travelling at high speeds, acceleration will be lower as a considerable amount of power has to be spent
overcoming air resistance. The acceleration during the early part must therefore be correspondingly higher. At
these levels of acceleration, what Deutsche Bahn refers to as the ‘comfort limit’ has long since been exceeded.
Unfortunately, as already mentioned, information on actual power levels applicable to the current generation
Transrapid09 is not available. The power requirement for the Transrapid is quoted to be a mere 6 MW at 400
km/h
12
However, the veracity of this figure is questionable as the vehicle is 3.7 m wide and 4.7 m high (though
its length is only a quarter of that of an ICE3), and because, once again, no information is provided about the
number of seats in the Transrapid.
There is also no information about the maximum track gradient at which the vehicle can maintain its top
speed. Had that information been available, it would have been possible to compute an estimate of the power
consumption level. The ICE3 (8 MW) can maintain its maximum design speed of 330 km/h only if the gradient
does not exceed 2 ‰. When running at its scheduled operating speed of 300 km/h, the ICE3 can cope with
gradients of 6.5 ‰. If a Transrapid train with a similar seating capacity to that of the ICE3 and a corresponding
mass of 200 t were to run up inclined track with a gradient of 10 % at a speed of 500 km/h, 27 MW of power
would be required simply for the lifting work. At least as much power would need to be supplied to overcome
air resistance. We will be taking a look at the question of efficiency later on, but these figures alone indicate
that a significant investment in infrastructure (cables, transformers, converters and the long stators in the
guideway) is required. How much would a converter for this power class cost? How many segments can one
converter supply? An upper limit to this latter question is provided by the headway (i.e. the distance between
consecutive vehicles travelling on the same route). Each train has its own individual power frequency, If this
were not the case, collisions could occur if a faster train entered the segment of a slower one. While this
makes rear-end collisions practically impossible, it is hardly an ideal situation for running multiple scheduled
maglev services. It is probably safe to assume that the upper frequency limit is even lower. In stationary
applications the distance between the converter and the motor is made as short as possible, if for no other
reason than to improve EMC, and the same design principle should apply here. At Hanover University of
Applied Sciences they still believe that the sections are longer than shown in the manufacturer’s own
published material, as longer sections would mean less frequent switching of the substations that are located
along the guideway and that are responsible for supplying power to the stator sections. But, viewed in terms
of the model of a rotating three-phase synchronous motor, longer stator sections would be equivalent to
having not only a drive motor in operation but also several other motors running in parallel and under no load.
IS TRANSRAPID TECHNOLOGY READY FOR COMMERCIAL USE?
As tilting technology is not part of the Transrapid system, curves in the guideway would have to have radii that
are several kilometres long. However, the goal is to achieve much tighter curves. Now by its very nature, a
Transrapid train is forced to run through curves at very high speeds, which means – in the absence of any
tilting technology – that such curves have to be steeply banked or ‘superelevated’ in the language of railway
engineering. Whether passengers feel comfortable with this is a matter of taste and is probably less of a
problem for those who always choose a ride on the roller coaster whenever they visit a fairground. One
possible consequence is that passengers will have to keep their seatbelts on during the journey, as is the case
12
Michael Witt; Fritz Eckert: ‘Maglev 2011 in Südkorea’ [Maglev 2011 in South Korea]. eb Elektrische Bahnen –
Elektrotechnik im Verkehrswesen 12/2011, p. 642

