Electrical Machines(Induction + Synchronous Machines ) Part-08

Electrical Machines(Induction + Synchronous Machines ) :

Q-186. In double cage induction motor, the rotor consists of two cages or two layers of bars short circuited by end rings. The upper cage and lower cage respectively has

a. High resistance and low reactance, high resistance and low reactance
b. High resistance and low reactance, low resistance and high reactance
c. Low resistance and high reactance, high resistance and low reactance
d. Low resistance and high reactance, low resistance and high reactance

Q-187. When the induction motor runs faster than the synchronous speed, the induction motor runs as

a. Asynchronous generator
b. Induction generator
c. Synchronous motor
d. Such condition is not possible

Q-188. The induction generator is

a. Self exciting
b. Separately exciting
c. Both (a) and (b)
d. None of these

Q-189. Which of the following is not true about induction generator?

a. Synchronization for induction generator is required
b. Induction generators are more suitable for high speeds
c. With the help of excitation supply and frequency, the voltage and frequency of induction motor are controlled
d. The construction is rugged for rotating parts

Q-190. While drawing the graph of no load losses versus voltage, the voltage is extrapolated to V = 0. The extrapolated region shows

a. Copper losses
b. Core losses
c. Friction and windage losses
d. None of these

Q-191. Applications of induction generators are

a. In railways for braking purpose
b. Wind mils
c. Drilling machines
d. Only (a) and (b)
e. None of these

Q-192. While using stator resistance starter with 3 phase induction motor, the resistances of the starter are kept at

a. Maximum
b. Minimum
c. Half of the maximum value
d. None of these

Q-193. When stator resistance starter is used, the factor by which stator voltage reduces is say x. If x<1, then due to stator resistance starter, the starting torque

a. Increases by fraction x
b. Reduces by fraction x ∧ 2
c. Reduces by fraction x
d. Increases by fraction x ∧ 2

Q-194. An autotransformer starter is suitable for

a. Star connected induction motor
b. Delta connected induction motor
c. Both (a) & (b)
d. None of these

Q-195. The cheapest starter for induction motor is

a. Stator resistance starter
b. Autotransformer starter
c. Star-delta starter
d. Rotor resistance starter

Q-196. Windings of star-delta starter while starting and during running are connected in

a. Star, delta
b. Delta, delta
c. VStar, star
d. Delta, star

Q-197. The advantages of star-delta starter over other types of starter is

a. Cheapest of all
b. Maintenance free
c. Both (a) & (b)
d. None of these

Q-198. When rotor resistance starter is used with induction motor then

a. Only starting current is limited
b. Only starting torque is limited
c. Both starting current and starting torque are limited
d. Neither starting current nor starting torque is limited

Q-199. Direct online starter also called D.O.L. starter is used for motors having capacity

a. Less than 5 h.p.
b. Less than 10 h.p.
c. Greater than 10 h.p.
d. For any capacity motor

Q-200. The NO contact and NC contact of D.O.L. starter is normally

a. Open, closed
b. Closed, open
c. Open, open
d. Closed, closed

Q-201. A machine without commutator, providing an ac emf to the external circuit is called as

a. D.C. generator
b. Alternator
c. Synchronous motor
d. Transformer

Q-202. Practically, most of the alternators prefer which type of construction?

a. Rotating field type
b. Rotating armature type
c. Both are equally important
d. None of these

Q-203. In synchronous machine poles are made up of thick steel laminations. In salient pole type rotor

a. Poles are projected out from the surface of the rotor
b. The unslotted portions of the cylinder itself acts as the poles
c. Both (a) and (b)
d. None of these

Q-204. Salient pole type rotors are generally used for the range of 125 rpm to 500 rpm. The limiting factor for the size of the rotor is the

a. Rotating field type construction
b. Rotating armature type construction
c. Centrifugal force acting on the rotating member
d. All of these

Q-205. The prime movers used to drive projected pole type alternator is

a. Water turbines
b. I.C. engines
c. Both (a) and (b)
d. None of these

Q-206. Separate damper winding is necessary in

a. Smooth cylindrical type rotor
b. Salient pole type rotor
c. Both (a) and (b)
d. None of these

Q-207. Generally, in alternator divided winding rotor is preferred to a conventional winding rotor, because it gives

a. Better damping
b. Less harmonics
c. More efficiency
d. None of these

Q-208. The air gap in a salient pole machine is

a. Maximum at the center of poles
b. Least at the center of poles and increases while moving away from the center
c. Maximum at the center and decreases while moving away from the center
d. Equally distributed

Q-209. The diameter and axial length of rotors of smooth cylindrical type rotors are

a. Small diameter, small axial length
b. Large diameter, large axial length
c. Large diameter, small axial length
d. Small diameter, large axial length

Q-210. Which type of rotor is generally used for high speed alternators or turbo alternators?

a. Salient pole type
b. Projected pole type
c. Smooth cylindrical type
d. Both (a) & (b)

Q-211. The phasor addition of stator and rotor mmfs is possible in a smooth cylindrical synchronous generator because

a. Mmfs of stator and rotor are stationary with respect to each other
b. Mmf of stator is stationary and that of rotor is rotating
c. Mmf of stator is rotating and that of rotor is stationary
d. None of these

Q-212. The prime movers which are used to drive non projected type rotors are

a. I.C. engines
b. Electric motors
c. Steam turbines
d. Both (b) & (c)

Q-213. In the brushless excitation system of synchronous generator, the feedback of output voltage of alternator is taken and given to the

a. Rotating armature a.c. excitor
b. Diode rectifier
c. Main alternator field
d. Magnetic amplifier

Q-214. When we plot the magnitude of the induced emf against the time, the nature of the induced emf will be

a. Sinusoidal
b. Square
c. Triangular
d. Parabolic

Q-215. During the one complete revolution of the conductor in a 4 pole synchronous generator, induced emf will experience

a. Two times maxima and two times zero
b. Two times maxima and four times zero
c. Eight times maxima and eight times zero
d. Four times maxima and four times zero