DEFINITION OF HYSTERESIS
LAGGING OF MAGNETIZATION OF A FERROMAGNETIC MATERIAL LIKE IRON
- RETENTION OF MAGNETISM
DRAW THE HYSTERESIS LOOP
REFER DIAGRAM
WHAT IS THE X AND Y AXIS OF B-H GRAPH?
X AXIS : MAGNETIZING FORCE (MAGNETIC FIELD)
Y AXIS : FLUX DENSITY (MEASURED IN TESLA)
WHAT ARE THE 3 IMPORTANT POINTS OF B-H GRAPH?
- POINT OF SATURATION
- C - COERCIVITY (X-INTERCEPT)
- R - RETENTIVITY (Y-INTERCEPT)
HOW IS B-H LOOP FORMED?
- WHEN A FERROMAGNETIC MATERIAL IS PLACED IN A MAGNETIC FIELD, IT IS MAGNETISED BY INDUCTION.
- IF WE VARY THE MAGNETIC INTENSITY OF THE FIELD, THE INTENSITY OF MAGNETISM AND FLUX DENSITY DO NOT VARY LINEARLY.
- POINT 1 IS THE INITIAL MAGNETIZATION STATE OF THE MATERIAL (m=0) AND ZERO FLUX DENSITY. AS H INCREASES, CURVE OP IS OBTAINED. THIS IS KNOWN AS THE VIRGIN CURVE.
- HOWEVER, AT P IT REACHES A MAGNETIC SATURATION WHERE FURTHER INCREASE OF H WILL NOT INCREASE IN M.
- WHEN H IS DECREASED, FLUX DENSITY (B) DOES NOT FOLLOW THE INITIAL CURVE BUT LAGS BEHIND H. THIS IS CALLED HYSTERESIS.
- WHEN H=0, IT IS SEEN THAT B DOES NOT EQUAL TO ZERO. THIS IS KNOWN AS RESIDUAL MAGNETISM. THE POWER OF RETAINING THEIR MAGNETISM IS CALLED THE RETENTIVITY OF THE MATERIAL.
- NOW, WHEN THE DIRECTION IS REVERSED, B DECREASES BUT H IS INCREASING IN REVERSE DIRECTION UP TO SATURATION POINT. THIS SHOWS THAT A MAGNETISING FORCE IS REQUIRED TO REDUCE THE RESIDUAL MAGNETISM. THIS VALUE IS CALLED COERCIVITY.
- FURTHER, WHEN H IS INCREASED IN REVERSE DIRECTION, THE HYSTERESIS LOOP IS OBTAINED.
- THROUGHOUT THE CYCLE, B LAGS BEHIND H.
WHAT DOES THE AREA OF HYSTERESIS LOOP REPRESENTS?
ENERGY LOSS
EXPLAIN ENERGY LOSS DUE TO HYSTERESIS.
- FROM THE CURVE, IT IS SEEN THAT WHEN H=0, SOME MAGNETIZATION IS RETAINED DUE TO HYSTERESIS EFFECT.
- THE ENERGY SUPPLIED TO THE MATERIAL IS NOT FULL Y RECOVERED.
- THE BALANCE OF ENERGY LEFT IN THE MATERIAL IS LOST AS HEAT.
DEFINITION OF COERCIVITY
- HARDNESS OR SOFTNESS OF THE MATERIAL TOWARDS MAGNETIZATION
- B- CURVE : MEASURE OF MAGNETIZING FIELD REQUIRED TO DESTROY THE RESIDUAL MAGNETISM
DEFINITITION OF RETENTIVITY
- MEASURE OF REMAINING MAGNETIZATION IN THE SUBSTANCE
IN THE CASE OF AC SUPPLY, HOW IS HYSTERESIS LOOP OBTAINED?
BY CHANGING THE DIRECTION OF H BY CHANGING THE DIRECTION OF AC SUPPLY TO THE COIL
RELATIONSHIP BETWEEN LARGE AREA AND SMALL AREA OF B-H LOOP
LARGE B-H AREA MEANS THAT THE MATERIAL HAS A BIG COERCIVITY.
THESE FERROMAGNETIC MATERIALS HAVE LARGE AREA AS ENERGY LOSS OVER B-H IS LARGE.
WHY DO HARD MAGNETS HAVE HIGH COERCIVITY?
CAN RETAIN ITS MAGNETIZATION IN AN ABSENCE OF AN APPLIED MAGNETIC FIELD.
APPLICATION OF SOFT MAGNETS (FREQUENT CHANGES IN MAGNETIZATION)
TRANSFORMER CORES
INDUCTORS
APPLICATION OF HARD MAGNETS (REQUIRE CERTAIN FORCE TO DEMAGNETIZE)
DISK DRIVES
ELECTRIC MOTORS
