RRB JE Electrical Solved Question paper 2012 | RRB JE Electrical

Answer.3. Stored Energy

Explanation:-

In electrical or electronic systems, the term transient designates a period of time during which the system under study is not at its equilibrium state. For instance, it can be the time necessary to let the system startup and reach its nominal output.

We knew that circuit elements like capacitors and inductors are energy storing devices. Energy storing elements like a capacitor or an inductor cannot change instantaneously. The time taken by a circuit to change from one steady-state to another steady state is called transient time. Severe transient behavior of a circuit or system having an energy storing element may cause excessive oscillations as the circuit changes from one energy state to the other. 

Answer.1. Free Electrons

Explanation:-

The process of adding either a pentavalent element or a trivalent element to a pure semiconductor is called doping. The doped semiconductor is called an extrinsic semiconductor. The doping material that is added to a pure semiconductor is also referred to as impurity material. Depending upon whether a pentavalent or trivalent doping material is added, extrinsic semiconductors are respectively called n-type semiconductors or p-type semiconductors. 

The number of charge carriers, either an excess of electrons or deficiency of electrons (causing holes in a material), can be altered by doping or adding very small quantities of impurities to a semiconductor material. A doped semiconductor always has an excess of one type of charge carrier. For example, excess electrons make it an N-type semiconductor “N” stands for negative. Excess holes make it a P-type semiconductor.”P” stands for positive. Each of these materials responds to positive or negative current flow oppositely.

Answer.1. Resistance is less

Explanation:-

Moving iron instruments are of two types: attraction type and repulsion type. In both types, the principle of operation is the force experienced by an iron piece in the presence of a magnetic field. The necessary magnetic field is produced by the ampere-turns of a current-carrying coil. When used as an ammeter the coil has fewer turns of thick wire so that the ammeter has low resistance.

When used as a voltmeter, the coil has a high impedance so as to draw a small current as possible since it is connected in parallel. Since the current through it would be small, it has to have a large number of turns to produce the necessary ampere-turns.

Answer.2. Infinite

Explanation:-

Since in an open circuit the value of current is zero this is only possible when the resistance of the circuit is infinite because the higher the resistance less current will flow through the circuit.

Answer.3. Amplitude is varied according to the instantaneous value of modulating wave

Explanation:-

Modulation is the process of modifying the characteristics of one signal in accordance with some characteristic of another signal. In most cases, the information signal, be it voice, video, binary data, or some other information, is normally used to modify a higher-frequency signal known as the carrier. The information signal is usually called the modulating signal, and the higher-frequency signal which is being modulated is called the carrier or modulated wave. The carrier is usually a sine wave, while the information signal can be of any shape, permitting both analog and digital signals to be transmitted. In most cases, the carrier frequency is considerably higher than the highest information frequency to be transmitted.

Amplitude Modulation

In AM, the information signal varies the amplitude of the carrier sine wave. In other words, the instantaneous value of the carrier amplitude changes in accordance with the amplitude and frequency variations of the modulating signal. The figure shows a single-frequency sine wave modulating a higher-frequency carrier signal. Note that the carrier frequency remains constant during the modulation process but that its amplitude varies in accordance with the modulating signal. An increase in the modulating signal amplitude causes the amplitude of the carrier to increase. Both the positive and negative peaks of the carrier wave vary with the modulating signal. An increase or decrease in the amplitude of the modulating signal causes a corresponding increase or decrease in both the positive and negative peaks of the carrier amplitude.

Answer.2. It gives a linear output

Explanation:-

• The low-frequency oscillators (sub-harmonic oscillators) are capable of producing signals in the range of about 1 Hz to 1 MHz.
• Most instruments in this category cover this total frequency range in a number of switched ranges, allowing fine-tuning within each smaller range with a potentiometer or similar variable device.
• The oscillator circuits traditionally used in low-frequency oscillator test equipment are sub-harmonic oscillators; that is, they produce sinusoidal output signals.
• In electronic terms, a harmonic oscillator is an amplifier that derives its input from its own output. The only part of the output signal is fed back to the input, the remainder is available as an output signal to the following equipment. 

Answer.3. Full load

Explanation:-

Hopkinson’s Test

Hopkinson’s test is also referred to as the regenerative or Back-to-Back test.  In Hopkinson’s test, we require two identical dc machines. This is a regenerative test. Both the dc machines are mechanically and electrically coupled and are tested simultaneously. Hopkinson’s test is conducted on full load.

