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Induction Melting Combustion furnaces and induction furnaces produce heat in two entirely different ways. In a combustion furnace, heat is created by burning a fuel such as coke, oil or natural gas. The burning fuel brings the interior temperature of the furnace above the melting point of the charge material placed inside. This heats the surface of the charge material, causing it to melt. Induction furnaces produce their heat cleanly, without combustion. Alternating electric current from an induction power unit flows into a furnace and through a coil made of hollow copper tubing. This creates an electromagnetic field that passes through the refractory material and couples with conductive metal charge inside the furnace. This induces electric current to flow inside the metal charge itself, producing heat that rapidly causes the metal to melt. Although some furnace surfaces may become hot enough to present a burn hazard, with induction, you heat the charge directly, not the furnace.
Current flowing in one direction in the induction coil induces a current flow in the opposite direction in the metal charge. This current heats the metal and causes it to melt. ________________________________________________ Induction Electrical System Configurations Induction furnaces require two separate electrical systems: one for the cooling system, furnace tilting and instrumentation, and the other for the induction coil power. A line to the plant’s power distribution panel typically furnishes power for the pumps in the induction coil cooling system, the hydraulic furnace tilting mechanism, and instrumentation and control systems. Electricity for the induction coils is furnished from a three-phase, high voltage, high amperage utility line. The complexity of the power supply connected to the induction coils varies with the type of furnace and its use. A channel furnace that holds and pours liquefied metal can operate efficiently using mains frequency provided by the local utility. By contrast, most coreless furnaces for melting require a medium to high frequency power supply. Raising the frequency of the alternating current flowing through the induction coils increases the amount of power that can be applied to a given size furnace. This, in turn, means faster melting. A 10 ton coreless furnace operating at 60 Hz can melt its capacity in two hours. At 275 Hz, the same furnace can melt the full 10 ton charge in 26 minutes, or four times faster. An added advantage of higher frequency operation is that furnaces can be started using less bulky scrap and can be emptied completely between heats. The transformers, inverters and capacitors needed to “tune” the frequency required for high-efficiency induction furnaces can pose a serious electrical hazard. For this reason, furnace power supplies are housed in key-locked steel enclosures, equipped with safety interlocks. Safety Implications ________________________________________________ Induction Furnaces Come In Many Varieties Coreless Furnaces - A coreless furnace has no inductor or core, unlike the channel furnace described below. Instead, the entire bath functions as the induction heating area. Copper coils encircle a layer of refractory material surrounding the entire length of the furnace interior. Running a powerful electric current through the coils creates a magnetic field that penetrates the refractory and quickly melts the metal charge material inside the furnace. The copper coil is kept from melting by cooling water flowing through it. Coreless furnaces range in size from just a few ounces to 100 tons of metal and more. A direct electric heat furnace is a unique type of highly efficient air-cooled coreless furnace that uses induction to heat a crucible rather than the metal itself. This furnace is used to melt most nonferrous metals.
Channel Furnaces - In a channel furnace, induction heating takes place in the “channel,” a relatively small and narrow area at the bottom of the main bath. The channel passes through a laminated steel core and around the coil assembly. The electric circuit formed by the core and coil is completed when the channel is filled with molten metal. Once the channel is filled with molten metal, power can be applied to the furnace coil. This produces an intense electromagnetic field which causes electric current to flow through and further heat the molten metal in the channel. Hotter metal leaving the channel circulates upward, raising the temperature of the entire bath. Foundries typically use channel furnaces to hold and dispense molten metal whenever it is needed. Channel furnaces are emptied only for relining. A pressure pour is, in essence, a channel furnace, as described above, that is carefully sealed so that the metal can be moved out of the furnace by way of pressurizing the chamber above the molten metal bath in the furnace. The recharge and pouring spouts reach below metal level and ensure that clean metal is raised out of the furnace and into a pouring launder. A high-speed digital camera sights on the mold pouring cup, controlling the stroke of a stopper rod to precisely control the flow of molten metal into the mold. This allows as many as 400 molds and more per hour to be poured precisely with no operator intervention required.
________________________________________________ Be Aware of Induction Hazards
A review of records of foundries that use induction furnaces reveals that, in almost every accident, injury and damage could have been prevented by observing basic safety precautions. Most melt shop precautions, such as wearing eye protection and nonflammable clothing, are simple common sense. Other safety measures, such as knowing how to deal with a bridging emergency, require specific knowledge of the induction melting process. This site will help you better understand and deal with both day-to-day hazards present in all foundries and many of the emergency situations you may one day encounter. Accident investigation reports indicate that most foundry
accidents happen due to one of the following reasons: This site will focus on what you can do to protect yourself and your co-workers from these hazards and others. However, this is not a substitute for the more detailed information found in your equipment manuals. The manuals must be your primary source of information.
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