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Arc Furnace Heating Effect
QUESTION:I am studying Electric Arc Furnaces and their heating effect.
One furnace specifically in mind uses a 4.5MVA continuously rated 3 phase AC
Start/Delta Transformer. The TX has 18 Tap settings that range from 30 kA @
70 Volts Tap 1 and 13 kA @ 200 V Tap 18. The Electrodes are about 400 mm in
dia.
If I assume that the furnace charge (substance under change) is purely
resistive and the heating effect of the furnace is directly related to P =
I^2Rt. Therefore if the resistance of furnace charge is basically constant,
the square of the current by time determines the amount of heat energy
delivered to the charge.
My question is, even though the input power (work) to furnace remains
constant for each tap setting, will the higher current taps produce a Hotter
charge (same work at a faster rate) as compared to lower current taps?
ANSWER: 1) Don't expect the resistance of an arc furnace to stay constant with
temperature, current or voltage.
2) Don't expect the transformer ratings at various taps to be
accurately reproduced for a given load.
2) Expect the energy applied to the furnace to be the time summation
of the instantaneous voltage X current product at the furnace.
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