Principle of transformers
In the transformer, supply is utilized by secondary.
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•Induction method is based on Principle of “Electromagnetic
Induction”
•When alternating Current flows in a conductor it produces
alternating flux.
•If any other conducting material is placed in this magnetic flux
emf gets induced in it
•This induced emf drives eddy current in that piece and power
loss due to eddy current appears as heat.
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Factors for Induction heating:-
1) it is proportional to relative permeability.
Heating produced in magnetic material is
more than non magnetic material.
2) Heating is proportional to MMF. Force can
be varied by changing current or number of
turns.
3) Heating effect can be increased by
employing high frequency supply.
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Types of Induction Furnace
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DIRECT INDUCTION HEATING
In this, currents are induced in the charge itself. This is usually used
in furnaces for smelting (extraction of metal from ore), melting of
metals etc.
This requires very high frequency supply.
They are classified as core and coreless type induction furnaces.
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INDIRECT INDUCTION HEATING
(Example :- Oven)
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Operation
In this, eddy currents are induced in the heating element.
Thus heat produced by heating element is then transferred
to the charge by radiation or convection.
When primary winding is connected to the supply , the
eddy currents gets induced in the metal container forming
the walls of the oven. Due to eddy currents, metal
container gets heated and then is transferred to the
charge by radiation. Secondary winding is metal container.
Below part is situated in the oven chamber which is made
up of special alloy which losses its magnetic property
when heated and regain when cooled.
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Advantages:1) Simple and foolproof method of temperature control
2) No external temperature control equipments required
Limitations:1) Poor Power factor
2) Complicated Construction
Applications:It is used for general heat treatment of metallic and other charges.
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CORE TYPE OF INDUCTION FURNACE
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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
winding and forms single-turn short circuited secondary.
Secondary channel should not be emptied.
Magnetic coupling between primary & secondary is weak resulting in
large leakage reactance poor p.f
Leakage reactance is nullified using low frequency supply
Drawbacks.
1)Leakage reactance is high & p.f is low.
2)Low frequency requirement.
3) Suffers from Pinching Effect.
Pinching Effect = current, density above 500 A/cm², flowing around the
melt interacts with the alternating field and produces force to squeeze
the melt of a section and results in complete interruption of current.
Use-preferable for non-intermittent service, are obsolete these days
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VERTICAL CORE TYPE INDUCTION FURNACE
Also known as AJAX-WYATT FURNACE.
Vertical crucible is used for the charge. Bottom is usually Vshaped or U-shaped. Molten metal is kept circulated round the
V. Pinch effect is counteracted by the weight of the charge.
Capable of continuous operation.
Normal frequency can be used.
Use-melting & refining of non-ferrous metals .
AdvantageHighly efficient.
Low operating cost.
Temperature control is simple.
Comparatively higher p.f
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Vertical Core Type Ajax-Wyatt
Furnace
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Construction :The limitation of core type induction furnace can be eliminated by use of vertical
type furnace called Ajax-wyatt Furnace.
The 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.
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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.
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Vertical crucible is used for 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
the charge. Capable of continuous operation. Normal
frequency can be used.
Use-melting & refining of non-ferrous metals .
AdvantageHighly efficient.
Low operating cost.
Temperature control is simple.
Comparatively higher p.f
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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 is done using different
materials for the detachable bar. P.f is comparatively
poor (w.r.t direct resistance furnace, approx-0.8)
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CORELESS TYPE INDUCTION FURNACE
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Coreless Type Consists of 3 parts
1) Primary coil 2) Ceramic crucible 3) Frame
*No iron core
*Primary coil is fed with ac supply
*Eddy current is induced within the charge
*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 low p.f (between 0.10.3)
*Static r is employed to improve p.f
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Use1) Steel production
2) Melting non-ferrous metals
3) Electronic industry
Advantages
Simple method to control
Semi skilled labor can handle
Fast operation
Required less space
Easy to maintain
Controllable by electrical clock or else
No oxidation
Can be used intermittently
Low cost / economical
Precise control of power
Low melting time
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Requirement of Good Electrode Material
Material Must have following properties:1)It should be good conductor of electricity.
2)It should have high resistance to thermal
shocks.
3)It should have sufficient mechanical
strength.
4)It should have low thermal conductivity.
5)It should be insoluble in the charge ,
chemically Inert and non-corrosive.
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Low & Medium Temperature (up to1150°C)
Alloy of nickel & chromium
Ni= 80%, Cr= 20%
Alloy of nickel, chromium & iron Ni= 65%, Cr=15%,Fe=20%
High Temperature (>1150 °C)
Silicon carbide
Molybdenum
Tungsten
Graphite
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ELECTRIC RESISTANCE FURNACE
These 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
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TEMPERATURE CONTROL OF HEATING
FURNACE
Temperature of Furnace depends upon–
Voltage (V) ±can be controlled 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 connected to supply or remain
isolated from the supply.
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LOSSES
Besides raising the temperature of the 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
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