Low frequency supply is necessary for direct core type induction furnaces because

Magnetic coupling between the primary and secondary circuit is poor.

With normal frequency supply the electro-magnetic forces cause severe stirring action in the molten metal.

Both Magnetic coupling between the primary and secondary circuit is poor and With normal frequency supply the electro-magnetic forces cause severe stirring action in the molten metal.

None of Magnetic coupling between the primary and secondary circuit is poor and With normal frequency supply the electro-magnetic forces cause severe stirring action in the molten metal.