Electrical Machines - DC Machines

This are  some questions which appeared in IAS Public exam.

1. Assertion (A) : Hopkinson's test is conducted on dc machines. Reason (R) : It is a load test. [IAS-1993]
a. Both A and R are true and R is the correct explanation of A
b. Both A and R are true but R is not the correct explanation of A
c. A is true but R is false
d. A is false but R is true

2. A separately excited dc machine, having an armature resistance of 2 ohms was working on a 220 V supply and drawing 10 A armature current from the source when the supply voltage suddenly dropped to 200 V. Assuming that the field circuit source voltage remained unaffected how will the armature current of the machine react to the change? [IAS-1994]
a. It will initially rise to 11A and then settle down to 10A
b. It will fall momentarily to 9.09A and then slowly attain 10A
c. it will reduce to zero first and then settle back to 10A
d. It will remain unaffected by the change and continue to 10A

3. A dc shunt generator builds up to a voltage of 220 V at no-load while running at 18 rated speed If the speed of the generator is raised by 25% keeping the circuit conditions unaltered, then the voltage to which the machine would build up will [IAS-1995]
a. not change and remain at 220V b. Increases 1.25 times of 220V
c. increase to a value lying between 220 V and 1.25 times
d. Increases a value greater then 1.25 times of 220V

4. In a dc generator operating on load with its brushes on the geometrical neutral axis (GNA), the magnetic neutral axis (MNA) is shifted in the direction of rotation Now, if the brushes are given a lead of (electrical), then the MNA  will [IAS-1996]
a. shift forward by 90 degrees
b. shift forward by more than 90 degrees
c. shift forward by less than 90 degrees
d. coincide with the GNA

5. A synchronous machine connected to infinite bus bars is initially delivering active power to and absorbing lagging reactive power from the bus bars. As increase in the power input to the synchronous machine will cause [IAS-1997]
a. an increase in both armature current and power factor
b. an increase in armature current and a decrease in power factor
c.  a decrease in both armature current and power factor
d. a decrease in armature current and an increase in power factor

6. A shunt motor is running at its rated speed on rated load. If the field circuit gets suddenly opened.
[IAS-1998]
a. torque developed by the motor will be almost zero and the motor will stop without causing any damage to the motor
b. the motor will draw very high armature current but it will continue to drive the load at a reduced speed
c. the motor would tend to race to a high speed while driving the load
d. the motor would draw very high armature current and will stall with consequent damage to the armature. 

7. A dc shunt motor is required to drive a constant power load at rated speed while drawing rated armature current. Neglecting saturation and all machine losses, if both the terminal voltage and the field current of the machine are halved, then  [IAS-1999]
a. the speed becomes 2 pu (per unit) but armature current remains at 1 pu
b. the speed remains at 1pu but armature current becomes 2 pu
c. both speed and armature current becomes 2 pu
d. both speed and armature current remain at 1 pu

8. Which one of the following statements for a dc machine which is provided with interpole windings (IW) as well as compensating winding (CW) is correct ? [IAS-2000]
a. Both IW and CW are connected in series with the armature winding
b. Both IW and CW are connected in parallel with the armature winding
c.  IW is connected in series but CW is connected in parallel with the armature winding
d.  IW is connected in parallel but CW is connected in series with the armature winding

9. Field winding of a 4-pole separately excited d.c. machine is excited. from a 50 Hz supply. Armature is rotated in at 1500 rpm and the brush-rocker is also rotated in the same direction at the same speed. Voltage across the brushes is  [IAS-2002]
a 50 HZ a.c
b. 100 HZ a.c
c. 50 HZ a.c with d.c component
d. Zero

10. Assertion (A) : A series-generator is used as a constant-current source by making use of its drooping part of the characteristic. Reason (R) : For special applications a series DC generator is designed to have a low value of armature reaction which produces the dropping part of its external characteristic. [IAS-2004]
a. Both A and R are true and R is the correct explanation of A
b. Both A and R are true but R is not the correct explanation of A
c. A is true but R is false
d. A is false but R is true

11. To present the shifting of the magnetic neutral axis, caused by the armature reaction in a d.c machine, the most effective method. to neutralize the armature-flux is to [IAS-2005]
a. shift the 'brush-axis'
b. provide high-reluctance main pole tips
c. cut horizontal slots in the main poles
d. place compensating windings on the main-pole faces

Answers:
1.D                        11.           
2.                           12.
3.                           13.
4.                           14.
5.                           15.
6.                           16.
7.                           17.A
8.
9.
10.

Explanations :