RBI electrical and Electronics questions

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 windingProduces

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 circuitThrough

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, inductionmotor. When the motor is delivering rated output, the slip is found to be 0.05. Thespeed 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 resistanceacross 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 ofpoles 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 secondaryside is connected in delta, what is the angle difference between phase voltage in thetwo 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 0.4mWb 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____________.

12⁰
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 thenumber 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 fieldcurrent 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 o50 , 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, 40Hzsupply 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 frequencydecreases.

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.