Page 28
on a plane, instead of being able to get up and visit the dining car or use the toilet facilities, as is the case on a
high-speed passenger train.
The required track infrastructure can be implemented in practice, provided that the Transrapid line is an
isolated system and not part of a network. However, if the Transrapid really wants to become the ‘future of
high-speed transport’, building a maglev network is absolutely essential. But with gigantic switches that are
reminiscent of floodgates, the construction of a major maglev network appears illusory at best.
If the Transrapid network was run as an isolated unit, it could be equipped with superelevated curves.
However, if there is any possibility that a train might be forced to make an unscheduled stop on open track,
the banking of the outside ‘rail’ of a curved section of track could never exceed 10 % – unless tilting technology
had been integrated into the vehicle design. And the effort and cost of developing tilting technology for such a
wide vehicle would be significant. It would also not be possible to locate stations in curves.
It has also been claimed that because of its powerful acceleration and deceleration, the Transrapid is suitable
not just for long-distance journeys, but also for local traffic services. As the Transrapid needs 5 km of track to
reach top speed, a line running from the city centre to the airport, whether in Munich or Shanghai, is just
about long enough for the train to reach its maximum speed before having to apply the brakes. The amount of
time saved then becomes more a matter of seconds rather than minutes. The ‘Metrorapid’ that has been
mooted for the Ruhr region in Germany, would be a prime example of this type of technological overkill.
Installing 100 linear motors in a long-distance maglev line, where each linear motor is active for only about
twelve seconds per train run, can hardly be regarded as economically or environmentally prudent compared to
installing 16 traction motors in a single ICE 3 multiple unit with each motor running for approximately 3000
hours per year. As long as the main reason for building Transrapid maglev systems remains the prestige of host
countries wanting to showcase themselves as high-tech societies, the technology overkill aspect will not be an
issue. An isolated project with a relatively short section of track covering only tens of kilometres is of little
practical utility and the level of investment is completely disproportionate to the transport benefits gained,
but from a marketing perspective, the view is of course very different.Germany no longer needs to market
itself in this way and is anyway from a geographical and demographic perspective not the best candidate for
such high-speed rail services. Even though high-speed trains in Germany only stop at those cities judged big
enough for the honour, the distances between such cities never really seem large enough to really benefit
from having high-speed services operating between them. Other countries are les densely populated and the
population tends to be concentrated in a few conurbations or even in some cases in a single capital city with
not much between except hundreds of miles of lowland countryside. This is the ideal landscape for high-speed
transport systems.
But even in such cases, reservations have been raised. A review
13
of the economic feasibility of the Transrapid
route once proposed between Hamburg and Berlin came to the following conclusion: ‘Fundamental doubts
have to be raised whether there is any sense in adopting magnetic levitation technology for long-distance
transport.’ In order to recover the very substantial investment costs associated with the planned Transrapid
line, the revenues generated – and therefore passenger numbers – will need to be correspondingly high.
However, high passenger numbers are only achievable over short distances and not on long-distance services
covering hundreds of kilometres. On the other hand, the high design speed of the maglev system can only be
effectively exploited over long distances. This exposes an inherent, insoluble contradiction of the Transrapid
technology.’ And it continues: ‘It is known empirically from transport and traffic studies that the number of
13
www.vr-transport.de/transrapid-wirtsch/n003.html

Page 29
passengers travelling between a point of source and a destination decreases with the square of the distance
from the origin.’
COSTS
Building a Transrapid maglev system does not come cheap. The cost of constructing a metre of guideway has
been estimated in one trade magazine at € 30,000. A single long-distance line covering 350 km would
therefore have a price tag of more than 10 billion euros. But as a high-speed railway line would cost about the
same, you could be forgiven for thinking that money is not an issue – especially when Europe doesn’t seem to
have a problem finding half a trillion euros to rescue parts of the banking sector. The difference is that any
money spent on infrastructure projects actually has to exist and has to be accessible as it is going to be used to
buy something real, something physically tangible. We could draw an analogy to active power and call this sort
of money ‘active money’. Its counterpart – ‘reactive money’ – rears its head whenever electrical engineers or
technicians try to understand the financial crisis, because ‘reactive money’ (like ‘reactive power’) just seems to
be transferred back and forth from one side to the other: ‘You give me a loan and I’ll give you one.’ Expanding
the analogy further, one could liken a creditor to a capacitance (the supplier of the ‘reactive power’) and one
could equate a borrower with an inductance (the recipient of the ‘reactive power’). And like reactive power,
the ‘reactive money’ being transferred back and forth between the two of them isn’t actually there in any real
sense and can’t be used to do useful work.
There have also been reports that the company that has been operating the maglev rapid transit system in
Shanghai since 2004 got into financial difficulties when passengers voted with their feet by deciding to use a
recently built conventional underground railway line running parallel to the maglev route.
The study mentioned above concludes that the original cost calculation contains numerous errors, all,
interestingly, of the same type. To quote the report: ‘The costs of building the Transrapid guideway were
hugely underestimated – the construction of a maglev track system is certainly comparable to conventional
bridge building’. Elsewhere the report states: ‘Unlike a railway, where bridges create significant extra
construction costs compared to building ground-level track, additional costs of this kind do not have to be
taken into account in the Transrapid system, as even a ground-level Transrapid guideway is essentially a bridge
built on pillars between 0.1 m and 2 m in height.’
The original analyses were also erroneous with respect to passenger numbers. It was assumed that once a
Transrapid system was in place, there would be no further competition from conventional railway services. At
that time, however, the German national railway operator was planning to continue to offer railway services at
15 pfennigs per passenger kilometre, while at the same time assuming that the Tranrapid would generate
revenue at 28 pfennigs per passenger kilometre. Passengers willing to pay this surcharge would certainly have
been in a minority despite the potentially large savings in travel time, though the shorter travel times would
not have been as dramatic as originally envisaged as the first high-speed ICE routes had already begun
operating. The fact that the costs of maintaining the maglev system had also suddenly risen from the original
estimate of 9 million deutschmarks to 131 million makes it hardly surprising, and more a blessing in disguise,
that the Transrapid was never actually built in Germany.
SAFETY
According to one marketing text: ‘The Transrapid is quiet, has low susceptibility to derailment and offers high
ride comfort in all speed ranges’
14
. The marketing brochure was clearly written before the tragic accident on
22 September 2006 in which 23 people were killed as a result of a collision with a track maintenance vehicle
14
www.thyssenkrupp.com/documents/transrapid/TRI_Flug_Hoehe_d_8_02.pdf

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