Fundamental Electrical and Electronic Principles, Third Edition

194 Fundamental Electrical and Electronic PrinciplesAssignment Questions35 Two air-cored coils, A and B, are wound with100 and 500 turns respectively. A current of 5 Ain A produces a flux of 15 μ Wb. Calculate (a)the self-inductance of coil A, (b) the mutualinductance, if 75% of the flux links with B, and(c) the emf induced in each of the coils, whenthe current in A is reversed in a time of 10 ms.36 Two coils, of self-inductance 50 mH and 85 mHrespectively, are placed parallel to each other.If the coupling coefficient is 0.9, calculate theirmutual inductance.37 The mutual inductance between two coilsis 250 mH. If the current in one coil changesfrom 14 A to 5 A in 15 ms, calculate (a) the emfinduced in the other coil, and (b) the change offlux linked with this coil if it is wound with 400turns.38 The mutual inductance between the twowindings of a car ignition coil is 5 H. Calculatethe average emf induced in the high tensionwinding, when a current of 2.5 A, in the lowtension winding, is reduced to zero in 1 ms. Youmay assume 100% flux linkage between thetwo windings.39 Sketch the circuit symbol for a transformer,and explain its principle of operation. Why isthe core made from laminations? Is the corematerial a ‘ hard ’ or a ‘ soft ’ magnetic material?Give the reason for this.40 A transformer with a turns ratio of 20:1 has240 V applied to its primary. Calculate thesecondary voltage.41 A 4 :1 voltage ‘ step-down ’ transformer isconnected to a 110 V a.c. supply. If the currentdrawn from this supply is 100 mA, calculate thesecondary voltage, current and power.42 A transformer has 450 primary turns and 80secondary turns. It is connected to a 240 Va.c. supply. Calculate (a) the secondaryvoltage, and (b) the primary current when thetransformer is supplying a 20 A load.43 A coil of self-inductance 0.04 H has a resistanceof 15 Ω . Calculate the energy stored when it isconnected to a 24 V d.c. supply.44 The energy stored in the magnetic field of aninductor is 68 mJ, when it carries a current of1.5 A. Calculate the value of self-inductance.45 What value of current must flow through a 20 Hinductor, if the energy stored in its magneticfield, under this condition, is 60 J?