MAGNETIC MEASUREMENTS

• The measurement of different magnetic phenomena is known as magnetic measurements

• The magnetic phenomena to be measured are – Flux

– Flux density

– Magnetising force

– Perrmeability

– B-H Curve

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TYPES OF TESTS

• Study of magnetic measurements are classified as:

– DC tests or ballistic tests

– AC tests – done at high frequencies

– Steady state tests – to measure air gap flux

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DC OR BALLISTICS TESTS

• For determining

– Flux/ flux density of a magnetic specimen

– B-H Curves

– Plotting Hysteresis loop

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MEASUREMENT OF FLUX/FLUX DENSITY

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MEASUREMENT OF FLUX/FLUX DENSITY

• Ring specimen is wound with a coil

• Coil carries current I

• Coil is connected in series with resistance R and calibrating coil to BG – Ballistic Galvanometer

• Current through the coil is reversed so that an emf is developed in the coil

• That emf is measured by BG

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MEASUREMENT OF FLUX/FLUX DENSITY

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MEASUREMENT OF FLUX/FLUX DENSITY

• Θ – throw of galvanometer

• Gq- galvanometer constant

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MEASUREMENT OF MAGNETISING

FORCE (H)

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MEASUREMENT OF MAGNETISING FORCE (H)

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MEASUREMENT OF MAGNETISING FORCE (H)

• Magnetizing force can be determined by measuring H on the surface of material

• It is due to tangential forces on the surface are equal in magnitude on both sides

• Coil is position to measure flux density B

• If permeability is known then H can be found

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Determination of B-H Curve

• Two methods:

– Method of reversals

– Step by step method

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Determination of B-H Curve by METHOD OF REVERSALS

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Determination of B-H Curve by METHOD OF REVERSALS

Construction

• Ring shaped specimen is used

• Thin tape is put on ring

• Coil is insulated by paraffined wax and is uniformly wound over the tape

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Determination of B-H Curve by METHOD OF REVERSALS

Working

• Specimen is demagnetised

• Magnetising current I is set to small vlue

• BG switch K is closed and switch S is operated 20 times backward and forward – to bring the specimen to reproducible magnetic state

• Key K is opened

• Value of flux corresponding to reproducible magnetic state ie H is taken

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Determination of B-H Curve by METHOD OF REVERSALS

Working

• Value of B can be formulated by knowing relative permeability and value of H

• Flux is the ration of B (flux density) to area ofspecimen

• The above steps are repeated by various values of H upto maximum testing point

• B-H Curve is plotted from values of B and H

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Determination of B-H Curve by STEP BY STEP METHOD

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Determination of B-H Curve by STEP BY STEP METHOD

• No reversal of magnetizing current

• Current is given to the coil by a potential divider having number of tappings

• By varying the position of tapping, current I can be varied so H can be increased upto desired maximum value

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Determination of B-H Curve by STEP BY STEP METHOD

Working

• Initially switch S1 is open

• Tapping is put at position 1

• Key K is open

• Switch S1 is closed

• Deflection in the galvanometer is noted – B(flux density)

• Current I is also noted

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Determination of B-H Curve by STEP BY STEP METHOD

Working

• Procedure is repeated in taping 2, 3, etc…

• Value of B and current is noted

• 𝐻 =𝑁𝐼

𝑙

• By knowing value of current I, H is found

• From the above values B-H Curve is plotted

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Determination of B-H Curve by STEP BY STEP METHOD

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COMPARISON OF BAR AND RING SPECIMENS

Ring specimen Bar specimen

Ballistic tests gives high accuracy Ballistic tetss gives less accuracy

End effects are absent End effects are present

Free from errors due to magnetic leakage Errors are there due to magnetic leakage

