Electrical Machines(Induction Motor) Part-07

Electrical Machines(Induction Motor) :

156. A 4 pole, 50 Hz, wound rotor inductor is supplied at rated voltage and frequency with slip ring open circuited. If the rotor is driven by an external means at 1500 rpm opposite to the direction of stator field, the frequency of voltage across slip rings will be

a. 50 Hz
b. 80 Hz
c. 100 Hz
d. 0 Hz

157. The approximate value of torque angle at which maximum pull out torque will occur under practical circumstances is

a. 90 degree
b. 75 degree
c. 60 degree
d. 30 degree

158. f P₂ is the rotor input power, P_m is the gross mechanical power developed and s is the slip of induction motor, then the gross mechanical power developed is

a. S P₂
b. (1 - s) P₂
c. P₂ / s
d. P₂ / (1 - s)

159. For slip s of an induction motor, the ratio of rotor copper loss to the gross mechanical output is

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

160. The torque equation of an induction motor is given as
ksE₂²R₂ / R₂² + (sX₂)²
The value of k in the above equation is,

a. 3 / (2 Π n_s)
b. 2 / (Π n_s)
c. 2 / (3 Π n_s)
d. 3 / (Π n_s)

161. While delivering an useful power of 20 kW to the full load, a 3 phase, 50 Hz, 6 pole induction motor draws a line current of 50 A. it is connected to supply of 420 V and runs at a speed of 950 rpm. If mechanical losses are 1000 W, then its rotor copper losses and shaft torque are

a. 1105.26 W, 201.03 N-m
b. 1125.26 W, 201.03 N-m
c. 1105.26 W, 208.09 N-m
d. 1125.26 W, 208.09 N-m

162. A 4 pole, 25 kW, three phase, 420 V, 50 Hz induction motor operates at an efficiency of 90% with a power factor of 0.8 lagging. The current drawn by the motor from the mains is

a. 42.89 A
b. 45.56 A
c. 47.73 A
d. None of these

163. In a three phase induction motor, the mechanical load should be such that the equivalent load resistance referred to stator is equal to the

a. Total leakage reactance of the motor referred to stator
b. Total leakage reactance of the motor referred to rotor
c. Total leakage impedance of the motor referred to stator
d. Total leakage impedance of the motor referred to rotor

164. Torque of an induction motor can sometimes be given in synchronous - watt. One synchronous - watt is equal to

a. 60 / (2 Π N_s) N - m
b. (2 Π N_s) / 60 N - m
c. 120 / (2 Π N_s) N - m
d. (2 Π N_s) / 120 N – m

165. When an induction motor is working under loaded condition, its induced voltage will

a. Leads the magnetic flux by 90 degree
b. Lags the magnetic flux by 90 degree
c. Lags the magnetic flux by 30 degree
d. In phase with the magnetic flux

166. The performance of induction motor is effected by the harmonics in the

a. Space variation of impressed voltage
b. Time variation of impressed voltage
c. Space variation of impressed current
d. Time variation of impressed current

167. The tooth or slot harmonics in an induction motor is caused due to

a. Variation of air gap reluctance
b. Leakage flux
c. Non sinusoidal nature of input voltage
d. None of these

168. The air gap flux of induction motor does not contain

a. 2^nd harmonics
b. 3^rd harmonics
c. 5^th harmonics
d. 7^th harmonics

169. A 50 Hz, 3 phase induction motor with 4 pole stator is supplied by mains, which contains a pronounced fifth time harmonic. The speed of the fifth space harmonic will be

a. 100 rpm
b. 300 rpm
c. 600 rpm
d. 7500

170. Crawling in an induction motor is caused

a. During starting of an induction motor
b. At the fraction of rated speed
c. Due to insulation failure
d. All of these

171. Cogging in an induction motor is caused

a. If the number of stator slots are integral multiple of rotor slots
b. If the motor is running at fraction of its rated speed
c. Due to fifth harmonic
d. All of these

172. Crawling is a phenomena mainly associated with

a. 3^rd harmonic
b. 5^th harmonic
c. 7^th harmonic
d. 2^nd harmonic

173. What happens if the number of stator slots in an induction motor is equal to the number of rotor slots or integral multiple of rotor slots?

a. Motor will run at very high speed
b. Motor will run at very low speed
c. Motor will fail to start
d. Does not get effected

174. The crawling and cogging is not predominant in

a. Slip ring induction motor
b. Squirrel cage induction motor
c. Both (a) and (b)
d. None of these

175. The Y - axis and X - axis in a circle diagram represents

a. Voltage and current
b. Current and power
c. Power and voltage
d. Current and voltage

176. The circle diagram can be used to

a. Predict the performance of an induction motor under various load condition
b. Find losses in an induction motor
c. Efficiency of an induction motor
d. All of these

177. For plotting circle diagram no load power factor is required. The value of no load power factor will be

a. Lesser than 0.5
b. Greater than 0.5
c. Equal to 0.5
d. 1

178. For conducting no load test on an induction motor two wattmeter’s are required. Readings of the meters

a. Both will be positive
b. One will be positive and other will be negative
c. Both will be negative
d. None of these

179. If I_c is the active component and I_m is the magnetizing component of no load current (I_o) obtained in the no load test of an induction motor. Then the no load branch resistance and no load branch reactance is

a. V_o / I_c , V_o / I_m
b. V_o / I_m , V_o / I_c
c. V_o / I_o , V_o / I_c
d. V_o / I_c , V_o / I_o

180. In the blocked rotor tests, slip is equal to

a. 0
b. 1
c. 0.5
d. 0.707

181. In a slip ring induction motor, the rotor resistance is

a. High during starting and low at normal operating speed
b. Low at starting and high at normal operating speed
c. Remains same during starting and normal operating speed
d. None of these

182. Deep bar or double cage rotors are used in squirrel cage induction motor to obtain

a. High starting current
b. High starting torque
c. Reduced starting current
d. Reduced rotor resistance

183. Deep bar or double cage rotors make use of skin effect. The skin effect depends upon

a. Nature of material
b. Diameter of wire
c. Shape of wire and frequency
d. All of these

184. In deep bar induction motor, the rotor consists of deep bars, short circuited by two end rings one on each side. The leakage inductance of bottom strip

a. Greater than that of top strip
b. Lesser than that of top strip
c. Equal to that of top strip
d. None of these

185. The skin effect in a deep bar induction motor is maximum when rotor is

a. At standstill
b. Running at its maximum speed
c. Running at no load
d. Running at half load