Electrical Engineering ⇒ Topic : Hysteresis Loop.

Mason
 
Hysteresis Loop. Consider an unmagnetised iron bar AB wound with N turns as shown in Fig. a (i). The magnetising force H (= NI/L) produced by this solenoid can be changed by varying the current through the coil. The doublepole, doublethrow switch (DPDT) is used to reverse the direction of current through the coil. We shall see that when the iron piece is subjected to a cycle of magnetisation, the resultant BH curve traces a loop abcdefa called hysteresis loop. We start with unmagnetised solenoid AB. When the current in the solenoid is zero,H= 0 and hence B in the iron piece is O. As H is increased (by increasing solenoid current), the flux density (+ B) also increases until the point of maximum flux density (+ B_{max}) is reached. The material is saturated and beyond this point, the flux density will not increase regardless of any increase in current or magnetising force. Note that BH curve of the iron follows the path oa.
figure (1)  
 
Kunal
 
HYSTERESIS LOOP Consider an iron ring which is completely demagnetised. If the magnetic flux density (B) is measured in the specimen with increasing values of magnetic field intensity (H), the relationship between the two quantities is given by the curve OPQ as shown in Fig. (a). If the value of H is now reduced, it is found that B follows the curve QRST and when H becomes zero, the flux density (B) in the specimen remains at OR. This quantity of flux density corresponding to OR is known as the remanent or residual flux density.If H is increased in the reverse direction, B decreases and becomes zero when H = OS. This corresponds to the magnetic field intensity required to wipe out the residual magnetism and is called the coercive force. Further increase of H beyond OS, increases B in the reverse direction as shown by the part of the curve ST. If the reversed value of H (corresponding to OB) is brought back to the original maximum value of H (corresponding to OA), the value of B follows the curve TUVQ. The magnitude of B at maximum and minimum values of H1, namely AQ and BT are equal. The complete curve is symmetrical about the origin 0 and the loop QRSTUVQ is known as the hysteresis loop. All magnetic materials exhibit the hysteresis loop when subjected to the cycle of magnetisation. The coercive force varies from metal to metal; varying from a value of 40,000 A/m (for Alnico) to 3 A/m (for Mu metal).
FIGURE (a) Hysteresis loop  
 
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