Electrical Heating 03-03

Mr. Vijay Balu Raskar - Electrical Engineer

CORE TYPE OF INDUCTION FURNACE


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

VERTICAL CORE TYPE INDUCTION FURNACE
Also known as AJAX-WYATT FURNACE.
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 .

Advantage-
Highly efficient.
Low operating cost.
Temperature control is simple.
Comparatively higher p.f Balu Raskar - Electrical Engineer
Mr. Vijay

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)


Vertical Core Type Ajax-Wyatt Furnace


The limitation of core type induction furnace can be eliminated
by use of vertical type furnace called Ajax-wyatt Furnace.
Construction :-
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.


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.

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 .

Advantage-
Highly efficient.
Low operating cost.
Temperature control is simple.
Comparatively higher p.f

CORELESS TYPE INDUCTION FURNACE


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 0.1-
0.3)
*Static r is employed to improve p.f

Use-
1) 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 Mr. Vijay Balu Raskar - Electrical Engineer

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.

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

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

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.

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

Electrical Heating 03-03

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