The power supply for the electric arc furnace is low-voltage and high current type.
1.
Power consumption of the arc furnace is very high.
2.
The secondary voltage (arc voltage) is of the order of 50 V- 150V.
3.
Heating effect is proportional to the square of the current, therefore to
achieve higher temperature heavy currents are essential.
4.
The secondary current will be of several hundred or thousand amperes for
melting the metals.
5.
Insulation and safety point of view the maximum secondary voltage is limited to
275V (Line to line open circuit voltage)
6.
The use of low voltage and high current the electrodes are kept very near to
the charge as the arc length is small.
7.
To take care of electro-mechanical and thermal stresses a special type of
transformer with high ruggedness and robustness is required.
8.
The transformer used with arc furnace is oil immersed type.
9.
Both core and shell types are used but shell type is preferred because the
secondary leads can be brought out.
10.
In the primary side of the transformer small amount of current is handled so
tappings should be provided for controlling secondary voltage.
11.
It is desirable to arrange the furnace and transformer in such way that the
secondary leads are of shorter length, so that inductance of the leads is
reduced.
12.
To minimize the skin effect the shape of the leads to be taken into account.
13.
The leads from the transformer to the furnace are to carry heavy currents so
leads consisting of rectangular strips spaced a few mm apart are mostly
employed as current carrying conductors.
14.
A typical specification for a 3-phase arc furnace transformer includes an
extended primary winding with taps there in for the secondary voltage range
235-220-205-190-175-160 volts with primary connected in delta.
15.
This voltage range is extended by changing the connection of the primary
winding from delta to star giving 58% voltage from each tap. [1/1.732 = 0.577 x
100 = 58%]
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