GAIL electronics and electrical questions with answers
1. A 3 stack stepper motor with 12 numbers of rotor teeth has a step angle of
Ans:DExplanation: Given m = 3, Nr = 12
Step angle = 360 / m x Nr = 360 /3 x 12 = 10°
2.Oil-filled cable has a working stress of __________ kV/mm
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
Explanation: Given: N=1000 rpm ; P= 6; s= 0.04;
and f = N_P/ 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 _________.
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
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.
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
Explanation: Torque _ Power
Power _ Voltage
Therefore, It has same angle as ‘V’ ha
8. 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.
Explanation: Wh = khfBm
1.6 and We = kef2Bm
Therefore, hysteresis and eddy current losses will be decreased when frequency decreases.
9. Electric ovens using heating elements of _______________ can produce temperature upto 3000°C.
Explanation: Chromium has high melting point.
10. Electric resistance seam welding uses __________ electrodes.
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 50 Hz power frequency signal
offers high impedance to carrier frequency signal
offers low impedance to carrier frequency signal
Both (A) & (C)
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
All of the above
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
Explanation: The rotor revolves synchronously because the rotor poles magnetically lock up with the revolving stator poles of opposite polarity
14. The armature of a dc machine is laminated to reduce:
Eddy current loss
friction and windage losses
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.
15. For LV applications (below 1 kV), ______________ cables are used.
Single core cables.
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
17. When a solenoid is activated, the force that moves the plunger is
an electromagnetic field
a permanent magnetic field
a steady current
18. Which of the following capacitors is polarized
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
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
21. An RC differentiator acts as a
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
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
capacitor start and capacitor run motor
capacitor start motor.
24. The drive motor used in a mixer-grinder is a
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
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
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
27 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.
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°.
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?
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?
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