Magnetic Flux Controllers in Induction Heating and Melting by Robert Goldstein of Fluxtrol Inc.

Magnetic Flux Controllers in Induction
Heating and Melting
Robert Goldstein, Fluxtrol, Inc.
MAGNETIC FLUX CONTROLLERS are
materials other than the copper coil that are used
in induction systems to alter the flow of the mag-
netic field. Magnetic flux controllers used in
power supplying components are not considered
in this article.
Magnetic flux controllers have been in exis-
tence since the development of the induction
technique. Michael Faraday used two coils of
wire wrapped around an iron core in his experi-
mentsthat led toFaraday’s law of electromagnetic
induction, which states that the electromotive
force (emf) induced in a circuit is directly propor-
tional to the time rate of change of the magnetic
flux through the circuit. After the development
of the induction principle, magnetic flux control-
lers, in the form of stacks of laminated steel, found
widespread use in the development of transfor-
mers for more efficient transmission of energy
(Ref 1, 2).
Magnetic cores gained widespread use in the
transformer industry because they increased
the amount of magnetic flux produced with
the same alternating current. The higher the
magnetic flux, the higher the emf, which results
in an increase in energy transfer efficiency from
the primary winding to the secondary winding.
Similar to transformers, magnetic cores were
used on early furnaces for induction melting
(Ref 1, 2). The benefits of magnetic flux control-
lers vary depending on the application. For
induction heating, magnetic flux controllers can
provide favorable and unfavorable paths for mag-
netic flux to flow, resulting in increased heating in
desired areas and reduced the heating in undesir-
able areas, respectively. Magnetic flux controllers
are not used in every induction heating applica-
tion, but their use has increased (Ref 3, 4).
Magnetic Circuits in Induction
Applications
Induction heating applications are similar to
transformers with a short circuited secondary
winding. The primary winding of the circuit
is the induction coil and the secondary is the
workpiece. For both cases, there are three closed
loops: flow of current in the coil, flow of mag-
netic flux, and flow of current in the workpiece.
In most cases, the difference between induc-
tion heating applications and transformers is that
the magnetic circuit is open. The magnetic field
path includes not only the area with the control-
ler, but also the workpiece surface layer and the
air between the surface and controller, which
cannot be changed. Therefore, the reluctance of
the magnetic path only partially depends on the
magnetic permeability of the controller (Ref 3).
Reluctance is a term used in magnetics analo-
gous to resistance in electricity.
Figure 1 shows a diagram of the magnetic
circuit for a single-turn induction coil with a
magnetic flux concentrator heating a cylindrical
workpiece from the outside. The magnetic flux
in the system is equal to the ampere turns of
the coil divided by the reluctance of the magnetic
circuit. The reluctance of the magnetic circuit
consists of three basic components: the back path
for magnetic flux, the coupling gap, and the
workpiece. When a magnetic flux concentrator
is applied, it strongly reduces the reluctance of
the back path for the magnetic flux (Ref 3). All
induction heating systems can be described in
this way.
The benefits of a magnetic flux concentrator
on the electrical parameters for a given applica-
tion depends on the ratio of the reluctance of
the back path for magnetic flux to the overall
reluctance in the system. It is also possible to
break down basic system components into sub-
components to determine the most economical
use of magnetic flux controllers in a given appli-
cation. Other benefits of magnetic flux control-
lers, such as shielding areas from heating, can
also be understood by describing induction sys-
tems in this manner.
Role of Magnetic Flux Controllers
in Induction Systems
Magnetic flux controllers are powerful tools
in induction heating technology. In general,
two primary reasons to use magnetic flux con-
trollers in induction systems are to reduce and
increase magnetic fields in a given region.
Reducing magnetic fields is done by using
either soft-magnetic materials or electrically
conductive materials in a closed loop perpen-
dicular to the flow of the magnetic flux between
the coil and the desired lower field area. Soft-
magnetic materials improve induction coil
parameters, while highly conductive materials
have a negative effect. Soft-magnetic materials
are primarily used to increase magnetic fields.
The benefits of using magnetic flux control-
lers in an induction heating system include:
Improvement of induction coil and process
efficiency
Improvement of coil power factor
Reduction in coil current
Reduction in unintended heating of machine
components
Reduction in undesired heating of areas of
the workpiece
ASM Handbook, Volume 4C, Induction Heating and Heat Treatment
V. Rudnev and G.E. Totten, editors
Copyright # 2014 ASM InternationalW
All rights reserved
www.asminternational.org
Fig. 1 Magnetic circuit in a single turn coil with
magnetic flux concentrator heating cylindrical
part from the outside: F = total. Source: Ref 3

