**RBI electronics and electrical questions with answers**

1.Resistance is measured in

henries

ohms

hertz

watts

Ans:B

2. A sine wave voltage is applied across an inductor. When the frequency of the voltage is decreased, the current

is increased

is decreased

does not change

momentarily goes to zero

Ans:A

3. The winding resistance of a coil can be increased by

increasing the number of turns

using a thinner wire

changing the core material

increasing the number of turns or using thinner wire

Ans:D

4. When the current through an inductor is cut in half, the amount of energy stored in the electromagnetic field

is halved

quadruples

doubles

does not change

Ans:A

5. In the complex plane, the number 14 – j5 is located in the

first quadrant

second quadrant

third quadrant

fourth quadrant

Ans:D

6. When the frequency of the source voltage decreases, the impedance of a parallel RC circuit

Increases

decreases

does not change

does not change

Ans:A

7. In a three-phase system, the voltages are separated by

45°

90°

120°

180°

Ans:C

8. A constant load power means a uniform conversion of

mechanical to electrical energy

electrical to mechanical energy

current to voltage

voltage to current

Ans:B

9. Polyphase generators produce simultaneous multiple sinusoidal voltages that are separated by

certain constant phase angles

certain constant frequencies

certain constant voltages

certain constant currents

Ans:A

10. Materials with lots of free electrons are called

Conductors

insulators

semiconductors

filters

Ans:A

11. Electrons in the outer orbit are called

nuclei

valences

waves

shells

Ans:B

12. A multimeter measures

current

voltage

resistance

current, voltage, and resistance

Ans:D

13. A wiper is the sliding contact in a

Switch

photoconductive cell

thermistor

potentiometer

Ans:D

14. A circuit breaker is a

Fuse

switch

resettable protective device

resistor

Ans:C

15. An ohmmeter is an instrument for measuring

current

voltage

resistance

wattage

Ans:C

16.The two windings of a transformer is

conductively linked.

inductively linked.

not linked at all.

electrically linked.

Ans:B

17. The d.c. series motor should always be started with load because

at no load, it will rotate at dangerously high speed.

it will fail to start.

it will not develop high starting torque

all are true.

Ans:A

18 In a stepper motor the angular displacement

can be precisely controlled.

it cannot be readily interfaced with micro computer based controller

the angular displacement cannot be precisely controlled.

it cannot be used for positioning of work tables and tools in NC machines

Ans:A

19 The power factor of a squirrel cage induction motor is

low at light load only.

low at heavy load only.

low at light and heavy load both.

low at rated load only.

Ans:A

20. The generation voltage is usually

between 11 KV and 33 KV.

between 132 KV and 400 KV.

between 400 KV and 700 KV.

None of the above.

Ans:A

21. When a synchronous motor is running at synchronous speed, the damper winding Produces

damping torque.

eddy current torque.

torque aiding the developed torque

no torque.

Ans:D

22. In a d.c. machine, the armature mmf is

stationary w.r.t. armature.

rotating w.r.t. field.

stationary w.r.t. field.

rotating w.r.t. brushes.

Ans:C

23. In a transformer the voltage regulation will be zero when it operates at

unity p.f.

leading p.f.

lagging p.f.

zero p.f. leading.

Ans:B

24 The emf induced in the primary of a transformer

is in phase with the flux

lags behind the flux by 90 degree.

leads the flux by 90 degree.

is in phase opposition to that of flux.

Ans:C

25. The current from the stator of an alternator is taken out to the external load circuit Through

slip rings.

commutator segments

solid connections

carbon brushes.

Ans:C

26. A hysteresis motor

is not a self-starting motor

is a constant speed motor

needs dc excitation

can not be run in reverse speed.

Ans:B

27. The most suitable servomotor for low power applications is

a dc series motor.

a dc shunt motor.

an ac two-phase induction motor.

