SSC JE electrical question paper 2019 with solution SET-2

Answer.3. 2

Explanation:-

A phase sequence is an order or sequence in which the currents or voltages of different phases attain their maximum values. In a three-phase system, there are only two possible phase sequences: R-Y-B (positive Phase Sequence) and R-B-Y (Negative Phase Sequence).

Answer.3. Solar Power Plant

Explanation:-

Conventional Energy:-  Those sources of energy, which have accumulated in nature over a period of millions of years and cannot be quickly replaced when exhausted are called non-renewable sources of energy. This non-renewable source of energy is also called a conventional source of energy. Conventional sources of energy include natural gas, coal, petroleum, electricity (Nuclear, Thermal & Hydro Power plants).

Non-Conventional Energy:- Non-conventional sources of energy are renewable resources of energy like biomass, solar energy, wind energy, tidal energy, etc.

Answer.1. 1:2

Explanation:-

In order to achieve uniform illumination over a surface, the number of luminaries must be placed suitably. The space-height ratio plays a vital role in deciding the height of lamps from floor level and their spacing.

Space to height ratio

It is defined as the ratio of horizontal distance between adjacent lamps to the height of their mountings.

In order to obtain almost uniform illumination over the working plane, the distance between the lamps should not be too much. An ideal scheme could be developed, when there is a large number of small size lamps. But this would increase the installation cost. To have uniform illumination, the ratio of the spacing between the lamps and their height above the working plane (i.e., space-height ratio) should normally lie between 1 and 2.

Answer.4. Same magnetic flux

Explanation:-

In the same way that we can have series, parallel or series-parallel electric circuits, we can also have equivalent series and series-parallel (but not parallel) magnetic circuits.

In practice, a magnetic circuit may consist of several parts in a series of different lengths, cross-sectional areas, and permeabilities. In such a series magnetic circuits, all the reluctances of several parts will get summed up together (as resistors in an electric circuit) to form the net reluctance of the circuit.

In series magnetic circuits the flux passing through each part will be the same (as current in the series electric circuits).

Answer.2. Watt-hour/ ton-km

Explanation:-

Specific energy consumption

It is the ratio of  Specific energy output of driving wheels to the overall efficiency of transmission gear and motor. It is measured in Watt-hour/ton-km.

Factors affecting specific energy consumption

The following are the factors to be considered for the calculation of specific energy consumption:

1. Distance between stops
2. Acceleration
3. Retardation
4. Maximum speed
5. Type of train and equipment used
6. Nature of track

At a given scheduled speed, the specific energy consumption will be lower for higher acceleration and retardation, as this will in the coasting period. Sufficient precautions should be taken to limit the weight of the equipment and its effect on energy consumption. Also, the overall efficiency will affect the specific energy consumption.

Answer.4. No starting torque

Explanation:-

Synchronous motors are not self-starting. The DC source is applied to the rotor of the synchronous Motor. Click here to read why synchronous motor is not self starting. When the supply is initially switched on, the rotor will experience a force, first in one direction and then in the other direction, every cycle as the stator flux rotates around the rotor at synchronous speed. Therefore, the synchronous motor is not self-starting. However, if the rotor is rotated at or near synchronous speed, then the stator and rotor poles of opposite polarity will ‘lock together’ producing a turning force or torque which will cause the rotor to rotate at synchronous speed. A synchronous motor is not self-starting, as it has no starting torque. It is, therefore, necessary to use external means to start the motor.

Some method must be designed into the motor to bring it up to speed before the excitation current is applied to the rotor. Several methods are used to bring the motor to synchronous speed. One of these is to use a separate induction motor, or DC motor, to drive the rotor of the synchronous motor until an RPM near the synchronous speed is achieved. At that point, DC excitation is applied to the rotor’s windings and it will lock with the rotating magnetic field of the stator.

Answer.1. 0.2 A, 0V

Explanation:-

I_o = V/R = 20/100

I_o = 0.2 A

Since the given diode is an ideal diode therefore there is no voltage drop across it.

V_o = I_o × R_D

V_o = 0.2 × 0

V_o = 0

Hence I_o = 0.2 A & V_o = 0

Answer.3. Graphite is used as a moderator

Explanation:-

In nuclear power plant the graphite is used as a moderator. The job of the moderator is to absorb some of the kinetic energy of the neutrons to slow them down. This is because slow neutrons are more easily absorbed by uranium-235. A neutron slowed In this way can start the fission process.

In the nuclear reactor, there are also control rods. made of boron or cadmium. These absorb the neutrons and take them out of the fission process completely. When the control rods are fully inserted into the core. the chain reaction is almost completely stopped and the rate of production of heat is low. As the control rods are withdrawn. the rate of fission increases producing heat at a greater rate.

Answer.2. Fringing effect

Explanation:-

Magnetic Fringing

When flux enters into the air gap, it passes through the air gap in terms of parallel flux lines. There exists a force of repulsion between the magnetic lines of force which are parallel and having the same direction. Due to this repulsive force, there is the tendency of the magnetic flux to  (spread out) at the edge of the air gap. This tendency of flux to spread out at the edges of the air gap is called magnetic fringing.

Magnetic fringing increases the effective cross-sectional area of the air gap reduces the flux density. So leakage, fringing, and reluctance should be as small as possible. This is possible by choosing a good magnetic material and making the air Magnetic fringing gap as narrow as possible.

Answer.1. Three-phase, four-wire

Explanation:-

The distribution system of electric power system is classified as a primary distribution and secondary distribution.

Primary Distribution: The overhead lines or underground cables are used as feeders to connect the distribution transformer. The high voltage side of a distribution transformer is known as primary distribution or primaries. Common primary distribution voltages are 11 kV, 33 kV etc.

Secondary Distribution: The primary distribution lines connect to the distribution transformer, which is usually installed near the consumer’s area. This distribution transformer steps down the delivered 11 kV voltage to 440 V line-to-line, and it distributes by the three-phase four wires system. Therefore, the secondary of a distribution transformer is known as secondary distribution or secondaries.

Secondary distribution voltages are used by the residential, commercial, and light industry. Some of the Asian countries utility companies use three-phase four wires for power supply to the residential consumers. Their phase voltage and line voltages are 220 V and 440 V, respectively.

In North America, both single-phase and three-phase systems are also used based on the connected load. A single-phase three wires are used in the most residential distribution. In this case, the phase and the line voltages are 120 V and 240 V, respectively. 120 V is used for lower-rated electrical appliances, and 240 V can be used for higher-rated electrical usages such as electric vehicle charging.

For high-density residential and commercial areas, single-phase four wires are used. In this case, high rating appliances like motors use 208V.