DC MACHINES 14

131. Rosenberg generator is a
a) constant voltage generator at low speeds and voltage polarity is independent of the direction of rotation.
b) a constant voltage generator at low speeds and voltage polarity changes with the direction of rotation.
c) a constant current generator at high speeds and voltage polarity is independent of the direction of rotation.
d) a constant current generator at high speeds and voltage polarity changes with the direction of rotation.
ans. c

132. If the number of poles in the first stage of a Rototrol is 2, the number of poles in the second stage is
a) 2
b) 4
c) 6
d) 8
ans. c

133. The inductor of a 3-wire generator should be iron-cored to
a) increase the current through it
b) reduce the current through it
c) reduce the voltage ripple in the output circuit
d) improve the voltage regulation of generator.
ans. b

134. In a dc machines, the armature windings are placed on the rotor because of the necessity for
a) electromechanical energy conversion
b) generation of voltage
c) communication
d) development of torque
ans. c

135. In dc machines , the space distribution of air-gap flux density wave at no load is
a) sinusoidal
b) co-sinusoidal
c) flat-topped
d) rectangular
ans. c

136. A commutator in dc machines can convert
a) ac to dc
b) dc to ac
c) both ac to dc and dc to ac
d) none of these
ans. b

137. In dc machines the space waveform for the air-gap flux distribution effects
a) torque but not the voltage
b) voltage but not the torque
c) neither the voltage nor the torque
d) both the torque and voltage
ans. c

138. In dc machines, the axis of armature mmf is along the
a) direct axis
b) interpolar axis
c) midway between the direct and interpolar axes
d) none of these
ans. c

139. A 4-pole dc generator is running at 1500 rpm. The frequency of current in the armature winding is
a) 25 Hz
b) 50 Hz
c) zero Hz
d) 100 Hz
ans. b

140. The direction of rotation of a dc shunt motor can be reversed by interchanging
a) the supply terminals
b) the field terminals only
c) the armature terminals only
d) either field or the armature terminals
ans. d