Download Work Coil Design used in Induction Hardening Machine

World Academy of Science, Engineering and Technology 18 2008
Work Coil Design used in Induction Hardening
Machine
Han Phyo Wai, Soe Sandar Aung, Jr., and Thidar Win
Keywords—Changing of applied frequencies, work coil type and
work piece size, design of helical multi-turn type of work coil and
induction hardening
heat is caused by the induction of current, the process is called
induction heating. And as the induction hardening is a branch
of induction heating system, the process of induction
hardening is the same as that of induction heating.
The heat development in the work piece depends on the
rating of the induction hardening machine, the electrical
resistivity of the work piece and the configuration of the work
coil and its relationship of the work piece. Although the
induction hardening system generates the heat energy on the
work piece, there is no tough between the work coil and work
piece. The source of heat is developed only in the work piece
but not on the work coil. Therefore, it is a non-contact method
of heating. In induction hardening process, the intense of heat
is highest at the surface of the work piece and lowest at the
centre of it. Depending upon the kind of hardening, the
development of heat must be from the surface of the work
piece to interior of the limited zone during heating cycles.
After being heated for some times, the work piece must be
quenched by suitable quenching medium to be desired
hardenability.
I. INTRODUCTION
II. INDUCTION HARDENING SYSTEM
N induction type hardening system, the essential
requirements are the coil called work coil and the alternating
current supply. The work coil may be wound the material to be
hardened called work piece or it may be near by the work
piece and the work coil is applied by the high-frequency
current. The flow of current in the work coil produces a
magnetic field or flux that surrounds each turn of the work coil
and this flux passes through air or any metal that is within or
near the work coil. The alternating current causes the flux to
change or the alternating magnetic field. And the change of
flux induces a voltage within the work piece. Due to this
induced voltage, the induced current is flown through the work
piece, and as the current passes through the resistance of the
work piece, the heat is developed in the work piece. Since this
There are AC power supply, rectifier circuit, inverter circuit,
work coil and quenching system for the whole system. The AC
power source may be single-phase or three-phase, and it
applies line frequency and line voltage. The control rectifier
converts the AC voltage to the DC values and applies the
desired DC current to the inverter circuit. The inverter changes
the DC signals to the AC signals with desired frequency to
apply the work coil. The work coil uses the input values and
makes the work piece develop heat. When the work piece has
been heated for a time, the quenching system is applied to the
work piece. Fig. 1 is the block diagram of induction hardening
system.
Abstract—The induction hardening machines are utilized in the
industries which produce machine parts and tools needed to achieve
high wear resistance. If they are constructed in local, the industries
can utilize them commonly and easily. As the machines are designed
and constructed in accordance with the desired products, the quality
of products is higher and the products can be used efficiently. In the
study of Design and Construction of Induction Hardening Machine,
the design of work coil is presented. A small model induction
hardening machine is designed for the output power 5 kW and
operation frequency 35 kHz. As the work coil is the heart of the
machine, a good coil design reduces power requirement and causes
good efficiency. The design of work coil that causes the most
magnetic flux density in the work piece, the least power consumption
and develops desired heat during limited time is studied. It has been
attempted to design the helical multi-turn type of work coil for any
size of cylindrical shape of work piece with uniform surface. The
design study is covered the changing of work piece size, the changing
of work coil turns and the changing the applied frequencies.
I
Han Phyo Wai is with the Electrical power Engineering Department, ,
Technology University Mandalay, Myanmar (e-mail: hanphyowai2007@
gmail.com).
Soe Sandar Aung is with the Electrical power Engineering Department,
Technology University Mandalay,
Myanmar (e-mail: soesandarag@
gmail.com).
Thidar Win is with the Electrical power Engineering Department,
Technology University Mandalay, Myanmar (e-mail: malthida80@
gmail.com).
