GAIL Model Papers

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

____________.

120
8°
24°
10°

AnsExplanation: 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

10
12
13
15
Ans

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

8
4
6
2
Ans

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

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

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.

Ans:A
Explanation: Wh = khfBm

1.6 and We = kef2Bm

2.k

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.

Nickel
Graphite
Chromium
Iron
Ans:C

Explanation: Chromium has high melting point.

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Answered By StudyChaCha Member