# ONLINE Multiple Choice Questions – Electric Machine Design PART 10

136. As compared to air, hydrogen has ___________
thermal conductivity and __________ density.
(A) lower ………. lower
(B) lower…….. higher
(C) higher……… lower
(D) higher ………. higher.

137. A machine is specified as 50 kW maximum continuous rating. The machine
(B) can be overloaded 10% only
(C) can be overloaded 20% for half an hour only
(D) can be overloaded 20% for two hours only

138. A short time intermittent rating of electric motors is considered while selecting motors for
(A) blowers
(B) punching machines
(C) reciprocating compressors
(D) sirens.

139. Short time rating machines have usually
(A) low operating times and longer shut down times
(B) low operating times and shorter shut down times
(C) longer operating times and shorter shut down times
(D) longer operating times and longer shut down times.

Questions 140 to 142 refer to data below :

The rate of temperature rise as measured from a temperature time
curve of a transformer is 0.0803 °C per minute and 00605 °C per minute,
when the temperature rise is 20.5 °C and 28.5 °C respectively.

140. The temperature rise at any time t is given by
(A) θ = θf ( 1- e -t/T )
(B) θ = θf ( 1+ e -t/T )
(C) θ = θf ( 1+ e t/T )
(D) θ = θf ( 1+ e -t/T )

141. The final steady temperature rise will be
(A) 15.5°C
(B) 35.5°C
(C) 52.5°C
(D) 72.5°C.

142. The heating time constant is
(A) 102 minutes
(B) 204 minutes
(C) 306 minutes
(D) 398 minutes.

143. A. 3 φ, 52 slot, 8 pole machine can have a coil-span of
(A) 4
(B) 71/2
(C) 6
(D) 8.

144. In case of induction motors the value of air gap flux density usually taken is
(A) 0.1 to 0.2 Wb/m2
(B) 0.35 to 0.6 Wb/m2
(C) 1.05 to 1.35 Wb/m2
(D) 2.15 to 2.55 Wb/m2.

145. Larger values of air gap flux density can be adopted while designing induction motors of
(A) larger output
(B) larger diameter of rotor
(C) both (A) and (B) above
(D) none of the above.

146. Higher values of flux density can be taken while designing machines for
(A) low voltages
(B) smaller number of poles
(C) both (A) and (B) above
(D) none of the above.

147.. A 5 HP cage induction motor has power factor
of 0.75 at 750 rpm. A motor of 100 HP at 750 rpm can be expected to
have a power factor of
(A) 0.88
(B) 0.75
(C) 0.72.

148. An induction motor of 100 HP at 750 rpm is
expected to have power factor of 0.88. A 100 HP induction motor at 1500
rpm can be expected to have power factor of
(A) 0.91
(B) 0.88
(C) 0.81.

149. An induction motor of 10 HP at 750 rpm has
efficiency of 83%. An induction motor of 100 HP at 750 rpm can be
expected to have an efficiency of
(A) 81%
(B) 83%
(C) 90%
(D) 99.9%.