Induction Technology
REPORTS
Energy-efficient power supply
for induction hardening and
heating processes
by Edmund Zok, Dirk M. Schibisch
Induction heating is an established, highly energy-efficient industrial process. The traditional areas of application include, among other things, induction hardening, forge heating, tube welding and annealing as well as the heating and
quenching followed by tempering of steel bars or tubes. More and more new applications are being added, as the
shortage of fossil fuels means that heating processes are being increasingly switched from oil or gas to electroheat. This
does not always mean that conventional furnaces should not be used, rather both combustion furnaces and induction
technology can be used intelligently side by side.
W
ith induction heating the energy is transferred from the inductor into the product to be
heated (workpiece). The inductor’s most basic
design is that of a cylindrical coil, however it can have
a variety of forms, depending on the application. If the
inductor is connected to an AC supply, an electromagnetic alternating field is created around it. This field
generates an alternating voltage within the electrically
conductive material, resulting in so-called eddy-currents
in the near-surface area of the workpiece. Heat is generated in the workpiece due to the electrical resistance of
the material. Heat conduction causes the temperature
of the workpiece to homogenize from the outside in
over a period of time.
The power supply for induction heating has to meet various requirements. If one looks at the typical applications from
the point of view of the power and frequencies required only,
it becomes apparent that both the power range and frequency range cover three to four logarithmic orders of magnitude.
Fig. 1 shows several widely used applications of induction
electroheat in the frequency-power diagram. The bordered
area shows the key areas of application. There are numerous
other applications shown outside the area marked.
It is clear that different power supplies are required for
the variety of applications. The difference is not just in
the electrical data of the equipment itself, but primarily
in the circuit topology.
Nevertheless, there are common features with almost
all types of power supplies used, and these can be attri-
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buted to the physical properties of the inductor-workpiece arrangement.
PHYSICAL PROPERTIES OF THE INDUCTOR-WORKPIECE ARRANGEMENT
The inductor-workpiece arrangement roughly corresponds
to a transformer with shorted secondary winding. In the
simplified equivalent circuit diagram it can be roughly
seen as the parallel or series connection of an equivalent
resistance and inductance. For a more detailed analysis
the parallel connection of the components is chosen, as
shown in Fig. 2.
It is theoretically possible to connect the inductor with the
workpiece directly to an AC power supply. This can be done
provided the frequency of the voltage source can be adjusted
to the requirements of the process. For most practical requirements it can be seen that the inductor’s reactive current IL,
which lags the feeding voltage by 90°, will be several times
greater (by a factor of 3 to 10) than the active current IR, which
is in phase with the voltage. This is shown in the right-hand
section of Fig. 2 as a vector diagram.
Only the active current IR causes the workpiece to heat
up. It is true that the inductor’s reactive current IL is absolutely essential for building the magnetic alternating field,
however it does not contribute to the energy expended in
the workpiece. Since the power source and the transmission paths are subject to extreme loads due to its intensity,
it may be potentially unfavourable for the energy balance
to connect the inductor directly to the power source. The
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Induction Technology
Converter for induction heating
Fig. 2: Equivalent circuit diagram and vector diagram
of the inductor-workpiece arrangement
Fig. 1: Power and frequency range of the power supply for induction electroheat
power source and the transmission paths would have to
be multiply over dimensioned to cover the reactive current
requirements.
ADVANTAGES OF A RESONANT LOAD
The reactive power of the inductor does not necessarily
have to be supplied by the power source, it can also be
generated within the load. A simple load configuration,
which has been tried and tested for years now and allows
this approach, is the so-called oscillating circuit. An oscillating circuit is created when a capacitor is connected in
parallel or in series to the inductor. For the sake of simplicity,
this study deals only with a parallel resonant circuit (Fig. 3) a
series resonant circuit behaves similar to the parallel resonant circuit.
The capacitor current IC leads the alternating voltage U
at the capacitor by 90°. It is also a reactive current, however
its phase is displaced by 180° with respect to the phase of
the inductor reactive current IL. The amounts of both reactive currents depend on the frequency. However, while the
capacitor reactive current rises as the frequency increases,
the inductor reactive current is reduced with an increase in
the frequency. For the arrangement shown in Figure 3 there
is exactly one frequency at which the absolute values of
both reactive currents are identical, as shown in the vector
diagram in Fig. 3 on the right-hand side. This frequency is
called resonant frequency.
Since the phases of both reactive currents are in opposition, in the case of resonance the inductor reactive current is fully compensated by the capacitor reactive current.
The external power source only has to supply the active
current I = IR, it is not loaded with any reactive power.
68 Fig. 3: Parallel resonant circuit and its vector diagram
The advantageous behaviour of the oscillating resonant
circuit described has resulted in it becoming the established method in almost all induction heating applications.
Several characteristic variables for the parallel resonant
circuit are shown below [1].
