Induction furnace coil
Coil of an induction furnace is considered its main part. The main function of the coil is to create magnetic fields required to conduct eddy currents that leads to heating or melting scrap metals or processed ores. Taking into account the role and importance of this part, the electrical and mechanical properties of coil remarkably affects the function and efficiency of the furnace. Therefore, considering diversity of material and quality of parts used for manufacturing coils, Araz Trans Co. provides a wide range of coil design for the induction furnace, whether designed in the company or as the drawings of the original coil provided by the costumers (in case an spare part is being provided). So, customer will be able to choose design, used material and even properties of the insulation on the intended basis.
Induction furnace Architecture
Generally, an induction furnace is composed of below parts:
- One or several active parts- that carries electric current of the furnace needed to generate magnetic field for melting and current flow for cooling. Active parts which are necessarily made of copper, are connected in parallel form to divide coil current and decrease coil inductance.
- One or several non-active cooling parts- that merely contain water flow to cool down refractory at the end parts of the furnace and sometimes are as single semi-turn to fill the gap between active parts. Cooling parts are mainly from non-magnetic stain-less steel or copper.
- If exists, one or two short-circuited copper rings at the ends of the coil- are exploited to adjust and decrease magnetic flux density at the end parts of the coil. These short-circuited rings are called Faraday ring and not necessarily present in any coil according to the coil design.
- Coil stabilizer- consisting couple of non-magnetic stainless steel or brass bolts and nuts directly welded to the coil turns in different angles handled by composite supports.
- Insulation and isolation system of coil- As most important parts of the coil, provides electric isolation between coil turns and objects around.
- Electrical terminals connecting coil to the power supply- including copper buses or threaded made out of copper or brass which delivers power from the source to the coil.
Induction furnace Coil types relative to winding approach
Induction furnace coils are categorized in two types from winding perspective: Helical and offset.
Helical coils are turns winded in spiral form. In this type, according to angular turns, magnetic field and hence heat is not imposed evenly on melt. So, this coil type is used mostly in low size and power furnaces and is not suggested for large size furnaces. Besides, non-even displacement between active parts and non-parallel upper and bottom coil surface make it difficult to assemble and utilization.
Offset coils, against helical ones, are composed of flat turns winded with offset at the ends of the turns to create levels. In contrary to helical coils, the upper and bottom side of the coil is completely parallel and hence, the exerted magnetic field and heat on the melt are even. Thus, this coils are mostly adopted to high power large furnaces.
Induction furnace Coil types relative to assembly approach
As told before, active parts of the coil are paralleled to each other in two forms: back-to-back and dual turn.
In coil with back-to-back connected active parts, the active parts are connected in coaxial successive way fixed in the same direction with supports and ultimately paralleled electrically with bused on the coil or water-cooled cables connection.
In this type, the mutual inductance between active parts is roughly around 20 to 40 percent.
Dual turn coils (with commonly two active parts), active parts are integrated in web form in a way that turns are in every other one form. Doing so, the mutual inductance of the active parts is around 100 percent.
It should be mentioned that, dual turn coils are winded mostly in helical form.
There is no preference between back-to-back and dual turn coils and relative to physical restrictions, size and desired electrical characteristics, both can be exerted for coil assembly.
Coil active parts
As mentioned above, active parts carry alternating electrical current to generate magnetic field needed to induce eddy currents in melt. There are mainly one or several (mostly two) active parts in induction furnace coils paralleled with each other with connections on the coil or water-cooled cables.
Active parts are rolled copper tubes under certain standards. Material, thermal operation and tube shape affects the operation and efficiency of coil, considerably. To reach more info on this, please refer to copper tubes section.
Cooling part coil
Cooling part of a coil does not carry electrical current and is solely for cooling up and down side of the crucible and not necessarily present in all the coils. If exist, it can consist of on or two parts at the up, bottom or both sides. Also, in helical coils, cooling parts are placed between active parts to cool down the refractory at that place.
