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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

Induction furnaces are further classified as

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i) Core type induction furnace

ii) Coreless induction furnace

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Core Type Induction Heating

Fig.1

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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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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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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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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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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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

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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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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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

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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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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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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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

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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PRIMARY WINDING

REFRACTORY CRUCIBLE

CHARGE

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

REFRACTORY CRUCIBLE

CHARGE

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

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PRIMARY WINDING

REFRACTORY CRUCIBLE

CHARGE

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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PRIMARY WINDING

REFRACTORY CRUCIBLE

CHARGE

Advantages High speed of heating Well suited for intermittent operation High quality of product Low operating cost

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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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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

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Large scrap melts with less oxidation loss.

Large scrap melts with more oxidation loss.

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MoreLess turbulence and stirring effect

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LessMaintenance cost is more3

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

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Not required.Frequency converter is necessary

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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

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Well suited for long production schedules of one alloy.

Well suited for intermittent operation of different alloys.

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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

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Comparison Between High Frequency And Power Frequency Furnaces

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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1.Coreless induction furnace operate on the principle of

a. Transformerb. Induction motorc. Generatord. 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

THANK YOU

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