One of the machines will act as a dc motor and the other as a dc generator. The motor will rotate the generator and the generator will supply power to the motor. Now the question arises, if both machines help each other, then which one will supply the losses. The answer is external dc supply, which means, the input of the external supply will provide the power losses of both the machines. Now the voltage across switch S should be zero and only then we can declare that similar polarities of the machines are connected. So, when the switch S is closed both the dc machines are in perfect parallel connection and there is no chance of the presence of circulating currents between the two machines.

Since in the case of the generator, the induced emf minus the armature drop is the terminal voltage and in the case of the motor the induced emf plus the armature drop is the terminal voltage, the excitation of the generator will be more than the excitation of the motor to maintain the terminal voltages equal to one another.

Obviously, the no-load iron loss and stray loss will not be equal for both the machines even though the machines are identical in all respects. Stray load loss is a very important factor in determining the efficiency of a dc machine. If we ignore this, we cannot determine the actual efficiency of the dc machine properly due to the following reasons. Stray load loss is the additional copper loss that occurs in the conductors on account of the non-uniform distribution of alternating currents. This increases the effective resistance of conductors and that is nothing but the skin effect.

When the conductors carry load current, the teeth of the core get saturated and as a result, more flux passes down the slots through the copper conductors and thus setting up the eddy current losses in them. Due to the flux distortion, the net increase in the core loss occurs and the extra core loss is nothing but the core stray load loss.

Advantages of Hopkinson’s Test

• The two machines are tested under loaded conditions so that stray-load losses are accounted for.
• Since it is a regenerative test, the power drawn from the mains is only that needed to supply losses. The test is, therefore, economical for a long-duration test like a “heat run”.
• There is no need to arrange for the actual load (loading resistors) which apart from the cost of energy consumed, would be prohibitive in size for large-size machines.
• By merely adjusting the field currents of the two machines, the load can be easily changed and a load test conducted over the complete load range in a short time.

Drawbacks of Hopkinson’sTest

• Both machines are not loaded equally and this is crucial in smaller machines.
• Since a large variation of field currents is required for small machines, the full-load set speed is usually higher than the rated speed and the speed varies with load. The full load in small machines is not obtained by cutting out all the external resistance of the generator field. Sufficient reduction in the motor field current is necessary to achieve full-load conditions resulting in speeds greater than the rated value.
• There is no way of separating the iron losses of the two machines, which are different because of different excitations. Thus the test is better suited for large machines.

Answer.3. Common collector

Explanation:-

Common-Collector Configuration

In a common-collector configuration, the collector terminal of a transistor is connected as a common terminal between input and output as shown in fig. The base and collector terminals are used to apply the input signal whereas the output signal is obtained. The CC configuration is also commonly known as the emitter follower or the voltage follower. This is because the output signal across the emitter is almost the replica of the input signal with little loss. In other words, there is no phase inversion between input and output in the emitter follower circuit. The output voltage is in phase with the input voltage and hence the voltage gain will be a maximum of one. The common-collector configuration is used primarily for impedance-matching purposes since it has a high input impedance and low output impedance, opposite to that of the common-base and common-emitter configuration.

Answer.3. All type of oscillator produce the sine wave

Explanation:-

 Generally, oscillators are classified into two broad categories according to wave shapes such as

1. Sinusoidal oscillators
2. Nonsinusoidal or relaxation oscillators

Sinusoidal oscillators produce a sine wave output whereas non-sinusoidal oscillators produce square or pulsed output. A multivibrator circuit is a type of non-sinusoidal oscillator.

Answer.1. Class C

Explanation:-

The frequency multiplier is used for converting the narrowband signal into a wideband signal and it raises the carrier frequency. The frequency multiplier stage consists of a non-linear device whose output is tuned to the desired harmonic. A frequency multiplier is a class C amplifier whose output frequency is some integer multiple of the input frequency. Most frequency multipliers increase the frequency by a factor of 2, 3, 4, or 5. Because they are class C amplifiers, most frequency multipliers also provide a modest amount of power amplification. 

Note:- The amplifiers that are, class A, class B, and class AB amplifiers, are linear amplifiers where the amplitude and phase of the output signal are directly related to the amplitude and phase of the input signal. In applications where linearity is not the main issue and efficiency is of primary concern, class C, class D, class E, and class F amplifiers are used.