Electrical Machine Design MCQ

81. No-load current in a large sized transformer is about

(a) 10% of full-load current.
(b) 7% of full-load current.
(c) 4% of full-load current.
(d) 1% of full-load current.

82. Typical value of per unit impedance for a transformer is

(a) 0.2
(b) 0.1
(c) 0.05
(d) 0.01 [A.M.I.E Sec B. 2006]

83. If the total iron loss in a transformer at full load is 600 W, the total iron loss at 25% of overload will be

(a) 600 W
(b) 750 W
(c) 1,000 W
(d) 400 W [A.M.I.E Sec B. 2006]

84. For minimum copper loss in the transformer windings

(a) the primary and secondary currents should be equal.
(b) the current densities in primary and secondary windings must be the same.
(c) copper losses should be equal to iron losses.
(d) none of the above.

85. Stacking factor will be minimum for…………core.

(a) four stepped
(b) three stepped
(c) square
(d) cruciform

86. In a transformer, the emf per turn is determined, in terms of its kVA output rating S from the relation

(a) E_t = KS.
(b) E_t= k√S.
(c) E_t = kS^3/2.
(d)E_t =k/√S.

87. The usual value of maximum flux densities for distribution transformers using hot rolled silicon steel are

(a) 0.5 to 0.75 T.
(b) 0.75 to 1.1 T.
(c) 1.1 to 1.4 T.
(d) 1.5 to 2.0 T.

88. Suitable value of flux density for design of large transformer is

(a) 1.1 T
(b) 1.4 T
(c) 1.6 T
(d) 1.9 T [A.M.I.E Sec B. 2006]

89. An iron cored transformer is working at a maximum flux density of 0.8 Wb/m². Its core is replaced by silicon steel core, working at a maximum flux density of 1.2 Wb/m². If the total flux is to remain the same, what is the reduction in volume expressed as a percentage of the original volume? The frequency and voltage per turn are the same in both cases.

(a) 33%
(b) 9%
(c) 22%
(d) 11% [I.E.S. E.E.-II, 2007]

90. The usual values of current densities for medium and large power transformers are

(a) 1.5 to 2.6 A/mm².
(b) 2.4 to 3.4 A/mm².
(c) 5.4 to 6.3 A/mm².
(d) 6.5 to 8.5 A/mm².

91. A transformer is considered to have a network of series capacitance due to

(a) laminated core.
(b) earthling.
(c) inter turn insulation.
(d) surrounding tank.

92. In a transformer, the series capacitance is increased and shunt capacitance is reduced by using………..winding.

(a) layer type
(b) shielded
(c) inter leaved
(d) any of these

93. ……….surge is the most damaging for a transformer.

(a) Steeper front flatter trail
(b) Flatter front steeper trail
(c) Uniform
(d) All will be equally damaging

94. The surge phenomenon is most important in case of……….voltage transformers.

(a) low
(b) medium
(c) high
(d) any of these

95. The tap changing facility is usually provided on………..transformers.

(a) high voltage
(b) distribution
(c) current
(d) all of these

96. Tappings of a transformer are provided

(a) at the phase end of lv side.
(b) at the phase end of hv side.
(c) at the neutral end of hv side.
(d) at the middle of the hv side. [A.M.I.E. Elec. Design 1993]

97. Under short-circuit conditions the windings of a transformer having 5% impedance will experience a radial force

(a) 5 times of full load value.
(b) 20 times of full load value.
(c) 25 times of full load value.
(d) 400 times of full load value. [A.M.I.E. Elec. Design 1993]

98. Addition of tubes to the transformer tank improves heat dissipation capacity because of

(a) additional cooling surface.
(b) additional dissipation by radiation only.
(c) additional dissipation by convection only.
(d) additional dissipation by radiation and convection both. [A.M.I.E. Elec. Design 1993]

99. In transformers, mechanical forces are developed due to

(a) interaction of currents flowing in the winding and leakage flux around it.
(b) vibrations.
(c) gap between laminations.
(d) none of the above.

100. The overload capacity of a transformer depends on

(a) supply frequency.
(b) core size.
(c) both (a) and (b).
(d) none of these.