Medium Frequency Induction Melting Furnace as a Load on the Power System

The variable frequency operation of the coreless induction melting furnace has been investigated in detail both theoretically and experimentally. The time variations of the operational impedance value of the induction melting furnace, owing to the time-varying resonance frequency and crucible conditions, have been derived for typical melting cycles by using a variable, parallel RLC model derived from extensive field measurements. The work coil and its crucible is supplied at variable frequency from a multi-pulse rectifier and a current-fed, load resonant inverter cascade. Uncharacteristic current harmonics caused by cross modulation phenomenon across the AC-DC-AC link have been derived theoretically, and verified by field tests. These theoretical results and extensive field measurements on a sample induction melting furnace installation have shown that i. the major power quality problem of an induction melting furnace as a load on the power system, is the injection of various characteristic and uncharacteristic current harmonics to the grid, ii. line currents on the supply side are nearly balanced, iii. flicker contribution is below the limits specified in the standards, iv. reactive power demand is relatively low and the variations in the magnitude and rate of change of active power are not drastic, and v. no voltage sag and swell phenomena arising from furnace operation occur.