SSC JE electrical question paper 2007

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1. For the circuit shown below, voltage V1 will be- ?
lets asume 






... Answer is C)
Kirchhoff's Current Law, (KCL) and Junction Rule

the total current entering a circuits junction is exactly equal to the total current leaving the same junction

Node A
10 =i1 +i2
10 = V1/1 + (V1-V2)/2
10 = (2 V1 +V1 -V2)/2
20 = 3V1 - V2 ------------(1)

Node B
i2 = i3+5+i4

(V1-V2)/2 = V2/4 + 5 + V2/4

(V1-V2)/2 = (V2 + V2 + 20) / 4
4/2*(V1-V2) = 2V2 + 20
2V1 - 2V2 = 2V2 + 20
2V1 - 4V2 = 20 -------------(2)

Solve equation 1 and 2 V1 = 6V V2 = -2V

2. The effective value of voltage given by v= 100 + 25 sin 3ωt + 10 sin 5ωt will be ?




... Answer is B)
effective value of voltage of = RMS value

Vrms = √[$I0^2$ + 1/2 ( $Im1^2$ +$Im2^2$)]

Vrms = √[$100^2$ + .5 ( $25^2$ +$10^2$)]

Vrms = √[10000 + .5 ( 625 +100)]

Vrms = √[10000 + 362.5]

Vrms = √[10362.5]

Vrms = 101.796 V
.

3. The voltage v and current i of a device are v= 100 sin 377t, i= 10 sin (377t + 30º) The power P indicated by watmeter will be ?




... Answer is D )
v= 100 sin 377t


i=10 sin (377t + 30º)


Wattmeter measure active power

active power = Vrms*Irms* cosθ

Vrms = 100/√2
Irms = 10/√2

θ = Angle between voltage and current = 30º

active power = Vrms*Irms* cosθ

active power = 100/√2*10/√2* cos30º

active power = 1000/2 *√3/2 =250*1.732 = 433 W

.

4. Zero power factor method for an alternator is generally used to determine ?




... Answer is C)
This Zero power factor (ZPF) method is used to determine the voltage regulation of synchronous generator or alternator.

5. Two alternators A and B are sharing the inductive load equally. What will happen when the excitation of A is increased ?




... Answer is A)
Two alternators A and B are sharing the inductive load equally. What will happen when the excitation of A is increased
Current delivered by A will increase and of B will decrease
.

6. The percentage slip in case of synchronous motor is- ?




... Answer is D)
slip is the difference between the speed of the rotating magnetic field of the stator and the actual speed of the rotor.


in case of synchronous motor, rotor running at same speed as (synchronous spped) the rotating magnetic field of the stator
so now slip is zero


7. Torque angle of synchronous motor is- ?




... Answer is C)
For the case of synchronous motor, the angle is called torque angle and the rotating field lags the stator field in this case.

For a generator, the power angle is the difference between the generator induced voltage and the generator terminal voltage. The value of the power angle is same as the load angle


8. Which of the following is preferred r.p.m. for 4-pole induction motor ?




... Answer is A)
4 pole induction motor synchronous speed
Ns =120f/p
in India we are using 50 Hz Ac supply
Ns =120*50/4 =1500 rpm
so in option A (1485 rpm ) motor will run at minimum slip value.
.

9. Maximum torque in a 3-phase induction motor depends on ?




... Answer is D)
The maximum torque is directly proportional to square of rotor induced emf at the standstill.
The maximum torque is inversely proportional to rotor reactance.
The maximum torque is independent of rotor resistance.
The slip at which maximum torque occur depends upon rotor resistance, R2.


10. Full-load current for a 3-phase, 440 volts, 20 HP induction motor will be closer to- ?




... Answer is C)
V = 440 volt
P = 20*746 watt
power factor = 0.8

Full load current = P / √3 * V * cosθ

=20*746 / √3*440*0.8 =24.5 A


11. How many different speeds can be obtained in cascade control of two induction motors ?




... Answer is B)
Cascade Operation

In this method of speed control, two motors are used. Both are mounted on a same shaft so that both run at same speed.
One motor is fed from a 3phase supply and the other motor is fed from the induced emf in first motor via slip-rings.
The arrangement is as shown in following figure..


12. A universal motor can be operated on 220 V d.c. as well 220 V single-phase a.c. which of the following will be same in the two case ?




... Answer is C)
A universal motor is a special type of motor which is designed to run on either DC or single phase AC supply. These motors are generally series wound (armature and field winding are in series), and hence produce high starting torque

13. A transformer has maximum efficiency at full load, when iron losses are 800 watts. Copper losses at half load will be- ?




... Answer is D)
At maximum efficiency

Copper losses = iron losses = 800 watts

Copper losses at half load =( 1/$2^2$ ) *800 = 800/4 =200 watts



14. Which of the following is not a standard transmission voltage in India ?




... Answer is C)
An extensive network of transmission lines has been developed over the years for evacuating power produced by different electricity generating stations and distributing the same to the consumers.