1
___
​ω0​ ​ = ​ ______
​
√
​ L · C ​
R
δ = ​ _____
​
2L  · ​ω​0​
Natural angular frequency
(1)
Damping
(2)
_____
​ωe​ ​ = ​ω​0​ · ​√1 − ​δ​2​ ​
Resonant angular frequency
(3)
With induction heating the resonant frequency changes
during the heating process. The eddy-current intensity
induced in the workpiece decreases from the surface of
the material to the interior. The penetration depth [2] is the
distance δ from the surface, at which the current density
has dropped to a 1/e-fold amount of the surface current
density. It depends on the frequency as well as on the
specific electrical conductivity and magnetic permeability
of the workpiece material.
_____
√
1
δ = ​ ______
​  ω · κ · μ
​ ​
(4)
δ = Penetration depth
κ = Specific electrical conductivity of the material
μ = Magnetic permeability of the material
ω = Induced angular frequency
Since the material parameters change with the
temperature and field intensity, the inductance of the
inductor-workpiece arrangement and therefore both,
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Induction Technology
the resonant frequency and oscillating circuit damping
also change. This change in the resonant frequency
needs to be picked up and evaluated by the power
source, in order that the operating frequency of the
power source must continuously track the resonant
frequency of the resonant load.
Fig. 4 shows the frequency characteristic [1] of a parallel resonant circuit. The top section shows the magnitude
of the impedance, and the bottom section shows the
phase response of the complex impedance as a function
of the frequency. The maximum magnitude of impedance, as well as a 0° phase shift between the voltage
and current at the resonant frequency are characteristic
of the parallel resonant circuit.
RESONANT CONVERTERS
For induction heating various types of frequency converters were developed over time which convert the mains
current into alternating current with the parameters suitable for the respective process. Most are resonant converters.
Resonant converters (Fig. 5) belong to a class of frequency converters used for supplying power to a resonant
load. Most resonant converters are indirect (dc link) converters, whereby the front-end rectifier and the back-end
inverter are connected by either an inductive or capacitive
energy buffer (dc link). With all resonant converters the
reactive power requirements of the inductor can be supplied locally from the capacitors of the oscillating circuit.
The most universal converters in this class are the parallel and series resonant converters. However, more complex systems are also used, such as the L-LC topology for
example. PWM converters may also be used in conjunction
with a resonant circuit. A direct converter can also be used
for feeding an oscillating circuit.
In general, the more complex converters
cannot be used as universally as the simple
parallel or series resonant converters. Their limitations are mostly attributable to a restricted
frequency range at a given converter configuration. Broadband applications are not possible
with these types of converters or are difficult
to implement.
REPORTS
Fig. 4: Frequency characteristics of a parallel resonant circuit
Reverse-blocking switches are required in the inverter
of a parallel resonant converter. For this reason, fast diodes
with soft recovery characteristics are connected in series
with the IGBTs or MOSFETs. A thyristor is naturally reverseblocking. Hence, a parallel resonant converter with thyristor
inverter was therefore easy to design.
Due to the inductive dc link, a parallel resonant converter has the characteristics of a power source. Its output
current almost has a rectangular wave form, the voltage
generated at the resonant load is sinusoidal. Fig. 7 shows
the typical wave form of the output current and voltage
of a parallel resonant converter.
The switching frequency of a parallel resonant inverter
is derived from the resonant frequency of the load and
must be quickly re-adjusted in the event of changes
in the load. Power control is usually performed by the
fully-controlled rectifier. The line-side power factor of
the converter depends on its output voltage and incre-
PARALLEL RESONANT CONVERTERS
The parallel resonant converter is an indirect
converter, assembled from a fully controlled,
line-commutated rectifier, a current dc link
and a load-controlled, self or load-commutated inverter. A parallel-compensated inductor is
connected to the output of the inverter. Fig. 6
shows a parallel resonant converter with an
IGBT inverter.
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Fig. 5: Resonant converters for induction heating
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Induction Technology
Fig. 6: Parallel resonant converter with IGBT inverter
asingly diminishes within the partial load range.
For a long time the parallel resonant converter
was the preferred system for induction heating
of material. When using thyristors a sophisticated
extinction-angle control system enabled the maximum power level to be attained. Power components
with switch-off capabilities, such as MOSFET transistors or IGBTs for example (Fig. 8) enable operation
both at the resonance frequency and with a slightly
capacitive or inductive phase shift at the inverter
output [2].
Parallel resonant converters can be used within
a very wide frequency range, achieving a high level
of operating efficiency with dynamically optimized
commutation control [3]. The circuit is robust and
short-circuits on the inductor can be easily managed.
As a rule, no output transformer is required. It is only
in the case of very short inductors, for example those
often used for induction hardening, that a transformer
is needed directly in front of the inductor to match
its impedance to the converter output impedance. A
poor power factor within the partial load range is one
disadvantage of the parallel resonant converter.
