Induction Heating Principle

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induction-heating


INDUCTION HEATING
was first noted when it was found that heat was produced in transformer and motor windings, as mentioned in the Chapter “Heat Treating of Metal” in this book. Accordingly, the Principle of induction heating was studied so that motors and transformers could be built for maximum efficiency by minimizing heating losses. The develop- ment of high-frequency induction power supplies provided a means of using induction heating for surface hardening. The early use of induction involved trial and error with built-up personal knowledge of specific applications, but a lack of understanding of the basic principles. Through- out the years the understanding of the basic principles has been expanded, extending currently into computer modeling of heating applications and processes. Knowledge of these basic theories of induction heating helps to understand the application of induction heating as applied to induction heat treating. Induction heating occurs due to electromagnetic force fields producing an electrical current in a part. The parts heat due to the resis- tance to the flow of this electric current.
Resistance
All metals conduct electricity, while offering resistance to the flow of this electricity. The resistance to this flow of current causes losses in power that show up in the form of heat. This is because, according to the law of conser- vation of energy, energy is transformed from one form to another—not lost The losses produced by resistance are based upon the basic electrical formu- la: P i2R, where i is the amount of current, and R is the resistance Because the amount of loss is proportional to the square of the current, dou- bling the current significantly increases the losses (or heat) produced. Some metals, such as silver and copper, have very low resistance and, consequent-
6 / Practical Induction Heat Treating are very good conductors. Silver is expensive and is not ordinarily used
for electrical wire (although there were some induction heaters built in WorldWar II that had silver wiring because of the copper shortage). Copper wires are used to carry electricity through power lines because of the low heat losses during transmission. Other metals, such as steel, have high resis- tance to an electric current, so that when an electric current is passed through steel, substantial heat is produced. The steel heating coil on top of an electric stove is an example of heating due to the resistance to the flow of the house- hold, 60 Hz electric current. In a similar manner, the heat produced in a part in an induction coil is due to the electrical current circulating in the part.
Alternating CurrentandElectromagnetism
Induction heaters are used to provide alternating electric current to an elec-
tric coil (the induction coil). The induction coil becomes the electrical (heat) source that induces an electrical current into the metal part to be heated (called the workpiece). No contact is required between the workpiece and the induction coil as the heat source, and the heat is restricted to localized areas or surface zones immediately adjacent to the coil. This is because the alternating current (ac) in an induction coil has an invisible force field (elec-
Fig. 2.1 Induction coil with electromagneticfield. OD, outside diameter; ID,
inside diameter.Source:Ref1
Theory of Heating by Induction
tromagnetic, or flux) around it.When the induction coil is placed next to or around a workpiece, the lines of force concentrate in the air gap between the coil and the workpiece. The induction coil actually functions as a trans- former primary, with the workpiece to be heated becoming the transformer secondary. The force field surrounding the induction coil induces an equal and opposing electric current in the workpiece, with the workpiece then heat- ing due to the resistance to the flow of this induced electric current. The rate of heating of the workpiece is dependent on the frequency of the induced current, the intensity of the induced current, the specific heat of the material, the magnetic permeability of the material, and the resistance of the material to the flow of current. Figure 2.1 shows an induction coil with the magnetic fields and induced currents produced by several coils. The induced currents are sometimes referred to as eddy-currents, with the highest intensity current being produced within the area of the intense magnetic fields.
Induction heat treating involves heating a workpiece from room tempera- ture to a higher temperature, such as is required for induction tempering or induction austenitizing. The rates and efficiencies of heating depend upon the physical properties of the workpieces as they are being heated. These properties are temperature dependent, and the specific heat, magnetic per- meability, and resistivity of metals change with temperature. Figure 2.2 shows the change in specific heat (ability to absorb heat) with temperature
Fig. 2.2 Changein specific heat with temperaturefor materials.Source:Ref2
8 / Practical Induction Heat Treating
for various materials. Steel has the ability to absorb more heat as temperature increases. This means that more energy is required to heat steel when it is hot than when it is cold. Table 2.1 shows the difference in resistivity at room temperature between copper and steel with steel showing about ten times higher resistance than copper. At 760 °C (1400 °F) steel exhibits an increase in resistivity of about ten times larger than when at room tempera- ture. Finally, the magnetic permeability of steel is high at room temperature, but at the Curie temperature, just above 760 °C (1400 °F), steels become nonmagnetic with the effect that the permeability becomes the same as air