Cooling turns are from copper or stain-less steel and are mostly in rectangular or circular (mostly circular from stain-less steel) form. Using stainless-steel cooling turns, three points are to be considered:
- The tubes must be from seamless type.
- The applied alloy must be non-magnetic (preferably from 304 or 316 grade)
- Cooling parts must be anealled after rolling process. Otherwise the turns would be deformed on the coil end parts.
Neutral single-turn faraday ring
Faraday ring or so called neutral single-ring is a single turn short-circuited copper ring with water channel at the middle and when exists, is the last ring on the top or bottom of the coil. İt is used to adjust magnetic flux the coil input and output. In other words, according to Faraday law (not connected to the coil power supply), by passing magnetic field through Faraday ring, the electric current flowing through it caused by induced voltage, counter magnetic field will be generated by the ring to counteract the main magnetic field which helps adjust the magnetic field at that places and prevents yokes and metalic parts from overloading.
Stabilizing system consists of equipments to prevent deformation and displacement of coil turns under magnetic field. It is mostly couple of non-magnetic bolts and nuts with composite material beside.
The material of the stablizer parts applied by Araz Trans Co. İs non-magnetic steel bolts and brass nuts. Using steel bolt and nut presents the possibilty of stiff nuts and totally brass bolt and nut leads in weak solidity of the stablizers. Above mentioned connections are done in two way:
- Direct welding of the steel bolts to the coil turns with nuts on them
- Wellding nuts on the coil turns with steel bolts on them
Both approaches can be exerted according to the costumer preference.
Aside from the mentioned fittings, to complete the stabilizing system, especial composite supports are installed at predefined angles. These supports are composed of fiberglass and epoxy resin and according to the grade of the fibers and resin, it offers various properties and characteristics. To reach more info on this, please refer to the insulation material for induction furnace coil electric isolation section.
Also using a Mika pelit is suggested to avoid excessive heat under urgent conditions.
Coil insulation and isolation
The most important aspect of constructing a coil is the coil insulation and isolation quality. According to the research done by Araz Trans Co R&D team, the main impairment factor for coils and production interruption is the weakness in insulation system and coil isolation quality. Though, Araz Trans Co. applies strict attention to quality and reliability of the coil insulation and three methods are suggested relative to the construction time and expenses:
First Method: American Inductotherm company has proposed a special insulation material called Inducto-Flex. The main property of this epoxy-based electrostatic powder is that it does not crack and peeled from the surface facing tensions while operating. High heat-resistance, considerable isolation capability and even coherence to the coil surface while sprayed with special apparatus are other characteristics of this insulation material. Using this material solely will provide desired isolation and with proper coil application, no other insulation like liquid or tape ones is needed. Isolating by Inductotherm-Flex, as number one option of Araz Trans Co. for isolation, is done after sandblasting the coil to remove the oxide layer.
Second Method: in this method a double-layered insulation tape and an epoxy-based liquid varnish layer is applied beside indcto-Flex insulation to alter the isolation quality. This method is only suggested when the conditions are hazardous and with normal conditions using only Inducto-flex is adequate.
Third Method: Not using Inducto-Flex in this method, after sandblasting the coil, the surface is sprayed with three layers of a special coated epoxy color and then wrapped with two layers of insulation tape covered with special epoxy –based varnishing liquid. This insulation method is mostly exerted for repaired coils or when there is a possibility for ultra tiny leaks.
With all discussed above, the isolation quality ranking will be like methods 2,1 and 3 and henceforth the expenses will increase in the same order.
Coil electrical connections to the power supply
Connecting coil to the power supply is done by water-cooled cables in two ways:
- Bus connections- In this way, copper bus fittings connect the coil and water-cooled cables.
- Threaded fittings- Threaded fittings connect the coil to the water-cooled cables in this method.
According to the conditions, one of the above methods is applied for coil construction. Below you can find schematic illustration of the both methods to be compared.
While using threaded fittings to connect the coil, for decreasing magnetic flux generated by water-cooled cables, the same phases are placed in cross configuration.