SERIES RESONANT CONVERTERS
Fig. 7: Voltage and current of a parallel resonant converter
Fig. 8: Parallel resonant converter 800kW, 450V, 200kHz, MOSFET inverter
(source: SMS Elotherm)
70 The inductor coil LL can also be connected with
the capacitor CL to a series resonant circuit, which
may be used as a series resonant converter load
(Fig. 9).
The series resonant converter is a voltage fed
converter. Its inverter is load controlled and it may be
operated either self-commutated or load-commutated. The power may be controlled by the frequency
of the inverter, modulation of the inverter control
pulses or a change in the dc link voltage.
Series resonant converters are also available for
a wide frequency and power range (Fig. 10). Since
high voltages can be attained relatively simply
with the aid of series resonant circuits, they are
ideally suited for high-power melting furnaces
and generally wherever a high inductor voltage is
required. In the case of both smaller inductor voltages and short inductors, an output transformer
must also be used.
Series resonant converters boast a high degree
of operating efficiency, as only a few components
are needed to set up a transistorized inverter, and
the switching losses in the load-commutated
inverter can be effectively reduced through optimized commutation [5]. If power control is done
by the inverter’s control system, an uncontrolled
rectifier may be used. In this way good power factor can be achieved under all operating conditions.
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Induction Technology
REPORTS
CONVERTERS WITH L-LC LOADS
The development of power electronics is largely driven by
converter technology for electrical drive systems. These converters generally have an uncontrolled rectifier, a voltage link
and an IGBT inverter. The uncontrolled rectifier has a better
power factor than the controlled rectifier, whose power factor
changes with its control angle. Therefore, topologies with an
uncontrolled rectifier are the preferred system also in terms
of resonant converters.
An uncontrolled rectifier and a voltage link are part of the
basic circuit of a series resonant converter. Since, however,
a parallel resonant circuit is preferred over a series resonant
circuit, solutions had to be found which made it possible to
feed the parallel resonant circuit from the voltage fed converter. This cannot be done directly. The task could be solved,
however, by adding a coupling inductor LS at the output of
the inverter (Fig. 11). This topology is often referred to in
technical literature as an L-LC circuit. The disadvantages of this
converter type is that the system is far more complex and,
more importantly, the useful frequency range is considerably
limited. This is because the LS value has to be designed tightly
for the operating frequency of the converter.
The L-LC circuit features two points of resonance (Fig. 12):
one with parallel and one with series resonance. Depending
on the desired circuit properties and application, both may
be used. To control the inverter, special algorithms have to be
used to find the desired point of resonance (parallel or serial)
and clearly establish the working point. Despite the L-LC circuit
has the advantage that both the frequency and power can
be controlled via the inverter. This technology has become
very widely used in recent years, although an L-LC converter
can only be used within a limited frequency range. There
are many applications for which a small operating frequency
range is sufficient.
CONVERTER WITH PWM INVERTER
In a voltage fed converter as shown in Fig. 11, the inverter
can be controlled according to various algorithms. The
Fig. 9: Series resonant converter with IGBT inverter
Fig. 11: C
onverter with L-LC load
IGBTs do not necessarily have to be switched once every
load voltage period, as with the L-LC converter. Therefore,
in the case of very low operating frequencies, for example
below 200 Hz, it may be beneficial to control the inverter
with a sine-weighted pulse pattern (Fig. 13), whereby
the modulation frequency is the same as the resonant
frequency of the parallel resonant circuit.
Fig. 10: Series resonant converter 2,400 kW, 800 V, 150 kHz, IGBT inverter (source: SMS Elotherm)
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Induction Technology
Fig. 12: F requency characteristics of an L-LC oscillating circuit
Fig. 13: Converter with PWM inverter; top: output voltage and current; bottom: load voltage
Under this condition the output current of the inverter is in phase with the fundamental component of the
output voltage and hardly subjects the inverter to reactive power. The load voltage is sinusoidal, just like the
inductor current. The control method described means
that applications with operating frequencies up to 200 Hz
can be used.
72 CONVERTER WITHOUT RESONANT CIRCUIT (DIRECT CONVERTER)
A voltage dc link converter with a PWM inverter can also
feed the inductor directly (Fig. 14). Since no resonant circuit and no resonant load exist, the output frequency of
the converter can be freely modified during operation.
This may be advantageous for certain applications [6]. The
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Induction Technology
Fig. 14: D
irect converter without resonant circuit
output power of the direct converter can be controlled by
changing the width of the inverter pulses (Fig. 15).
This circuit, however, has one serious disadvantage; the
entire inductor current, including the full proportion of reactive current, flows through the inverter and the connecting
cables between the inductor and the inverter. See also the
explanations given in Chapter 2 et seq. in this article.
