Mumbai University > Electronics Engineering > Sem7 > Power Electronics 2

Marks: 10M

1) Electrical heating is superior to other methods of heating because of the following reasons:

• Its maintenance is easy as there are no moving parts.
• Electrical heating is clean and efficient
• It offers compact and reliable heating system.

2) Electrical heating is done either at high frequency or at power frequency. At Power frequency heating, heat is transferred to the material to be heated by convection, conduction or radiation. At high-frequency heating, electrical energy is converted into heat within the material itself. High frequency heating is, therefore, more efficient.

3) Principle of Induction Heating: When a transformer primary is energized , iron loos occurs in the core of transformer. This core loss, made up of eddy-current loss and hysteresis loss, is undesirable and efforts are made to minimize this loss in transformer. This eddy current loss is, however, exploited in producing heat in the metals to be heated and this is what is called induction heating.

4) Factors Affecting Induction Heating δ

a) Consider a metal cylinder of diameter‘d’ and length ‘l’ as shown in fig . This cylinder is to be heated, is usually called workpiece.

b) A workpiece coil having N turns and carrying a current I is wound over the workpiece. For the direction of current indicated, the flux created by mmf IN is vertically up.

c) As per Lenz’s law an eddy current must be induced in the metal cylinder such that this current would oppose the vertical flux. This eddy current $I_2$ can be treated as the secondary current of a transformer having one circular turn near the surface as shown in fig 11.33(a).

d) Assuming no end effects, primary ampere turns, IN = secondary ampere turns $I_2$ = I , Power loss in workpiece , is given by P = $I^2R$ = [$(IN)^2$ * ρ * length] / area Here, length along the direction of current flow = circumference of circular workpiece = πd , where d= diameter of the cylindrical metal

e) Consider δ as the depth off heat penetration, then

$P=(IN)^2\frac{p.\pi .d}{1.\delta}....W$

f) The power loss in the cylindrical workpiece is confined to a thin layer near the surface of the work piece due to skin effect. Tendency of the high frequency current to flow in a thin layer near the surface of the workpiece is called skin effect.

g) The magnitude of eddy current in the workpiece varies exponentially and is given by

$$I(x) = I_0 e^{- x/ δ}$$

$I_0$ = current induced at the surface of workpiece , A

x = distance measure from the surfaxe towards the center of workpiece ,

δ = skin depth peneteration, measured from the surface, m

h) If $P_s$ = power entering the workpiece per square metere of cylindrical surface area πdl,

$$P_0=\frac{P}{πdl}=(IN)^2\frac{ρ.πd}{1.δ} \times \frac{1}{πdl} \\ = \big(\frac{IN}{l}\big)^2\frac{ρ}{δ}=H^2\frac{ρ}{δ} .... W/m^2 $$

i) In addition to eddy current loss, hysteresis loss also leads to heating of the metal object. But hysteresis loss is much less as compared to eddy current loss, hence it is usually neglected.

j) Factors governing induction heating are magnetic field intensity, relative permeability, frequency and resistivity. Higher the values the greater is the heat generated by induction heating.

5) Induction heating Circuit

For frequencies from 250Hz to 10KHz, rectifier – inverter circuits are used. The two basic circuit configurations are as under:

• Voltage source series resonant inverter

  (i) Fig shown in 11.35 (a) , diode rectifier converts utility voltage to direct voltage. The dc link voltage is kept level by the use of capacitor C1 . Inductor L1 limits the current pulsations.

  (ii) Load inductance L is large, therefore C should be suitably chosen so that RLC circuit is underdamped.

  (iii) For load commutation to occur, switching frequency should be below the circuit resonant or circuit ringing, frequency.

• Current source parallel-resonant inverter

  i) Fig shown in 11.35 (b) , inductor L1 is quite large.

  ii) With the turning on of starting SCR Ts a constant current flows through L1 .

  ii) In this configuration, the switching frequency is kept somewhat more than the circuit resonant frequency. Load power is regulated by controlling the firing angle of thyristor rectifier.

6) Applications of induction heating

i. Induction hardening :

Induction hardening uses induced heat and rapid cooling (quenching) to increase the hardness and durability of steel. Induction is used to harden gears, crankshafts, camshafts, drive shafts, output shafts, torsion bars, rocker arms, CV joints, tulips, valves, rock drills, slewing rings, inner and outer races.

ii. Induction tempering :

Induction tempering is a heating process that optimizes mechanical properties such as toughness and ductility in workpieces that have already been hardened. Induction tempering is widely employed in the automotive industry to temper surface-hardened components such as shafts, bars and joints.

iii. Induction brazing

Brazing is a materials-joining process that uses a filler metal (and usually an anti-oxidizing solvent called flux) to join two pieces of close-fitting metal together without melting the base materials. The aeronautics sector uses induction to braze fan blades, blades for casings, and fuel and hydraulic systems.

iv. Induction bonding

Induction bonding uses induction heating to cure bonding adhesives. Induction is the main method for curing adhesives and sealants for car components such as doors, hoods, fenders, rearview mirrors and magnets.

v. Induction welding

With induction welding the heat is electromagnetically induced in the workpiece. Induction welding is used in the tube and pipe industry for the longitudinal welding of stainless steel (magnetic and non-magnetic), aluminum, low-carbon and high strength low-alloy (HSLA) steels and many other conductive materials.