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Friday, 2 August 2019

Magnetic Materials for Electrical Machines

Wrought Iron :

Wrought iron is a ferrous material. In the form of bar, it is widely used as cores for relays after annealing. Annealing is done after all shaping, forming, or machining operations.

Low Carbon Steel : 

Open hearth steel of very low carbon content has magnetic properties comparable with those of wrought iron or cast steel. It is used to a considerable extent for laminated and solid magnetic parts. It is inferior to silicon steel as regards ageing and core losses.

Cast Steel: 

Cast steel is extensively used for those portions of magnetic cults which carry uniform or continuous flux. Annealing improves the quality of cast steel.

Nickel Iron Alloy : 

Where Nickel is alloyed to iron in various proportions, it produces a series of alloys with wide range of magnetic properties with 30% of Nickel, the alloy is practically non-magnetic and with 78% Nickel the alloy has a very high permeability, Mainly there are three types of Nickel iron alloys.

(a) Perminvar (b) Permalloy (c) Mu-Metal

(a) Perminvar:

This alloy is made of 50% Nickel, 25% Cobalt and 25% iron. It is a soft magnetic material. Permeability is constant over a low flux density range. Hysteresis loss is low. Resistivity is very low, 18.6 x 10-8 ohm-m. This is used for armatures of motors, loudspeakers etc.

(b) Permalloy:

This alloy contains 78.5% Nickel, 21% iron, and a small quantities of copper, molybdenum, chromium, manganese etc. Hysteresis loss is low at low magnetization. Permeability of this material is about 8 x 105 and resistivity is 17 x 10-8 ohm-m. It is used as thin tape wrapped round the conductors of continuously loaded sub marine cables.

(c) Mu-Metal:

It is a soft magnetic material with 75% Nickel 1.4% Copper 1.5% chromium and 19.5% iron. Magnetic properties are stable than permalloy. For better properties it is annealed in pure and dry hydrogen at 1100oC for several hours and then cooled slowly. Its permeability is equal to 1 x 105 and resistivity 5.6 x 10-8 ohm-m. It is used for the cores of current transformers where coercivity is to be kept small and in moving iron instruments.

Effect of Nickel content in Iron as an Alloy : The percentage of nickel content in the iron alloy varies widely the magnetic properties. With 14% Nickel the alloy is practically non magnetic and has a resistivity of 86% ohm per cm. With 78% Nickel, the alloy property when heat treated, has the highest permeability.

Ferrites Cores :

It is a mixture of moulded metallic oxide powder and are used as deflection transformers, yokes, in antenna rods, pulse transformers, delay lines and wave guide elements. It has low hysteresis loss and eddy current loss. Permeability is in the range of 1000 to 3000.

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