Precise control of the magnetic field and
resulting heat pattern
Improvement in efficiency of high-frequency
power supplying circuitry
Reduction of external magnetic fields in
close proximity to the coil
In most applications, more than one of these
benefits usually occurs (Ref 3–7). Recently,
better understanding of the role magnetic flux
controllers in induction heating systems has been
obtained through comprehensive studies using
computer simulation and experiments, such as
those conducted at Fluxtrol, Inc. Results show
that a magnetic flux controller properly used
is typically beneficial in an induction heating
system (Ref 3). Magnetic flux controllers play
different roles in induction heating installations.
Depending on the application, they are referred
to as concentrators, controllers, diverters, cores,
impeders, yokes, shunts, and screens. The effects
of controllers in different types of induction heat-
ing applications are described here.
Materials for Magnetic
Flux Control
Two main categories of materials for mag-
netic flux control are electrically conductive
materials and magnetic materials. Electrically
conductive materials typically are used in the
form of shunts and screens to reduce external
magnetic fields.
Two main forms of magnetic materials are hard
and soft. The difference is the amount of flux den-
sity that remains after they are no longer exposed
to a magnetic field. Hard magnetic materials
retain a significant amount of the magnetic field,
while soft materials retain almost no magnetic
field when the source is turned off. Soft magnetic
materials are used almost exclusively as con-
centrators, controllers, diverters, cores, impeders,
yokes, shunts, and screens for magnetic flux con-
trol in induction systems.
Highly Conductive Materials
for Field Reduction
Highly conductive materials in the form of
closed loops are commonly used in induction
systems to reduce the level of magnetic fields
in certain areas. The effect on the distribution
of the magnetic field is described by Lenz’s
Law, which states an induced electromotive
force always produces a current whose mag-
netic field opposes the original change in
magnetic flux. This current creates a field of
reaction that influences the distribution of the
magnetic field, and increases both the reluc-
tance of the magnetic circuit and the current
in the coil to produce the same amount of heat
in the workpiece. These closed loops, com-
monly referred to as Faraday, or “robber” rings
carry high frequency currents, which result in
heating by Joule losses. The losses reduce the
efficiency of the system, and are not desirable.
Therefore, nonmagnetic materials with high
electrical conductivity are preferred for use as
Faraday rings. Copper is the most common
material used, but other materials, such as alu-
minum, are used due to cost and/or weight
considerations.
Soft Magnetic Materials for
Magnetic Flux Control
Soft magnetic materials most commonly used
in induction systems are laminations and soft
magnetic composites. Soft magnetic ferrites are
used occasionally in some high-frequency appli-
cations. The main requirements are that it should
have a relative magnetic permeability 1 and
should not have a good electrically conductive
path for strong eddy current generation.
Placing the material in the path of magnetic
flux lowers the reluctance of that part of the
magnetic circuit. Therefore, it requires less cur-
rent to drive the magnetic flux through that
part of the circuit, and a higher percentage of
flux flows in the magnetic material than would
flow in the same space containing only air.
However, this positive effect has some limita-
tions because the magnetic circuit is almost
always open.
For the same coil current, workpiece power
increases with increasing concentrator per-
meability very fast initially, then approaches
the threshold value asymptotically. At the
same time, losses in the induction coil often
increase slowly with increasing concentrator
permeability.
Computer simulation was used to demon-
strate the diminishing positive effect of very
high permeabilities in induction applications.
In a real induction heating application, the mag-
netic permeability of a magnetic flux controller
depends on magnetic flux density, frequency,
and temperature of the controller itself (not
the part it is heating). In the study, permeability
in the magnetic flux controller is considered to
be constant in the cross-section and at a fixed
value for each calculation. The study was not
conducted for a particular material, but is used
only to show the effect of conductor permeabil-
ity on induction coil parameters.
Figure 2 shows the effects of magnetic per-
meability on coil current and efficiency for
heating a flat plate using a single leg of an
inductor at frequencies of 3 and 10 kHz. Simi-
lar studies show that in most induction heating
applications, the threshold value for workpiece
power occurs when concentrator permeability
is 100. In high-frequency induction heating
applications, the threshold value occurs at
lower levels of permeability. Therefore, incre-
asing permeability to higher values will not
improve coil parameters significantly (Ref 3).
In some cases, use of materials with higher
than optimal magnetic permeability reduces
induction coil lifetimes without any benefits
(Ref 3, 8, 9).
Other important properties of soft magnetic
materials, such as good saturation flux density,
stable mechanical properties, low magnetic
losses, chemical resistance, and resistance to
elevated temperature depend on the application.
Laminations are commonly made of coated
thin sheets of silicon electrical steel with 3 or
4% silicon. The laminates are cut (by water
jet, laser, CNC, and electrical discharge machin-
ing) or stamped to the required shape for use in
induction coils.
For induction coils, multiple laminations are
stacked between mechanical supports called
keepers (Fig. 3). The large lamination cross
section is oriented so it is in the plane of the
flow of the magnetic field; intense eddy current
heating occurs if it is not in the plane.
Lamination thickness varies based on fre-
quency used to limit eddy-current losses from
the in-plane magnetic field. The lamination coat-
ing prevents an electrical connection between
individual laminations. In line-frequency appli-
cations, individual laminations are typically
between 0.020 to 0.040 in. (0.5 to 1 mm) thick.
Laminations can be as thin as 0.002 in. in appli-
cations using higher frequencies.
Laminations have very high magnetic perme-
ability and saturation flux density. They also
have high temperature resistance limited pri-
marily by the coating. The primary drawbacks
of laminations are intense heating in 3-D mag-
netic fields and limited frequency range (up to
about 30 kHz).
Soft Magnetic Composites consist of a
soft-magnetic component (typically iron and
iron-base alloy powder metal) and a dielectric
component (usually an organic polymer binder).
The soft-magnetic component provides a favor-
able path in which the magnetic field can flow.
The dielectric component electrically insulates
magnetic particles from each other to limit eddy
current losses.
Two main forms of soft-magnetic composites
are machinable and formable. Machinable
materials are commonly produced via powder
metallurgy compaction techniques and heat
treated to improve magnetic and mechanical
properties. Machining soft-magnetic compo-
sites is easier than machining laminations.
The composites are applied on induction coils
using an adhesive and mechanical supports
(Fig. 4).
Machinable-soft magnetic composites have
good magnetic permeability and saturation flux
density, as well as good temperature resistance,
which is limited primarily by the polymer
binder. Unlike laminations, soft-magnetic com-
posites can be tailored to work in the entire
range of frequencies for induction heating and
melting, and perform well in 3-D magnetic
fields (Ref 3–7).
Formable soft-magnetic composites are shaped
around the induction-coil surfaces, held in place
mechanically, and cured in an oven, which fixes
their shape. Magnetic properties of formable
soft magnetic composites are not as favorable as
those of machinable composites, and are used in
634 / Equipment

instances of irregular geometries and use of a
machined material is difficult.
Figure 5 shows relative magnetic permeabil-
ity versus magnetic field strength of some com-
mon soft-magnetic composites (Ref 4). Their
diverse properties offer the opportunity for fine
control using different materials on the same
induction coil.
Design Guidelines for Using
Magnetic Flux Controllers on
Induction Coils
The decision of where to use a magnetic flux
controller depends strongly on the shape of the
induction heating coil. Many induction coils are
hybrids and have components made of more
than one of the basic coil types. Therefore, it
is important to understand how magnetic flux
flows and how it is affected by a magnetic flux
controller to properly apply it on more complex
induction coils.
Effects of Magnetic Flux Controllers
on Common Coil Styles
Outer Diameter (OD) Coils. The basic
magnetic circuit for an OD coil (Fig. 1) was
described previously. Computer simulation is
used to visualize and quantify the effects of
a magnetic flux controller on an induction heat-
ing system (Ref 3–7). Figure 6 shows magnetic
field lines and current density in a single-turn
OD coil for heating a cylindrical copper work-
piece with and without the use of a magnetic
flux controller. The same voltage (similar value
of magnetic flux) is applied to each coil. The
process was simulated using Cedrat Technolo-
gies Flux 2D software. A copper workpiece
was used so current density values in the induc-
tion coil and workpiece were similar to make
visualization of the effects easier.
The magnetic field is distributed in a smaller
area for the coil with the magnetic flux control-
ler, and the resulting current density in the part
is concentrated under the coil heating face.
Nearly all of the power induced into the part
is useful, and leads to an increase in tempera-
ture in the desired heating area. Nearly all of
the current in the coil with the magnetic flux
controller flows on the heating face.
In the coil without a magnetic flux controller, a
significant portion of the current flows in areas
outside of the heating face, which does not help
heating the part, but instead, draws additional
power from the power supply. A significant por-
tion of current in the coil flows up the sides of the
turn and around the back; this current is not use-
ful and only leads to additional losses in the coil,
busswork, and matching components.
The reduction in current in the coil and con-
centration of power under the heating face is
called the concentrator effect (Ref 3, 10, 11),
and can be explained by considering the system
from a magnetic circuit point of view (see
Fig. 1). The concentrator effect is a result of
the magnetic flux controller lowering the
Fig. 2 Effect of magnetic permeability on coil current (a) and efficiency (b); curves generated from computer simulation of heating a flat plate using a single leg of an inductor;
50 kW in the part under the coil face. Source: Ref 3.
Fig. 3 Induction coil with laminations stacked bet-
ween mechanical supports. Courtesy of Tucker
Induction.
Fig. 4 Induction coil with soft magnetic composites.
Source: Ref 3
Magnetic Flux Controllers in Induction Heating and Melting / 635

