Electrical Machine Design MCQ

101. For a pulse transformer, the material used for its core and the possible turns ratio from primary to secondary are respectively as

(a) ferrite; 20 : 1
(b) ferrite; 1 : 1
(c) laminated iron; 1 : 1
(d) powdered iron; 1 : 1. [A.M.I.E. Power Electronics 2007]

102. Normally for the design of a transformer the ambient temperature is taken as

(a) 20 °C.
(b) 25 °C.
(c) 45 °C.
(d) 65 °C.

103. A transformer is designed for certain ambient temperature. If it is operated at a temperature 10°C above the temperature for which the transformer has been designed, its kVA rating should be

(a) reduced by 20%.
(b) reduced by 10%.
(c) reduced by 5%.
(d) the same as designed.

104. Heat dissipation by radiation in oil immersed transformers with cooling tubes is

(a) about 0.6 W/m²/°C.
(b) about 2 W/m²/°C.
(c) about 10 W/m²/°C.
(d) about 50 W/m²/°C. [A.M.I.E. Design of Electrical Systems 2006]

105. The heat dissipation capacity of transformer of rating exceeding 50 kVA is increased by providing

(a) fins.
(b) tubes.
(c) radiator tanks.
(d) corrugations.
(e) all of the above.

106. Transformers with a capacity of up to…………have a cooling radiator system with natural cooling.

(a) 2 MVA
(b) 5 MVA
(c) 10 MVA
(d) 25 MVA

107. The forced oil and air circulation method is usually employed for cooling of power transformers of rating exceeding

(a) 10 MVA.
(b) 20 MVA.
(c) 30 MVA.
(d) 25 MVA.

108. Synchronous compensators are used for controlling the reactive power (kVAR) of power supply networks with rating up to about

(a) 100 MVAR.
(b) 50 MVAR.
(c) 25 MVAR.
(d) 10 MVAR.

109. In turbo-alternators the rotor diameter is limited to about 1.2 m due to

(a) centrifugal forces.
(b) deflection.
(c) critical speed.
(d) all of these.

110. The stator core of turbo-alternators is made of

(a) segmental laminations of low loss silicon steel.
(b) cold rolled non-oriented steel with about 3% silicon.
(c) cold rolled grain oriented steel with about 4% silicon.
(d) both (a) and (b).

111. Multiple inlet system of air cooling of turbo-alternators can be employed for machines of rating up to

(a) 20 MW
(b) 100 MW
(c) 60 MW
(d) 250 MW

112………cooling is most commonly used in hydro-generation.

(a) Closed circuit air
(b) Hydrogen gas
(c) Direct water
(d) Any of these

113. The diameter of hydro-generator is as large as

(a) 2.5 m
(b) 5 m
(c) 10 m
(d) 15 m

114. Turbo-alternators are designed for a maximum peripheral speed of about

(a) 140 m/s
(b) 175 m/s
(c) 275 m/s
(d) 400 m/s

115. Hydro-generators are designed for a maximum peripheral speed of about

(a) 140 m/s
(b) 175 m/s
(c) 75 m/s
(d) 250 m/s

116. The slot leakage flux is more in case of

(a) open type of slots.
(b) semi-closed slots.
(c) closed slots.
(d) open slots with aluminium conductors.

117. For a phase sequence of RYB, the phase windings under a pole for 60° phase spread are to be arranged in the order

(a) RYB
(b) BYR
(c) RBY
(d) YRB [A.M.I.E. Design of Electrical Systems 2005]

118. An ac winding has 2 slots per pole per phase. The slot harmonics will be

(a) 5th and 7th.
(b) 11th and 13th.
(c) 17th and 19th.
(d) none of the above. [Design of Electrical Systems 2005]

119. Damper winding is used to

(a) reduce air gap harmonic flux.
(b) reduce oscillations.
(c) increase stability limits.
(d) resist the moisture.

120. Use of higher specific magnetic loading (B_av) in synchronous machines is restricted due to

(a) excessive iron losses.
(b) higher short-circuit current.
(c) reduced space available for armature teeth in high voltage machines.
(d) all of the above.