Principle of Induction Heating (Induction Heating Principle)

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

induction coil design

induction Principle

induction heating theory

• 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.