EEL1196 Instrumentation and Measurement Techniques
Chapter 5
Tutorial 5: Magnetic Measurements
5.1
An iron ring of 3.5cm2 cross-sectional area with a mean length of 100cm is wound
with a magnetising winding of 100 turns. A secondary coil of 200 turns of wire is
connected to a ballistic galvanometer having a constant of 1 micro-coulomb per scale
division, the total resistance of the secondary circuit being 2000 ohm. On reversing a
current of 10A in the magnetising coil, the galvanometer gave a throw of 100 scale
divisions. Calculate the flux density in the specimen and the value of the permeability at
this flux density. (Answer: B = 1.43 wb/m2, µ = 1.43 × 10-3 H/m)
5.2
A magnetic circuit consists of an annular ring of mean diameter 15cm with a single
narrow saw-cut through the ring. The width of the saw-cut is 0.1cm and the cross
sectional area of the ring is 4cm2 with a magnetization curve given by the following table:
H (At/m)
40
80
120
160
800
1600
3200
2
B (Wb/m ) 0.37
0.72
0.92
1.04
1.4
1.47
1.55
A coil 0f 400 turns is uniformly wound on the ring. Estimate the current in the coil
required causing a flux of 0.28 mWb in the air gap formed by the saw-cut. (Answer: I =
1.49 A)
5.3. A fluxmeter with a single turn search coil and an area of 50cm2 shows a 100
divisions difference between two successive deflections. Assuming that the air damping
effect and the resistance, both are small:
i) calculate the change of flux linkages of the search coil, when the produced flux density
is 1 Wb/m2.
ii) It is proposed to extend the range of fluxmeter to measure flux densities 5 times the
above using the same search coil and method of reversals. If the resistance of search coil
is 1, find the resistance of the shunt to be connected in parallel with the fluxmeter.
(Answer: i) Δ = 0.5 wb, ii) Rs = 0.25Ω)
5.4
An iron ring has a mean diameter of 0.1 m and a cross section of 33.5 × 10-6 m2. It
is wound with a magnetizing winding of 340 turns and the secondary winding of 220
turns. On reversing a current of 10 A in the magnetizing winding, a ballistic
galvanometer gives a throw of 272 scale divisions, while a Hilbert Magnetic Standard
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EEL1196 Instrumentation and Measurement Techniques
Chapter 5
(Bar Sample) with 10 turns and a flux of 2.5 × 10-4 gives a reading of 102 scale divisions,
other conditions remaining the same. Find the relative permeability of the specimens.
(Answer: µr = 33.089)
5.5
A fluxmeter with 5 turns of search coil and an area of 40 cm2, is showing a 20
divisions difference between two successive deflections. Calculate the change of flux
linkages of the search coil if the produced flux density is 0.1 Wb/m2. Assume that the air
damping effect and the resistance, both are small. [Final Exam 2014] (Answer: Δ = 8
mwb)
5.6 A current of 10 A is flowing through the magnetizing winding of 314 turns that
wound on an iron ring specimen. The specimen has a mean diameter of 10 mm and a
cross section of 40 mm2. If the flux is 16 × 10-6 wb, find the relative permeability of the
specimen. (µo=4 × 10-7). [Final Exam 2015]
(Answer: µr = 10)
Self-Test Question
5.7
A magnetizing winding of 60-turns wound uniformly on an iron ring having a mean
diameter of 0.1m and a cross-sectional area of 1cm2. A secondary winding of 20-turns is
connected to a fluxmeter having a constant of 0.1×10-3 weber turn per division. When a
current of 2A is reversed in the primary winding, a deflection of 33 divisions is observed.
Determine:
(i)
the applied magnetizing force, H.
(ii)
the flux density, B.
(iii) total flux linkages in the iron ring, .
(iv) the relative permeability of the specimen, µr.
[Final Exam Trimester 2, 2016/2017]
(Answer : i)
H = 381.92 A-turns/m, ii) B= 0.825 wb/m2, iii)  =8.25×10-5 Wb, iv) µr =
1719)
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