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Tuesday, 6 August 2019

Soft Magnetic Materials Examples


Soft Magnetic Materials with Examples:


These materials are used for the construction of core plates of transformers, electro-magnets electric machines, reactors etc. as they have the property to quickly acquiring magnetism and loosing them when the applied field is removed. These soft magnetic materials should have the following important properties

(a) They have high permeability and low coercivity which means they require very little force to remove magnetism from the material.
(b) The magnetic energy stored in them is not high and so the residual magnetism is low.
(c) They can be easily magnetised and demagnetized.
(d) The area of hysteresis loss is small and so the energy loss is also small.
(e) They have a high saturation value of flux density
(f) The dipoles align easily and quickly.

In order to produce more magnetic flux in less area, flux density f the material chosen should be more. When the flux path area is ss. the volume and thus the cost of the core becomes less. Some examples of commercially used soft magnetic materials are :

(a) Low Carbon Steel : These have 0.08% to 0.45% carbon and so is similar to pure iron. Higher percentage of carbon increases resistivity, decreases permeability, lowers saturation point, improves the mechanical strength, increases the coercive force. It has a Fibrous structure with melting point at about 1400°C. They are used where mechanical strength is primary consideration (Ultimate stress 4200 kg / cm2) i.e., in the manufacture of the rotor of turbo-alternators.

(b) Pure Iron: It is ferrous material with very low carbon content. As it has low resistivity, it gives rise to eddy current when operated at high flux densities in alternating magnetic fields. It has high permeability. Its magnetic properties can be improved by reducing traces of carbon, oxygen and hydrogen to minimum. It has low hysteresis losses. Though it is cheap, its application is limited only to use in weak magnetic fields.



(c) Silicon Steel: When Silicon is alloyed with steel it is called silicon steel. Pure iron is added with small quantities of silicon of about 0.5 to 5% by weight in order to reduce eddy current loss. This is used for transformer core plates, rotating machines, electro magnets and relays. It increases the permeability at low and moderate flux densities but decreases at high densities. Silicon steel have narrow hysteresis loops which keep the hysteresis loss low and heat produced is also less. Addition of silicon reduces hysteresis loss and magneto-striction. Two percent of Silicon, 1% of manganese and 0.4% to 0.6% of carbon with steel has good tensile strength. This is used for poles of motors and transformer core plates. With silicon more than 5% the steel is brittle and hard work. 14% silicon content steel is used in chemical industries due to high corrosion resistance. Transformer core plates have 4% silicon that operate at 1 Wb / m2.

Since the cores of the rotating electrical machines are slotted with the teeth having small cross-sectional area, the flux density in the teeth is much higher of the order of 1.8 Wb / m2. To obtain high flux density, the magnetising current should be more. Also punching slots in high silicon steel would be difficult because it is harder and more brittle. For these reasons steel with 2% silicon with higher saturation is used. The thickness of the plates varies from 0.35 mm to 0.5 mm. For high frequency equipments thin laminations of 0.1 mm are used. Figure gives the magnetising curve for low and high silicon and Table gives the characteristics of some grades of Silicon steel.

Magnetization Curve of Low and High Silicon

Characteristics of some grade of Silicon Steel

Grade
Nominal Silicon content
Max. core loss in watts / kg at 50 cps and 1 Wb/ m2
Thickness of plates (mm)
Stacking factor (min)
RSP 230 - 50
1.8%
2.0
0.5
0.94
RSP 260 - 50
1.6%
2.6
0.5
0.94
RSP 300 - 50
1.2%
3.0
0.5
0.94
RSP 360 - 50
0.9%
3.6
0.5
0.94
RSP 400 - 100
2.0%
4.0
1.0
0.94
RSP 500 - 100
1.6%
5.0
1.0
0.94


Some other examples of Soft Magnetic Materials are Perminvar, Permalloy and Mu-Metal.

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