Induction furnace yoke (shunt)

Induction furnace yoke (shunt)

Yokes are to collect and conduct magnetic flux generated by coil. Design and quality of the yokes has a major role in furnace operation, energy waste and ultimately furnace efficiency. Also, geometrical architecture of the yokes plays an effective role in evenly spreading of magnetic flux inside furnace, turbulence caused by magnetic field and finally melting quality. Araz Trans Co. benefiting high tech knowledge and sophisticated personnel, constructs yokes for induction furnaces according to design plans provided by costumers or our own designs in relation to operation conditions. Factors affecting yoke’s operation quality can be categorized as below:

Fully-welded yoke:

In this design plan, core connection to the body and cooling system is done by welding and no threaded fitting is used. This approach is mostly applied by some Chinese and Turkish companies.

The main issue of fully-welded yokes is the loosening of the bottom. As core sheets welding is done on the upper surface facing the furnace body, tension caused by welding will result in core sheets to make distance from each other on the bottom side and hence become loosen. This consequently generates magnetic properties in sheets collecting dust and filing particles of yoke’s surrounding area. Collected particles gradually establish slight electrical connection and arc between yoke and coil. Also, noise caused by yoke’s core sheets’ vibration is the other side of yoke bottom loosening.

Apart from the disadvantages mentioned for this category of yokes of induction furnaces in terms of operation and performance of the yoke, apart from the simplicity of production and relatively low initial construction price, no special advantage can be considered for this design. Therefore, the use of yokes made in this design is not recommended.

Welded and bolted yokes:

In this design scheme, core connection to the body and cooling system is done through both welding and threaded fittings. This plan is very common among average and high level induction furnace constructors like Indian Megatherm, German Otto Junker and some Turkish and Chinese companies.

While yoke bottom loosening is highly mitigated in comparison to fully welded scheme but problems like no capability to dissemble and heat loss duo to welding and causing electrical connection between core sheets, remains unsolved.

Fully-bolted yokes:

In this design plan, core connection to body and cooling system is done fully by threaded fittings and there is no welding between sheets and body. Now, according to construction difficulties mainly caused by time and money consuming process of sheets punching and providing insulated fittings, constructors are not eager to produce such yokes and the only company to do so is Araz Trans Co..

The main difference between this yokes and previous category is using more threaded bolts and fittings closely instead of welding which will remove all the issues in previous designs.

Yoke core

Yoke core, as the main element of induction furnace yoke, is comprised of magnetic core sheets, same as sheets in transformers core. Quality and thickness of the sheets plays a major role in magnetic permeability and flux integration, losses and consequently furnace efficiency. To reach more info on quality and characteristics of various core sheet grades, please refer transformer core sheets.

Yoke’s cooling system

Cooling system is mainly to mitigate the heat caused magnetic field passing yoke while operating. There are several cooling system designs for yokes highly relative to yoke design scheme. Below various cooling systems are discussed with their benefits and drawbacks:

Considering above table, each approach has its own drawbacks. Hence, Araz Trans’ design plan at the last row, avoids all the weak points and aside from cooling quality, by applying an aluminum plate, a magnetic shield is formed preventing magnetic field leakage and loss.

Dou to compression of the magnetic field around yoke, the surrounding space is divided into power side and normal side. Power side is the area with high magnetic field density relative to coil’s input and output water-cooled cables. The remaining space is called normal side containing lower levels of magnetic field. Considering high density of agnetic field in comparison to normal side of the yoke, it is suggested to apply more efficient cooling system at normal side.

Yoke’s stabilizing system

The stabilizing system consists of fittings connecting core and body. In welded type, the connection is created by Argon welding of sheets from one side and ultimately welding them by non-magnetic steel pelits as holder.

In bolted-and-welded and fully bolted yoke types, stabilizing system is consisted of holding non-magnetic steel bolts. Although some constructors use normal (magnetic) bolts and pelits to lower the expenses, but Araz Trans Co. uses non-magnetic steel bolts (mainly stain-less 304 or 316 grade steel) to avoid fittings warming-up in the magnetic field.

The main point to be considered while using non-magnetic stain-less steel bolts, is insulation and isolating these bolts from core and yoke body, electrically. Applying low-level insulation, hence insulation failure and consequently connection of the bolts to the core or body, induced currents in the bolts and other parts of the yoke will cause considerable heat and loss duo to short-circuit creation. Aside from losses, this can lead in redhot or melted core and bolts. To avoid this issue and further problems, Araz Trans Co. applies three insulating methods for bolts, discussed in below table with details.

Yoke Support

Yoke support is the resting-place for yoke on the coil mainly to fix coil placement and stability while operating. A considerable point is to cover the empty spaces between yoke and coil to prevent dust and filing collection. As mentioned before, these particles may gradually lead in electrical short-circuit between coil and yoke. To attain both stabilizing and filling empty spaces simultaneously, two main methods are suggested: First, making curvy resting-place in the yoke support while final assembly and second, exploiting side insulation connections in flat-back yokes to have more connection points. The second approach proposed by Araz Trans Co. is highly suggested, because applying first approach makes it hard for vapor caused by sintering to exit that sometimes maintains vapor as drops at that place. Against common wrong beliefs, using flat-back yoke and hence presence of a gap between yoke and coil at some places of support will not lead in electrical loss of the furnace and comparing this to the gep between rotor and stator in rotary machines is not reasonable.

In addition to support shape in the connection point to the coil, support corners shape is important too. A curvy corner shape without covering yoke body, will absorb wandering magnetic field around coil altering yoke efficiency.

Beside all the info on yoke support, there is possibility to assemble and install insulation pads in Araz Trans Co.’s suggested design scheme. Insulation pads are a set of insulation layers separately placed between yoke and coil (in common recent designs). Applying this approach presents more connection surface between yoke and coil especially in flat-back ones and also offers a possibility for vapor of sintering process to exit yoke back properly. Below illustration presents details and benefits of this approach.

Yoke forehead

Yoke forehead as a medium between yoke and structure-stabilizing bolts, is constructed according to physical condition of the crucible or Araz Trans’ suggested designs. Below you can find some construction designs for yoke forehead exploited by Araz Trans Co. which are categorized as pull type and push-pull type related to their application. Pull type is used to only press the yoke to the coil while the push-pull type is used to separate the yoke from coil, too.

With all discussed above, induction furnace yoke supports will be of the upcoming four categories:

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