electrical engineering objective questions

electrical engineering objective questions
Electrical Engineering Questions and Answers
multiple choice questions in electrical engineering

1. A shunt motor is running at its rated speed on rated load. If the field circuit gets suddenly opened
(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

Ans:-B
2. A single-phase alternator has a synchronous reactance of 2 ohms and negligible resistance. If it supplies 10 A to a purely capacitive load at 200 V, then the generated emf will be.
(a) 240 volts
(b) 220 volts
(c) 200 volts
(d) 180 volts

Ans:-D
3. Suppose a synchronous generator connected to an infinite bus it supplying electrical power at unity PF to the bus. If its field current is now increased
(a) Both the active and reactive power output of the generator will remain unchanged
(b) The active power supplied will remain unchanged but the machine will also supply lagging reactive power
(c) The active power supplied will increase and the machine will draw leading reactive power
(d) The active power supplied will decrease and the machine will supply leading reactive power

Ans:-B



4. In the phasor-diagram of a round-rotor synchronous generator, the volt age equations is E f = V t + I e ( r a + j X a ), where E f = excitation voltage, V t = terminal voltage, Ie = armature current at lagging power factor, r a = armature resistance; Xs= synchronous reactance. While remaining synchronized to the bus-bars (infinite bus), f the power input from the prime mover is gradually decreased and finally stopped, it will result in
(a) Reversal of ‘I a’ and E lagging V t
(b) Reversal of I a but E f leading V t
(c) The sign of I a remaining unchanged, but E f leading V t
(d) The sign of I a remaining unchanged, but E f lagging V t

Ans:-C

5. A 3-phase Induction motor has a full-load slip of 3 per cent at normal voltage. Which one of the following will be the value of the slip of the motor if ft develops the same torque theoretically while operating at 110 per cent of its normal voltage?
(a) 2.48%
(b) 0.248%
(c) 0.483%
(d) 4.83%

Ans:-A


6. A 6-pole, 3-phase, 60 Hz Induction motor runs at 1000 rpm developing maximum torque. Rotor resistance per phase is 1.2 ohms. Neglecting stator impedance, then for developing maximum torque, the external resistance to be connected in series with each rotor phase will be
(a) 7.2 ohms
(b) 6 ohms
(c) 1.44 ohms
(d) 1.2 ohms

Ans:-A


7. The capacitor-start single-phase Induction motors develop much larger starting torque in comparison with the “split-phase” motors, because the use of capacitors in the auxiliary winding enables
(a) Provision of larger number of turns in the auxiliary winding
(b) A larger starting current to be drawn from the supply
(c) The torque-slip characteristic in general to get a shape to give a large starting torque
(d) The current division between the main and auxiliary windings to be conducive for the development of large starting torque

Ans:-D
8. If the load of a synchronous motor is increases while keeping the field excitation constant, then
(a) Power factor will increase and power angle will decrease
(b) Power factor will decrease and power angle will increase
(c) Both power factor and power angle will increase
(d) Both power factor and power angle will decrease

Ans:-C


9. A synchronous motor operates as a synchronous condenser when it is
(a) Operated at unity power factor
(b) under-excited
(c) Over-excited
(d) Connected in parallel with condensers

Ans:-C


10. An equipment has 0.8 per unit impedance on a base of 66KV, 100 MVA. It’s per unit impedances on a base of 33 KV, 100 MVA will be
(a) 0.4
(b) 0.8
(c) 1.6
(d) 3.2

Ans:-A


11. In a 3-phase power transformer, 5-limbed construction is adopted to
(a) Suppress the 5th and 7th harmonics
(b) Suppress the 3rd and its multiple harmonics
(c) Suppress all the orders of harmonics
(d) Increase the capacity of the transformer

Ans:-A

12. A transformer has a resistance of 2% and reactance of 4%. Its regulations at 0.8 power factor lagging a leading respectively are
(a) 4 per cent and — 0.8 per cent
(b) —4 per cent and 0.8 per cent
(c) — 0.8 per cent and 4 per cent
(d) 0.8 per cent and —4 per cent

Ans:-A

13. Consider the following statements:
The use of Delta-connected tertiary windings in star-star connected power transformers
1. Makes available supply for single-phase loads.
2. Suppresses harmonic voltages.
3. Allows flow of earth fault current for operation of protective devices
4. Provides low-reactance paths for zeros sequence currents.
Of the statements
(a) 1, 2 and 3 are correct
(b) 1, 3 and 4 are correct
(c) 2 and 4 are correct
(d) 1, 2, 3 and 4 are correct

Ans:-D


14. If dv / dt occurring across a thyristor exceeds its permissible value, then the thyristor will
(a) Remain in the forward blocking state without any damage
(b) Turn on only to get damaged
(c) Turn on but whether it gets damaged or not will depend on the final follow up current
(d) Not turn on but get damaged

Ans:-C



15. In a dc machine, the armature mmf is
(a) Rectangular and directed along the inter- polar axis
(b) Triangular and directed along the inter-polar axis
(c) Triangular and directed along the brush axis
(d) Rectangular and directed along the brush axis

Ans:-C


16. The average load current of a dc chopper feeding a pure resistive load is I amps. If a variable inductance connected in the load circuit Is progressively increased from zero value, keeping the duty ratio unchanged, then the average load current will
(a) Increase starting from 1
(b) Decrease with a starting value of 1
(c) Remain the same at 1
(d) Increase to some highest value of current and then decrease again to 1

Ans:-D

17. Consider the following statements:
The source inductance of an ac to dc line commutated phase controlled converter
1. limits dv / dt capability of thyristors.
2. Causes a voltage drop in the de terminal voltage.
3. Improves the line side power factor.
4. limits the range of firing angles.
5. reduces the line side power factor.
Of these statements
(a) 1, 3 and 4 are correct
(b) 1, 2 and 4 are correct
(c) 2 and 5 are correct
(d) 2, 4 and 5 are correct

Ans:-B




18. When starting a differentially connected compound motor, it Is desirable to short-circuit the series field winding to
(a) Avoid very high starting time
(b) Avoid excessive starting speed
(c) Prevent the motor from starting in the reverse direction
(d) Avoid heavy in-rush of current

Ans:-B

19. A single-phase voltage controller is employed for controlling the power flow from 206 V, 50 Hz source into a load consisting of R = 5 Ω and ω L = 12 Ω The value of maximum mis load current and the firing angle are respectively
(a) 260 / 10.9 A and 00
(b) 20 A and 0°
(c) 20 A and 90°
(d) 260 / 10.9 A and 90°

Ans:-C

20. The number of thyristors required for single- phase to single-phase cycloconverter of the midpoint and for three-phase to three-phase 3-pulse type of cycloconverter are respectively
(a) 4and6
(b) 4and4
(c) 4andl8
(d) None of the above

Ans:-C