Electrical Engineering ⇒ Topic : Magnetisation (B-H) Curves

MAGNIETISATION (B-H) CURVES

Consider a toroidal solenoid wound on a non-magnetic core, such as shown in Fig. (a). If the flux density is measured on the centre line of the toroid. the relationship between B and H is given by the straight line OA in Fig. (b) If now the space within toroid is filled with an unmagnetised ferromagnetic material, the well known magnetisation curve OBCDE, is obtained. The megnetisation has many names being referred to as:

                                       Fig.(a) Toroid.                                                     Fig.(b). Magnetising curve for a ferromagnetic material

B-H curve, the magnetic saturation curve, the virgin curve or simply the saturation curve.

   The difference in flux between the saturation curve and the air line, OA at any magnetising force, is due to the contribution of the magnetic material, This flux is known as the intri nsic flux and gives a true measure of magnetic properties than does the total flux, especially at very high m.m.fs The point at which the intrinsic flux density curve becomes horizontal gives the intrinsic saturation

                                                                   Fig.(c) B-H curves

The magnetisation curves can be determined by the following methods provided the materials are in the form of a ring:

• By means of a ballistic galvanometer
• By means of a fluxmeter.

The graphs in Fig.(c) show the relationship between the flux density (B) and the magnetic field strength (H) obtained for different qualities of iron. The data for mild steel, wrought iron and sheet steel are so similar that they can be represented by a common graph. Stalloy is an alloy of iron and silicon commonly used in the construction of transformers and A-C machines.