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Determination of B-H curve

The two methods available for the determination of B-H curve of a ring specimen are,

1. Method of reversal

2. Step b step method

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.  )ow the #e ( is opened and the value of flu% corresponding to this value of H can be

measured from the galvanometer deflection *ust b reversing the reversible switch.

!ig. +etermination of B-H curve b the method of reversal

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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

  is the flu% measured in 'b.ɸ

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 thegalvanometer.

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&ingforce is then increased to H2  b suddenl switching S2  to tapping 2, and the

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corresponding increase in flu% densit B can be determined from the deflection of the

galvanometer.

!lu% densit B2 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.

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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 s4uaresare

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

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!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 othercut 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 lossin 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.

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