Saturday, August 20, 2011

Induction Heating Transformers

       The principle of induction can be used for heating purposes. The principle of induction heating is shown in the Fig.1.
Fig. 1
       When alternating current flows in a conductor it produces alternating flux. If any other conducting material is placed in this magnetic flux which is alternating, the e.m.f. gets induced in the conducting material. This induced e.m.f. drives eddy currents in that piece. The power loss due to such eddy currents appears as heat. The action of inducing e.m.f. in other material due to alternating flux produced by a current carrying conductor is a transformer action.
Note : The only difference between transformer and induction heating is in a transformer electrical energy available in secondary is utilised outside secondary when in induction it is used to heat the charge itself which acts as a short circuited secondary.
       The heating effect due to such principle depends on following factors :
1. It is proportional to relative permeability. So heating produced in magnetic material is more than non magnetic material. In addition to the eddy currents, there is loss due to hysteresis in magnetic material which contributes to generate more than heat. But hysteresis loss becomes negligible at high fequencies.
2. For a given material and supply frequency, heating is proportional to magnetomotive force (m.m.f.). The force can be varied either by changing current or by varying number of turns of coil as m.m.f. is NI.
3. For a given material and magnetizing force, heating effect can be increased by employing high frequency supply.
       The high power induction furnaces used to produce high quality steel and other alloys based on principle of induction are often called induction heating transformer.
1.1 Coreless Type Induction Heating Transformer
        The Fig. 2 shows coreless type induction furnace using the transformer principle.
Fig. 2  coreless type induction heating transformer

       It consists of a crucible of cylindrical in shape with refractory lining. Primary winding is wound around a crucible. The molten metal acts as a short circuited secondary. The crucible can be infact of any convenient shape. The furnace is generally operated at high frequency. Due to this, skin effect exists which increase effective resistance of primary winding. Due to this there are large copper losses and winding gets heated. So primary winding is made hollow and water is circuited inside for cooling purpose.
Operation
       This furnace also operates on the principle of transformer. The primary is connected to an alternating supply of frequency as high as 500 to 600 Hz. The magnetic flux produced by primary sets up eddy current in the molten. This heat is used to melt the molten metal. The l\electromagnetic forces produced does the stirring action. The operating voltage is about 1000 to 2000 V for larger sizes and upto 10000 V for the smaller sizes.
        Such induction heating furnaces have ratings varying from 15 KVA to 40000 KVA. As power rating increases, the operating frequency decreases. The power factor of such furnaces is very low (about 0.2) as magnetising current required is large to drive the flux through molten metal and air. To improve the power factor capacitor bank is required.
1.2 Channel Type Induction Heating Transformer
       This type of induction heating furnace uses the core type transformer. The iron core is linked to channel of molten iron as shown in the Fig. 3.
Fig. 3  Channel type induction heating motor

       The ceramic pipe is used as a channel which is fitted to the bottom of the crucible. A 50 Hz supply is used to excite the primary winding. The induced current I2 flows through the channel and the molten metal. The channel acts as single turn short circuited on itself. The magnetising current is low as flux is required to be driven through iron core having high permeability. The magnetic coupling between primary and secondary is poor which results into large leakage flux and hence leakage reactance. To keep its effect minimum, the normal frequency supply is used. The power factor is higher than coreless type (about 0.6 to 0.7). Thus smaller capacitor banks are required to improve the power factor. Due to high temperatures the primary windings are made hollow and water cooled.
1.3 Advantages of Induction Heating Transformer
       The various advantages of induction heating transformer are,
1. No danger of contamination of molten metal
2. The product composition can be accurately controlled.
3. The intermittent operation is possible.
4. No special starting procedure is necessary for such furnaces.
5. The method of heating is very clean.
6. The uniform heating of metal is possible.
1.4 Limitations of Induction Heating Transformer 
       The various limitations of induction heating transformer are,
1. The power factor is low thus capacitor banks are required to improve the power factor.
2. Due to capacitor banks and need of high frequency supply, initial cost is high.
1.5 Applications 
       The induction heating furnaces are often used,
1. To manufacturer high grade steel.
2. To melt the various alloys.
3. To melt the metals like aluminium, copper and iron.

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  9. In induction heating, a coil of copper is wound around an object to be heated. The coil of copper can be compared to the transformer primary, and the object to be heated can be compared to the secondary of the transformer.
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