Objective Type Questions for SSC JE Electrical

1. Which one of the following is correct ?

(a) The effect of field current on main flux of a synchronous machine is called armature reaction
(b) The effect of air gap flux on armature current of a synchronous machine is called armature reaction
(c) The effect of armature current on main flux of a synchronous machine is called armature reaction
(d) The effect of armature current on air gap flux of a synchronous machine is called armature reaction

2. A lap wound DC generator having 250 armature conductor runs at 1200 rpm. If the generator emf is 200 V, then the operating flux of the DC generator is

(a) 0.02 Wb
(b) 0.08 Wb
(c) 0.04 Wb
(d) 0.06 Wb

3. The highest speed attainable by DC shunt motor is

(a) equal to no-load speed
(b) much higher than no-load speed
(c) much lower than no-load speed
(d) ideally infinite

4. The direction of rotation of a DC shunt motor can be reversed by interchanging

(a) the field terminals only
(b) the armature terminals only
(c) either field or armature terminals
(d) the supply terminals

5. AC series motors are built with as few turns as possible to reduce

(a) speed
(b) flux
(c) reactance
(d) iron losses

6. Based on revolving field theory, the forward and backward frequencies of the rotor emf of a 4-pole, 50 Hz, single-phase induction motor when running at 1300 rpm in the same direction of the forward field are respectively

(a) 6.67 Hz, 93.33 Hz
(b) 107.69 Hz, 7.69 Hz
(c) 93.33 Hz, 6.67 Hz
(d) 7.69 Hz, 107.69 Hz

Answer: (a) 6.67 Hz, 93.33 Hz

7. Three lamps are in circuit as shown in figure. At what condition 100 W lamp will have the maximum brightness?

(a) Key K₁ is open, K₂ is closed and K₃ is open
(b) K₁ is closed, K₂ is open and K₃ is also open
(c) Both (a) and (b)
(d) Key K₁ is closed, K₂ is open and K₃ is closed

8. Given two coupled inductors L₁ and L₂ having their mutual inductance M. The relationship among them must satisfy

Answer:

9. If the length of a bar of magnetic material is increased by 20% and the cross-sectional area is decreased by 20% then the reluctance is

(a) increased by 67%
(b) increased by 50%
(c) remaining same
(d) decreased by 33%

Answer: (b) increased by 50%

10. Two coupled inductors L₁ = 0.2 H and L₂ = 0.8 H, have coefficient of coupling K = 0.8, The mutual inductance M is

(a) 0.24 H
(b) 0.16 H
(c) 0.02 H
(d) 0.32 H

11. A coil with a certain number of turns has a specified time constant. If the number of turns is doubled, its time constant would

(a) become doubled
(b) become four fold
(c) get halved
(d) remain unaffected

12. The iron loss per unit frequency in a ferromagnetic core, when plotted against frequency, is a

(a) Constant
(b) Straight line with positive slope
(c) Straight line with negative slope
(d) Parabola

Answer: (b) Straight line with positive slope

13. The mutual inductance between two closely coupled coils is 1 H. If the turns of one coil is decreased to half and those of the other is doubled, the new value of the mutual inductance would be

(a) 1/2 H
(b) 1/4 H
(c) 1 H
(d) 2 H

Answer: (c) 1 H

14. Following graph shows the loss characteristics of a sheet of ferromagnetic material against varying frequency f. P_i  is the iron loss at frequency f, Hysteresis and eddy current losses of the sheet at 100 Hz are

(a) 1 W, 10 W
(b) 10 W, 100 W
(c) 10 W, 50 W
(d) 1 W, 5 W

Answer: (a) 1 W, 10 W

15. Hysteresis losses are present in iron core coil when

(a) the current in the coil is sinusoidal only
(b) the current in the coil is alternating
(c) the current is unsymmetrical alternation only
(d) the current in the coil is DC only

16. Eddy current loss in ferromagnetic core is proportional to

(a) reciprocal frequency
(b) square of frequency
(c) square root of frequency
(d) frequency

17. Two inductors have self inductances of 9 mH and 25 mH. The mutual inductance between the two is 12 mH. The coefficient of inductive coupling between the two inductors is

(a) 1.25
(b) 18.75
(c) 0.25
(d) 0.8

18. The magnetic materials that are used to prepare permanent magnets should have

(a) low coercive force
(b) steeply rising magnetisation curve
(c) small hysteresis loop
(d) high retentivity

19. Poles of dc machines are often laminated to reduce

(a) hysteresis loss
(b) eddy current loss
(c) iron weight
(d) armature reaction

20. Universal motor is a _______ motor

(a) shunt
(b) series
(c) single phase induction
(d) synchronous