reluctance of this portion of the magnetic cir-
cuit and reducing the need for current in this
area of the induction coil to drive the magnetic
flux around the back path.
The primary benefits of magnetic flux con-
trollers on OD induction coils are:
Higher efficiency
Improved heat pattern control (ability to heat
fillets, not overheat shoulders, obtain sharper
transition zones, etc.)
Better use of power in the workpiece (energy
savings)
Reduction in the induction coil current
(reduced losses in power supplying circuitry)
Reduction in heating of unintended areas of
the part
Reduction in heating of machine/structural
components
Reduction in external magnetic fields
The aspect ratio of the induction coil must be
considered to determine the impact of a mag-
netic flux controller on an OD coil. Two key
variables are the ratio of coil length to diameter,
and the ratio of the coupling gap to the length
of the coil. Magnetic flux controllers are most
benefitial when the length-to-diameter ratio and
coupling gap-to-length ratio are small. The effects
of the magnetic flux controller are reduced as
these ratios increase. For very large coupling gaps
relative to coil length, a magnetic flux controller
can result in lower electrical efficiency.
Inner Diameter (ID) Coils. The basic mag-
netic circuit for an ID coil is shown in Fig. 7.
The return path for magnetic flux is on the
inside of the coil. Power density in the workpiece
is proportional to the magnetic flux density
squared, and flux density is magnetic flux divided
by the cross-sectional area through which it is
flowing. The magnetic impedance of an area is
directly proportional to the length of the region
and inversely proportional to the cross-sectional
area. Therefore, the magnetic resistance (Rm)
component is a higher percentage of the total
reluctance on an ID coil than it is on an OD coil.
Due to the larger influence of Rm, the effect
of a magnetic flux controller on ID coils is much
larger than on OD coils (Ref 3, 12). Benefits of
magnetic flux controllers on ID induction coils
include:
Improved coil efficiency (energy savings)
Better use of power in the workpiece (energy
savings)
Reduction in induction-coil current (reduced
losses in power supplying circuitry, coil leads)
Reduction in heating of unintended areas of
the part
Improved heat pattern control
Magnetic flux controllers significantly improve
nearly all ID coils. To determine the impact
Fig. 5 Magnetic permeability of some Fluxtrol soft-magnetic composite materials as a function of magnetic field
strength.
Color Shade Results
Quantity : |Current density| A/(square mm)
Phase (Deg): 0
Scale / Color
0 / 14.8125
14.8125 / 29.625
29.625 / 44.4375
44.4375 / 59.25
59.25 / 74.0625
74.0625 / 88.875
88.875 / 103.6875
103.6875 / 118.5
118.5 / 133.3125
133.3125 / 148.125
148.125 / 162.9375
162.9375 / 177.75
177.75 / 192.5625
192.5625 / 207.375
207.375 / 222.1875
222.1875 / 237
Color Shade Results
Quantity : |Current density| A/(square mm)
Phase (Deg): 0
Scale / Color
0 / 14.8125
14.8125 / 29.625
29.625 / 44.4375
44.4375 / 59.25
59.25 / 74.0625
74.0625 / 88.875
88.875 / 103.6875
103.6875 / 118.5
118.5 / 133.3125
133.3125 / 148.125
148.125 / 162.9375
162.9375 / 177.75
177.75 / 192.5625
192.5625 / 207.375
207.375 / 222.1875
222.1875 / 237
Fig. 6 Magnetic field lines and current density for a single-turn OD coil with (left) and without (right) a magnetic flux
controller. Courtesy of Fluxtrol, Inc. Fig. 7 Magnetic circuit for ID coil. Source: Ref 3
636 / Equipment