Fig. 1 Block diagram of induction hardening system
Typically the other requirements are the protection system,
choke coil or filter circuit, load matching system and cooling
system. In most of the induction hardening systems, the
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World Academy of Science, Engineering and Technology 18 2008
6)
7)
8)
9)
protection circuits, over voltage protection and over current
protection are matched with the rectifying triggering circuit
because of the complicated operation functions of inverting
triggering circuit. The voltage and current sampling circuit
senses the work coil voltage and current. It uses the current
transformer to detect the over current and uses the voltage
transformer to detect the over voltage. The over voltage and
current are mainly caused when the operated work piece is
changed. For a constant size and shape of work piece, all
settings do not need to change, but according to the various
size and shape of work piece it is needed to change the voltage
and current setting. The power control of work coil is done by
several ways.
The first one is varying the DC link voltage. The reducing
of supply voltage to the inverter can decrease the power at the
work coil. As the inverter is supplied from the rectifier, by
controlling the rectifying operation the variable voltage DC
supply fulfils the required voltage of inverter. Varying the DC
link voltage allows full control of the power. The second one
is varying the duty ratio of the devices in the inverter. By
reducing the on-time of the switches in the inverter the power
processed by the inverter can be decreased. At the time of the
devices are switched on, power is only sourced to the work
coil. When the devices are tuned off, the load current is left to
freewheel through the diodes. So, varying the duty ratio of the
switches allows full control of the power.
The next one is varying the operating frequency of the
inverter. If the operating frequency of the inverter is moved
away from the resonant frequency of the load circuit, there is
less resonant rise in the load circuit and the current in the coil
diminishes. Therefore, less circulating current is induced in the
work piece. The last one is varying the value of inductance in
the matching system. Altering the inductance of the matching
system adjusts the value to which the load impedance is
translated. In general, decreasing the inductance of the
matching system causes the work coil impedance to be
transformed down to a lower impedance. This lower load
impedance being presented to the inverter causes more power
to be sourced from the inverter. Conversely, increasing the
inductance of the matching system causes a higher load
impedance to be presented to the inverter and causes less
power to be sourced from the inverter.
Oxidization is less because of short time heating.
Distraction is smaller compared to that of other methods.
Easy for automation.
Skilled operators will not be required to operate the
machine.
IV. DEPTH OF HARDNESS
For work piece, the depth of penetration is that a distance
from the surface to the interior of work piece that to be
hardened. And it is also called the depth of hardness. It is
expressed as
δw =
1
2π
1
µ r fσ
(1)
.
Where, δ = depth of hardness, m
f = applied frequency, Hz
µr = relative permeability, H/m
σ = electric conductivity of work piece, mhos/m
V. ROLE OF WORK COIL
The work coil is the heart of induction hardening system. It
is the main part of the system that transfers the electric energy
to the heat energy. So, to be the maximum transfer of heat
energy to the work piece the role of work coil is very
important. The work coil is generally formed to be the largest
possible number of magnetic flux lines inserted in the work
piece at the area to be heated. The denser the flux at this point,
the higher will be the current generated in the work piece. As
the current generated in the work piece causes the heat energy,
the heating rate and the depth of heat penetration absolutely
depend upon the work coil. The work coil faces various types,
sizes and shapes of work pieces for a wide variety of heating
operations. So, the formation of work coil is the most
important one for the whole system to be a good efficiency.
A. Types of Work Coil
Work coil for induction hardening is made in a wide variety
of styles, shapes and sizes. Depending upon the natures of
work piece the styles of work coil are changed. According to
the geometry of work piece surface the shape of work coil is
varied. And the size of work coil is governed by the length of
work piece. The work coils are mainly divided into helical,
pancake and internal and also they are shown in Fig. 2. A
helical coil is generally used for the part or area to be heated
located within the coil and, thus, in the area of greatest
magnetic flux. A pancake coil is used for heating flat surfaces
and so flux from only one surface intersects the work piece.
An internal coil is used for heating inner surfaces of holes. In
general, helical coils used to heat round work pieces have the
highest values of coil efficiency and internal coils have lowest
values. Coil efficiency is that part of the energy delivered to
the coil that is transferred to the work piece. This should not
be confused with overall system efficiency.
Besides coil efficiency, heating pattern, part motion relative
III. BENEFITS OF INDUCTION HARDENING
The benefits of induction hardening are as follow:
1) Depth of hardness can be minutely controlled.
2) Different degrees of hardness can be obtained within a
single work piece.
3) As there is no flame or heating element, there is no smoke,
ash or pollution during operation, it is a clean operation
process.
