is based on magnetism in conjunction with electrical voltages and currents. The following rules apply, as used in the design and construction of motors and for transformers: when an electrical conductor is placed in an alternating magnetic field, an electrical voltage is induced in the conductor. When a current flows through the conductor, a thrust is imparted to the conductor, which acts at an angle of 90° to the direction of the current. This property is used in the construc- tion of motors.
In the generator, a voltage is generated in the rotor or stator winding of a con-
ductor.
In the transformer, a core holds 2 separate coils, each with a different number of
windings. The voltage is transformed to a higher or lower level, depending on
the number of windings, i.e. the transmission ratio. Because the actual output
remains the same, the current changes in the inverse proportion.
e.g.
Primary: 10 kV 200 windings 50 A 500 kVA
Secondary: 1 kV 20 windings 500 A 500 kVA
The crucible induction furnace is based on the induction principle of transform- ers, which is explained below by means of an example.
A medium-frequency furnace with a 5 t capacity has an output of 3,000kW at
3,000 Volt, 11 windings and a crucible wall thickness of 140 mm. An effective cur-
rent of 1,000 A flows in the coil.
If the melt or scrap charge is now applied as a secondary winding, this gives a
theoretical voltage in the crucible of 273 Volt and a total current as a product in
the melt of approx. 11,000 A. This high current acts as an eddy current, which
heats and melts the charge.
A connection between the coil and bath (lining leak) can also be identified with
an earth short or imbalance monitoring device. In this way, a warning signal can
be generated if the melt penetrates into the vicinity of the coil.
There is also the danger that a winding short caused by this metal will damage
the conductor to such an extent that cooling water escapes.
It is essential at all times to avoid water getting into the melt. The voltage must therefore be switched off automatically if the bath gets dangerously close to the coil. This danger can also be significantly reduced by the appropriate design of the coil. The furnace itself also includes the additional electrical devices shown in Figure 114 for a medium-frequency furnace; the only difference in a mains frequency furnace is the absence of the frequency converters.