RRB JE Electrical solved question paper 2015 | RRB JE Electrical

Ques.21. A Transformer is working at its maximum efficiency. Its iron loss is 1 kW. Its copper loss will be

  1. 0.2 kW
  2. 0.25 kW
  3. 0.5 kW
  4. 1 kW

Answer 4. 1 kW

Explanation:

The efficiency of the transformer will be maximum at a load in which the total copper loss in the windings is equal to the core-loss

Since the Iron loss is 1 kW, therefore, the core loss will be also equal to 1 kW

 

Ques.22. For arc heating electrodes are made up of

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

Answer.3. Graphite

Explanation:-

Electrodes used in an arc furnace

The following three types of electrodes are used in arc furnaces:

(i) Carbon electrodes:

  • These electrodes are used with small furnaces for the manufacture of ferroalloys, aluminum, calcium carbide, phosphorus, etc.
  • These are made of anthracite coal and coke. 
  • These are very cheap and cost less than one-half as much for the same weight ah graphite electrodes.
  • The large area of carbon electrodes allows for more uniform heating.

(ii) Graphite electrode :

  • These electrodes are obtained by heating the carbon electrodes to a very high temperature.
  • Owing to the lower resistivity of graphite (one-fourth that of carbon), graphite is required half in size for the same current resulting in easy replacement.
  • Graphite begins to oxidize at about 600°C whereas carbon at about 400°C. Under average conditions, the consumption of graphite electrodes is about one-half that of the carbon electrode.

(iii) Self-baking electrodes:

  • These electrodes are employed in Ferro-alloys and electrochemical furnaces and the electrolytic production of aluminum. 
  • These electrodes are made of special paste, whose composition depends upon the types of process for which it is used, it is contained in the thin steel cylinder. The flow of current produces heat and the paste is baked and formed into an electrode.

Features of carbon and graphite electrodes are:

  • Resistivity: The graphite electrodes have low-specific resistance than the carbon electrodes, so the graphite required half in size for the same current resulting in easy replacement.
  • Oxidation: Graphite begins to oxides at 600°C whereas carbon at 400°C.
  • Electrode consumption: For steel-melting furnaces, the consumption of the carbon electrodes is about 4.5 kg of electrodes per tonne of steel and 2.3-to 6.8 kg of electrodes per tonne of steel for the graphite electrodes.
  • Cost: The graphite electrodes cost about twice as much per kg as the carbon electrodes. The choice of electrodes depends chiefly on the question of the total cost. In general, if the processes require large quantities of the electrode, carbon is used but for other processes, the choice depends on local conditions.

 

Ques.23. Sodium lamps are used in

  1. Reading rooms
  2. Street lights
  3. Auditorium
  4. Libraries

Answer.2. Street lights

Explanation:-

A sodium-vapor lamp is a gas-discharge lamp that uses sodium in an excited state to produce light at a characteristic wavelength near 589 nm.

There are two varieties of such lamps:

  1. Low-pressure sodium lamp
  2. High-pressure sodium lamp

Both the low pressure and high pressure are filled with both sodium and Neon gas.

Low-pressure sodium lamps are highly efficient electrical light sources, but their yellow light restricts applications to outdoor lighting such as street lamps.

High-pressure sodium lamps produce a broader spectrum of light than low-pressure lamps, but they still have poorer color rendering producing the monochromatic yellow light 

On turning on a sodium vapor lamp, initially, it emits a dim red/ pink light, which warms up sodium metal. After a few minutes, the light turns bright yellow, due to the vaporization of sodium, which is a nearly monochromatic light. Objects, lighted by sodium vapor lamps are not easily distinguished in color, because they appear almost entirely yellow due to the reflection of this narrow bandwidth of yellow light.

 

Ques.24. The fuse wire is made up of

  1. Nichrome
  2. Lead-tin alloy
  3. Copper
  4. Tungsten

Answer.2. Lead-tin alloy

Explanation:-

Fuse is the current interrupting device that breaks or opens the circuit (in which it is inserted) by fusing the elements when the current in the circuit exceeds a certain value.