4) As the localized heating is developed, the dissipation of
heat is small and the efficiency of the heating system is
good.
5) Uniform results after a heating cycle has been established.
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World Academy of Science, Engineering and Technology 18 2008
considerations are as follows:
Single-turn coils are preferred when the heated area is
narrow or restricted. Single-turn coils are more practical where
the height does not exceed the diameter. Multi-turn coils are to
be preferred for heating long areas.
When the length of a coil exceeds eight times its diameter,
uniform heating may be difficult. Long area should be heated
by progressive feed through short coils.
to the coil, and production rates are also important. Because
the heating pattern reflects the coil geometry, inductor shape is
probably the most important of these factors. Quite often, the
method by which the part is moved into or out of the coil can
necessitate large modifications of optimum design. If one part
is needed every 30 seconds but a 50 seconds heating time is
required, it will be necessary to heat parts in multiples to meet
the desired production rate. It is important to look at a wide
range of coil techniques to find the most appropriate one.
C. Inductance Losses
When connecting the leads of a heating coil to a generator,
especially those of the quick change type, it is desirable to
keep them as close together as possible in order to avoid
inductance losses between the leads, as might be represented
by the example as shown in Fig. 4. The spacing would result in
unwanted inductance, so the maximum heating of the work
within the coil could not be attained. By providing leads as
shown in Fig. 5, where the space is held to a minimum, there is
better assurance of maximum heating of the work.
Fig. 2 Types of work coil
B. Helical Coil Type
The copper conductor is wound or formed either
symmetrical in contour or shaped to suit the outline of the part
to be heated. This type is suited to surface heating of shafts
and bars. The shape of helical coil mainly depends on the
shape of work piece. So, the coil may be in the shape of round,
rectangular, formed, spiral helical and others. Of all, the round
coils are commonly used and suitable for several shapes of
work piece. The coil includes single-turn or multi-turn.
Depending upon the application, it is a single-turn, singleplace coil or a multi-turn, single-place coil. Both coils are used
for heating a single part at a time. And it is a single-turn, multiplace coil or a multi-turn, multi-place coil for heating the multi
parts at a time. The following figure shows typical
configurations for helical coils.
Fig. 4 Large coil lead spacing
Fig. 5 Narrow coil lead spacing
VI. WORK COIL CHARACTERIZATION
To achieve uniform heating along the work piece length, the
work coil must be modified to provide better uniformity.
Adjusting the work coil turns, coil pitch and coupling distance
with the work piece to achieve a uniform heating pattern is
known as work coil characterization.
In applying multi-turn coils, the pitch of the turn windings
has a direct relation to the depth of heat penetration. In multiturn work coils, the spacing between turns should not be more
than one half of the diameter of the tubing itself. Beyond this
limit, or with loosely wound coils, nonuniform heating is likely
to occur. If the work coil is placed closely to the work piece,
the heated zone is the exact shape of the coil winding but not
uniform throughout the surface of work piece and the large
heating zone as shown in Fig. 6. By increasing the coil
coupling as shown in Fig. 7, the heating becomes more
uniform but the less heated layer. A good relation is that the
Fig. 3 Typical configurations for helical coils
The use of single-turn coil and the use of multi-turn coil are
based on usually the area of the zone to be heated. The basic
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World Academy of Science, Engineering and Technology 18 2008
VIII. DESIGN RESULTS
coupling distance should be equal to the coil pitch and not
more than two times coil pitch to be uniform heating on the
work piece.
The number of work coil turns is mainly based on the length
of work piece and the pitch of coil windings. Then it is
expressed as follow.
(2)
lw
N =
dc + C p
Where, N = number of. turns of work coil
Lw = length of work piece to be hardened
Cp = pitch of coil winding
The results for work coil, conductor, work piece and
electrical properties of the system are calculated. These results
are presented by the following tables.
