Core type furnace: The core type furnace is just like a transformer having primarily connected to the supply and the charge to be heated as secondary as shown in Fig. given below.
Question: Explanation of various types of core type induction furnaces or indirect induction oven in details?
Types Of Core Type Induction Furnaces or Indirect Induction Oven are three
- Direct Core Type Induction Furnace
- The Vertical Core Type Induction Furnace
- Indirect Core Type Induction Furnace
All type are explained below –
Direct Core Type Induction Furnace
- Direct core type induction furnace is shown in Fig. which consists of an iron core, the crucible of some insulating material, and primary winding connected to an a.c. supply.
- The charge is kept in the crucible, which forms a single turn short-circuited secondary circuit.
- This type of furnace has the following drawbacks :
- As magnetic coupling between the primary and secondary circuit is poor, therefore, leakage reactance is high and the power factor is low.
- This difficulty, however, is overcome by employing a supply of frequencies as low as 10 Hz for the operation of Direct Core Type Induction Furnace.
- For obtaining low-frequency supply. motor-generator set or frequency changer is required, which involves an extra cost in Direct Core Type Induction Furnace.
- If normal frequency supply is employed for the operation of such furnaces, the electromagnetic forces cause severe stirring action in the molten metal, low-frequency supply, therefore is also necessary from this point of view in Direct Core Type Induction Furnace.
- If the current density exceeds about 5 A/mm2 the pinch effect (formation of bubbles and voids etc) due to electromagnetic forces may cause complete interruption of the secondary circuit and the supply.
- The crucible for the charge is of odd shape and inconvenient from the metallurgical point of view.
- For the functioning of the furnace, the closing of the secondary circuit is essential which necessitates the formation of a complete ring of the charge around the core.
- Such furnaces are not suitable for intermittent services.
- On account of the drawbacks mentioned above such furnaces have become obsolete nowadays.
The Vertical Core Type Induction Furnace
- Vertical Core Type Induction Furnace is an improved form of the furnace described above. The furnace of this type, known as Ajax Wyatt vertical core type furnace employs a vertical channel instead of a horizontal one for the charge.
- The convection currents keep the circulation of molten metal around the V portion. As the V channel is narrow, so even a small quantity of charge is sufficient to keep the secondary circuit closed. Hence, the chances of discontinuity of the circuit are less in Vertical Core Type Induction Furnace.
- Due to the pinch effect, the adjoining molecules carrying current in the same direction will try to repel each other, but because of the weight of the charge they will remain in contact, and chances of interruption will be reduced.
- The inside of the Vertical Core Type Induction Furnace is lined depending upon the charge. Clay lining is used for yellow brass. For red brass and bronze, an alloy of magnesia and alumina or corundum having high contents of alumina is employed.
- The top of the Vertical Core Type Induction Furnace is covered with an insulated cover which can be removed for charging. Necessary hydraulic arrangements are usually made for tilting the furnace to take out the molten metal.
- This type of furnace is suitable only for continuous operation. These furnaces are widely used for melting and refining of brass and other heavy non-ferrous metals.
- The furnace of Indirect Core Type Induction type has an average power factor between and 0.85 with normal supply frequency its efficiency is about 75 percent.
- Standard sizes of Vertical Core Type Induction Furnace range 60 to 300 kW, all single-phase, 50 Hz in this country tor standard voltage up to 600V.
Indirect Core Type Induction Furnace
- In Indirect Core Type Induction Furnace an inductively heated element is made to transfer its heat to the charge by radiation, Furnace of this type is shown in fig. given below.
- Indirect Core Type Induction Furnace consists of an iron core linking with the primary and secondary winding. In this case, secondary consists of a metal container forming the walls of the furnace proper.
- The primary winding in the Indirect Core Type Induction Furnace is connected to the supply, inducing current and heating the metal container. Heat is transmitted to the charge by radiation.
- It is advantageous in respect of temperature control without the use of external control equipment in Indirect Core Type Induction Furnace.
- It consists of part AB of the magnetic circuit situated in the oven chamber and consisting of a special alloy that looses its magnetic properties at a particular temperature and regains them when cooled to the same temperature.
- As soon as the oven attains the critical temperature, the reluctance of the magnetic circuit increases many times, and the inductive effect correspondingly decreases, thereby cutting off the heat supply.
- The bar AB is the detachable type and can be replaced by others having different critical temperatures between 4000C and 1,0000C.
- Simple and foolproof method of temperature control.
- NO external temperature control equipment is necessary.
- Poor power factor.
- Complicated construction.
- Indirect induction oven is used for general heat treatment of metallic and other charges.
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