Fuse wire is made with an alloy of lead and tin having a low melting point and low resistance (although the resistance of fuse wire is higher than that of electrical appliances). If due to any malfunction or fault, excessive current begins to flow through the circuit, the fuse wire immediately melts due to the heat generated by the flowing current. The circuit is broken and the excess current, which may damage equipment is prevented from flowing. 

 

Ques.25. There are _______  number of modulation techniques.

  1. 1
  2. 2
  3. 3
  4. 4

Answer.3.3

Explanation:-

When it is required to transmit digital data over a band-pass channel, it is necessary to modulate the incoming data onto a carrier wave (usually sinusoidal with fixed frequency limits imposed by the channel. The data may represent digital computer outputs or PCM waves generated by digitizing voice or video signals. The channel may be a telephone channel, microwave radio link, satellite channel, or optical fiber.

 In any event, the modulation process involves switching or keying the amplitude, frequency, or phase of the carrier in accordance with the incoming data. Thus there are three basic modulation techniques for the transmission of digital data; they are known as

  1. Amplitude shift keying (ASK)
  2. Frequency-shift keying (FSK)
  3. Phase-shift keying (PSK)

The above techniques can be viewed as special cases of amplitude modulation, frequency modulation, and phase modulation respectively.

 

Ques.26. In frequency modulation

  1. Amplitude of carrier wave remains the same
  2. Frequency of carrier wave remains constant
  3. Both frequency and amplitude of carrier wave vary
  4. Signal gets distorted

Answer.1. Amplitude of carrier wave remains the same

Explanation:-

Frequency Modulation (FM)

The process by which the frequency of a carrier wave is varied in accordance with the modulating signal is called frequency modulation. The process of frequency modulation is shown graphically in Fig

The three waves shown in the figure are:

  1. Signal
  2. Carrier
  3. The resultant frequency-modulated wave.

When a signal of frequency fs is modulated with the carrier wave of frequency fC, a resultant modulated wave is produced. When a sinusoidal carrier is frequency-modulated, its instantaneous frequency is varied in accordance with the characteristics of the modulating signal. When the modulating signal is also sinusoidal, the frequency of the modulated carrier wave will vary sinusoidally.

frequency modulation

The figure illustrates the effect of frequency modulation. The carrier wave has its highest frequency when the modulating waveform is at its maximum amplitude and has its lowest frequency when the modulating waveform is at its minimum value. When the modulation has zero amplitude the carrier wave is at its resting frequency. In between these values, the frequency of the carrier wave increases or decreases by an amount proportional to the amplitude of the modulating wave. The amount by which the carrier frequency changes from its resting frequency is called the frequency deviation.

The following points are worth noting:

(i) The amplitude of the modulated wave is the same as that of the carrier wave.

(ii) The frequency of the modulated wave varies in accordance with the message signal.

Advantages:

1. Better audio quality.

2. High transmission efficiency.

3. Noiseless reception.

Limitations:

1. A smaller area of reception.

2. The wider channel is needed.

3. Equipment used is more complex and costly.

 

Ques.27. In mercury arc rectifier the mercury is used as

  1. Cathode
  2. Conducting medium
  3. Ionizing medium
  4. Electron accelerator

Answer.2.Conducting medium

Explanation:-

Similar to a common diode, a mercury arc rectifier consists of a cathode and an anode. Mercury is used as the cathode and the anode is made of iron.

When the rectifier is in operation, mercury forms an arc between the mercury surface and the iron anode. This arc contains ionized mercury vapor consisting of electrons and positive ions. The area on the surface of the mercury where the arc concentrates is known as the cathode spot. This is a region of high temperature and most of the ionization (formation of irons by the loss of electrons), takes place here. In the arc, the electrons are attracted by the positive anode whereas positive ions are attracted by the negative cathode. Since the positive ions are heavy, they carry a lot of kinetic energy which is released in the Bonn of heat energy when they strike the cathode.

