Department of Technical Education
Andhra Pradesh
Name
:
V. Narsimha Reddy
Designation
:
Lecturer in EEE
Branch
:
D.E.C.E.
Institute
:
GMR. Polytechnic Gajwel
Semester
:
VI Semester
Subject
:
Electrical Utilization & Automation
Subject Code
:
EE605A
Topic
:
Electric Heating
Duration
:
100 Minutes
Sub topic
:
Principle and Operation of Induction Furnace
Teaching Aids
:
PPT, Diagrams, Animation
Revised By
:
K. Chandra Sekhar, L/EEE, GPT, HYD
9EE605A.17to18
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Recap
In the last class you have learnt about
• Electric Arc Heating
• Types of Arc Furnaces
•
Direct
• Indirect
• Applications of Direct and Indirect Arc Furnaces
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Objectives
On completion of this topic you would be able to know
• Operation of Induction Heating
• Core type and Coreless Induction Heating
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Induction Heating
• Induction heating is also known as high frequency heating
• Induction heating process makes use of currents. induced by
electromagnetic action in the material to be heated
• It uses transformer principle
• Conversion of electromagnetic energy into heat energy takes
place in the material, it self
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Induction Heating
• Heat transfer by high frequency heating is as much as
10,000 W/cm2
• The high frequency heating can be applied mainly to two
classes of materials firstly, conducting materials &
secondary insulating materials
• Heating of first type of materials is called induction
heating and heating of Second type of materials is
dielectric heating
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Induction Heating
Induction furnaces are further classified as
i) Core type induction furnace
ii) Coreless induction furnace
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Core Type Induction Heating
Fig.1
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Core Type Induction Heating
• Fig 1 shows a core type
induction surface
• The Core type furnace is
essentially a transformer
• Primary side is
connected to supply
• Secondary side contains
charge (materials) in iron
core annular hearth
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Core Type Induction Heating
• The charge is
magnetically coupled to
the primary by an iron
core
• The magnetic coupling
between primary and
secondary is very poor
resulting in high leakage
current and a low power
factor. For this reason the
furnace is operated at low
frequencies of the order
of 10Hz or so
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Core Type Induction Heating
• Hence it is called low
frequency furnace i.e.,
10Hz
• The low frequency
necessiates an additional
MG set or frequency
converter
• To start the furnace
Molten metal is poured in
the annular hearth before
start
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Core Type Induction Heating
• Otherwise there is no
material and the
secondary side is open
and no current will be
induced
• Hence no heating will
take place
• This is convenient where
the furnace is to be used
for melting different types
of charges
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Core Type Induction Heating
• If the current density
exceeds about 5A/mm2
the pinch effect due to
electro magnetic forces,
may cause a complete
interruption of the
secondary circuit
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Core Type Induction Heating
Disadvantages
• A crucible of inconvenient shape is required
• Low power factor due to poor magnetic
• A M.G. or frequency converter is required
• It is bulky due to the presence of core
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Ajax Wyatt Vertical Core Furnace
Fig.2
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Ajax Wyatt Vertical Core Furnace
• Fig 2 shows Ajax Wyatt
Vertical Core Furnace
• It is an improvement over
the core type induction
furnace
• The magnetic coupling in
this furnace is better than
core type furnace
• Leakage reactance is
comparatively low and
frequency is high, hence it
is high frequency furnace
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Ajax Wyatt Vertical Core Furnace
• It employs vertical
crucible instead of
horizontal
• The system avoids the
pinch effect due to the
weight of the charge in
the main body of the
crucible
• The circulation of Molten
metal takes place round
the ‘V’ portion by
convection currents
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Ajax Wyatt Vertical Core Furnace
• Inside the furnace is
lined depending upon the
charge
• The top of the furnace is
covered with an insulated
cover which can be
removed for charging
• Hydraulic arrangements
are usually made for
tilting the furnace to
remove the molten metal
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Indirect Core Type Induction Furnace
Fig.3
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Indirect Core Type Induction Furnace
• This type of furnace is
used for heat treatment of
metals
• The wall of the container
