Application of Magnetics E-shaped cores in welding equipment

Author: Vasilyeva K.

The actual features concerning application of powder e-shaped cores with distributed gap in power supplies of welding invertors and in other devices are analyzed in this article. The method of calculation of typical chokes made of Magnetics permalloy cores is studied.

It is a well-known fact that no one power module is used without coils. When searching for solutions in reliability increasing, dimensions decreasing and device characteristics improving without regard to its field of application leading engineers always paid special attention to the materials choice.

Among companies studying permalloys and manufacturing cores on the base of permalloys and Powdered Iron one can note well-known foreign enterprises such as Micrometals, Magnetics, Arnold, etc.

These companice produce a wide range of toroids on the base of permalloys (MPP, Kool Mμ, HighFlux и др.), as it is the toroid form that ensures a high resistance to external influences of electromagnetic fields and the lowest value of leakage inductance. Nevertheless, some companies pay close attention to the production of E-cores. Due to the high cost of many alloys, magnetic cores made of less expensive material Kool Mμ (Alsifer) and Powdered Iron are of interest (Figure 1), as they are an inexpensive alternative to permalloy.

Figure 1 – Е-cores on the base of Kool Mμ (Magnetics) [3]

So, Magnetics is one of the leaders in the development and production of permalloy magnetic cores producing a range of E-shaped cores made of Kool Mμ material with a permeability 26μ, 40μ, 60μ и 90μ.

Among advantages of Е-form one can note the possibility to use them in welding equipment, for example, in inverter-type welding machines. As known, a lot of modern series of welding inverters are equipped with output chokes [4,5]. In this case, the choke can serve as a key element of the welding machine, which uses electrodes in the welding process, and also can be used as a part of the welding semiautomatic device. The application pf coils in the inverter-type welding machines circuits allows not only to stabilize the current, but also to smooth out the pulsating voltage due to their magnetic field. In addition, the choke can be used in self-made welding machines, and similar equipment manufactured at the factory.

When creating welding machines engineers also take into account that the choke including the E-shaped core has a non-magnetic gap (0.5 - 1.0 mm), which allows to avoid saturation and more effectively control the inductance. As shown in Figure 1, one can create the gap using one of three ways. The first method is about to form an air gap on the central leg of the E-core by reducing its height. The gap can also be created simultaneously in the central and side-wall legs of the magnetic core. Cores with a discrete gap retain a high inductance up to the inflection point on the curve, which leads to a sharp saturation. In this regard, the attention is paid to the formation of a distributed gap in the E-shaped magnetic cores of powder material, which consists of a large number of small air gaps that appear as a result of the fact that powder particles of the soft-magnetic material are not fully sintered into a single composition. In such a case, the saturation is achieved smoothly, which contributes to increasing of the internal resistance to a device's functional loss. In such cores, the optimal value of B _max is maintained and DC bias is ensured at high temperatures [6].

Figure 2 - Е-shaped core: а) Magnetics ferrite core on the base of P material with initial permeability 2500 with a discrete gap along a central leg b) Magnetics ferrite core on the base of P material with initial permeability 2500 with a discrete gap along central and side-wall legs c) Kool Mμ ferrite core with a distributed gap[6]

Since chokes for inverter-type welding machines made of powder materials can improve not only technological effectiveness of the final product, but also reduce overall dimensions and decrease the intensity of dispersion fields [7]. It is actual to compare the characteristics of permalloy and Powdered Iron that are of greatest interest for using in such schemes.

Comparative characteristics of some alloys , such as Kool Mμ (Magnetics) and Powdered Iron are shown at figures 3, 4 and table 1.

Material	Kool Мμ® (Magnetics)	Powdered Iron
Alloy's composition	Fe Si Al	Fe
Current loss	Low	High
Initial permebility	26...125	10...100
Temperature Curie, °С	500	770
Operating temperstures, °С	-55...200	-30...75
Saturation induction, T	1	-1,2...-1,5

As is seen from the presented data (Figure 3), the main advantage of Kool Мμ material is lower loss in comparison with Powdered Iron . The cores made of Kool Мμ® material have almost zero magnetostriction and can be used at temperatures up to 500°C, which corresponds to Curie temperature (Table 1). In addition, the alloy based on Al, Si, Fe is made without organic binders. As a result it is not influenced by the effect of thermal aging.