The nominal Extra High Voltage lines in vogue are ± 800 kV HVDC & 765 kV, 400 kV, 230/220 kV, 110 kV,66KV and 33 kV AC lines.

15. A load power factor of 0.95 lagging implies reactive power demand of- ?




... Answer is C)
A load of 0.95 pf lagging implies a Q demand of 0.33 kVar per kW of power.
In planning a network it is desirable to assess the Q requirements to ascertain whether the generators are able to operate at the required power factors for the extremes of load to be expected..

We know:

KVAR/KW = VI sinΘ / VI CosΘ = SinΘ/CosΘ

SinΘ = √(1-$CosΘ^2$) = √(1-$.95^2$) = 0.3122

now

KVAR/KW =SinΘ/CosΘ = 0.3122/0.95 = 0.328

Almost 0.33 KVAR/KW implies a Q demand



16. Which of the following processes essentially needs direct current ?




... Answer is D)
Dc power needs for electroplating Power supply selection must ... dc power supplies provide a well-regulated current or voltage level that is preset ... Plating is essentially a molecule-by-molecule process where soluble metal ..



Electroplating involves passing an electric current through a solution called an electrolyte. This is done by dipping two terminals called electrodes into the electrolyte and connecting them into a circuit with a battery or other power supply.

17. The power factor of a spot welding machine is expected to be around- ?




... Answer is D)
Spot welding (also known as resistance spot welding) is a resistance welding process.
This welding process is used primarily for welding two or more metal sheets together by applying pressure and heat from an electric current to the weld area.

The power factor of a spot welding machine is expected to be around 0.3 to 0.5 lagging

18. In helium arc welding, the electrode is made of- ?




... Answer is D)
Tungsten Electrode.

The electrode used in GTAW is made of tungsten or a tungsten alloy, because tungsten has the highest melting temperature among pure metals, at 3,422 °C (6,192 °F). As a result, the electrode is not consumed during welding, though some erosion (called burn-off) can occur.



19. A dynamometer type wattmeter can be used on- ?




... Answer is D)
Dynamometer type wattmeter is used for the measurement of A.C. as well as D.C. power. It consists of a fixed coil forming the current coil whereas the moving coil forms the potential coil.

20. Which of the following instruments is an integrating type instrument ?




... Answer is B)
An integrating instrument is an analog device which is used to measure the total energy supplied by the circuit in a given amount of time and is independent of total energy consumed.
Example: The best example of integrating systems is the watt-hour meter (Energy Meter).

21. Which of the following a.c. bridges is performed for the measurement of inductance having high Q-factor ?




... Answer is A)
Hay's bridge is used to determine the Inductance of an inductor with a high Q factor. Maxwell's bridge is only appropriate for measuring the values for inductors with a medium quality factor.

De Sauty Bridge is a very simple type of AC Bridge used to measure capacitance.

Wien's Bridge. The Wien's bridge use in AC circuits for determining the value of unknown frequency.

22. Two d.c. series motors are connected in series to produce a torque T. Now if the motors are commected in parallel, the torque produced will be- ?




... Answer is D)
Watch the Video.

23. Which motor has the poorest speed control ?




... Answer is D)
Watch the Video.

24. Which d.c. motor is generally preferred for cranes and hoist ?




... Answer is A)
Watch the Video.

25. The unit of luminous flux ?




... Answer is C)
Watch the Video.

26. According to Lambert's law, the illumination on a surface is proportional to ?




... Answer is B)
Watch the Video.

27. If the fault current is 2000 amp, the relay setting at 50% and CT ratio is 400/5, then the plug setting multiplier will be- ?




... Answer is D)
Watch the Video.

28. If a combination of HRC fuse and circuit breaker is used, the circuit breaker operates- ?




... Answer is A)
Watch the Video.

29. A synchronous phase modifier as compared to a synchronous motor used for mechanical load has- ?




... Answer is B)
Watch the Video.

30. A 100 V/10 V, 50 VA transformer is converted to 100V/110V auto transformer, the rating of the auto transformer will be ?




... Answer is A)
Watch the Video.

31. Induction generator works betwen the ship of- ?




... Answer is C)
Watch the Video.

32. . Mho really is normally used for the protection of ?




... Answer is A)
Watch the Video.

33. A lightning arrestor is usually located near ?




... Answer is B)
Watch the Video.

34. For traction work, which motor is preferred ?




... Answer is C)
Watch the Video.

35. Which motor has widest variety of methods for speed control ?




... Answer is A)
Watch the Video.

36. Which of the following electric discharge lamps gives highest lumens per watt ?




... Answer is A)
Watch the Video.

37. The function of choke in the fluorescent tube circuit is to- ?




... Answer is D)
Watch the Video.

38. Electirc power for illumination in locomotive is provided by– ?




... Answer is B)
Electirc power for illumination in locomotive is provided by small turbo generator.

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