forms the secondary
winding
• Iron core links the primary
as well as secondary
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Indirect Core Type Induction Furnace
• Heat produced in the
secondary due to induced
current is transmitted to
the charge by radiation
• A detachable magnetic
circuit made of a special
alloy is kept inside the
chamber
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Indirect Core Type Induction Furnace
• The alloy will loose
magnetic properties at a
particular temperature
and are regained when it
cools down
• On reaching the critical
temperature the
reluctance of the alloy
increases and here by
decreasing induction
effect
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Indirect Core Type Induction Furnace
• Critical temperature
varying 4000c to 10000c
can be employed for
heating different materials
• The furnace operates at a
PF of around 0.8
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Coreless Induction Heating
REFRACTORY
CRUCIBLE
PRIMARY
WINDING
CHARGE
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Coreless Induction Heating
• The coreless
induction furnace
operates on the
principle of an electric
transformer
• If there is no core, the
flux density will be low
• For compensating the
low flux density, the
primary supply should
have high frequency
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REFRACTORY
CRUCIBLE
PRIMARY
WINDING
CHARGE
24
Coreless Induction Heating
REFRACTORY
CRUCIBLE
• The furnace consists of a
refractory or ceramic
crucible cylindrical in
shape enclosed with in a
coil which forms the
primary of a transformer
• The furnace also may
have a conducting or
non-conducting container
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PRIMARY
WINDING
CHARGE
25
Coreless Induction Heating
• When high frequency of
REFRACTORY
CRUCIBLE
500 of 1000 Hz supply is
given to primary windings
• The eddy currents are set
PRIMARY
WINDING
up in charge or container
CHARGE
by transformer action
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Coreless Induction Heating
• There currents heat the
charge to melting point
and they also set up the
electromagnetic force
which produce a stirring
action to the charge
• The furnace becomes
relatively light in weight
and can be easily tilted
for pouring the metal
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REFRACTORY
CRUCIBLE
PRIMARY
WINDING
CHARGE
27
Coreless Induction Furnace
Advantages
• High speed of heating
• Well suited for intermittent operation
• High quality of product
• Low operating cost
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Core type Induction Furnace
Industrial applications
• Used in foundries for melting and refining brass,
zinc and other non-ferrous metals
• Used for heat treatment of metals
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Coreless Induction Furnace
Industrial applications
• These are used for steel production
• These are used for melting of non-ferrous metals like
brass , copper, aluminium along with various alloys of
these elements
• The production of carbon from ferrous alloys
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Comparison Between High Frequency And
Power Frequency Furnaces
S.
No.
1
High Frequency
Power frequency
Frequency converter is
necessary
Not required.
2
More energy required i.e, 20%
to 30% for same rating.
Less energy required.
3
Maintenance cost is more
Less
4
Less turbulence and stirring
effect
More
5
Large scrap melts with more
oxidation loss.
Large scrap melts with less
oxidation loss.
6
No special starting procedure is
required
Special starting procedure is
required.
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Comparison Between High Frequency And Power
Frequency Furnaces
Contd..
7
Low capacity (few kgs. To 15
tons) furnaces are used
Large capacity (above 1
tone)
8
Initial cost is more
Less cost
9
High frequency (500 to 1000 Hz) Only 50Hz.
10
Well suited for intermittent
operation of different alloys.
Well suited for long
production schedules of one
alloy.
11
Care should be taken against
stray field
Negligible.
12
Overall efficiency is low.
Overall efficient is more.
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Summary
In this class we have discussed about
• Principle and operation of induction heating
• Core type induction heating
• Core less induction heating
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Quiz
1.Coreless induction furnace operate on the principle of
a. Transformer
b. Induction motor
c. Generator
d. Motor
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Frequently Asked Questions
1) What is induction heating ?
2) Describe briefly with a neat sketch the core type induction
furnace
3) Explain the vertical core type of furnace (Ajax Wyatt
furnace) with a neat diagram
4) Explain the construction and operation of coreless induction
furnace
5) Explain the indirect core type induction furnace with sketch
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THANK YOU
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