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  • Physical Characteristics:

    Elementary
    symbol

    Name

    Atomic weight

    Specific
    weight

    Melting point

    Boiling point

    Specific heat

    Coefficientof
    heat conduction

    Element
    number

    Ag

    silver

    107.880

    10.49

    960.80

    2210

    0.056(0')

    1.0(0'C)

    47

    Al

    aluminum

    26.97

    2.699

    660.2

    2060

    0.223

    0.53

    13

    As

    arsenic

    74.91

    5.73

    814

    610

    0.082

    -

    33

    Au

    gold

    197.21

    9.32

    1063.0

    2970

    0.031

    0.71

    79

    B

    boron

    10.82

    2.3

    2300+-300

    2550

    0.309

    -

    5

    Be

    beryllium

    9.02

    1.848

    1277

    2770

    0.52

    0.038

    4

    Ba

    barium

    137.36

    33.74

    704+-20

    1640

    0.068

    -

    56

    Bi

    bismuth

    209.0

    9.80

    271.30

    1420

    0.034

    0.020

    83

    C

    carbon

    12.010

    2.22

    3700+-100

    4830

    0.165

    0.057

    6

    Ca

    calcium

    40.8

    1.55

    850+-20

    1440

    0.149

    0.30

    20

    Cd

    cadmium

    112.41

    8.65

    320.9

    765

    0.055

    0.22

    48

    Ce

    cerium

    140.13

    6.9

    600+-50

    1440

    0.042

    -

    58

    Co

    cobalt

    58.94

    8.85

    1499+-1

    2900

    0.099

    0.165

    27

    Cr

    chromium

    52.01

    77.19

    1875

    2500

    0.11

    0.16

    24

    Cs

    cesium

    132.91

    1.9

    28+2

    690

    0.052

    -

    55

    Cu

    copper

    63.54

    8.96

    1083.0

    2600

    0.092

    0.94

    29

    Fe

    iron

    55.85

    7.896

    1536.0

    2740

    0.11

    0.18

    26

    Ga

    gallium

    69.73

    5.91

    29.87

    2070

    0.079

    -

    31

    Ge

    germanium

    72.60

    5.36

    958+-10

    2700

    0.073

    -

    32

    Hg

    mercury

    200.61

    13.546

    38.36

    357

    0.033

    0.0201

    80

    In

    indium

    114.76

    7.31

    156.4

    1450

    0.057

    0.057

    49

    Ir

    iridium

    193.1

    22.5

    2454+-3

    5300

    0.031

    0.147

    7

    K

    potassium

    39.096

    0.86

    63.7

    770

    0.177

    0.24

    19

    La

    lanthaduim

    138.92

    6.15

    826+-5

    1800

    0.045

    -

    57

    Li

    lithium

    6.940

    0.535

    186+-5

    1370

    0.79

    0.17

    3

    Mg

    hydrogen

    24.32

    1.74

    650+-2

    1110

    0.25

    0.38

    12

    Mn

    manganess

    54.93

    7.43

    1245

    2150

    0.115

    -

    25

    Mo

    molybdenum

    95.95

    10.22

    2610

    3700

    0.061

    0.35

    42

    Na

    sodium

    22.997

    0.971

    92.82

    892

    0.295

    0.32

    11

    Nb

    niobium

    92.91

    8.57

    2468+-10

    >3300

    0.065(0'C)

