1. The brushes of a dc motor, are shifted by 5′, from the main field axis, then __________
a) demagnetizing or magnetizing effect may occur depending on machine
b) cross-magnetization will be pronounced
c) both cross-magnetization as well as magnetization and demagnetization will occur
d) none of the mentioned
Explanation: When the brushes are shifted from GNA, there will either be magnetizing or demagnetizing effect will be observed.
2. A 100KW, 250 V, 400 A, a long shunt compound generator has an armature resistance of 0.025 ohms. There are 1000 shunt fields turns per pole and 3 series field turns per pole. The series field is connected in a such a fashion that positive armature current produces direct-axis MMF which adds to that of the shunt field. Compute the gross MMF at the rated terminal current when shunt field current is 4.7A and speed is 1150 rpm.
a) 5.9AT
b) 3.5AT
c) 4.7AT
d) 1.2AT
Explanation: Series field current = Is = I(l)+I(f)
= 400+4.7
= 405 A(approx)
Main field mmf = I(f)+(Ns/Nf)*I(s)
= 4.7+(3/1000)*405
= 5.9 AT.
3. Compensating windings are embedded in pole faces and having the same polarity as that of adjoining armature winding.
a) True
b) False
Explanation: Compensating windings are embedded in pole faces and having an opposite polarity as that of adjoining armature winding.
4. Compensating winding has an advantage of __________
a) eliminating flux distortion
b) importing speed of response
c) protecting overloading
d) any of the mentioned
Explanation: Any of the corrective measures will be applicable.
5. A dc shunt motor is connected to the source through 3-point starter. If the field id kept open and starter handle is moved from off to on position, then __________
a) motor will not start
b) armature will draw large current from source
c) no sparking would occur
d) all of the mentioned
Explanation: Leaving the field winding open, the machine will not start as the exciting flux is zero.
6. A dc shunt motor is running at 1000 rpm at the rated load torque. If we reverse the filed winding terminals, then the __________
a) direction of rotation reverses and commutation will be unaffected
b) direction of rotation will be same and commutation will be unaffected
c) direction of ration reverses and commutation will be affected
d) direction of ration will be same and commutation will be affected
Explanation: Reversing the field will change the direction of rotation but will not affect the commutation.
7. A dc shunt motor is running at 1000 rpm at the rated load torque. If we reverse the supply terminals, then the __________
a) direction of rotation reverses and commutation will be unaffected
b) direction of rotation will be same and commutation will be unaffected
c) direction of ration reverses and commutation will be affected
d) direction of ration will be same and commutation will be affected
Explanation: Reversing the supply in dc machine will not change the direction of rotation and will not affect the commutation too.
8. A dc shunt motor is running at 1000 rpm at the rated load torque. If few of the field windings get shot circuited then the __________
a) motor speed will increase and more armature current is drawn
b) motor speed will decrease and more armature current is drawn
c) motor speed will increase and lesser armature current is drawn
d) motor speed will decrease and lesser armature current is drawn
Explanation: If field winding get short circuited, field excitation reduces causing rise in speed and more armature current.
9. The brushes are placed __________
a) along geometrical neutral axis
b) perpendicular to geometrical neutral axis
c) along magnetic neutral axis
d) perpendicular to magnetic neutral axis
Explanation: The brushes are placed along geometrical neutral axis so that in ideal conditions commutations take place without sparking. Also placing brushes along the magnetic neutral axis requires consistent variations as load varies which is not desirable.
10. The brushes should be shifted in _______ direction in generator for the satisfactory operation of commutation.
a) forward
b) reverse to rotation
c) all of the mentioned
d) none of the mentioned
Explanation: For the generator, the electromagnetic torque produced is in the opposite direction and so the induced current. So to eliminate the sparking the brushes should be shifted in the forward direction of rotation.