The induction heating works on the transformer principle. It is also known as eddy current heating. The currents are induced by the principle of electromagnetic induction. The induction heating may be low frequency as in the case of core type induction furnace or high frequency as the case with coreless induction furnaces.
Types of Induction Heating
- Direct Induction Heating.
- Indirect Induction Heating.
Direct Induction Heating
In direct induction heating, the eddy currents are produced within the material itself that is to be heated. The examples of direct induction heating are the high-frequency eddy current heating used for case hardening or tempering of various machine parts, annealing of steel strip and soldering. The core type induction furnace used for melting non-ferrous metals such as copper, zinc, brass. Coreless induction furnace used for preparing various high-grade steels also work on the same principle.
Indirect Induction Heating
The example of indirect induction heating is the indirect induction oven which is preferred over resistance oven due to its fine temperature control. It is used for the same purpose as the resistance oven. Moreover, in the indirect induction heating method the eddy current are induced in the heating elements by electromagnetic induction which produces heat in heating elements. The heat thus produced is transferred to the body to be heated by radiation.
Types of Induction Furnaces
There are basically two types of induction furnaces
- Core type induction furnace.
- Coreless induction furnace.
Core Type Induction Furnace
The figure shows a core type induction furnace. It 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 the 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, core type induction furnace is operated at a very low frequency say 10 Hz.
The main disadvantage is that electromagnetic forces produce great turbulence of the molten metal which is useful to a certain point, but it becomes too severe unless the frequency is kept low. Another disadvantage of this furnace is that a crucible is of inconvenient shape from the metallurgical point of view.
At the time of starting, a complete ring of metal must be present 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 requires frequency changer, which involves extra cost. On account of the above drawbacks mentioned above core type induction furnaces have been obsolete nowadays.
Ajax Wyatt Furnace Working
An improved form of core type induction furnace is Ajax Wyatt furnace. 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 the 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 Ajax Wyatt vertical core type 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 the molten state.
Uses of Ajax Wyatt Furnace
Therefore, Ajax Wyatt vertical core furnace 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 1 to 2 hours for large sizes and in lesser time for smaller sizes.
Ajax Wyatt Vertical Core 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.
Advantages of Ajax Wyatt Furnace
- Control is simple.
- An ideal working condition in the cool atmosphere with no dirt, noise or smoke.
- High power factor 0.8 to 0.85 comparatively
- The absence of crucible.
- Uniform stirring of charge gives the homogeneous composition.
Thanks for reading about ajax wyatt furnace working and vertical core type induction furnace.
Electrical Heating – 2 | Objective Type Question Answers
#1 In an electric press mica is used
as an insulator
#2 Induction heating takes place in which of the following?
Conducting materials which may or may not be magnetic
#3 For heating element high resistivity material is chosen to
reduce the length of heating element
#4 In resistance heating highest working temperature is obtained from heating elements made of
#5 For intermittent work which of the following furnaces is suitable?
Indirect arc furnace
#6 Due to which of the following it is desirable to have short arc length ?
All of the above
#7 In the indirect resistance heating method, maximum heat transfer takes place by
#8 Which of the following methods is used to control temperature in resistance furnaces?
All of the above methods
#9 It is desirable to operate the arc furnaces at power factor of
#10 Radiations from a black body are proportional to
#11 In arc furnace the function of choke is
to stabilize the arc
#12 Ajax Wyatt furnace is started when
it is filled above core level
#13 In electric press, mica ie used because it is……….conductor of heat but/and………..conductor of electricity.
#14 Resistance variation method of temperature control is done by connecting resistance elements in
all of the above ways
#15 Hysteresis loss and eddy current loss are used
induction heating of steel
#16 In heating the ferromagnetic material by induction heating, heat is produced due to
due to hysteresis as well as eddy current loss taking place in the charge
#17 Radiant heating is used for which of the following?
Drying of paints and varnishes
#18 Which of the following devices is necessarily required for automatic temperature control in a furnace ?
#19 For radiant heating around 2250°C the heating elements are made of
#20 Which of the following is an advantage of eddy current heating ?
All of the above.
#21 The electrode of a direct arc furnace is made of
#22 Direct arc furnaces have which of the following power factor?
#23 In direct arc furnace, which of the following has high value?
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