When magnetic materials are subjected to an alternating flux, two types of losses occur which are: 1. Hysteresis Loss 2. Eddy Current Loss
The above losses comprise the total core loss.
Type # 1. Hysteresis Loss:
Below curie temperature (it is the rising temperature at which the given material ceases to be ferromagnetic, or the falling temperature at which it becomes ferromagnetic) all ferromagnetic materials exhibit the phenomenon called hysteresis which is defined as the lagging of magnetisation or induction flux density (B) behind the magnetising force (H) or it is that quality of a magnetic substance due to which energy is dissipated in it on the reversal of its magnetism.
Fig. 8.28 shows a typical hysteresis loop. It is a curve plotted between B and H for various values of H from a maximum value in the positive direction to maximum value in the negative direction and back again.
Starting at zero with a coil wound round a toroid of unmagnetised iron, the magnetisation curve follows curve OD. If the m.m.f. is gradually reduced, the flux curve follows the line DE. As the m.m.f. is reversed the flux falls into the point L and hence to M. As the m.m.f. is returned to zero, the flux traces out the path MN.
Then, as m.m.f. is again increased, curve NPD is followed. The area within the closed loop is a measure of energy lost during the cycle. This energy is, in effect, a frictional loss and shows up as heat in the material. The distance OE is a measure of the residual flux left in the closed magnetic circuit when the current is zero.
It is known as Br, the residual induction. Br describes circuits in which there are no air gaps, e.g., the iron toroid, should not be confused with remanance, a more general term which refers to magnetic induction remaining in the magnetic circuit (usually in the air gap when one is present) after magnetising force has been removed.
The distance OL is known as -Hc, the coercive force, and is the value of the demagnetising m.m.f. required to bring the residual or remanent magnetic induction to zero when such a loop is being traced out.
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If a ferromagnetic substance is subjected to an alternating m.m.f., the first hysteresis loops traced out do not necessarily fall upon each other. When successive loops retrace preceding ones, the material is said to be in a cyclically magnetised condition. For electromagnet core materials, values of Br and – Hc are determined from a hysteresis loop taken when material is cyclically magnetised. Permanent magnet values, however, are taken from the first hysteresis loop, since permanent magnets need be magnetised only once.
The hysteresis loop equals the work which is necessary to reverse the direction of magnetisation. The actual shape and area of loop depend on the internal structure and composition of the ferromagnetic substance.
Thus work done (W) = (area of B-H loop) joules/m3/cycle.
While calculating the actual area, scales of B and H should be taken into consideration.
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For example, if scales are-
The value of Steinmetz coefficient k is approximately 2 for all modern magnetic materials.
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Note:
The transformers and generators cores and armatures of the electric motors etc., which are subjected to rapid reversals of magnetisation should be made of such substances which have low coefficient in order to reduce the hysteresis loss.
Type # 2. Eddy Current Losses:
The term “eddy currents” is applied to those electric currents which circulate within a mass of conducting material when the latter is situated in a varying magnetic field. The conducting material may be considered as consisting of large number of closed conducting paths, each of which behaves like a short-circuited winding.
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The varying magnetic field induces eddy e.m.fs in these closed elemental paths giving rise to eddy currents. These eddy currents produce loss in power resulting in heating of materials. This loss is of considerable importance as it affects the efficiency and heating of electrical machines.
The eddy currents produce a magnetic field of their own which opposes the main magnetic field. As the effect of eddy currents is not uniform over the cross-section of the material, this result in a flux distribution which is not uniform, the flux density in the outer portions being greater than that at the centre. Thus there is a reduction in effective cross- section. The effect of eddy currents upon flux distribution is chiefly of importance in transformers where the material otherwise would be worked at a uniform flux density.
The magnetic materials used for varying magnetic fields are laminated (made up of thin sheets insulated from each other) so as to reduce eddy currents and associated losses, as by laminating, the area of path of eddy currents is reduced giving rise to a large value of resistance.
Eddy Current Loss in Thin Sheets:
Fig. 8.29 shows a thin plate of thickness t and width b, thickness being considerably smaller than the width. Let us suppose that this sheet carries a flux, B = Bmax sin t, the field running parallel to the axis of the sheet. Eddy currents would flow in the sheet in the elemental paths as shown in Fig. 8.29.
The thickness t of laminated material is of considerable importance as regards to eddy current loss. Eqn. (8.16) shows that eddy current losses are reduced by using thinner plates and a material of higher resistivity.
