Starting resistors are usually connected single-phase into one furnace phase in mains-frequency furnaces. In installations up to 1000 kW, three-phase starting resistors have also been used. For a single-phase resistor, its resistance is roughly equal to the apparent impedance of the induction furnace. At low furnace voltages the value is around 0.4 Ohm, and at higher voltages, such as 2,600 V, the value is around 1.2 Ohm. The rated current is 300 to 800 Amperes, depending on the intended duty cycle. With PLC control, significantly shorter switch-on times are required.
The energy at the starting resistor is converted into heat and, with frequent switching on at short intervals, leads to destruction of the cast-iron resistor elements. When 800 A flows through a 1 Ohm resistor, this produces a power of 640 kW, which over a duty cycle of 5 seconds converts approx. 0.9 kWh into heat.[1] A 50 kg resistor heats up by about 120 to 140 K in this time.[2]
Switching on induction crucible furnaces without a starting resistor causes impermissible current spikes in the mains supply, which must be avoided. Switching on the furnace without a starting resistor at 50% of rated voltage allows operation at 25% of rated power with approximately the same inrush current as with a starting resistor. Switching on at higher voltages can cause the switchgear to fail.
References
- Herbert H. Netzel: Induktionsofenhandbuch. 3rd ed., Eigenverlag IES, Witten 2003, p. 18 (power and energy).
- AZoNetwork: Properties: An Introduction to Grey Iron, specific heat capacity. Material data sheet, accessed 27 August 2026.