RRB JE Electrical question paper with solution 2013 | RRB JE Electrical

Ques.11. The main function of a transformer is to change

  1. Power
  2. Frequency
  3. Voltage Level
  4. Power factor

Answer.3. Voltage level

Explanation:-

The transformer is a static device which is used to convert the voltage level i.e it step-up or step-down the voltage.

 

Ques.12. Which of the following is a cold cathode lamp?

  1. Sodium lamp
  2. Neon Lamp
  3. CSL Lamp
  4. Tubelight

Answer.1,2. Neon Lamp and sodium vapor lamp

Explanation:-

SODIUM VAPOR LAMP

A sodium vapor lamp is a low pressure and cold cathode lamp i.e its electrode is not electrically heated by a filament. A sodium vapor discharge lamp consists of a U-shaped tube enclosed in a double-walled vacuum flask, to keep the temperature of the tube within the working region. The inner U-tube consists of two oxide-coated electrodes, which are sealed with the ends. These electrodes are connected to a pin type base construction of a sodium vapor lamp is shown in Fig.

This sodium vapor lamp is low luminosity lamp so that the length of the lamp should be more. In order to get the desired length, it is made in the form of a U-shaped tube. This long U-tube consists of a small amount of neon gas and metallic sodium. At the time of start, the neon gas vaporizes and develops sufficient heat to vaporize metallic sodium in the U-shaped tube.

Neon Lamp

This is a cold cathode lamp, in which no filament is used to heat the electrode for starting. The neon lamp consists of two electrodes placed at the two ends of a long discharge tube.

The discharge tube is filled with neon gas. A low voltage of 150 V on DC or 110 V on AC is impressed across the two electrodes; the discharge takes place through the neon gas that emits light or electromagnetic radiation reddish in color. The sizes of electrodes used are equal for both AC and DC supplies. On DC, neon glow appear nearer to the negative electrode; therefore, the negative electrode is made larger in size. Neon lamp electric circuit consists of a transformer with high leakage reactance in order to stabilize the arc. The capacitor is used to improve the power factor. Neon lamp efficiency is approximately 15-40 lumens/W. The power consumption of the neon lamp is 5 W

If the helium gas is used instead of neon, pinkish white light is obtained. These lamps are used as night lamps and as indicator lamps and used for the determination of the polarity of DC mains and for advertising purpose.

 

Ques.13. The V.I characteristics of the diode lie in the

  1. 1st and 2nd Quadrant
  2. 1st and 3rd Quadrant
  3. 1st and 4th Quadrant
  4. Only in 1st Quadrant

Answer.2. 1st and 3rd Quadrant

Explanation:-

V. I Characteristics of Diodes

The combined forward and reverse characteristics is called V-I characteristics of a diode. The potential at which current starts increasing exponentially is also called offset potential, threshold potential or firing potential of a diode. 

  • The graph of forward current I and forward voltage across diode Vf is called forward characteristics of a diode and is plotted in the first quadrant. The forward resistance of a diode is always very small. The voltage at which forward current through the diode starts increasing rapidly is called cut-in voltage.
  • When the positive side of the diode is connected to the negative side of the battery and negative side of the diode is connected to the positive side of the battery the diode is said to be reversed diode. The reverse characteristics lie in the third quadrant. Since in the first quadrant (X and Y ) is positive and in the third quadrant ( X and Y are negative i.e reversed).

 

Ques.14. Which of the following is used in a semiconductor device?

  1. Silicon
  2. Graphite
  3. Copper
  4. Aluminum

Answer.1. Silicon

Explanation:-

Silicon is the most widely used semiconductor material because it has a useful temperature range and is abundant, cheap, and easy to manufacture. Germanium was among the first semiconductor materials to be developed and is less widely used than silicon, but it is useful in very high-speed devices when alloyed with silicon.

 

Ques.15. Generator transformer are

  1. Step-up transformer
  2. Step-down transformer
  3. Auto transformer
  4. One-one transformer

Answer.1. Step-up transformer

Explanation:-

A step-up transformer associated with the turbine generator systems is called a generator transformer. Its role is to raise the voltage of the electric energy generated which can be efficiently transmitted to the substation via buried cables or overhead lines. Generator transformers step the voltage up from the generator level of typically 10-20 kV to the transmission or sub-transmission voltage. 

 

Ques.16. Two electric bulbs designed to operate with a power of 500 watts in 220-volt line are connected in series with a 110 volt line. The power generated by each bulb will be

  1. 31.25 watts
  2. 3.125 watts
  3. 22 watts
  4. 62.5 watts

Answer.4. 62.5 watts

Explanation:-

Given

P = Power = 500 W

Voltage V = 220 volt

Resistance R = 

The power is given as

P = V2/R

500 = (220)2/R

R = 96.8 ohms

Now bulb is connected to 110 V supply. Therefore power will be

P = V2/R

P = (110)2/96.8

P = 125 watts

Hence power generated by each bulb as they are connected in series will be

P = 125/2 = 62.5 watts

 

Ques.17. The resistance of an ideal ammeter is

  1. Low
  2. High
  3. Infinite
  4. Zero

Answer.4. Zero

Explanation:-

An ammeter is used to measure the current in a circuit. It is connected in series with the circuit to avoid division of current, but ‘when placed in series with the circuit, it increases the resistance and decreases the current being measured by it. Hence, an ideal ammeter should have zero resistance.