an ac series motor

Ans:B

28. For a given wirewound core, an increase in current through the coil

reverses the flux lines

decreases the flux density

increases the flux density

causes no change in flux density

Ans:C

29. When the current through the coil of an electromagnet reverses, the

direction of the magnetic field reverses

direction of the magnetic field remains unchanged

magnetic field expands

magnetic field collapses

Ans:A

30. The unit for permeability is

Wb/At × m

At/m

At/Wb

Wb

Ans:A

31. The ability of a material to remain magnetized after removal of the magnetizing force is known as

Permeability

reluctance

hysteresis

retentivity

Ans:D

32. When a solenoid is activated, the force that moves the plunger is

an electromagnetic field

a permanent magnetic field

varying voltage

a steady current

Ans:A

33. Which of the following capacitors is polarized

mica

ceramic

plastic-film

electrolytic

Ans:D

34. In a series resonant band-pass filter, a lower value of Q results in

a higher resonant frequency

a smaller bandwidth

a higher impedance

a larger bandwidth

Ans:D

35. A steady-state condition is reached when

the output voltage reaches the average value of the input voltage

the output voltage reaches the input voltage

the output voltage reaches approximately 63% of the input voltage

the output voltage reaches the effective value of the input voltage

Ans:A

36. An RC differentiator acts as a

low-pass filter

high-pass filter

band-pass filter

band-stop filter

Ans:B

37. A balanced three-phase, 50 Hz voltage is applied to a 3 phase, 4 pole, induction motor. When the motor is delivering rated output, the slip is found to be 0.05. The speed of the rotor m.m.f. relative to the rotor structure is

1500 r.p.m.

1425 r.p.m.

25 r.p.m.

75 r.p.m.

Ans:D

Explanation: NS = 120f /P = 120 x 50 /4 =1500rpm

N = NS ( 1-s) = 1500 (1-0.05) = 1425

\relative speed = 1500 – 1425 = 75 rpm

38. A ceiling fan uses

split-phase motor.

capacitor start and capacitor run motor

universal motor.

capacitor start motor.

Ans:D

39. The drive motor used in a mixer-grinder is a

dc motor.

induction motor.

synchronous motor.

universal motor.

Ans:D

40. A 1:5 step-up transformer has 120V across the primary and 600 ohms resistance across the secondary. Assuming 100% efficiency, the primary current equals

0.2 Amp.

5 Amps

10 Amps.

20 Amps.

Ans:A

Explanation: I1= V1 /R1 = 120/600 = 0.2 (h = 100%, losses are zero \V1 = VR = I1R1)

41. A 50 Hz, 3-phase induction motor has a full load speed of 1440 r.p.m. The number of poles of the motor are

4.

6.

12

8.

Ans:A

Explanation: N= Ns (1-S) = NS –NS x S

1440 = Ns (1-S)

Ns = 1440 / (1-S)

Ns = (120 f/ p) = 120 x 50/p = 6000 p

Ns will be closer to N i.e 1440

When P=2 ; Ns = 3000 rpm , not close to N

When P=4 ; Ns = 1500 rpm , it is closer to N

Therefore P =4 for N=1440

42. In a 3-phase synchronous motor

the speed of stator MMF is always more than that of rotor MMF.

the speed of stator MMF is always less than that of rotor MMF.

the speed of stator MMF is synchronous speed while that of rotor MMF is zero

rotor and stator MMF are stationary with respect to each other.

Ans:D

Explanation: Because, Motor is magnetically locked into position with stator, the rotor poles are engaged with stator poles and both run synchronously in same direction Therefore,

rotor & stator mmf are stationary w.r.t each other.

43. In a three phase transformer, if the primary side is connected in star and secondary side is connected in delta, what is the angle difference between phase voltage in the two cases.

delta side lags by -30°.

star side lags by -30°.

delta side leads by 30°.

star side leads by -30°.

Ans:C

Explanation: This is vector group and has +30° displacement. Therefore, delta side leads by +30°.

44. Slip of the induction machine is 0.02 and the stator supply frequency is 50 Hz. What will be the frequency of the rotor induced emf?

10 Hz.

50 Hz.

1 Hz.

2500 Hz.

Ans:C

Explanation: Given : s = 0.02 ; f = 50 Hz

Therefore, frequency of rotor induced emf = s f

= 0.02 x 50 = 1.0 Hz

45. A 4 pole lap wound dc shunt motor rotates at the speed of 1500 rpm, has a flux of 0.4 mWb and the total number of conductors are 1000. What is the value of emf?

100 Volts.

0.1 Volts.

1 Volts.

10 Volts.

Ans:D

Explanation: Given N = 1500 rpm, F = 0.4 mWb, Z = 1000, P = 4, & A= 4

Therefore, Eb = NFPZ / 60 A

= 1500 x 0.4 x 4 x 1000 x 10-3 / 60 x 4

= 60/6 = 10 volts

46. A 3 stack stepper motor with 12 numbers of rotor teeth has a step angle of ____________.

12^{0}

8^{}

24^{}

10^{ }

Ans:D

Explanation: Given m = 3, Nr = 12

Step angle = 360 / m x Nr = 360 /3 x 12 = 10^{}

47 .Oil-filled cable has a working stress of __________ kV/mm

10

12

13

15

Ans:D

Explanation: This is defined by dielectric strength of mineral oil i.e. 15 kV/mm.

