Electrical Machines Questions and Answers_pag63

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b) 960 rad/s 
c) 320 rad/s 
d) 490 rad/s 
View Answer 
Answer: a 
Explanation: v = V/(2*r*l*B) 
= 120/(2*0.0005*1*0.25) 
 = 480 rad/s. 
8. If the torque is in the direction of rotation, the DC machine acts as 
a) generator 
b) motor 
c) amplidyne 
d) any of the mentioned 
View Answer 
Answer: a 
Explanation: The electromagnetic torque generated in the dc generator is in the direction in 
which the prime mover is rotated.It is produced due to the action of armature current and stator 
field. 
9. The cogging torque is absent in the permanent magnet dc machine is due to 
a) non-magnetic nature of rotor 
b) magnetic nature of rotor 
c) absence of ac supply 
d) any of the mentioned 
View Answer 
Answer: a 
Explanation: It is due to the fact that no field poles are induced in PMDC to create magnetic flux 
leading to harmonics. 
10. DC Motor torque depends on 
a) geometry 
b) magnetic properties 
c) any of the mentioned 
d) both geometry and magnetic properties of the structure 
View Answer 
Answer: d 
Explanation: The torque will indeed depend on the geometry as well the magnetic materials 
used in the construction of the dc machine. Though this remains constant with the machine. 
11. For a universal motor, the rated supply current is 10A giving a rated torque of 100 N-m. If a 
current of 5A is applied, what will be generated torque vary to? 
a) 250 N-m 
b) 400 N-m 
c) 500 N-m 
d) 1000 N-m 
View Answer