on the coil, it is necessary to consider the coil
aspect ratio. Similar to OD coils, two key vari-
ables are the coil length-to-diameter ratio and
the coupling gap-coil length ratio. Unlike OD
coils, aspect ratios that benefit most from a
magnetic controller are inverted. Magnetic flux
controllers provide the greatest benefit when the
two ratios are larger.
Linear Coils. Three basic classes of linear
induction coils are:
Coils with return current on the opposite
side of the part (e.g., single-shot coils, oval
coils, channel coils)
Coils with one primary heating leg and
distributed return on the same side of the
part (vertical loop, split-n-return coils)
Coils with two active heating directions
and return on the same side of the part
(hairpin)
The difference between linear induction coils
and cylindrical coils is there is no natural flux
concentrating/de-concentrating effect from the
change in radius in linear coils. Therefore, the
only other electromagnetic effects present are
the proximity effect between coil and work-
piece and the proximity effect between the coil
turns themselves in the case where the return is
on the same side of the part.
A hairpin inductor (magnetic circuit shown
in Fig. 8) is used to illustrate the concept of a
magnetic circuit for linear coils. If the coil legs
are the same size and part properties under each
leg are the same, each leg produces one half of
the magnetic flux in the system. Magnetic flux
flows around the legs of each turn. In the region
between the two turns, magnetic flux from each
turn are additive, and the all magnetic flux from
the system flows through this area. At the same
time, the cross-section through which the mag-
netic flux must flow is smaller than the areas
of the back path for magnetic flux. Thus, the
Rm value is high on hairpin coils, and the bulk
of Rm resides in the region between the two
turns (Ref 3).
This phenomenon also occurs in other types
of linear inductors (e.g., vertical loop, split-n-
return) where the return is on the same side
of the inductor; the bulk of Rm resides in the
area between the turns that are in the opposite
direction. Use of a magnetic flux controller
in this area is very beneficial. The benefit is
greatest for aspect ratios where the distance
between turns relative to the coupling gap is
small. The benefit is less when the heat face
is long and the return is on the opposite side
of the part.
Cooling of Magnetic Flux
Controllers
The service life of induction heating coils
with magnetic flux controllers varies from days
to years. Cooling the magnetic flux controller
minimizes mechanical damage and increases
service life.
Heat Sources in Magnetic Flux Control-
lers. All magnetic flux controllers generate
and absorb heat when exposed to an alternating
magnetic field. The amount of heat depends
on induction system parameters, induction coil
and part geometry, magnetic flux controller
material, part temperature, and the atmosphere
surrounding the coil.
Highly conductive nonmagnetic materials are
heated by eddy currents, radiation and conduc-
tion from the hot part, and convection from
the hot environment. The main heat source is
from eddy currents. Because Faraday rings use
the eddy-current reaction field, the primary
ways to reduce losses is to use low-resistivity
materials and increase the length of the shield-
ing face.
Soft-magnetic materials are heated by eddy
currents, heat from thermal sources, hysteresis
losses, and sometimes by conductive heat trans-
fer from the copper coil. Unlike highly conduc-
tive nonmagnetic materials, shielding in soft-
magnetic materials is by concentration of fields
of reaction rather than by reduction of the fields.
Therefore, soft-magnetic materials are segmented
perpendicular to the plane of the magnetic field
at a small fraction of reference depth to minimize
eddy currents.
The electrical resistivities of laminations
are similar to those of steels, so individual lam-
ination thickness should be between 0.002 to
0.040 in. depending on frequency. With proper
lamination thickness, eddy current losses in
2-D magnetic fields make up a small portion
of the heat that needs to be removed. In 3-D
magnetic fields, eddy current losses grow rapidly
in laminations and are larger than the other heat
sources (Ref 3).
Soft-magnetic composites typically have
electrical resistivities several orders of mag-
nitude higher than those for laminations. In
general, composites that will be used in higher
frequency applications should have higher
electrical resistivities. Therefore, the allowable
thickness of soft-magnetic composites is much
larger, enabling fabrication of the concentrator
from one or only a few components. Global
eddy current losses in soft-magnetic composites
generally make up a small fraction of the total
heat (Ref 3).
For soft-magnetic composites, besides global
eddy current losses, there are also eddy currents
in the individual magnetic particles. Similar to
eddy currents in larger bodies, losses in individ-
ual particles are controlled by particle size and
composition. Smaller particles are typically
used for higher frequencies. In most induction
applications, eddy current losses in individual
particles make up only a small portion of the
total heat (Ref 3).
When the correct magnetic flux controller is
used, hysteresis losses make up the largest com-
ponent of heat that must be removed. Hysteresis
losses are due to internal friction of magnetic
domains as they reorient in the presence of an
alternating magnetic field.
A hysteresis loop is used to characterize mag-
netic hysteresis for a given magnetic material
(Fig. 9). Key points on the curve are saturation
flux density, remanence, and coercive force. Sat-
uration flux density is the threshold value for
the magnetization of the material. Remanence
is the magnetism that remains after removingFig. 8 Magnetic circuit for a hairpin inductor coil. Source: Ref 3.

the source of the magnetic field. Coercive force
is a measure of how much magnetic field must
be applied in the opposite direction to remove
the remanence.
Losses in a magnetic material due to hystere-
sis are proportional to the product of the area of
the hysteresis loop and the frequency. For good
soft-magnetic materials, the hysteresis loop
should be as narrow as possible. Minimizing
hysteresis losses is accomplished through alloy-
ing elements and by stress relieving the mate-
rial (Ref 3).
Heat Removal in Magnetic Flux Control-
lers. The cooling method depends on the
type of magnetic flux controller used and the
process environment. Power density losses in
Faraday rings are higher than in soft-magnetic
materials. However, removing this heat is eas-
ier due to the nature of the materials. Depend-
ing on the magnitude of eddy current losses,
Faraday rings can be cooled using internal
water cooling, free convection, and conduction
to a large structure.
Losses in soft-magnetic materials (approxi-
mately directly proportional to frequency) are
small compared with those in the copper coil,
which are usually proportional to the square root
of frequency. For low frequencies, the ratio
between copper losses and soft magnetic losses
is large. It is smaller at higher frequencies, but
copper losses are still the dominant factor in
total coil losses below several MHz.
For soft-magnetic materials operating at low
frequencies or low power densities, cooling by
free convection is sufficient to maintain a satis-
factory temperature provided the atmosphere is at
a temperature low enough to remove heat and the
magnetic core is not continuously exposed to
direct radiation from a hot part.
For medium frequencies and power densities,
the most common method to remove heat is con-
duction to the water-cooled copper (Fig. 10).
A thermally conductive adhesive is placed
between the soft-magnetic material and copper
tubing to facilitate heat removal. Desirable
thermal conductivities of laminations and soft-
magnetic composites are 15 to 100 times
lower than that for copper, and the thermal con-
ductivity of adhesives is even lower. Adhesive
layer thickness, consistency of gap filling,
and adhesive thermal conductivity each play a
critical role in heat removal from the magnetic
flux controller. The use of a 0.010-in. (0.25-mm)
layer of an epoxy adhesive (1 W/mK thermal
conductivity) with Fluxtrol materials is suffi-
cient for heat removal in most cases (Ref 4).
In instances of heavy loading, a supplemental
water-cooling plate applied to the backside
of the magnetic flux controller increases heat
extraction.
Direct water cooling could be required in appli-
cations involving very heavy loading. Cooling
methods include water channels in the concentra-
tor (for soft-magnetic composite only, Fig. 11),
supplemental water drizzle and quench head for
quench-in-place applications. A coating on water
cooling channels is recommended when using
direct water cooling of magnetic flux controllers
to prevent water cooling-system contamination.
Computer simulation shows that Fluxtrol 100
material can withstand continuous operation at
a magnetic flux density of 1 T, a frequency of
1 kHz, and direct exposure to radiation from a
1200
C (2190
F) part using direct water cooling
(Fig. 12).
Fig. 11 Soft magnetic composite material with channels for direct water cooling. Source: Ref 4
Fig. 10 The most common method to remove heat from a magnetic flux controller is by conduction to the water-
cooled copper coil. Source: Ref 3
Fig. 9 Hysteresis loop for magnetic materials. Source: Ref 3
638 / Equipment

