Questions and answers for exam Preparation

MCQs

Total Questions : 1232 | Page 123 of 124 pages

Question 1221. A DC shunt machine generates 250 V on open circuit at 1000 RPM. Armature resistance is 0.5 Ω, field resistance 250 Ω. When running as a motor takes 4 A at 250 V. Calculate the efficiency when running as a motor taking 40 A at 250 V. Armature reaction weakens the field by 4%.

82.5%
78.5%
67.5%
72.5%

Discuss Question

Answer: Option A. -> 82.5%

Question 1222. Consider the following statements: To control the speed of a dc shunt motor above the base speed over a reasonably wide range, the motor must 1. have compensating winding 2. have interpole winding 3. be started using a 3-point starter 4. be started using a 4-point starter Of these statements

2 and 4 are correct
1, 2 and 3 are correct
1, 2 and 4 are correct
1 and 3 are correct

Discuss Question

Answer: Option C. -> 1, 2 and 4 are correct

Question 1223. For a DC machine having P no of pole pairs Z no of conductors having length L of resistivity ρ and cross section S are arranged. what is the armature resistance for lap-winding ?

PZL/4Sa²
PZL/ 4Sa
PZL/4SP²
PZL/4S

Discuss Question

Answer: Option C. -> PZL/4SP²

Question 1224. For a DC machine having P no of pole pairs, Z no of conductors having length L of resistivity ρ and cross section S are arranged. what is the armature resistance for simple wave-winding ?

PZL/4S
PZL/4Sa²
PZL/ 4Sa
PZL/4Sp²

Discuss Question

Answer: Option A. -> PZL/4S

Question 1225. A 200 V DC machine supplies 20 A at 200 V as a generator. The armature resistance is 0.2 Ω. If the machine is now operated as a motor at same terminal voltage and current but with the flux increased by 10%, the ratio of motor speed to generator speed is

0.87
1.06
0.96
0.95

Discuss Question

Answer: Option A. -> 0.87

Question 1226. A 220 ,V 15 , kW 100 rpm shunt motor with armature resistance of 0.25 , W has a rated line current of 68 A and a rated field current of 2.2 A. The change in field flux required to obtain a speed of 1600 rpm while drawing a line current of 52.8 A and a field current of 1.8 A is

36.36% increase
36.36% increase
18.18% increase
36.36% decrease

Discuss Question

Answer: Option D. -> 36.36% decrease

Question 1227. A 220 V, DC shunt motor is operating at a speed of 1440 rpm. The armature resistance is 1.0 Ω and armature current is 10 A. of the excitation of the machine is reduced by 10%, the extra resistance to be put in the armature circuit to maintain the same speed and torque will be

1.8 9 Ω
1.79 Ω
3.1 Ω
2 .1 Ω

Discuss Question

Answer: Option B. -> 1.79 Ω

Question 1228. A 240 V, dc shunt motor draws 15 A while supplying the rated load at a speed of 80 rad/s. The armature resistance is 0.5 W and the field winding resistance is 80 W. The external resistance to be added in the armature circuit to limit the armature current to 125% of its rated value is

31.1 Ω
15.1 Ω
15.9 Ω
31.9 Ω

Discuss Question

Answer: Option A. -> 31.1 Ω

Question 1229. A 240 V, dc shunt motor draws 15 A while supplying the rated load at a speed of 80 rad/s. The armature resistance is 0.5 W and the field winding resistance is 80 W. The net voltage across the armature resistance at the time of plugging will be

234 V
474 V
240 V
6 V

Discuss Question

Answer: Option B. -> 474 V

Question 1230. A dc series motor driving and electric train faces a constant power load. It is running at rated speed and rated voltage. If the speed has to be brought down to 0.25 p.u. the supply voltage has to be approximately brought down to