Determination of B-H curve The two methods available for the determination d etermination of B-H curve of a ring specimen are, 1. Meth Method od of reve revers rsal al 2. Step Step b step step metho ethod d Method of reversal The !ig shows the circuit of the determination of B-H curve b the method of reversal. "t consists of a ring specimen with #nown dimensions. $ search coil insulated b paraffined wa% is wound over the tape put on the ring. $nother laer of tape is put over the search coil and the magneti&ing winding is uniforml wound over the tape. $fter demagneti&ing completel, the test is started b passing a ver low value of current through the magneti&ing circuit. 'ith the galvanometer #e ( closed, the ring specimen is brought into a reproducible cclic magnetic state b throwing the reversing switch S bac#ward and forward about twent times. v alue of H can be )ow the #e ( is opened and the value of flu% corresponding to this value measured from the galvanometer deflection *ust b reversing the reversible switch.
!ig. +etermination of B-H curve b the method of reversal
The value of flu% densit corresponding to this H can be calculated using formula,
'here $s is the cross-sectional area of the specimen in m2
ɸ is the flu% measured in 'b. B is the flu% densit in 'bm2 The above procedure is repeated for various values of H upto ma%imum testing point. The B-H curve can be plotted from the various measured value of H and the calculated value of B as shown in fig.
!ig. B-H curve Step by step method "t consists of a potential divider with large number of tappings. The tappings are so
arranged that the magneti&ing force H can be increased up to the ma%imum value in a number of suitable steps. Before performing the test, the ring specimen is completel demagneti&ed. "nitiall the switch S2 is set on tapping 1. Then the switch S1 and the corresponding increase in the flu% densit of the specimen is measured from the deflection of the galvanometer. et it be B1of magneti&ing force, corresponding to B1 can be calculated from the value of current flowing in the magneti&ing winding at tapping 1. et it be H 1, the magneti&ing force is then increased to H2 b suddenl switching S2 to tapping 2, and the
corresponding increase in flu% densit B can be determined from the deflection of the galvanometer. !lu% densit B 2 corresponding to magneti&ing force H2 is given b B1 /B. The above procedure is repeated for various values of H upto the ma%imum testing point and the complete B-H curve is plotted as shown in figure
!ig. +etermination of B-H curve b step b step method
Measurement of iron losses The three methods used for the measurement of iron loss in ferromagnetic materials are 1. 'attmeter method 2. Bridge method 0. otentiometer method Wattmeter method Constructional details This method is most commonl used for measurement of iron loss in strip sheet3 material.
The strip material to be tested is assembled as a closed magnetic circuit in the form of a
s4uare which is #nown as magnetic s4uare.The two common forms of the magnetic s4uares are i3 5pstein s4uare ii3 lod-!isher s4uare Epstein square
!ig. 5pstein s4uare 5pstein s4uare which consists of four stac#s of strips that are bounded and then taped.
The individual strips are insulated from each other and each strip is in the plane of the s4uare. The stac# s are slipped into four magneti&ing coils with the strips pro*ecting beond the coils. The ends of the four strips are interleaved and clamped at corners. Lloyd fisher square
!ig.lod fisher s4uare
This is the most commonl used magnetic s4uare. whose strips are usuall 6.27 m long and 76
to 86mm wide and built up into four stac#s. 5ach stac# is made up of two tpes of strips one cut in the direction of rolling and the other cut perpendicular to the direction of rolling. The stac#s are placed inside four similar magneti&ing coils of large cross sectional area which are connected in series to form the primar winding. 5ach magneti&ing coil has two similar coils called secondar coils. Hence a magnetic s4uare has eight secondar coils which are connected in series and formed in to as two groups to form two separate secondar windings. The ends of the strips pro*ect beond the magneti&ing coil and are so arranged that the plane of each strip is perpendicular to the plane of the s4uare. These four stac#s are *oined at the corners b a set of right angled corner pieces, to form a magnetic circuit. There is an overlapping of corner pieces and strips at the corners due to which cross section of iron is doubled at the corners. Therefore the 1neasured loss should be corrected for the loss in the corner pieces. rinciple of operation The test specimen is weighed separatel and its effective cross-sectional area is determined. The !igure shows the connection diagram of wattmeter method for the determination of iron loss. 9ne end of the primar winding is connected to a sinusoidal suppl voltage through a variable ratio transformer in order to appl variable voltage to the primar winding. $n ammeter is connected in series with the primar winding in order to measure the primar current. The other end of the primar winding is connected to the current coil of the wattmeter . $s the power factor of the test is as low as 6.2, a speciall designed low power factor wattmeter is used for the measurement. Two secondar:s with e4ual number of turns are wound S1 is connected to the pressure coil of the wattmeter. The second secondar winding S2 is connected to an electrostatic voltmeter or an electrodnamic voltmeter of ver high impedance3.
!ig. ;onnection diagram of wattmeter method for the determination of iron loss
"nitiall the suppl fre4uenc is ad*usted to a correct value. The voltage applied to the primar winding is ad*usted using a variable ratio transformer, till the magneti&ing current is ad*usted to get the re4uired value of Bm and the corresponding voltmeter, ammeter and wattmeter readings are noted down. The electrostatic voltmeter connected across secondar winding S2 measures the rms value of induced voltage in S2 whose value is given b, <<.13
Since
ɸm =Bm:$s
<<<<..23
where, 5 = rms value induced voltage in secondar winding S2 in > ( f = form factor which is 1.11 for sinusoidal voltage ɸm = ma%imum value of flu% in 'b Bm: = apparent value of ma%imum flu% densit in 'bm2 $s = cross-sectional area of test specimen in m f = Suppl fre4uenc in H&. )2 = )umber of? turns in secondar winding S2 54uation23 becomes
<<.03
This apparent value of ma%imum flu% densit includes the flu% densit in the airgap between the test spcimen and secondar winding along with the actual value ma%imum flu% densit in the spcimen. Hence, $ctual value of ma%imum nu% densit = $pparent value of ma%imum flu% densit - flu% densit in the air gap Hence,
<<@3
where, B =$ctual value of ma%imum flu% densit in the test specimen in 'bm2 $c = ;ross-sectional area of the secondar winding in m2 H = Magnetising force corresponding to ma%imum flu% densit in $m obtained from the B-H curve of test specimen. The wattmeter reading includes the iron loss in the specimen and the copper loss in the secondar winding. Hence, "ron loss in the speci1nen = 'attmeter reading - copper loss in the secondar winding<..73 " = ;urrent. through the pressure coil of wattmeter which is proportional to voltage applied
to pressure coil of wattmeter i.e. "
V
∝
$s both the secondar windings have e4ual number of turns,>oltage induced in S1= >oltage induced in S2 Hence, voltage induced in S1 = 5 <<<<<<<.83 B neglecting the lea#age reactances of secondar winding S1 and pressure coil of wattmeter, we have, 5 =" p r p / r c3 <<..A3 Total iron loss in the specimen / total copper loss in the secondar winding = 5> <..3 Hence, e4uation A3, Total iron loss in the specimen, i
<<.C3
Since <<<<.163
Specific iron loss of the specimen can be calculated b dividing the total iron loss b the weight of the specimen. Specific iron loss is the iron loss#g. Specific iron loss of the specimen = Total iron loss in the specimen 'eight of the specimen