Determining the Thickness of Soft
Magnetic Material
The amount of soft-magnetic material
required is determined by considering magnetic
flux density, mechanical strength, and physical
space available for the application. Magnetic
flux density is important because both magnetic
permeability and magnetic losses are dependent
on it. Magnetic flux density should not exceed
the saturation flux density of the magnetic con-
troller unless there is limited available space
and only partial shielding is required.
To determine magnetic loading, it is neces-
sary to determine the magnetic flux density B
in the magnetic material. For a long solenoid,
it can be determined from Eq 1 if the magnitude
of voltage on the coil head is known, or from
Eq 2 if the magnitude of the coil current is
known. In an electrical circuit, the magnetic
permeability of the material is a function of
magnetic flux density:
B ¼
U
oSN
(Eq 1)
B ¼
IN
l
(Eq 2)
where B is magnetic flux density, U is coil volt-
age, o is angular frequency, S is cross-sectional
area, N is number of turns, m is magnetic per-
meability, and l is length.
For most other coil styles, Eq 1 and 2 can
provide a rough estimate of flux density. Eq 1
is better for ID coils. Computer simulation pro-
vides the best information.
Magnetic Flux Control in Induction
Melting Systems
Three types of induction melting systems are
channel-type, crucible-type, and cold crucible.
Only the role of magnetic flux controllers in
an induction melting system is discussed here.
Details of each type of induction melting sys-
tem are discussed elsewhere in this Volume.
Channel Type Furnaces
Channel melting furnaces are similar to either
a transformer or an ID coil (Fig. 13). They should
always have a magnetic flux controller or better
system efficiency. Channel furnaces operate at
low frequencies and are used primarily to pro-
duce large volumes of metal. For this reason,
laminations are the material of choice due to
lower cost and availability in large sizes.
Crucible Type Furnaces
Crucible type furnaces consist of a solenoid
induction coil and a ceramic refractory or con-
ductive (graphite or metal) crucible. Magnetic
flux controllers are used on these furnaces pri-
marily to reduce external magnetic fields, which
can cause heating of the equipment structure, or
can necessitate a very large footprint due to occu-
pational safety limits for human exposure to
electromagnetic fields.
Large Low-Frequency Melting Furnaces.
Faraday rings and shunts are used in many large
furnaces; shunts are used on the OD of the
induction coil and a combination of Faraday
rings and counterwound turns are used on the
top and bottom of the coil. A crucible melting
furnace is shown in Fig. 14. The design is typical
of low-frequency (tens to hundreds of Hertz) fur-
naces used to melt large volumes of metal.
Shunts used in these furnaces generally are
silicon steel laminations contained in a metal
support structure. Silicon-steel shunts work
well on the back of the induction coil because
the field is regular, in one direction, and of a
simple geometry. Shunt assemblies with lami-
nation shunts are less expensive than alternative
materials such as soft-magnetic composites for
large low-frequency furnaces.
Incorporating laminations on the top and bot-
tom of the induction coil is more difficult,
which is why a combination of counterwound
turns and/or Faraday rings is used to reduce
the level of magnetic fields in this area. These
methods effectively control external fields, but
reduce coil efficiency.
Fig. 12 Computer simulation of heat removal from soft-magnetic material exposed to a magnetic flux density of 1 T,
a frequency of 1 kHz, and a 1200
C part by direct water cooling. Source: Ref 4
Fig. 13 Schematic of channel-type induction melting
furnace. Source: Ref 13
Fig. 14 Sketch of induction crucible-type (coreless)
melting furnace. Source: Ref 2

An alternative method is to use a soft-magnetic
composite ring on the top of the induction coil
backed by a Faraday ring. The composite strongly
reduces the level of magnetic field to which the
Faraday ring is exposed with lower losses than
using counterwound turns. Losses of this type of
assembly are much lower while providing the
same level of shielding. A drawback of this
approach is higher initial cost. Capital versus
operating costs should be considered when choos-
ing between top and bottom shielding.
Medium-Size Medium-Frequency Melting
Furnaces. Magnetic flux controllers are used
for the same purposes on 1 to 10-kHz frequency
medium-size furnaces as for larger ones, but
different materials are used. Many of these fur-
naces operate in a controlled atmosphere, where
magnetic flux controllers are more important
than in open air furnaces. The controllers could
be used to shield the chamber from heating.
Furnace size must be larger without the use of
magnetic flux controllers, increasing the capital
cost of the installation.
Lamination thickness decreases with increas-
ing frequency making them more difficult to
work with. Also, shount pack structural materials
heat up more at higher frequencies, resulting is
an increased cost of using laminate shunt packs.
Due to these factors, the use of soft-magnetic
composites is becoming more popular for
medium-size melting furnaces. Figure 15 shows
a medium-size melting coil with a Fluxtrol 100
soft-magnetic composite. This makes it possible
to incorporate magnetic poles on the top and
bottom of the induction coil, so it is not neces-
sary to counterwind the upper and lower turns
or use a Faraday ring. Therefore, the entire
length of the induction coil works together to
create the internal magnetic field.
Another benefit of magnetic flux controllers
in these furnaces is when multiple melting pots
are placed in close proximity to each other. The
furnaces can be controlled using either one
power supply or multiple power supplies. How-
ever, the magnetic fields of different pots inter-
act if they are placed too close to one another.
This phenomenon is called cross-talking, and it
can cause nonuniform melting pot performance.
Magnetic flux controllers greatly reduces the
amount of cross-talking.
Cold-Crucible Melting Furnaces
In cold-crucible furnaces, a water-cooled seg-
mented copper ring is placed between the
induction coil and the melt. Magnetic flux flows
through the slots of the cold crucible and heats
the melt. Water-cooled Faraday rings are often
used on the bottom of the furnace. A metal skull
typically forms between the melt and the copper
components, which creates a barrier that prevents
contamination of the melt by impurities.
These furnaces are less efficient than coreless
melting furnaces due to electrical losses in
the segmented copper ring, the lower Faraday
Fig. 15 Medium-sized melting coil with soft-
magnetic shunts. Courtesy of Fluxtrol, Inc
Traditional design
PDmax = 216 MW/m3
Color Shade Results
Quantity : Power density W/(cubic m)
Phase (Deg): 0
Scale / Color
0 / 13.50879E6
3.50879E6 / 27.01758E6
27.01758E6 / 40.52636E6
40.52636E6 / 54.03515E6
54.03515E6 / 67.54394E6
67.54394E6 / 81.05274E6
81.05274E6 / 94.56152E6
94.56152E6 / 108.0703E6
108.0703E6 / 121.57909E6
121.57909E6 / 135.08789E6
135.08789E6 / 148.59667E6
148.59667E6 / 162.10546E6
162.10546E6 / 175.61426E6
175.61426E6 / 189.12304E6
189.12304E6 / 202.63182E6
202.63182E6 / 216.14061E6
Modified design
PDmax = 279 MW/m3
Color Shade Results
Quantity : Power density W/(cubic m)
Phase (Deg): 0
Scale / Color
0 / 17.47247E6
17.47247E6 / 34.94494E6
34.94494E6 / 52.41741E6
52.41741E6 / 69.88988E6
69.88988E6 / 87.36234E6
87.36234E6 / 104.83482E6
104.83482E6 / 122.30729E6
122.30729E6 / 139.77976E6
139.77976E6 / 157.25224E6
157.25224E6 / 174.7247E6
174.7247E6 / 192.19717E6
192.19717E6 / 209.66965E6
209.66965E6 / 227.14211E6
227.14211E6 / 244.61459E6
244.61459E6 / 262.08706E6
262.08706E6 / 279.55952E6
Fig. 16 Comparison of power density in the melt charge derived from a 2-D simulation using Flux 2D software: (a)
traditional induction coil design with laminate shunts, and (b) modified design with a soft-magnetic
composite controller with top and bottom shunts. Source: Ref 14
640 / Equipment

