RRB JE Electrical Solved Question paper 2012 | RRB JE Electrical

Answer.3. Limestone

Explanation:-

Any substance which promotes the melting of another is called a flux. The term is especially applied to those materials which promote the melting of earth and the separation of the metals from their oxides.

Fluxes, in practical use in the blast furnace, are lime, magnesia, clay, silex, and the foreign matter in the fuel.

Lime:- Limestones are generally applied in the blast furnace instead of burnt lime, though a weaker alkali. Limestone is not only an excellent flux, but it also has the proper that it can remove sulfur (S) from the liquid iron.

Answer.1. Separately excited generator

Explanation:-

The exciter supplies direct current to the field winding of the generator, at any voltage required to overcome the resistance of the winding. The rating of the exciter is specified as its output power, current, and voltage corresponding to the rating of the generator, recognizing the temperature limits of the generator’s field-winding. The exciter rating generally has some margin over this requirement, as defined when the generator is designed. The most common type of exciter used in the early years was the commutator type dc generator. This is very rarely used for new generators today. Either one of the following systems usually supplies the newer turbine generators:

• A shaft-driven alternator with solid-state diode rectifiers
• A solid-state thyristor-based rectifier supplied by a transformer, deriving its power from the power system or from the generator output
• A shaft-driven alternator with its output winding on the rotor, its output rectified by rotating solid-state rectifiers (commonly called a “brushless exciter”)

The normal function of the exciter is to provide the proper level of direct current to the generator field-winding, as required for the apparent power being supplied to the system, the terminal voltage, and the power factor of the generator load. In addition, the exciter must also be able to produce a ceiling voltage (which is maximum exciter voltage) and to operate at that condition for a specified brief period, as required by the voltage response ratio, which is specified in the excitation system’s specification. The voltage response ratio is a measure of the change of exciter output voltage in 0.5 s when a change in this voltage is suddenly demanded.

Answer.4. 1:3

Explanation:-

EMF induced in the Generator is given as

E = PφZN/60a

where

P = Number of poles

φ = flux per pole

N = Speed

Z = Number of conductor

a = Number of parallel path

For wave winding number of parallel path is always equal to 2 hence Generator A

a = 2

For Lap winding Number of parallel paths is equal to the number of poles, therefore, Generator B

a = p = 6

There the ratio of Generator A and B is

G_A/G_B = PφZN/60 × 2 ⁄ PφZN/60 × 6

G_A/G_B = 1/3

Answer.4. Bending moment, unbalanced magnetic pull and twisting stains

Explanation:-

Direct current generators and motors are similar in structure. Consequently, with few adjustments, they can interchange. A dc generator converts mechanical energy into electrical energy when driven by a prime mover and may be made to function as a motor by applying a voltage across the output terminals.

Armature Shaft

The shaft of the D.C machine is made from mild steel having a maximum breaking strength and should be able to withstand unbalanced magnetic pull on the armature core. The shaft is used to transfer mechanical power from or to the machine. The rotating parts like armature core, commutator, cooling fan etc. are keyed to the shaft.

An armature steel shaft is thicker in the middle than at the ends. Its construction enables the shaft to withstand the bending effects that the weight of the armature, the electromagnetic side-pull and the sideways drag of the pulley exert on the shaft. In addition, the shaft has to withstand the twisting strain that it is subject to and the out-of-balance stresses that occur on the shaft if an unbalanced armature core attaches. 

Answer.4. Both DC and AC signals

Explanation:-

CRO is an electronic instrument used for measuring voltage, time, frequency, phase shifts, and for observing the shape of an input waveform. It is also known as a fast x-y plotter which displays an input signal versus another signal versus time.

Measurement of voltage: CROs are voltage-dependent instruments and can be used for the measurement of voltages at any frequency within the range of the operation of the CRO. Either ac or dc voltages can be measured. The dc voltages are displayed as horizontal straight lines whereas the exact waveform is displayed for that of the ac signal. From the peak value of the ac signal, the RMS and other parameters can be determined. Not only can perfect sinusoidal waveforms be measured, but since the waveform is displayed, the amount of distortion and other parameters of interest can also be determined.

Answer.3. A fixed AC to a variable magnitude AC

Explanation:-

A cyclo converter is ac to ac converter that directly converts ac power at input frequency (usually the supply frequency) to an output ac power at a different (usually lower) frequency without employing an intermediate dc stage. It is, therefore, a single-stage frequency converter.

Answer.1. Magnetic field

Explanation:-

An electromechanical energy conversion device is essentially a medium of transfer between electrical energy and mechanical energy and vice versa. The structures of these devices may be different depending upon the functions they perform. Some devices are used for continuous energy conversions like motors and generators, while some devices are used to produce translational forces whenever necessary like solenoids, relays, electromagnets, etc. In the case of a motor, the input is electrical energy and the output is mechanical energy, while in generators the mechanical energy is converted into electrical energy. Various conversion devices may be different structurally, but they all operate on similar principles.

The coupling between the electrical energy and mechanical energy of these devices is either through a magnetic field or an electric field. In commercial applications, the use of a magnetic field as the coupling medium is more common as the energy storing capacity of the magnetic field is much more than that of the electric field.

Answer.3. III

Explanation:-

A synchronous machine rotates at a constant speed in a steady state. Unlike induction machines, the rotating air gap field and the rotor in the synchronous machine rotate at the same speed, called the synchronous speed. The synchronous speed depends on the frequency, f. of armature currents, and the number of field poles, P, dictated by the relation.

Ns = 120f/P

Answer.1. Resistance of the order of 1 ohm

Explanation:-

The low resistance refers to the resistance of the order of 1 ohm while medium resistance ranges from above 1 ohm to few kiloohms.