    -

    41

    Ni

    nickel

    58.69

    8.902

    1453

    2730

    0.112

    0.198

    28

    Os

    osmium

    190.2

    22.5

    2700+-200

    5500

    0.031

    -

    76

    P

    phosphorus

    30.98

    1.82

    441

    280

    0.017

    -

    15

    Pb

    lead

    207.21

    11.36

    327.4258

    1740

    0.031

    0.08

    82

    Pd

    palladium

    106.7

    12.03

    1544

    4000

    0.058(0'C)

    0.17

    46

    Pt

    platinium

    195.23

    21.45

    1769

    4410

    0.032

    0.17

    78

    Rb

    rubidium

    85.48

    1.53

    39+-1

    680

    0.080

    -

    37

    Rn

    radon

    102.91

    12.44

    1966+-3

    4500

    0.059

    0.21

    45

    Ru

    ruthenium

    101.7

    12.2

    2500+-100

    4900

    0.057(0'C)

    -

    44

    S

    sulfer

    32.066

    2.07

    119.0

    444.6

    0.175

    -

    16

    Sb

    antimony

    121.76

    6.62

    630.5

    1440

    0.049

    0.045

    51

    Se

    selenium

    78.96

    4.81

    220+-5

    680

    0.084

    -

    34

    Si

    selicon

    28.06

    2.33

    1430+-20

    2300

    0.162(0'C)

    0.20

    14

    Sn

    tin

    118.70

    7.298

    231.9

    2270

    0.054

    0.16

    50

    Sr

    strontium

    87.63

    2.6

    770+-10

    1380

    0.176

    -

    38

    Ta

    tantalum

    180.88

    16.654

    2996+-50

    >4100

    0.036(0'C)

    0.13

    73

    Tc

    technetium

    127.61

    6.235

    450+-10

    1390

    0.047

    0.014

    52

    Th

    thorium

    232.12

    11.66

    1750

    >3000

    0.126

    -

    22

    Ti

    titanium

    47.90

    4.507

    1688+-10

    >3000

    0.126

    -

    22

    Tl

    thallium

    204.39

    11.85

    300+-3

    1460

    0.031

    0.093

    81

    U

    uranium

    238.07

    19.07

    1132+-5

    -

    0.028

    0.064

    92

    V

    vanadium

    50.95

    6.1

    1900+25

    3460

    0.120

    -

    23

    W

    tungsten

    183.92

    19.03

    3410

    5930

    0.032

    0.48

    74

    Zn

    zinc

    65.38

    7.133

    419.505

    906

    0.0915

    0.27

    30

    Zr

    zirconium

    91.22

    6.489

    6.489

    >2900

    0.066

    -

    40


Ohm's Law

Ohm's Law states that in a simple electrical circuit, the strength of a current (I) flowing through a resistance (R) is proportional to the applied voltage (E). It is expressed by the formula:


 

Thus, if you increase the voltage, and resistance remains the same, the current will increase proportionately.

Resistive Heating
Resistance is well named, for it opposes current flow. The lower the resistance, the higher the current flow in the curcuit, and hence the greater the power. This power (P) is the rate at which electrical energy is transformed into heat. It is expressed by the formula:

This heat can be put to good purpose and is the principle behind heating elements such as you will find in hair dryers and baseboard heaters. However, such direct production of heat is inefficient, localized, and difficult to control. For industrial purposes it is preferable to produce heat by using an induced current rather than a direct one.

Induction Heating
With induction heating, we substitute an induced current for a direct one, which is of course the principle of the transformer. It works this way. Alternating current flowing through the primary coils of the transformer creates an electromagnetic alternating field. Since the reverse is also true, by placing secondary coils within that field, we will induce a current to flow through them. And depending on the respective number of electrical turns in the primary and the secondary, we can step up or step down the voltage levels. It is the voltage in the secondary turns which, when applied to heating elements, creates the energy to heat or melt metals.

Resistive Heating Illustrated

Induction Heating Illustrated

Current flow is induced in the secondary circuit by placing the secondary turns within the changing magnetic field created by the primary turns.

PDF: Induction Heating Principle

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