Aug 8, 2014

Magnetic materials for Electrical Machines



magnetic-materials-of-electrical-machines
The magnet properties of the magnetic materials rely on the direction of the quartzes of the material and choose the dimensions of the machines or instrumentation for a given rating, excitation needed, operation efficiency etc.

The certain properties of the better magnetic material ought to possess are discussed under.
  • Less reluctance or ought to be extremely permeable or ought to have a large value of relative permeability µr.
  • High permeation induction (to reduce volume of iron parts and weight).
  • High electric resistivity so the eddy voltage and also the therefore eddy current loss is a smaller amount. 
  • Tapered hysteresis loop or less Coercivity so hysteresis loss is a smaller value and effectiveness of operation is great. 
  • A high temperature.
  • Must have a great value of energy product.  It is expressed in joules / m3.
Magnetic materials will generally be classified as Paramagnetic, Diamagnetic, Ferromagnetic, Ferrimagnetic and Antiferromagnetic materials.  Solely Ferro-magnetic materials have the magnetic properties that are well appropriate for electrical machines. Ferro-magnetic properties are cramped virtually entirely to Cobalt iron, and nickel and their alloys. The sole exceptions are some alloys of metallic element and a few of the group parts.
The relative permeableness µr of Ferro-magnetic material is way bigger than 1.0.  Once magnetic force (Ferromagnetic) materials are exposed to the force field, the di-poles arrange in a line themselves within the direction of the smeared field and obtain powerfully magnetized. 

Further the magnetic force materials may be classified as arduous or Soft Magnetic materials and Permanent Magnetic materials.

Hard or permanent magnetic material has massive size hysteresis loop and bit by bit rising magnetization curve.
Ex:  tungsten steel, Carbon steel, Cobalt steel, rigid ferrite, alnico etc.

Soft magnetic material

It has tiny size hysteresis loop and a steep magnetization curve.
Ex: i) forged iron, cast steel, rolled steel, solid steel etc., (in the solid form).
Commonly used for yokes poles of D.C. machines, rotors of turbo generator etc., wherever d.c. flux is concerned. 

Silicon steel 

It is (Iron +0.3% to 4.5% silicon) within the laminated type. Adding of silicon in appropriate share reduces ageing and cut back core loss. Small silicon contented steels or dynamo grade steel is employed in revolving electric machines and is worked at high flux density. Great contented silicon steel (4% to 5% of silicon) or transformer rating steel (or high resistance steel) is employed in transformers. More sheet steel is also hot or cold revolved. Cold rolled grain oriented steel (CRGOS) is expensive and superior to hot rolled. CRGO steel is mostly utilized in transformers. 

Special purpose Alloy: 

Nickel iron alloy has high permeableness and addition of chromium ends up in better magnetic material. Nickel with iron in several proportion ends up in:
  • High nickel nickel-base alloy (iron + molybdenum + copper or chromium), utilized in magnetic amplifiers, current transformers, etc.
  • Low nickel nickel-base alloy (iron + silicon + chromium or manganese), utilized in induction coils, chokes, transformers, etc. 
  • Perminvor (iron + nickel + cobalt).
  • Mumetal (Copper + iron)
  • Pemendur (iron + cobalt + vanadium), used for oscilloscopes, microphones, etc.

Amorphous alloys

Amorphous alloy is made by fast set of the alloy at cooling rates of a couple of million degrees centi-grade per sec. The alloy solidifies with a glass like atomic structure that is non-crystalline frozen liquid. The fast cooling is achieved by inflicting the molten alloy to flow through an orifice onto a rotating water cooled drum. This will manufacture sheets as skinny as 10µm and a meter or additional wide.


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