Uniform winding should be there Uniform winding is easy

Magnetic flux density is not same at inner and outer portions

Uniform magnetic flux density all over the specimen

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PERMEAMETERS

• Permeameters are instruments used to plot B-H

curves of straight magnetic specimens

• It gives true value of H in the relation

𝐵 = μ𝐻

• It generally consists of steel frame to which bar

type magnetic specimen is connected

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PERMEAMETERS

Perm

eam

eter

s Hopkinson

permeameter

Ewing double bar permeameter

Illovici Permeameter

Burrow’s permeameter

Fahy’s Simplex Permeameter

National physical laboratory (England) form Permeamter

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Hopkinson’s Permeameter (Bar and Yoke Method)

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Hopkinson’s Permeameter (Bar and Yoke Method)

• Test coil is wound on central part of specimen

• Bar is clamped between two halves of massive iron yoke

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Hopkinson’s Permeameter (Bar and Yoke Method)

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Hopkinson’s Permeameter (Bar and Yoke Method)

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Hopkinson’s Permeameter (Bar and Yoke Method)

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Hopkinson’s Permeameter (Bar and Yoke Method)

• Flux density B is measured by Ballistic Galvanometer

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EWING DOUBLE BAR PERMEAMETER

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EWING DOUBLE BAR PERMEAMETER

• Two similar bar specimens under test

• Two pairs of magnetisng coils, one pair exactly half length of other, axial length is same

• Bar specimens are fixed into two yokes

• Position of yokes on the bars can be adjusted

• Tests are done at length l and l/2

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EWING DOUBLE BAR PERMEAMETER

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EWING DOUBLE BAR PERMEAMETER

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EWING DOUBLE BAR PERMEAMETER

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EWING DOUBLE BAR PERMEAMETER

• Flux density B is measured by ballistic galvanometer

• B-H curves are plotted for full and half lenghts

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BURROW’S PERMEAMETER

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BURROW’S PERMEAMETER

• S1- bar specimen to be tested

• S2- similar specimen

• M1,M2- magnetising windings

• A1,A2, B2,B2- compensating windings for elimination of leakage at joints

• C, C’- similar search coils

• D1, D2 – search coils having half number of turns of C, C’

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BURROW’S PERMEAMETER

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BURROW’S PERMEAMETER Working

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BURROW’S PERMEAMETER

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FLUXMETERS

• A special type of ballistic galvanometer

– Very small control torque

– High electromagnetic damping

• Grassot fluxmeter

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FLUXMETERS Grassot flux meter

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FLUXMETERS Grassot flux meter

Construction

• Small cross section carrying coil C

• Cross section is suspended in narrow air gap of permanent magnet

• Current is injected to the coil through annealed silver springs

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FLUXMETERS Grassot flux meter

Construction

• Pointer is fixed to moving system

• Scale of pointer is calibrated in terms of flux turns

• Flux meter is designed by Grassot

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FLUXMETERS Grassot flux meter

• Spirals of silver spring makes controlling torque minimum

• Since controlling torque is minimum, pointer takes some time to come back to zero position

• Readings are obtained by taking by the deflection between initial and final positions of pointer

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FLUXMETERS Grassot flux meter

• Resistance of coil is minimum

• Inductance of coil is large

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FLUXMETERS Grassot flux meter

• Let ϕ1 − 𝑖𝑛𝑡𝑒𝑟𝑙𝑖𝑛𝑘𝑖𝑛𝑔 𝑓𝑙𝑢𝑥 𝑖𝑛 𝑖𝑛𝑖𝑡𝑖𝑎𝑙

position

ϕ2 − 𝑖𝑛𝑡𝑒𝑟𝑙𝑖𝑛𝑘𝑖𝑛𝑔 𝑓𝑙𝑢𝑥 𝑖𝑛 𝑓𝑖𝑛𝑎𝑙

position

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FLUXMETERS Grassot flux meter

• Let Ɵ1 − 𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑑𝑒𝑓𝑙𝑒𝑐𝑡𝑖𝑜𝑛

Ɵ2 − 𝑓𝑖𝑛𝑎𝑙 𝑑𝑒𝑓𝑙𝑒𝑐𝑡𝑖𝑜𝑛

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FLUXMETERS Grassot flux meter

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FLUXMETERS Grassot flux meter

Advantages

• Deflection is same irrespective of change in flux

• Portable

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Differences

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