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GAIL electronics and electrical questions with answers
1. A 3 stack stepper motor with 12 numbers of rotor teeth has a step angle of
____________.
120
8°
24°
10°
Ans:D
Step angle = 360 / m x Nr = 360 /3 x 12 = 10°
2.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.
3. The rotor frequency for a 3 phase 1000 RPM 6 pole induction motor with a slip of 0.04 is________Hz
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
4. 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
5. The efficiency of a transformer is mainly dependent on
core losses.
copper losses
stray losses.
dielectric losses.
Ans:A
6. 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 CosF < I
7. 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
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.
Nickel
Graphite
Chromium
Iron
Ans:C
Explanation: Chromium has high melting point.
10. Electric resistance seam welding uses __________ electrodes.
Pointed
Disc.
Flat
Domed
Ans:B
Explanation: Disc type electrodes are used for electric resistance seam welding.
11. The line trap unit employed in carrier current relaying:
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
12. The temperature of resistance furnaces can be controlled by changing the:
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.
13. 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
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.
Plastic.
Single core cables.
Oil filled.
Ans:C
Explanation: For low voltage applications single core cables are suitable.
16. The ability of a material to remain magnetized after removal of the magnetizing force is known as
Permeability
reluctance
hysteresis
retentivity
Ans:D
17. When a solenoid is activated, the force that moves the plunger is
a permanent magnetic field
varying voltage
a steady current
Ans:A
18. Which of the following capacitors is polarized
mica
ceramic
plastic-film
electrolytic
Ans:D
19. 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
20. 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
21. An RC differentiator acts as a
low-pass filter
high-pass filter
band-pass filter
band-stop filter
Ans:B
22. 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
23. A ceiling fan uses
split-phase motor.
capacitor start and capacitor run motor
universal motor.
capacitor start motor.
Ans:D
24. The drive motor used in a mixer-grinder is a
dc motor.
induction motor.
synchronous motor.
universal motor.
Ans:D
25. 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)
26 . 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
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.
28 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°.
29. 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
30. 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
{C}{C}