 

Ques.19. Power factor can be improved by connecting which among the following?

  1. Static capacitor
  2. Resistor
  3. Synchronous condenser
  4. Both 1 and 3

Answer. 4. Both 1 and 3

Explanation:-

Normally, the power factor of the whole load on a large generating station is in the region of 0.8 to 0.9. However, sometimes it is lower and in such cases In order to improve the power factor, some device taking leading power should be connected in parallel with the load. This can be achieved by the following equipment

  1. Static capacitors
  2. Synchronous condenser
  3. Phase advancers

Static Capacitor:-

  • The power factor can be improved by connecting capacitors in parallel with the equipment operating at lagging power factor.
  • The capacitor (generally known as the static capacitor) draws a leading current and partly or completely neutralizes the lagging reactive component of load current.
  • This raises the power factor of the load. For three-phase loads, the capacitors can be connected in delta or star as shown in Fig. Static capacitors are invariably used for power factor improvement in factories.

  • Static capacitors are connected across the mains at the load end.
  • This supplies a reactive component of the current to reduce the out-of-phase component of current required by an inductive load i.e., it modifies the characteristics of an inductive load by drawing a leading current which counteracts or opposes the lagging component of the inductive load current at the point of installation.
  • So the reactive VAr’s transmitted over the line is reduced, thereby the voltage across the load is maintained within the specified limits.
  • By the application of the shunt capacitor to a line the magnitude of source current can be reduced, the power factor can be improved and consequently the voltage drop between the sending and receiving ends is also reduced.
  • It is important to note that it does not affect the current or the power factor beyond their point of installation.

Advantages

  1. They have low losses.
  2. They require little maintenance as there are no rotating parts.
  3. They can be easily installed as they are light and require no foundation.
  4. They can work under ordinary atmospheric conditions.

Disadvantages

  1. They have short service life ranging from 8 to 10 years.
  2. They are easily damaged if the voltage exceeds the rated value.
  3. Once the capacitors are damaged, their repair is uneconomical.

Synchronous condenser

A synchronous condenser is a synchronous motor running without the mechanical load. The Synchronous motor takes a leading current when over-excited and therefore behave as a capacitor. When such a machine is connected in parallel with the supply, it takes a leading current which partly neutralizes the lagging reactive component of the load. Thus the power factor is improved.

Advantages

  1. A synchronous condenser has an inherently sinusoidal waveform and the voltage does not exist.
  2. It can supply as well as absorb kVAr.
  3. The PF can be varied in smoothly.
  4. It allows the overloading for short periods.
  5. The high inertia of the synchronous condenser reduces the effect of sudden changes in the system load and improves the stability of the system.
  6. It reduces the switching surges due to sudden connection or disconnection of lines in the system.
  7. The motor windings have high thermal stability to short-circuit currents.
  8. By varying the field excitation, the magnitude of current drawn by the motor can be changed by any amount. This helps in achieving stepless control of power factor.
  9. The faults can be removed easily.

Disadvantages

  1. There are considerable losses in the motor.
  2. The maintenance cost is high.
  3. It is not possible to add or take away the units and to alter the rating of the synchronous condenser.
  4. For small rating it is uneconomical.
  5. As a synchronous motor has no self-starting torque, therefore, auxiliary equipment has to be provided for this purpose

Phase Advancer:

There are special commutator machines, which are used to improve the power factor of the induction motor. When the supply is given to the stator of an induction motor, it takes a lagging current. So, the induction motor has low lagging power factor. For compensating this lagging current, a phase advancer (mounted on the same shaft) is used. It supplies mmf to the rotor circuit at slip frequency.

Advantages

  1. The lagging kVAr drawn by the motor is reduced by compensating the stator lagging current at slip frequency.
  2. Where the use of synchronous motors is not suitable, phase advancer can be used. Generally, these compensators are not recommended for the economical operation of motors of low rating below 200 HP.

Advantages of power factor improvement.

  1. The kW capacity of the prime movers is better utilized due to decreased reactive power.
  2. This increases the kilowatt capacity of the alternators, transformers, and the lines.
  3. The efficiency of the system is increased.
  4. The cost per unit decreases.
  5. Improves the voltage regulation of the lines.
  6. Reduction in power losses in the system due to the reduction in load current.

 

Ques.20. Which of the following rotor is used in thermal power plants?

  1. Cylindrical rotor
  2. Salient pole rotor
  3. Either of these
  4. Square cage rotor

Answer.1. Cylindrical rotor

Explanation:-

The rotor of the alternator is of two types

  1. Cylindrical rotor
  2. Salient pole

Cylindrical Rotor

Rotor and stator are the main parts of a generator. The rotor is placed inside the stator. The air gap in between stator and rotor is very less. In power generators of thermal power plants, the cylindrical rotor is used because they run at very high speed. This type of rotor is also called non-salient or drum type rotor.

Rotor of the generator carries field winding. When DC power supply is applied to this winding, it becomes magnetic. Rotor also carries a fan which circulates cooling medium inside the generator to cool down the generator.

The design of a rotor is critical, as the field winding is put inside the slots provided and it has to rotate at a speed of normally 1500 rpm or 3000 rpm. Also, the mass of a rotor is high.

Normally, cylindrical rotors for two or four-magnetic pole design are used for the generation of power at power plants. The number of poles of a rotor depends upon the speed of the generator.

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