MERCURY ARC RECTIFIER

A part of or all the mercury vaporizes and the vapor atoms ionize and serve as the conducting medium in the tube. Hence the mercury vapor may be used as the conducting medium in a diode of the gas-filled type and is widely used in heavy-duty rectifiers. Just as in other types of diodes, the cathode of the mercury-vapor rectifier must be heated before conduction can take place. The use of mercury as the cathode permits the rectifier to withstand heavy momentary overloads without damage. The heat of the arc produces the necessary ionization of the mercury vapor, and conduction takes place through the external anode circuit if complete. 

 

Ques.28. For single-phase rotary converters, when the power factor is unity, the ac to dc current ratio will be

  1. 1
  2. 0.707
  3. 1.4
  4. 0.5

Answer.3. 1.4

Explanation:-

A rotary converter transforms an ac voltage into a dc voltage through rotation of a single winding in a constant stationary field. The stator winding carries both ac and dc voltages superimposed over each other.

The synchronous or rotary converter is a single machine with one armature and one field. It combines the function of a synchronous motor and a D.C generator. It receives alternating current through a set of slip ring at one end of an armature that rotates at synchronous speed and delivers direct current from the opposite end through a commutator and a set of brushes.

The principle of operation of the rotary converter is based on the fact that an alternating e.m.f. is induced in the armature of a D.C. machine and is rectified only by means of a commutator; therefore, when the armature winding is connected directly to slip rings, A.C voltage is obtained across the rings and the machine may be connected through the latter to an A C. circuits.

The formula of the relation between AC current and DC current is given as

Isr = 2√2.Idc.cosφ/m

Where

Isr = A.C line current at slip ring

Idc = Dc current

m = number if rings

Since in the given question, the power factor is unity, therefore, cos φ = 1

2π/m = 1

For single phase 2 ring, the ratio between AC current and DC current will be

Isr = 2√2.Idc/2

Isr/Idc = 1.414

 

Ques.29. When a material becomes a superconductor, its resistivity becomes?

  1. Very small
  2. Zero
  3. About 10%
  4. About 20% of the normal value

Answer.2. Zero

Explanation:-

Superconductivity is a phenomenon in which certain metals, alloys, and ceramics conduct electricity without resistance when it is cooled below a certain temperature called the critical temperature.

A superconductor is a material that loses all its resistance (offers zero resistance) to the flow of electric current when it is cooled below a certain temperature called the critical temperature or transition temperature TC. Examples: Mercury (Hg), Zinc (Zn), Vanadium (V), Tin (Sn), and Niobium (Nb).

 

Ques.30. The INPUT of the logic gate is 0. The output is 1. The gate is

  1. NAND or XOR
  2. OR or XOR
  3. AND or XOR
  4. NOR or Ex-NOR

Answer.4. NOR or Ex-NOR

Explanation:-

NOR gate

The NOR gate has two or more input signals but only one output signal. All inputs must be low and get a high output. In other words, the NOR gate recognizes only the input words whose bits are all 0S.

Nor circuit

NOR means NOT OR, i.e. the OR output is NOTed. So, a NOR gate is a combination of an OR gate and a NOT gate.
Fig shows the logical structure of the NOR gate which is an OR gate followed by a NOT gate. Therefore, the final output is NOT the OR of the inputs. A small invert bubble has been added to the OR symbol to form the NOR symbol.

Nor gate

Exclusive NOR (Ex-NOR) Gate

The exclusive-NOR gate, abbreviated Ex-NOR, is an Ex-OR gate, followed by an inverter. An Exclusive-NOR gate has two or more inputs and one output. The output of a two-input Ex-NOR gate assumes a HIGH state if both the inputs assume the same logic state or have an even number of 1s, and its output is LOW when the inputs assume different logic states or have an odd number of 1s.

Ex nor symbol

The logic symbol of the Ex-NOR gate is shown in above Fig. and its truth table is given in Table. From the truth table, it is clear that the Ex-NOR output is the complement of the Ex-OR gate.

Ex nor output

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