In this article we will discuss about the design of core and coreless induction furnace.
Design of Core Induction Furnace:
Fig. 3.12 shows a core type furnace. Core type furnace is essentially a transformer with the charge of metal to be heated as single turn short circuited secondary and magnetically coupled to the primary winding by iron core.
The current flowing through the charge is very high may be of the order of several thousand amperes. From the figure it is clear that the coupling is loose and leakage reactance is high hence the power factor is low. To overcome this difficulty, this furnace is operated at a very low frequency say 10 Hz.
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The main disadvantages is that electromagnetic forces produces great turbulence of the molten metal which is useful to a certain point, but it becomes too severe unless frequency is kept low.
Another disadvantage of this furnace is that a crucible is of inconvenient shape from metallurgical point of view is used.
At the time of starting, a complete ring of metal must be present in the crucible otherwise the secondary circuit will remain open and heat will not be produced. Yet another disadvantage is that it requires low frequency (10 Hz) supply which can be had by using motor generator set or frequency changer, which involves extra cost.
On account of the above draw backs mentioned above such furnaces have been obsolete nowadays.
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Ajax Wyatt Furnace or Vertical Core Furnace:
An improved form of core type furnace is Ajax-Wyatt vertical core type furnace which is explained as below:
This furnace is very suitable for continuous operation and it can be operated on normal supply frequency i.e. 50 Hz. The core is made of laminated steel and the secondary circuit is formed by single turn of molten metal. The charge is given from above and the molten metal is taken out through the spout.
The stirring action is produced by the pinch effect which is proportional to the square of amp-turns. The secondary circuit is only complete when there is sufficient molten metal in the ‘V’ to complete the secondary circuit. In case the furnace is to be kept overnight without operation, a reduced voltage is applied which supplies just enough energy to keep the metal in ‘V’ in molten condition.
The furnace, therefore, is very suitable for continuous operation. A proper arrangement is made for fitting the furnace to take out molten metal.
The power factor of the circuit is about 0.8 and the power required for the furnace in order to complete melt in a reasonable time is about 200 KW per ton. The melt is completed in about 1½ to 2 hours for large sizes and in lesser time for smaller sizes
Uses:
The furnace is used for melting and refining of non-ferrous metal like brass, copper and zinc. Its efficiency is about 75 percent. Standard sizes of these furnaces are 60 – 300 KW, single phase 50 Hz working at 600 volts.
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Advantages:
(1) Control is simple.
(2) Ideal working condition in cool atmosphere with no dirt, noise or smoke.
(3) High power factor 0.8 to 0.85 comparatively
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(4) Absence of crucible.
(5) Uniform stirring of charge gives homogeneous composition.
Design of Coreless Induction Furnace:
No core is provided in the coreless induction furnace. A crucible of more convenient shape can be employed. In this case also the charge to be melted is made the secondary of the transformer. The primary is wound over the crucible itself.
The eddy currents produced in the charge not only heat it up but also account for the stirring action. Iron laminations are provided outside the primary winding to create a low reluctance path for flux and also contain the stray field which may otherwise induce heavy current in supporting steel structure.
The frequency employed depends upon the size of the furnace. For lower capacities of furnaces higher frequency of the order of 3000 Hz are employed whereas for higher capacities frequencies are down to 600 Hz. To reduce copper losses, hollow copper tubes are used in which cold water is circulated.
The furnace is chiefly used for melting of steel and other ferrous metals. The capacities available vary from 50 Kg to about 20 tonnes. The initial cost is more as compared to arc furnace.
Advantages of Coreless Induction Furnaces:
The advantages of coreless induction furnace over other types are given below:
(i) Low operating cost.
(ii) Automatic stirring action produced by eddy currents.
(iii) Low erection lost.
(iv) Absence of dirt, smoke noise etc.
(v) Less melting time.
(vi) Simple charging and pouring, precise control of power.
(vii) Most suitable for production of high grade alloy steels.
Methods of obtaining of Power Supply for Coreless Induction Furnaces:
1. For coreless induction furnaces power supply is obtained from ordinary supply system and its frequency is converted to higher value by means of oscillators. The oscillators can generated very high frequency in the order of megahertz.
The ac supply is stepped up by a transformer and then rectified by using a bridge rectifier circuit. The rectified voltage is applied to the oscillator and high frequency output is fed to the charge to be heated through an output transformer.