is required when connecting a 1-phase crucible induction furnace to a 3-phase power supply. Through the use of Steinmetz switching, the 1-phase load is converted into 3-phase of the same size. The power suppliers allow an imbalance of 10% of the highest phase current, depending on the stability of the network. In case of the failure of a balancing choke, a crucible induction furnace can also be operated without balancing devices. It may be necessary to obtain the permis- sion of the power supply company for this imbalanced load at 60% of the nominal current with 2 phases.
The important factor in this method of operation is that the nominal current of the transformer is not exceeded. an example is given below for a 12.5-t furnace with 2,000 V and 2,600 kW nominal output.
1. the balanced phase must be disconnected from the network.
2. the choke must be completely removed.
3. the maximum phase current is determined from the rating plate of the transformer.
4. by slowly increasing the output at 70% of the crucible filling level, the maximum voltage in practice can be determined. If for example at 1,700 V the phase current is over 1,000 A, the next lower step at 1,400 V must be defined as the maximum voltage step, in order not to overload the transformer. The output will then be 1,275 kW and 49% of the nominal output.
- This current is given for the lowest melting stage for constant output. As a rule, the top 5 steps are designed for this purpose, which in this case means at 1,700 V and Step 6 with 930 A.
- at 1,700 V and 930 A, this gives an output of 1,580 kW, or 60% of the nominal output.