Eqn. (8.16) is derived by assuming that magnetic field is uniform throughout and parallel to the axis and also the sheets are very thin. Therefore, as such, it should not be applied to cases where frequency is abnormal, the sheets thick or the variation of flux density is not sinusoidal with time.
The eddy current loss can be reduced in the following ways:
1. It has been found that this loss can be minimised by building up the required cross-section for the flux path by stacking thin pieces known as “laminations”. Since the e.m.fs set up in the material by the varying flux are usually of small magnitude, the natural oxide on the surface of the sheet iron or steel from which the laminations are punched will effectively insulate the laminations from one another, and thus limit each eddy current path to a single lamination.
2. This loss may also be reduced by grinding the ferromagnetic material to a powder and mixing it with a binder that effectively insulates the particles from one another. This mixture is then formed under pressure into the desired shape and heat treated. Magnetic cores for use in communication equipment are frequently made by this process.
Iron losses (hysteresis and eddy current losses) if allowed to take place unchecked, not only reduce the efficiency of electrical equipment but also raise the temperature of the core. Hence these losses should be kept as small as is economically possible.
Other Factors Affecting Core Loss:
1. Rivets and Bolts:
Laminations must be held together. A common method is to use rivets or bolts, but they tend to short circuit lamination insulation. Rivets and bolts very often form part of a closed magnetic loop which, under A.C. operation, may cause a serious loss of electrical energy.
2. Burrs:
As dies become worn, the burrs that they form on punched edges of lamination become larger and larger. Thus burrs are not always removed by deburring operation and cut through sheet insulation forming contacts between laminations and allow excessive eddy currents to flow. These are commonly referred to as inter-sheet eddy currents.
Machining operations that are performed on stacked laminations, such as the grinding of induction motor stators or broaching of solenoid faces, often smear the lamination together, forming good connections between sheets. Many schemes, such as treating the faces with acid or alkali, have been tried in an attempt to eat away the metal particles after laminations have been assembled into apparatus, but this attempted cure often causes other troubles, such as rusting. Burning of the burrs with a flame has also been attempted, with varying success.
3. Pressure:
Mechanical strains set up in magnetic materials either from cold working during rolling or punching or distortion during final assembly operations, affect magnetic properties in several ways. The hysteresis and eddy current losses as well as the magnetising current are increased.
The increased hysteresis loss due to punching depends upon the material, its thickness, and the configuration of the grains. The loss may be as high as 300 percent of the loss in the un-punched sheet. The strains produced by punching operations increase the loss primarily in the narrow section next to the punched edge. The percent increase in hysteresis loss is much lower for high flux densities than for low ones.
The effect of tension on silicon steel is to increase the permeability until high flux densities are reached. The effect of compression, however, is much more marked than that of tension and reduces the permeability considerably.
Total iron loss is the sum of hysteresis and eddy current loss and is given by the relation:
The total iron loss can be calculated with the aid of above expression but it gives good results only in the case of static machinery and not in the case of dynamo electric machinery. The reason being that this expression is applicable when the field is simply alternating, but when the reversals of flux are due to rotational magnetic field, the losses do not follow the same laws. Therefore, in practice, iron loss curves are used which represent the loss in watt per kg as a function of flux density.
Factors Affecting Permeability and Hysteresis Loss:
Generally if the initial permeability is high, the hysteresis loss is low and vice versa.
The permeability and the hysteresis loss depend upon the following conditions:
(i) Physical condition of the sample.
(ii) Chemical purity of the sample.
When the crystals of a ferromagnetic material are cold worked, they experience deformation as a result of which the material has very poor magnetic properties. Due to the internal strains on the domains, greater magnetic field is required to give a definite magnetisation, as a consequence the permeability decreases and the hysteresis loss is increased. A material which has suffered magnetic damage due to cold work may be treated to a sufficiently high temperature when the magnetic properties will be restored.
The impurity content of the material exercises a limit on the highest magnetic permeability and the lowest hysteresis loss that can be obtained. Impurities bring harm to the magnetic properties by affecting the regular geometric pattern of the crystals.
The main impurities in the magnetic materials used for transformer cores and electrical machinery are carbon, sulphur, oxygen and nitrogen. Carbon is the most detrimental and the amount of carbon in commercial materials is kept as low as 0.01 percent.