# ONLINE Multiple Choice Questions – Electric Machine Design PART 9

121. In case of thyrite the resistance is
(A) inversely proportional to the applied voltage
(B) directly proportional to the square of the applied voltage
(C) directly proportional to the cube of the applied voltage
(D) none of the above.

122. The ‘hum’ in a transformer is due to
(A) vibrations in cooling oil
(B) vibration in laminations
(C) sinusoidal voltage waveform
(D) all of the above.

123. Which test is conducted on transformers to check its ability to withstand the transient voltages due to lightening etc. ?
(A) Induced over voltage test
(B) Partial discharge test
(C) Impulse test
(D) Any of the above.

124. The density of transformer oil is around
(A)1.2gm / cc
(B) 1.0 gm/cc
(C) 0.95 gm/cc
(D)J0.80 gm/cc.

125. The flash point of transformer oil is usually
(A) 50°C
(B) 75°C
(C) 160°C
(D) 150°C.

126. The maximum permissible water content in transformer oil is
(A) zero
(B) 10 ppm
(C) 25 ppm
(D) 50 ppm.

127. The kinetic viscosity of transformer oil should be
(A) 5 cst
(B) 2.5 cst
(C) 50 cst
(D) 125 cst.

128. The maximum pour point of transformer oil should be
(A)10°C
(B) 0°C
(C) 4°C
(D) 14.4°C.

129. The dielectric strength of transformer oil’ should not be less than
(A)10kV
(B) 20 kV
(C) 30 kV
(D) 50 kV.

130. The specific resistance of the transformer oil is of the order of
(A) 104 ohms cm
(B) 106 ohms cm
(C) 1012 ohms cm
(D) 1024 ohms cm.

131. Phase spread in ac machines is almost always
(A) 90°
(B) 60°
(C)120°
(D) 50°

132. The electric motor provided on refrigerators is usually
(A) open type
(B) protected type
(C) screen protected type
(D) totally enclosed type.

133. Flame proof motors are used in
(A) explosive atmosphere
(B) moisture free atmosphere
(C) dust free atmosphere
(D) moisture and dust free atmosphere.

134. Which of the following machine is invariably provided with forced ventilation ?
(A) Fractional horse power slow speed motor
(B) Fractional horse power high speed motor
(C) Hermetically sealed motor of refrigerator
(D) 50 MW generator in a power plant.

135. Since hydrogen is 14 times lighter than air its use as cooling medium is mainly to
(A) reduces noise
(B) reduces windage losses
(C) increase maintenance
(D) none of the above.