ring, and thermal losses of the melt to the water-
cooled copper components. Therefore, cold-
crucible melting furnaces are typically used only
in the production of metals that are highly reactive
when molten or that require very low impurities.
Cold-crucible processes almost always are carried
out in a controlled atmosphere.
Magnetic flux controllers improve the effi-
ciency of cold-crucible furnaces. Similar to other
controlled-atmospheremelting applications,mag-
netic flux controllers are used on the OD of the
induction coil to shield the chamber from heating.
Additional improvements are achieved by
using magnetic poles on the top and bottom of
the induction coil to reduce the losses associated
with the Faraday rings on the top and bottom
of the cold crucible. Studies show up to 50%
improvement in induction coil efficiency using
an induction coil with soft-magnetic composite
top and bottom poles compared with a standard
coil design (Fig. 16, 17) (Ref 14).
Magnetic Flux Control in
Mass-Heating Applications
The role of magnetic flux controllers in an
induction mass-heating system depends on the
application. Of the many types of induction
mass-heating installations, the largest applica-
tion is heating component part prior to a shap-
ing operation, such as forging, forming, and
rolling. The role of magnetic flux controllers
whole-body and local area mass-heating appli-
cations is discussed here. Detailed discussion
of these types of installations are presented
elsewhere in this Handbook.
Whole-Body Mass Heating
Workpieces are large in most whole-body
mass-heating applications, and the part passes
through the induction coil. Part shape and induc-
tion coil length are the primary determinants of
the type of magnetic flux controller used in these
systems.
Long solenoids are used for long cylindrical
or tubular body workpieces. Soft-magnetic con-
trollers do not provide significant benefits for
long solenoids (length = several times the diam-
eter). Therefore, several induction coils are
used to bring the billet up to the desired temper-
ature distribution. In these installations, Fara-
day rings at the end of the coils are the only
magnetic flux controllers used (Fig. 18). They
perform two functions: shielding mechanical
handling rolls from heating and preventing
cross-talking among multiple coils used in the
billet-heating line.
For rectangular workpieces, the benefits of a
magnetic flux controller depend upon the aspect
ratio of the cross-section to be heated. For
square and nearly square parts, guidelines for
magnetic flux controller and coil selection are
similar to those for cylindrical and tubular
parts. Magnetic flux controllers play a greater
role for rectangular parts with one dimension
significantly larger than the other (e.g., slabs,
sheet, strip). In this case, both longitudinal and
transverse flux heating methods are used.
Longitudinal coils generate a uniform tem-
perature in the cross-section. Soft-magnetic
materials are beneficial when the length of the
induction coil is small relative to the perimeter
of the coil inside cross-section.
Soft-magnetic materials should be used on
transverse flux heating coils (Ref 3, 10, 15).
Material selection depends on the frequency.
Silicon steel laminations are used for low fre-
quencies, and soft magnetic composites are
used for middle and higher frequencies. Soft-
magnetic composites are also used near the
ends of low frequency transverse flux coils
where 3-D magnetic fields are present.
Local Area Mass Heating
In many cases, it is desirable to heat only a
portion of the component for forming. Local
area mass heating can be achieved using both
static and continuous heating arrangements.
For static heating, both single-turn and multi-
turn induction coils are used, where the coil l is
comparable to the coil diameter. Soft-magnetic
controllers applied to the induction coil improves
temperature distribution and coil efficiency due to
better use of power in the workpiece together with
reduced coil current demand.
Continuous heating applications use a chan-
nel coil, which can be one turn to heat fasten-
ers, for example (Fig. 19) or several turns as
in a bar end heater (Fig. 20). Soft-magnetic
controllers help significantly in both cases.
For single-turn channel coils, use of soft-
magnetic material reduces the power required
to heat the desired area of the part by 20 to
50% (Ref 3, 4). Soft-magnetic composites are
the material of choice because these applica-
tions usually operate at 10 kHz or higher.
Fluxtrol Design
Ptotal
(kW)
Ptotal
(kW)
Pleads Pmelt Pcoil Pcrucible I(A)
122.5 6% 47% 15.5% 31% 1600
Traditional Coil Design
Pleads Pmelt Pcoil Pcrucible I(A)
135 6.5% 31% 21% 41.5% 1900
Fig. 17 Comparison of integral power values for traditional induction coil design with laminate shunts and a Fluxtrol-designed coil with soft-magnetic composite controller with
top and bottom shunts. Source: Ref 14
Fig. 18 Billet-heating coil with Faraday rings on both
ends. Courtesy of Pillar Induction, Inc.