Figure 3 - The dependence curve "core loss vs magnetic flux density" for powder materials (Kool Мμ® and Powdered Iron)

According to the data shown at figure 4 one can note relatively close curves "permeability-DC bias" for powder materials Kool Мμ® and Powdered Iron. Along with bias, caused by the constant component of the current, alternating current also flows in pulse regulators chokes. In the frequency range 10kHz-300kHz the variable component can cause core loss and its heating to temperatures which are above the Curie temperature. The choice of Kool Мμ® as core's material helps to reduce the influence of the variable component and improve the efficiency of the choke.

Figure 4 - Permeability vs DC bias for Kool Мμ® (Magnetics) and Powdered Iron

It is well-known fact, that for most of the coils spray radiation can appear [8]. In materials with low magnetic permeability this effect operates in such a manner: the measured induction is higher than the value calculated by formula 1 according to the specified parameters of the core.
L – induction, mH
μ – permeability
N – number of turns
A_e –cross-section square, mm²
I_e – path length, mm.

In addition, the scattering effect is also influenced by the number of turns, the design of the choke, the size and configuration of the core. For example, in case of E-shaped core increasing of the winding length, magnetic field scattering will exhibit itself lesser, unlike other technical designs. When determining the required number of turns, the size and configuration of the core in the process of power supplies designing, it is often necessary to carry out appropriate calculations. Some approaches for hoke parameters determining in case of an inverter welding machine are described in the literature [9, 10].

Many companies producing cores on the base of powder materials offer their customers both technical documentation and free software. [9]. As a example one can mention free Magnetics programs which allow to make required calculations for power supplies chokes using known parameters. When determining cores dimensions and number of turns the following simplified method also can be used [11]:

3)After determination of core size the number of turns can be calculated by using the following procedure:
The inductance factor (AL in nH/T²) for the core is obtained from the table 3 or datasheet.Then A_Lmin by using the worst case negative tolerance (generally -8%) is detrmined. With this information, one can calculate the number of turns needed to obtain the required inductance from:

where L – required inductance (µH) ;
A_Lmin – minimal value of inductance factor, nH/T².
- The calculation of the bias in A•T/cm from [3]:

где N – turns number;
I – DC current, А;
L_e – core magnetic path length , sm.
- From the curves (figure 5) the rolloff in per unit of initial permeability for the previously calculated bias level should be determined. Curve fit equations shown in the catalog can simplify this step.
One can multiply the required inductance by the per unit rolloff to find the inductance with bias current applied.
Next the number of turns is increased by dividing the initial number of turns (from formula 2) by the per unit initial value of permeability. This will yield an inductance close to the required value after steps listed eariler are repeated.

Figure 5 - Permeability vs dC BIAS curves for Kool Mμ® cores[11]

Thus, the main characteristics of Kool Mμ® permalloy and the application of E configuration in inverter-type power supplies of welding machines were considered. Comparative properties of powder materials such as Kool Мμ® and Powdered Iron are presented. It is noted that Kool Mμ® cores have lower loss in comparison with Powdered Iron. The using of E-cores with a distributed gap in the output chokes of welding devices allows not only to reduce dimensions and increase the reliability of the final product, but also to reduce the intensity of dispersion fields.

Table 3 – Characteristics of Е-cores and other shapes produced by Magnetics [11]

Literature
1. Bessonov L.А. Theoretical foundations of electrical engineering. Electrical circuits. – 10-е issue., reworked and added.– М.: Garadariki, 2002. – 638 p.
2. Mishin D.D. Magnetic materials, Training manual for universities. 2-issue, eworked and added. – М.: High school, 1991. 384 p.
3. http://www.mag-inc.com/products/powder-cores/kool-mu/large-kool-mu-core-shapes
4. S. Petrov. Entry Level Welding Inverter: Development Example / Power Electronics.2010 - №5 – P. 82-89
5. S. Petrov. Circuit design of industrial welding inverters / Modern electronics. 2007 - №8 –P.42-47.
6. Michael W. Leakage flux considerations on kool Mμ “E” cores. Bulletin No. KMC-E2
7.Volodin V. Modeling of inductors based on powder cores using the LTspice simulator. Power Electronics Magazine - 2010 г. - №2. - P.84-90.
8. V.V. Shirokhin, Y.M. Кhazantsev. Interference emission reducing for power chokes based on toroid-shaped cores/ Bulletin of Tomsk Polytechnic University. 2010. - V. 316. № 4. – P.107-110.
9. Volodin V. Free versions of programs for calculating a choke with a powder core / Power electronics. 2010 - №3. – P.92-99.
10. Volodin V. Invertor source of welding current. Experience in the repair and calculation of electromagnetic elements. Radio Magazine №8, 9, 10 - 2003 .
11. http://www.mag-inc.com/products/powder-cores/kool-mu/large-kool-mu-core-shapes