Page 170 - Electromechanical Devices and Components Illustrated Sourcebook

132 Electromechanical Devices & Components Illustrated Sourcebook

Arc Furnaces Thermostats

Electric arcs provide an excellent heat source for melting met- Thermostats are the most common device used to control a
als. Figure 7-16 shows a small laboratory vacuum arc furnace. heater. These units are extremely common and can be found
These units are used to alloy small amounts of metals for test- in nearly every home and office in the world. Figure 7-18
ing and prototyping. The system is constructed inside of a bell shows a typical coiled bimetal strip thermostat. A mercury
jar. The base is the negative terminal and the electrode holder switch is mounted to the outside end of a coiled bimetal strip.
is the positive terminal. The bases are generally water-cooled The inside end is attached to a fixed mount. As the tempera-
and carry a port to bleed in reaction gases. The vacuum port ture changes, the bimetal strip deflects and the mercury switch
is connected to an appropriate vacuum system. rotates into an actuation angle (dotted lines).
Large scrap steel processing is done almost exclusively Figure 7-19 shows a typical control circuit for a high-
with electric arc furnaces. Figure 7-17 shows a schematic power heater controlled with a thermostat. The thermostat
representation of a commercial arc furnace. These units use operates on a 24-VAC signal and controls the contactor that
a three-phase arc set and the electrodes may be as large as switches the heater element.
36 inches in diameter.

Switch Mount
Mercury Switch
Actuation
Angle
Flexible Cable
Coiled
Electrode Bimetal Strip
Terminals
Electrode Clamp
Electrode Pivot
Arc Fixed Mount
+ Column Crucible
Figure 7-18 Coiled Bimetal Strip Thermostat
Molten Metal
Insulator Crucible Socket
Bell Jar
Jar Seal Main Fuses
Heater Elements
Primary
Contactor
Disconnect
Reaction Cooling
Gas + − Water 240/480 VAC
Three-Phase
Negative Terminal
Positive Terminal C
Heater Lamp
Vacuum Port Base Plate
L
Figure 7-16 Vacuum Arc Furnace
L Power Lamp
Transformer Fuses Power Switch
Control Transformer Thermostat Switch
Thermostat Probe
Three-Phase Power Control Circuit Fuse
Figure 7-19 Heater Control Schematic

Electrodes
Figure 7-20 shows a few commercial thermostats.
Thermostats are available for nearly any control environment
and installation imaginable. They are typically inexpensive
Arcs and highly reliable pieces of equipment.

Temperature Controllers
Molten Steel
Temperature controllers perform the same basic function as a
thermostat, except that they provide a higher degree of con-
trol. Temperature controllers generally provide some control
Crucible
over the temperature the heater generates. Mechanical con-
trollers provide fairly rudimentary control while digital con-
trollers provide a high degree of control. Figure 7-21 shows
Figure 7-17 Three-Phase Arc Furnace Schematic two common commercial temperature controllers.