The process of induction heating is based on the principle of transformer working. The material to be heated make use of induced currents due to changing electric current in the primary. Generally, there are two types of induction furnaces, core type induction furnace, and coreless type induction furnace. In this article let us learn about core type induction furnaces.
Core Type Induction Furnace :
The core type induction furnace consists of primary and secondary windings which are magnetically linked together through an iron core that facilitates a low reluctance path for the flux linkage between primary and secondary. The primary of the furnace is connected to the supply and the charge to be heated as secondary. The core type induction furnaces are again further classified into three types,
- Direct core type induction furnace,
- Vertical core type induction furnace, and
- Indirect core type induction furnace.
Direct Core Type Induction Furnace :
The direct core type induction furnace is similar to a two-winding transformer. In this furnace, the primary consists of a number of turns, and the charge to be heated forms the short-circuited secondary. Both primary and secondary are magnetically coupled by an iron core as shown below.
The furnace consists of a circular hearth in the form of a trough. The charge to be heated is kept in the trough. The flux set up by the primary will induce a heavy current in the charge to melt it. In case of no charge in the trough then the secondary will remain open and thus no current flows through it. Hence before starting the furnace, molten metal is kept in the trough, or some molted charge from the previous cycle is left in the trough.
In this type of furnace, the magnetic coupling between primary and secondary is weak which results in high leakage reactance thereby decreasing the power factor. This drawback can be overcome by operating the furnace at a low frequency in the order of 10Hz.
Also, the low-frequency operation of the furnace will eliminate the production of high electromagnetic forces, severe turbulence of molten metal, and pinch effect (since the pinch effect also depends upon frequency). But the requirement of a frequency converter to obtain low frequency increases the cost of the furnace.
Vertical Core Type Induction Furnace :
The vertical core type induction furnace is the improved version of the direct core type induction furnace in which a vertical core is used instead of a horizontal core for the charge. Also, some of the drawbacks of the direct core type furnace are eliminated in this furnace. This furnace is also called as Ajax Wyatt furnace.
The core of this furnace is made up of heavy steel. The top surface is covered with an insulating material and is made in such a way that it can be removed for charging the furnace. In order to take out the molten metal, the furnace is attached with a hydraulic mechanism. The primary of the furnace is an inductor coil that is wound on the central limb of a laminated core, and the secondary is a closed loop of molten metal.
Due to high magnetic coupling compared to direct core type furnaces, the power factor will be high. Due to the vertical arrangement of the core, the pinch effect due to the weight of the charge is eliminated. The inner surface of the furnace is lined with clay when the charge is of yellow brass and a furnace with a lining of magnesia and alumina alloy is used for red brass.
When an ac supply is given to the furnace, a strong alternating electromagnetic field is produced in the laminated core. The molten metal in the V channel acts as a short-circuited secondary coil carrying a heavy induced current which generates heat in the charge. As the V channel resistance is high the heat is produced and distributed evenly due to the convection currents and by stirring action of electromagnetic force.
Due to the narrow V-shaped channel, the molten metal (charge) even if it is in small quantity will accumulate at the bottom which keeps the secondary circuit closed. But it is recommended that the V-channel should be kept full of charge to maintain continuity of the secondary circuit.
The advantages of a vertical core type induction furnace are,- The power factor of a vertical core type induction furnace is high in the range of 0.8 to 0.85.
- This type of furnace is suitable for continuous operation services.
- A simple and accurate temperature control.
- Due to better magnetic coupling, the furnace can be operated at power frequency and its efficiency is about 75%.
- Pinch effect can be avoided.
Indirect Core Type Induction Furnace :
In an indirect core type furnace, an inductively heated element that forms the secondary is made to transfer its heat to the charge by radiation. This type of induction furnace is used for providing heat treatment to metal.
It consists of an iron core linking with primary winding and secondary. The secondary winding consists of a metal container forming the walls of the furnace. The primary winding is connected to the supply, inducing current and heating the metal container, thereby transmitting the heat to the charge by radiation.
It consists of part AB of the magnetic circuit situated in the oven chamber. It is made up of a special alloy that loses 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 corresponding inductive effect decreases thereby cutting off the heat supply.
The magnetic circuit AB is made so that it can be replaced by other magnetic circuits. The operating power factor of this type of furnace is around 0.8. One of the main advantage of an indirect core type induction furnace is that wide temperature variations with effective control is possible.