48 . The rotor frequency for a 3 phase 1000 RPM 6 pole induction motor with a slip of 0.04 is________Hz

8

4

6

2

Ans:D

Explanation: Given: N=1000 rpm ; P= 6; s= 0.04;

and f = N_P/ 120

= 1000_6/120

= 50 Hz

Rotor frequency fr=s_f = 0.04_50

= 2.0 Hz

49 The synchronous speed for a 3 phase 6-pole induction motor is 1200 rpm. If the number of poles is now reduced to 4 with the frequency remaining constant, the rotor speed with a slip of 5% will be _________.

1690 rpm

1750 rpm

1500 rpm

1710 rpm

Ans:D

Explanation: Given : Ns1 =1200 , P1= 6,

P2 = 4, s = 0.05,

Frequency f = Ns_P/120

= 120_6/120 = 60 Hz

rotor frequency f/ = s.f = 0.05 _60 = 3.0 Hz

Now, Ns2 = 120 _60 /4 = 1800 and Ns – N = 120 f / P2

Therefore, N=Ns- 120 f / P2 = 1800-120_0.05_60/4 = 1800-90 = 1710

50. The efficiency of a transformer is mainly dependent on

core losses.

copper losses

stray losses.

dielectric losses.

Ans:A

51. A synchronous motor is operating on no-load at unity power factor. If the field current is increased, power factor will become

Leading & current will decrease

Lagging & current will increase.

Lagging & current will decrease

Leading & current will increase.

Ans:A

Explanation: Initially synchronous motor is operating at no load and unity power factor. When field current increases, the excitation will increase. Therefore, p.f will be leading and

current will be I Cos< I

52. If the phase angle of the voltage coil of a directional relay is o 50 , the maximum torque angle of the relay is

130

100

50

25

Ans:C

Explanation: Torque _ Power

Power _ Voltage

Therefore, It has same angle as ‘V’ ha

53. A 220/440 V, 50 Hz, 5 KVA, single phase transformer operates on 220V, 40Hz supply with secondary winding open circuited. Then

Both eddy current and hysteresis losses decreases.

Both eddy current and hysteresis losses increases.

Eddy current loss remains the same but hysteresis loss increases.

Eddy current loss increases but hysteresis loss remains the same.

Ans:A

Explanation: Wh = khfBm

1.6 and We = kef2Bm

2.k

Therefore, hysteresis and eddy current losses will be decreased when frequency decreases.

54. Electric ovens using heating elements of _______________ can produce temperature

upto 3000°C.

Nickel

Graphite

Chromium

Iron

Ans:C

Explanation: Chromium has high melting point.

55. Electric resistance seam welding uses __________ electrodes.

Pointed

Disc.

Flat

Domed

Ans:B

Explanation: Disc type electrodes are used for electric resistance seam welding.

56 The line trap unit employed in carrier current relaying:

offers high impedance to 50 Hz power frequency signal

offers high impedance to carrier frequency signal

offers low impedance to carrier frequency signal

Both (A) & (C)

Ans:B

Explanation: The line trap unit employed in carrier current relaying offers high impedance to

carrier frequency signal.

Because carrier frequency range is 35 km – 500 kHz

XL = 2_ f l

Where f increases XL will also increases

57. The temperature of resistance furnaces can be controlled by changing the:

applied voltage

number of heating elements

circuit configuration

All of the above

Ans:D

Explanation: Temperature of resistance furnaces can be controlled by changing either applied voltage or by number of heating elements or by circuit configuration.

58. A single phase Hysteresis motor

can run at synchronous speed only

can run at sub synchronous speed only

can run at synchronous and super synchronous speed

can run at synchronous and sub synchronous speed

Ans:A

Explanation: The rotor revolves synchronously because the rotor poles magnetically lock up with the revolving stator poles of opposite polarity

59. The armature of a dc machine is laminated to reduce:

Eddy current loss

Hysteresis loss

copper losses

friction and windage losses

Ans:A

Explanation: Thinner the laminations, greater is the resistance offered to the induced e.m.f., smaller the current and hence lesser the I2R loss in the core.

60 For LV applications (below 1 kV), ______________ cables are used.

Paper insulated.

Plastic.

Single core cables.

Oil filled.

Ans:C

Explanation: For low voltage applications single core cables are suitable.