Electrical Machines(DC Machines) :

101. Which method is suitable for the speed control, below and above the normal rated speed of d.c. shunt motor?

a. Flux control method
b. Rheostatic control method
c. Voltage control method
d. All of these

102. If a resistance is added in series with the field winding of d.c. shunt motor, then its

a. Both speed and torque decreases
b. Both speed and torque increases
c. Speed decreases, torque increases
d. Speed increases, torque decreases

103. Ward-Leonard system is used for

a. Wide range of speed control
b. Very sensitive speed control
c. Both (a) & (b)
d. None of these

104. For frequent starting, stopping and reversals, which motor is commonly used?

a. Permanent d.c. motor
b. Ward-Leonard system
c. Brushless d.c. motor
d. All of these

105. The basic difference between three point starter and four point starter is

a. No volt coil is connected in series with the field winding in three point starter while connected independently across the supply in four point starter
b. No volt coil is connected across supply in three point starter while in series with field winding in four point starter.
c. No volt coil is connected across field winding in three point starter while connected across supply in four point starter.
d. None of these

106. Which starter does not provide high speed protection to the d.c. shunt motor?

a. Three point starter
b. Four point starter
c. Two point starter
d. None of these

107. For the protection of d.c. series motor, which starter is commonly used?

a. Two point starter
b. Three point starter
c. Four point starter
d. None of these

108. The torque constant (K_m), of a permanent magnet d.c. motor is 0.25 V sec / radian and armature resistance is 2 ohm. If the applied armature voltage is 90 V, then its zero speed torque is

a. 10.45 N-m
b. 11.25 N-m
c. 13.65 N-m
d. 15.56 N-m

109. The speed of permanent magnet d.c. motor cannot be controlled by

a. Flux control method
b. Rheostatic control method
c. Electronic circuits
d. None of these

110. The number of windings on the field of permanent magnet d.c. motor is

a. More compared to shunt motor
b. Less compared to shunt motor
c. Absent
d. None of these

111. Permanent magnet d.c. motors are extensively used in

a. Automobiles
b. Heaters
c. Air conditioners
d. All of these

112. In the biomedical instruments like artificial heart pumps, the commonly used motor is

a. Permanent magnet d.c. motor
b. Brushless d.c. motor
c. Ward-Leonard system
d. Series motor

113. A machine operating as motor may go into regenerative braking mode if its speed becomes

a. Sufficiently high to make back emf greater than supply voltage
b. Sufficiently low to make back emf greater than supply voltage
c. Sufficiently high to make back emf half of the supply voltage
d. Sufficiently low to make back emf half of the supply voltage

114. In a three phase induction motor, the resultant of three alternating fluxes, separated from each other by 120 degree has constant amplitude

a. Equal to the maximum amplitude of an individual flux
b. Twice of the maximum amplitude of an individual flux
c. 1.5 times of the maximum amplitude of an individual flux
d. All of these

115. If the frequency of a three phase induction motor is doubled and number of poles is halved, then its synchronous speed is given by

a. Remains same
b. Becomes double of the original synchronous speed
c. Becomes four times of the original synchronous speed
d. Becomes half of the original synchronous speed

116. A three phase 4 pole, induction motor is supplied from 50 Hz supply. Then its synchronous speed will be

a. 1000 rpm
b. 1500 rpm
c. 1600 rpm
d. 2000 rpm

117. In a three phase induction motor, the direction of rotation of rotating magnetic field can be reversed by

a. Interchanging any two terminals of three phase winding while connecting it to ac supply
b. by reversing the direction of rotating magnetic field
c. Cannot be changed
d. None of these

118. For connecting the rotating member of machine to the external stationary circuit,

a. Slip rings and brush assembly is used
b. Only slip ring is used
c. Only brushes are used
d. Directly connected to the external stationary circuit

119. If the air gap of the induction motor increases then its power factor

a. Increase
b. Decreases
c. Remains same
d. Either increases or decreases

120. When the air gap is smaller in induction motor then its

a. magnetizing current reduces
b. starting torque increases
c. both a and b
d. neither a nor b

121. In squirrel cage rotor, bars are usually brazed to the end rings. The end rings are used to

a. Prevent rotor leakage current
b. Give reduced starting torque
c. Provide good mechanical strength
d. All of these

122. Slots of squirrel cage rotor are generally skewed to

a. Reduce magnetic hum
b. Reduce magnetic locking
c. Make motor operation smooth
d. All of these

123. The wound rotor induction motor gives

a. high stating torque
b. moderate starting torque
c. reduced starting torque
d. all of these

124. The slip ring or wound rotor induction motor find its applications in

a. lifts, cranes
b. lathes
c. drilling machines
d. printing machines