Fig. 6 Nonuniform heat penetration
TABLE II
RESULTS FOR WORK COIL
specification
shape
Number of turns
Inner diameter (m)
Outer diameter (m)
Length (m)
Coil pitch (m)
Coupling distance (m)
Fig. 7 Uniform heat penetration
If the work piece is not straight, coupling must decrease. At
high frequencies, work coil currents are lower and coupling
must be increased. With low and medium frequencies, work
coil currents are considerably higher and coupling must be
decreased.
specification
material
shape
thickness (m)
diameter (m)
Length (m)
To design the work coil, it is needed some specifications of
work piece, conductor and operation. The specifications for
operating process are the ambient temperature is assumed
300.15 K, the desired hardened temperature is 1116.48 K, the
duration of hardened time is about 10 sec of apply frequency is
35 kHz. Table I is for the specifications of conductor used as
work coil and work piece material.
specification
Material
Shape
Nature of surface
Depth of hardness (m)
diameter (m)
length (m)
Cross sectional area (m2)
Surface area (m2)
Volume (10-6)
SPECIFICATIONS OF CONDUCTOR AND WORK PIECE
value
copper
1.7 × 10−8(at 293.15 K)
1
1040 carbon steel
Resistivity (Ωm)
12.7 × 10−8(at 293.15 K)
115.6 7 × 10−8(at 1253.15 K)
Permeability (H/m)
Specific heat (J/kg.K)
1
434 (at 300 K)
1169 (at 1000 K)
1749.26
1116.48 – 1172.03
7861.13
copper
round
0.000702
0.00635
1.282781
Design value
1040 carbon steel
cylindrical
uniform
0.0009587
0.067008
0.033504
0.000199
0.007053
6.665071
TABLE V
RESULTS FOR ELECTRICAL PROPERTIES OF THE SYSTEM
Specification
Melting temperature (K)
Hardened temperature (K)
Density (kg/m3)
Design value
TABLE IV
RESULTS FOR WORK PIECE
TABLE I
material
resistivity (Ωm)
Permeability (H/m)
material
round
4
0.070184
0.082884
0.0381
0.03175
0.01588
TABLE III
RESULTS FOR CONDUCTOR
VII. REQUIRED SPECIFICATIONS FOR DESIGN CALCULATION
specification
Design value
Resistance of work coil (Ω)
Resistance of work piece (Ω)
Inductance of work coil (µH)
Magnetizing Inductance (µH)
Power factor
Resonated capacitance (µF)
Quality factor
Total impedance (Ω)
supply current (A)
Supply voltage (V)
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Design value
0.003114
0.121220
1.434858
0.551223
0.273791
10.411355
3.512809
1.658596
71.479484
118.555586
World Academy of Science, Engineering and Technology 18 2008
IX. CONCLUSION
The study of work coil design for induction hardening
machine is for the work piece of cylindrical shape with the
uniform surface and the type of work coil is helical multi-turn
coil. And the method of hardening is stationary hardening
method. The design study does not match with the work piece
with irregular surface and the work piece its length less than its
radius. So, the further study should be the design consideration
of work coil for the work piece with irregular surface and
single-turn work coil because helical type single-turn coils are
commonly in used. And it should be for the study of the
several shape of work piece. As multi-turn coils have a number
of coil pitch, rotating and moving of work piece during
operation has advantages. So, the scanning method of
hardening should be studied in future.
ACKNOWLEDGMENT
Firstly the author would like to thank her parents: U Ba Thin
and Daw San Yee for their best wishes to join the Ph.D
research. Special thanks are due to her supervisor Dr. Ni Ni
Win, Lecturer, Electrical Power engineering Department,
MTU, Myanmar. The author greatly expresses her thanks to all
person whom will concern to support in preparing this paper.
REFERENCES
[1]
[2]
[3]
Curits, F.W.1944. High-frequency Induction Heating. 1st ed. New
York:: McGraw-Hill Book Company, Inc.
Zinn, S., and Semiatin, S. L. 1988. Coil Design and Fabrication: Basic
Design and Modifications. July 2005 <http://www.ameritherm.com/
techonotes. html >
Herbert B. D wight. 1945. Electrical Coils and Conductors. 1st Ed. Mc
Graw-Hill Publishing Co.Ltd.
Han Phyo Wai received her M.E in Electrical Power Engineering from
Yangon Technological University, then following two years teaching in
Technological University, Myanmar. Her interests include Power Electronic
Devices and its applications.
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