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Department of Technical EducationDepartment of Technical Education

Andhra PradeshAndhra PradeshName : 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

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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

PRIMARY WINDING

REFRACTORY CRUCIBLE

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

PRIMARY WINDING

REFRACTORY CRUCIBLE

CHARGE

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Coreless Induction Heating

• 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

PRIMARY WINDING

REFRACTORY CRUCIBLE

CHARGE

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Coreless Induction Heating

• When high frequency of

500 of 1000 Hz supply is

given to primary windings

• The eddy currents are set

up in charge or container

by transformer action

PRIMARY WINDING

REFRACTORY CRUCIBLE

CHARGE

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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

PRIMARY WINDING

REFRACTORY CRUCIBLE

CHARGE

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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

Special starting procedure is required.

No special starting procedure is required

6

Large scrap melts with less oxidation loss.

Large scrap melts with more oxidation loss.

5

MoreLess turbulence and stirring effect

4

LessMaintenance cost is more3

Less energy required.More energy required i.e, 20% to 30% for same rating.

2

Not required.Frequency converter is necessary

1

Power frequencyHigh FrequencyS. No.

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Overall efficient is more.Overall efficiency is low.12

Negligible.Care should be taken against stray field

11

Well suited for long production schedules of one alloy.

Well suited for intermittent operation of different alloys.

10

Only 50Hz.High frequency (500 to 1000 Hz)9

Less costInitial cost is more8

Large capacity (above 1 tone)

Low capacity (few kgs. To 15 tons) furnaces are used

7Contd..

Comparison Between High Frequency And Power Frequency Furnaces

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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