Electrical Machines(Induction Motor) Part-06

Electrical Machines(Induction Motor):

126. “The direction of induced current in the rotor is so as to oppose the cause producing it”. This is given by

a. Faraday’s law
b. Lenz’s law
c. Electromagnetic law
d. Ampere’s law

127. If the rotor of the machine catches the speed of the rotating magnetic field, then the motor will

a. Rotate at the speed of rotating magnetic field
b. Rotate at double the speed of the rotating magnetic field
c. Eventually stop
d. None of these

128. Slip speed of the motor decides the magnitude of the induced emf and the rotor current, which in turn decides the torque produced. If N_s is the synchronous speed and N is the motor speed in rpm, then the slip speed is given by

a. N_s
b. N_s – N
c. N_s + N
d. N - N_s

129. At start, the slip of the induction motor is

a. 1
b. 0
c. 0.5
d. None of these

130. Slip of a induction motor will be zero,

a. At start
b. When machine is rotating at its maximum speed
c. When it is rotating at half of the maximum speed
d. Cannot be zero

131. A 4 pole, 3 phase, 50 Hz star connected induction motor has a full load slip of 5%. Then full load speed of the motor is

a. 1500
b. 1400
c. 1405
d. 1425

132. A 50 Hz, 3 phase slip ring induction motor, has 6 poles on stator and 4 poles on rotor. Then the machine will run at

a. 1000 rpm
b. 1500 rpm
c. 1400 rpm
d. Machine will not run at all

133. A 3 phase, 50 Hz squirrel cage induction motor, has 6 poles on stator and 4 poles on rotor. Then the machine will run at

a. 1000 rpm
b. 1500 rpm
c. 1750 rpm
d. Machine will not run

134. The rotor frequency of a three phase induction motor is

a. Slip times the supply frequency
b. 1 / slip times the supply frequency
c. Equal to the supply frequency
d. None of these

135. The rotor resistance per phase on standstill and rotor reactance per phase on standstill of induction motor is

a. Slip times the rotor resistance at standstill and slip times rotor reactance at standstill
b. Same as rotor resistance at standstill and slip times rotor reactance at standstill
c. Same as rotor resistance at standstill and same as rotor reactance at standstill
d. Slip times the rotor resistance at standstill and same as rotor reactance at standstill

136. Nature of the rotor power factor in running condition is always

a. Leading
b. Lagging
c. Both (a) and (b)
d. None of these

137. n terms of slip, the actual speed (N) of the induction motor can be expressed as

a. N_s (1-s)
b. N_s (s-1)
c. N_s / (1-s)
d. N_s / (s-1)

138. If a voltmeter when connected to the rotor of an induction motor gives 150 oscillations per minute and stator frequency is 50 Hz. Then the slip of induction motor will be

a. 3%
b. 4.5%
c. 5%
d. 5.5%

139. For a three phase induction motor, the two rotor frequencies are given by

a. sf, (1 - s) f
b. sf, (2 - s) f
c. (1 - s) f, (2 - s) f
d. Sf , sf

140. The torque produced in the induction motor depends on the following factors

1. The part of rotating magnetic field which reacts with rotor and is responsible for producing emf.
2. The magnitude of rotor current in running condition.
3. The power factor of the rotor circuit in running condition.

Which of the above statement is true?

a. Only 1
b. 1 & 2
c. 2 & 3
d. 1, 2 and 3

141. The torque produced in a three phase induction motor is

a. Directly proportional to the square of the induced emf in rotor
b. Directly proportional to the induced emf in rotor
c. Inversely proportional to the square of the induced emf in rotor
d. Inversely proportional to the induced emf in rotor

142. For maximum torque, the only parameter which controls the torque is slip. The maximum torque is

a. directly proportional to the rotor resistance
b. inversely proportional to the rotor resistance
c. directly proportional to the slip times the rotor resistance
d. independent of the rotor resistance

143. When the torque produced by the induction motor is at its maximum, the slip is given by

a. X2 / R2
b. (R2 + X2) / X2
c. R2 / X2
d. R2 / (R2 + X2)

144. A 400v, 4 pole, 3 phase, 50 Hz star connected induction motor has a rotor resistance and reactance per phase equal to 0.02 ohm and 0.2 ohm respectively. Then the slip at which maximum torque occurs and the corresponding maximum torque is (assuming stator to rotor turns ratio as 6)

a. 10%, 159.08 N - m
b. 20%, 318.16 N - m
c. 11%, 159.08 N - m
d. 10%, 318.16 N – m

145. When an induction motor is loaded from no load to full load, its speed and slip

a. Increases, decreases
b. Decreases, increases
c. Both increases
d. Both decreases

146. An induction motor when loaded from no load to full load, its slip also varies from s = 1 to s = 0. Then the torques in the low slip region and in high slip region is

a. Both are directly proportional to the slip
b. Both are inversely proportional to the slip
c. Directly proportional to the slip, inversely proportional to the slip
d. Independent of slip, proportional to the slip

147. The curve obtained by plotting torque against slip from s = 1 to s = 0 is called torque slip characteristics. The nature of the graph in the low slip region and in the high slip region is

a. Rising exponential, decaying exponential
b. Both will be straight line
c. Straight line, rectangular parabola
d. Straight line, decaying exponential

148. If an induction motor is operating at a point in the high slip region, then the motor will be

a. Stable
b. Unstable
c. Either stable or unstable depending on the torque
d. None of these

149. The value of slip for motoring, generating and braking region respectively are

a. S > 1, 0 = s = 1, s < 0
b. S < 0, 0 = s = 1, s > 1
c. 0 = s = 1, s > 1, s < 1
d. 0 = s = 1, s < 0, s > 1

150. When the slip of an induction machine is negative, it is running as a

a. Motor
b. Generator
c. Synchronous machine
d. None of these

151. If s_f is the full load slip and s_m is the slip for maximum torque, then the ratio of full load torque to the maximum torque is given by

a. 2s_fs_m / ( s_m² + s_f² )
b. 2s_f / ( s_m² + s_f² )
c. 2s_m / ( s_m² + s_f² )
d. s_fs_m / ( s_m² + s_f² )

152. If s_st is the starting slip and s_m is the slip for maximum torque, then the ratio of starting torque to the maximum torque is

a. s_m / ( s_m² + 1 )
b. 2s_m / ( s_m² + 2 )
c. 2s_m / ( s_m² + 1 )
d. 1 / ( 1 + s_m² )

153. A 16 pole, 50 Hz, star connected three phase induction motor has rotor resistance of 0.012 ohm per phase and rotor reactance of 0.220 ohm per phase at standstill. Its full load torque is at speed of 220 rpm. Ratio of its starting torque to maximum torque is

a. 0.1208
b. 0.1087
c. 0.1102
d. 0.1012

154. If the rotor resistance of an induction motor is doubled, keeping the other parameters constant, then the maximum torque of the induction motor will become

a. Double
b. Halved
c. One fourth
d. Remains same

155. “By adding external resistance to rotor till it becomes equal to rotor reactance, the maximum torque can be achieved at start”. This statement holds for

a. Slip ring induction motor
b. Squirrel cage induction motor
c. Both a and b
d. Neither a nor b