With this type of topology the inverter has to be
heavily over dimensioned. Since the losses in the connecting cables show a square-law increase with the
current, the efficiency of the system decreases overall,
compared to resonant converters with the same level
of power. Only the need to continuously change the
operating frequency during the heating process may
justify the use of this circuit.
CONCLUSION
Looking to the future, resonant converters will continue to
maintain their position as highly efficient standard power
supply systems for induction heating applications. Parallel
and series resonant converters will still be used as universal
systems. Modern power semiconductors as well as optimized control algorithms will result in a further reduction
in frequency-dependent switching losses in the inverter.
This means the maximum operating frequencies of such
converters can be further increased.
New converter topologies, such as the voltage link converter with an L-LC resonant load, have gained considerably
in importance, in spite of some limitations compared to
parallel and series resonant converters. For system-related
reasons, they can only be used within a very narrow frequency range without changing their components. Additional
components, such as the coupling choke, contribute to an
increase in the losses.
The direct converter without resonant circuit will continue to be used only for applications where continuous
adjustment of the operating frequency is required during
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REPORTS
Fig. 15: PWM voltage control of a converter without resonant circuit
operation. It is otherwise so inferior compared to resonant
converters in terms of its efficiency that it is really no alternative when it comes to energy efficiency.
More and more importance is being attached to a reduction in the system perturbation of frequency converters. It
is not only the cos φ power factor of the fundamental that
plays an important role here. The loading of the system with
harmonic currents may result in a variety of problems and
needs to be kept in check. Higher pulse rectifiers, power
factor correction circuits or active filter technologies may be
useful here. The improvement in the cos φ and the reduction
in harmonic current both result in the enhancement of the
energy efficiency of the power supply.
Frequency thyristors, the further development of which
was virtually stopped, are now only being used by a handful
of converter manufacturers in high-power inverters and at low
operating frequencies. More and more frequency thyristors
are being discontinued by the manufacturers, with the result
that their use may continue to dwindle.
Due to their advantageous properties IGBTs are ideal for
use in voltage dc link converters. This topology forms the
basis of many of the types of converters described. The most
well-known of these are the series resonant converter and
the converter with L-LC load. Converters with sine-weighted
PWM modulation of the inverter can also be used to feed a
parallel resonant circuit via a coupling choke.
IGBTs and MOSFETs are semiconductor technologies
which, to some extent, compete with each other. For high
frequencies MOSFETs still offer more benefits than IGBTs,
even though the range of IGBT-applications is continuously
growing.
It will still be some time before power semiconductors
from silicon carbide (SiC) are used in medium and highpower frequency converters. Silicon-based IGBTs and MOSFETs will remain the components of choice for a long time
to come. Future areas of application for SiC components
may first be found in high-frequency converters.
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LITERATURE
[1] F ricke, H. ; Vaske, P.: Grundlagen der Elektrotechnik Teil 1. Elektrische Netzwerke, B. G. Teubner Verlag Stuttgart 1982
[2] D
ede, E.: Static Inverters for Induction Heating: From the Fundamentals to the Analysis and Design. PCIM International
Conference 1998 Seminar Notes
AUTHORS
[3] P
atent DE 101 15 326 B4 2009.10.15 Verfahren zur Ansteuerung eines Schwingkreis-Wechselrichters. SchwingkreisWechselrichter und Regler.
Dipl.-Ing. Edmund Zok
[4] M
ohan; Undeland; Robbins: Power Electronics. Converters,
Applications and Design. John Wiley & Sons, Inc. 1995
Tel.: +49 (0) 2191/ 891-639
[5] Z
ok, E.; Matthes, H.G.: Kritische Halbleiterbelastungen im
Schwingkreisumrichter, VDE Konferenz Leistungselektronik
und ihre Bauelemente in Bad Nauheim 2002
[6] N
uding, M.: MF-Umrichtertechnologie zur Vereinfachung
induktiver Erwärmprozesse, Elektrowärme International,
issue 1/2009
SMS Elotherm GmbH
Remscheid, Germany
e.zok@sms-elotherm.com
Dipl.-Wirtsch.-Ing. Dirk M. Schibisch
SMS Elotherm GmbH
Remscheid, Germany
Tel.: +49 (0)2191/ 891-300
d.schibisch@sms-elotherm.com
56th INTERNATIONAL COLLOQUIUM ON REFRACTORIES 2013
September 25th and 26th, 2013 . EUROGRESS, Aachen, Germany
C o n f e r e n c e To p i c
Re fract or ies fo r Indu s tri al s
• Glass
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/plaster
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refractories
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lining service
•
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•
•
Quality management
Wear and Corrossion
Recycling
Environmental
Protection
The deadline for submission of abstracts is 8th March 2013.
For further information please contact:
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– Feuerfest-Kolloquium –
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