The effect of soft-magnetic materials on
channel coils with multiple turns for heating
longer sections is not as strong as for shorter
single-turn coils; energy savings is typically
between 5 and 20% (Ref 3, 4).
Magnetic Flux Control in Induction
Tube-Welding Applications
Induction tube welding is a major application
of induction heating, used in high volume pro-
duction of carbon-steel tubes. It is also used to
produce stainless steel, copper, and aluminum
tubing. Magnetic flux controllers play a crucial
role in tube-welding lines, both in the welding
process and post weld heat treatment.
Magnetic Flux Controllers for
Continuous Induction Tube Welding
In an induction tube welding process, steel
strip is rolled into the form of a tube and passed
through a cylindrical induction coil where the
strip edges are heated. The strip passes through
a final set of rollers that close the profile of the
tube and welding occurs. Induction welding
systems for steel tubing operate at high fre-
quencies (100À500 kHz) and high power (in
the range of 100 kW to a few MWs).
Figure 21 shows a schematic of current flow
in the tube in a typical induction welding appli-
cation. The induction coil induces a current that
flows around the tube OD under the face of the
induction coil. When the current reaches the open
edge of the tube, it turns to close the current loop.
Three options for the current to flow are:
1. Along the edges of the steel strip toward the
rolls, connecting at the weld apex and return-
ing back on the opposite edge of the tube
2. Along the edges away from the V-shape
groove, returning on the tube OD of the tube
farther up the line
3. Along the tube ID
The first current path is the most desirable
one, the second path is somewhat useful, and
the third path is undesirable. In induction
welding applications, a soft-magnetic material
is placed inside the tube to reduce heating
along the ID of the tube and to enhance current
flow along the tube edges toward the V
groove. The soft-magnetic material on the tube
ID of the tube is referred to as an impeder,
because it impedes current flow along the
tube ID.
Impeders are made of soft magnetic ferrites
encased in a fiberglass tube with water cooling.
The tube protects the ferrite from metal spume
and provides mechanical support for the brittle
ferrites. The tube also keeps the ferrites in place
as they often fracture in production due to ther-
mal shock. Figure 22 shows some typical impe-
ders used in the welding industry. For large pipe
Fig. 20 Continuous bar-end heater with magnetic flux concentrator. Courtesy of Fluxtrol, Inc.
Fig. 19 Channel coil for fastener heating. Courtesy of
Fluxtrol, Inc.
Fig. 21 Schematic of a continuous high-frequency induction longitudinal seam tube-welding system. Source: Ref 13
Fig. 22 Impeders for small-tube welding; used for
current flow along the tube ID. Source: Ref 16
642 / Equipment

welding systems, the impeder consists of multi-
ple impeders assembled together, referred to as
a cluster impeder (Fig. 23).
Ferrites are the material of choice in most
tube-welding applications due to their low cost,
high magnetic permeability at lower flux densi-
ties, and low losses at high frequencies. The
drawbacks of ferrites are low-saturation mag-
netic flux density, sensitivity to thermal shock,
and sensitivity of magnetic properties to tem-
perature change.
The life of ferrites in the case of small tubes
ranges from several hours to a week. The mag-
netic flux density in the impeder core is higher
due to the smaller cross-section inside the tube,
and ferrites become saturated. In some instances
of short impeder life, ferrites have been replaced
with Fluxtrol 75 soft-magnetic composite, result-
ing in 10 to 30% energy savings and increased
life (Ref 17).
The efficiency of tube welding systems is
improved using an external magnetic bridge
(Ref 18). A soft-magnetic composite material
placed above the tube opening helps increase
current flow along the weld “V” and helps
balance heating of the top and bottom edges
of the tube wall (Fig. 24). In some applications,
the benefits of using an external magnetic
bridge are 20 to 30% energy savings and
improved weld quality. System improvements
from using a magnetic bridge depend on the
configuration of the weld rolls. To achieve sig-
nificant benefits, the magnetic bridge must be
close to the weld apex without moving the coil
farther away from it.
Magnetic Flux Controllers for
Seam-Annealing Inductors
In some cases, the weld seam requires anneal-
ing to relieve stresses from the welding process
or to improve the microstructure. To accomplish
this, induction heating is commonly used,
referred to as seam annealing. In instances of
high production rates and for heavy-walled pipe,
several seam annealers are placed in series.
Seam-annealing inductors are linear induc-
tors, either vertical loop or split-n-return,
operating at 1 or 3 kHz frequency and power
levels of hundreds of kWs. Figure 25 shows a
typical seam-annealing induction coil in the
field. C-shaped soft-magnetic materials (lamina-
tions or soft-magnetic composites) are placed
over the central turn to concentrate heat on the
weld seam. Magnetic flux controllers are not
used on the return leg (vertical loop) or legs
(split-n-return), because it is desirable to widely
distribute the return current to minimize heat
generated by those turns.
Magnetic Flux Control in
Local Heat Treating Applications
Surface hardening of machinery components
using induction is a common application. The
main advantage of induction heat treating com-
pared with other heat treatment methods is
the ability to quickly and selectively heat only
the areas of the workpiece where property
modification is desired. This results in lower
distortion, more favorable residual stress distri-
bution, better microstructure, significant energy
savings, and the ability to place the induct-
ion system in-line with other manufacturing
operations. Application of magnetic flux con-
trollers, primarily in the form of soft-magnetic
Fig. 23 Cluster impeder (multiple impeders assembled
together) for large diameter pipe welding.
Courtesy of Electronic Heating Equipment, Inc.
Fig. 24 Influence of a magnetic bridge on weld temperature distribution for a 40 mm OD by 2.5 mm thick tube
traveling at a line velocity of 75 m/min; the bridge helps balance heating of the top and bottom edges of
the tube wall. Source: Ref 4
Fig. 25 Seam annealer for tube-welding line. Courtesy of EFD Induction

