electrical heating 03-03

INTRODUCTION

Mr. Vijay Balu Raskar - Electrical Engineer

CORE TYPE OF INDUCTION FURNACEMr. Vijay Balu Raskar - Electrical Engineer

DIRECT CORE TYPE INDUCTION FURNACE:-

Consists of a vertical furnace with a V shaped portion at the bottom.Consists of a Transformer charge is magnetically coupled to the primary windingandforms single-turnshort circuitedsecondary.Secondary channel should not be emptied.Magnetic coupling between primary & secondary is weakresulting in large leakage reactance poor p.fLeakage reactance is nullified using low frequency supply

Drawbacks.1)Leakage reactance is high & p.f is low.2)Low frequency requirement.3) Suffersfrom PinchingEffect.PinchingEffect = current, density above 500 A/cm2, flowing around the melt interacts with the alternating field and produces force to squeeze the melt ofa sectionandresults incomplete interruptionof current.Use-preferablefor non-intermittent service, areobsolete these days


VERTICAL CORE TYPE INDUCTION FURNACEAlsoknownas AJAX-WYATT FURNACE.Verticalcrucible is usedfor the charge.Bottom is usually V-shaped or U-shaped.Molten metal is kept circulated round the V.Pinch effect is counteracted by the weight of thecharge.Capable of continuous operation.Normal frequency can be used.Use-melting & refining of non-ferrous metals .Advantage-Highlyefficient.Lowoperatingcost.Temperaturecontrolissimple.Comparatively higher p.f


INDIRECT CORE TYPE INDUCTION FURNACE

Charge is heated indirectly.A metal container forms the secondary of the transformer.Container is heated up by transformer action.Temperature control isdone using different materials for the detachable bar.P.f iscomparatively poor(w.r.t direct resistancefurnace, approx-0.8)


Vertical Core Type Ajax-Wyatt FurnaceMr. Vijay Balu Raskar - Electrical EngineerIMP

The limitation of core type induction furnace can be eliminated by use of vertical type furnace called Ajax-wyatt Furnace.Construction :-Mr. Vijay Balu Raskar - Electrical EngineerThe furnace is narrow V-shaped.V shaped tendency of the molten metal is to get itself accumulated at the bottom and this helps at least a small amount of charge to be present for the secondary circuit to be completed.The probability of the discontinuity of the circuit is thus found to be less.Pinch effects occurs in it.

OPERATION:-The primary inductor coil is connected to the A.C. Supply.This causes an intense electromagnetic field to fluctuate in the iron core.The metal acts a short circuited secondary and carries the induced currents. Heat is mainly generated in the V shaped portion as it has highest resistance due to small narrow section. This heat is rapidly distributed to the metal with the help of convection current and by electromagnetic forces.The heat ultimately results into melting of a charge.


Verticalcrucible is usedfor the charge.Bottom is usually V-shaped or U-shaped.Molten metal is kept circulated round the V.Pinch effect is counteracted by the weight of thecharge.Capable of continuous operation.Normal frequency can be used.Use-melting & refining of non-ferrous metals .Advantage-Highlyefficient.Lowoperatingcost.Temperaturecontrolissimple.Comparatively higher p.f


CORELESS TYPE INDUCTION FURNACE Mr. Vijay Balu Raskar - Electrical Engineer

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Coreless Type Consists of 3 parts1) Primary coil 2) Ceramic crucible 3) Frame*No iron core*Primary coil is fed with ac supply*Eddycurrent isinducedwithin thecharge*Eddy current produces heating of the charge*High frequency supply is to be used (flux density is low)*High frequency increases resistance due to skin effect resulting in high primary copper loss*No magnetic core results in very lowp.f (between 0.1-0.3)*Static r is employed to improve p.f


Use- 1) Steel production2) Melting non-ferrous metals 3) Electronic industryAdvantagesSimple method to controlSemi skilled labor can handleFast operationRequired less spaceEasy to maintain Controllable by electrical clock or elseNo oxidation Can be used intermittentlyLow cost / economical Precise control of powerLow melting time


Requirement of Good Electrode Material Mr. Vijay Balu Raskar - Electrical EngineerMaterial Must have following properties:-It should be good conductor of electricity.It should have high resistance to thermal shocks.It should have sufficient mechanical strength.It should have low thermal conductivity.It should be insoluble in the charge , chemically Inert and non-corrosive.

Low & Medium Temperature (up to1150C)Alloy of nickel & chromium Ni= 80%, Cr= 20% Alloy of nickel, chromium & iron Ni= 65%, Cr=15%,Fe=20%

High Temperature (>1150 C)Silicon carbideMolybdenumTungstenGraphite


ELECTRIC RESISTANCE FURNACEThese are insulated closed chambers with a provision for ventilation

Heating elements may consists of circulars wire or rectangular ribbons

Heating elements placed either on top or sides of the oven

Charge is placed inside a heating chamber


TEMPERATURECONTROL OF HEATING FURNACETemperatureofFurnace depends upon Voltage (V) can becontrolled by tapped transformer.Resistance (R)-can be controlled by switching in various combinations of groups of resistance of the furnace.Time (t)-can be controlled by an on-off switch, which determines the time for which the furnace is connectedto supply or remain isolated from the supply.


LOSSESBesidesraising thetemperature ofthe Charge, heat energy is lost in following operations:- In raising the temperature of furnace- In raising the temperature of the container/carrier- Heat conducted through wall- Escapement of heat due to opening of door



electrical heating 03-03

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