ELECTRICAL HEATING – PART – 12 - CORE TYPE OR LOW FREQUENCY INDUCTION FURNACES

Low frequency furnaces are Direct core, Indirect core and Vertical core (or) Ajax Wyatt furnace.

DIRECT CORE TYPE
In this furnace, the charge forms the short-circuited secondary which is magnetically coupled to the primary by an iron core.

CONSTRUCTION
1. The furnace consists of a circular hearth in the form of a trough (a concave shape with an open top) which contains the charge to be melted in the form of an annular (shaped like a ring) ring.
2. The metal ring quite large in diametre is magnetically inter-linked with an electrical winding.
3. The ring is energized from an a.c. source.
4. This furnace is like a transformer in which the charge to be heated forms a single turn short circuited secondary and is magnetically coupled to the primary by an iron core.
5. When there is no molten metal in the ring, the secondary becomes open-circuited thereby cutting off the secondary current.
6. So, to start the furnace, molten metal has to be poured in the annular.
7. The magnetic coupling between the primary and secondary is very poor, it results in high leakage reactance and low power factor.
8. To minimize the leakage reactance, low primary frequency of the order of 10 Hz is used.
9. For producing low frequencies a special Motor-Generator set is required which increases cost.
10. If the current density increases beyond limit, it produces ‘PINCH EFFECT’ which can cause temporary interruption of the secondary circuit.

DISADVANTAGES
1. This furnace cannot function if the secondary circuit is not closed.
2. The crucible used is of very odd shape and size. This is not convenient from metallurgical point of view.
3. This furnace is not suitable for intermittent services.

PINCH EFFECT
The current density of the charge is to be limited by 480-500 Amps per centimetre square. High current density will produce high electro-magnetic force in the molten metal and that will cause adjacent particles or molecules of the molten metal carrying current in the same direction to repel each other since similar charges repel each others. This repulsion may cause interruption of the secondary circuit". This effect is known as pinch effect.

INDIRECT CORE TYPE INDUCTION  FURNACE
In this type of furnace, a suitable element is heated by induction which, in turn, transfer the heat to the charge by radiation.

CONSTRUCTION
1. In this furnace, the secondary winding is formed by the walls of a metal cylinder.
2. An iron core links the primary and secondary winding.
3. When primary winding is connected to a.c. supply secondary current is induced in the metal container by transformer action which heats up the container.
4. Heat produced, due to induced currents, is transferred to the charge by radiation.
5. The part AB of the magnetic circuit situated inside the oven chamber consists of a special alloy which loses its magnetic properties at a particular temperature but regains them when cooled back to the same temperature.
6. If the oven temperature attains the critical temperature, the reluctance of the magnetic circuit increases highly there by cutting off the supply of heat.
7. The bar AB is detachable and can be replaced by other bars having different critical temperature.
9. It is similar to the resistance furnace but with a poor power factor about 0.7.
10. The temperature of furnace can be very easily controlled and a temperature up-to 1000 degree centigrade can be obtained.