materials, are particularly well suited for use in
heat treating applications, because they strongly
influence the power density value and its distri-
bution along the surface of the workpiece,
enabling faster heating and better heat pattern
control (Ref 3–6).
All induction heat treating processes can be
classified in two categories: single shot and
scanning, based on whether the induction coil
is moving (excluding rotation) relative to the
part during the heating process. A brief discus-
sion of the benefits of magnetic flux controllers
in these cases follows.
Single-Shot Heat Treating
In single-shot induction heat treating applica-
tions, the position of the induction coil relative
to the heated length of the part does not move.
In many single-shot heating applications, the
part rotates while heating and quenching occur
to ensure uniformity of pattern. Within the fam-
ily of single-shot heat treating applications, there
are several varieties of induction coil configura-
tions. Common induction coil styles used for
heat treating include linear coils, ID coils, encir-
cling coils, and encircling/non encircling coils.
The most important role of soft-magnetic mate-
rials in these applications is to control the distribu-
tion of temperature and ensure good part quality.
This is especially important near the ends of the
induction coil to control run-out or in central areas
where there are geometry changes.
Secondary benefits of soft-magnetic materials
in these applications are energy savings and lower
coil currents. This is especially important in app-
lications such as simultaneous dual-frequency
hardening of gears, where power levels at high
frequency tend to be large. Lower currents reduce
the coil kVAs, and allow for smaller transformers
and matching components, which reduce overall
system cost.
Soft-magnetic materials most commonly used
for induction heat treating applications are lami-
nations and soft-magnetic composites. In some
applications, a combination of laminations and
soft-magnetic composites are used on the same
coil. Laminations are more common on longer
linear coil sections at lower heat treating fre-
quencies (1–10 kHz). Soft-magnetic composites
are more common on medium to high frequency
linear coils (10 kHz and up) and coils with short
linear sections. For coils with circular sections,
soft-magnetic composites are more popular in
most cases due to ease of application.
Induction Scan Heat Treatment
In induction scan heat treatment applications,
the position of the induction coil relative to the
heated length of the part does move. In many
scanning applications, the part rotates during
heating and quenching to ensure uniformity of
pattern. Within the family of scanning applica-
tions, induction coil configurations are not as
diverse as for single shot. Common induction
coil styles used for scanning include encircling
coils, short linear coils, and channel coils.
Encircling coils likely are the most common
for induction scanning (Fig. 26). In induction
scanning applications with encircling coils, the
main benefit of a magnetic flux controller is in
heat pattern control at the beginning and end
of the part. Energy savings from the application
of magnetic flux controllers is less than in the
case of single-shot coils, because much of the
stray heating before and after the coil is still
used in the scanning process to contribute to
the case depth. In some instances, such as axle
scan hardening, where the heat treatment in
the fillet area drives the design, magnetic flux
controllers improve the pattern and achieve
energy savings between 15 and 50% (Ref 4,
17, 19). Figure 27 shows some induction scan
hardening coils with magnetic flux controllers
used for axle scan hardening.
Linear coils commonly are used for scan-
ning large components, such as hardening
bearing races on slewing rings, and tooth-by-
tooth hardening of large gears. Other applications
are scan hardening of components where there is
a large geometrical change. Nearly all short lin-
ear scanning coils use soft magnetic materials,
because they are necessary for heat pattern con-
trol and provide large energy savings.
Channel coils are commonly used for in-line
heat treatment and brazing of smaller components,
such as hardening or tempering of nuts and
bolts, brazing of cutting bits, and hardening
of saw blade teeth. Magnetic flux controllers
are used for heat pattern control and efficiency
improvement. Most of these installations oper-
ate at higher frequencies, so soft-magnetic compo-
sites are the material of choice when a magnetic
flux controller is used.
REFERENCES
1. A. Muhlbauer, History of Induction Heat-
ing, Vulcan Verlag GmbH, Huyssenallee,
Essen, Germany, 2008
2. V. Rudnev, et al., Handbook of Induction
Heating, Marcel Dekker, New York, 2003
3. V.S. Nemkov, “Resource Guide for Induc-
tion Heating, CD-R,” Fluxtrol Inc., 2006
4. www.fluxtrol.com
5. R.S. Ruffini, R.T. Ruffini, and V.S. Nemkov,
Advanced Design of Induction Heat Treating
Coils, Part I: Design Principles, Industrial
Heating, June 1998.
6. R.S. Ruffini, R.T. Ruffini, and V.S. Nemkov,
Advanced Design of Induction Heat Treating
Coils, Part II: Magnetic Flux Concentration
and Control, Industrial Heating, Nov 1998
7. R.T. Ruffini, V.S. Nemkov, and R.C. Goldstein,
“Prospective for Improved Magnetic Flux
Control in the Induction Heating Tech-
nique,” presented at 20th Heat Treating Soci-
ety Conference and Exposition, St. Louis,
2001
8. R.C. Goldstein, et al., “Virtual Prototyping
of Induction Heat Treating,” 25th Heat
Treating Society Conference and Exposi-
tion, Indianapolis, Sept, 2009
Fig. 26 Induction scan-hardening coils made of formed
tubing. Source: Ref 4 Fig. 27 Two-turn scan hardening coil with magnetic flux controllers used for axle scan hardening. Source: Ref 4
644 / Equipment

9. Myers, et al., Optimizing Performance of
Crankshaft Hardening Inductors, Industrial
Heating, Dec 2006
10. S. Lupi, et al., Induction Heating Industrial
Applications, France 1992
11. V.S. Nemkov, and R.C. Goldstein, Design
Principles for Induction Heating and Hard-
ening, Handbook of Metallurgical Process
Design, G. Totten, K. Funatani, and L. Xie,
Eds., Marcel Dekker, New York 2004
12. V.S. Nemkov, et al., “Optimal Design of
Internal Induction Coils,” Intl. Symposium
on Heating by Electromagnetic Sources,
Padua, Italy, June 2004
13. Bialod, et al., Induction Heating: Industrial
Applications, UIE, 1991
14. Heidlhoff, et al., “Advancements in Ti
Alloy Powder Production by Close-
Coupled Gas Atomization,” Advances in
Powder Metallurgy Particulate Materi-
als, 2011
15. Spagnolo, et al., “Space Control Optimi-
zation of Multi-Coil Transverse Flux
Induction Heating of Metal Strips,” Intl.
Symposium on Heating by Electromagnetic
Sources, Padua, Italy, May 2010
16. www.startechnologica.com.br
17. Fluxtrol internal documents
18. V.S. Nemkov, Magnetic Flux Guide for
Continuous High Frequency Welding of
Closed Proﬁles, U.S. Patent Application
20080308550, Dec, 2008
19. R.C. Goldstein, et al., “Optimizing Axle
Scan Hardening Inductors,” 24th Heat
Treating Society Conference and Exposi-
tion, Detroit, Sept 2007

Magnetic Flux Controllers in Induction Heating and Melting by Robert Goldstein of Fluxtrol Inc.

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Magnetic Flux Controllers in Induction Heating and Melting by Robert Goldstein of Fluxtrol Inc.