Correct Answer - Option 1 : by lamination
- When an alternating magnetic field is applied to a magnetic material, an emf is induced in the material itself according to Faraday’s law of Electromagnetic induction.
- Since the magnetic material is a conducting material, these EMF’s circulates current within the body of the material. These circulating currents are called Eddy currents. They are produced when the conductor experiences a changing magnetic field.
- The process of lamination involves dividing the core into thin layers held together by insulating materials.
- Due to lamination effective cross-section area of each layer reduces and hence the effective resistance increases.
- As effective resistance increases, the eddy current losses will get decrease.
Mathematically, the eddy current loss is given by:
Pe = Ke Bm2. t2. f2. V Watts
K - coefficient of eddy current. Its value depends upon the nature of magnetic material
Bm - Maximum value of flux density in Wb/m2
t - Thickness of lamination in meters
f - Frequency of reversal of the magnetic field in Hz
V - Volume of magnetic material in m3
From the above formula, we conclude that the Eddy current loss is proportional to the square of the frequency.
- To minimize the eddy current loss we increases the resistance in the path of eddy current by laminating it.
- Eddy current losses are directly proportional to area of armature or more precisely the path of motion.
- In laminated armature eddy current losses are reduced to very less or '0' quantity. That is why armature of DC machines (either motor or generator) is laminated.