SSC JE electrical question paper 2019 with solution SET-2

SSC JE electrical question paper 2019 with solution SET-2

Ques.1. The type of insulator used on lines up to 33 kV is?

  1. Pin insulator
  2. Reel insulator
  3. Post insulator
  4. Strain insulator

Choose the correct options

  1. A, D
  2. B, C
  3. C, D
  4. B, D

Ans.1.  A,D

Explanation:-

Pin Insulator:- Pin insulators are commonly used on rural and urban primary distribution lines. These can be used up to 33 kV. Pin insulators can be of a single piece or multipiece. The pin type of insulators is uneconomical beyond 33 kV operating voltage. Also, the replacement of these insulators is expensive. For these reasons for insulating overhead lines against higher voltages, suspension insulators are used.

Strain Insulator:- The strain insulators are exactly identical in shape with the suspension insulators.  These are used to take the tension of the conductors at line terminals, at angle towers, at road crossings, and at the junction of overhead lines with cables. These insulators are, therefore, known as tension or strain insulators. For low voltage lines (less than 11 kV), shackle insulators are used as strain insulators.

 

Ques.2. The water hammer effect is expected in a

  1. Draft tube
  2. Penstock
  3. Surge tank
  4. Turbine casing

Ans.2. Penstock

Explanation

When the generator of a water turbine is disconnected from a power network, the turbine speed starts to increase. Consequently, the turbine controller closes the inflow to the turbine, thus creating a water hammer in the penstock.

Water hammer effects in the penstock are created by any changes in discharge through the turbine, caused by changes in the connected power network. Sometimes, the entire system becomes unstable due to the mutual influence of a turbine equipped with a controller and to unsteady flow in the penstock. In such a case, even small variations in pressure in the penstock may increase steadily and perilously.

 

Ques.3. The change in DC excitation of a synchronous motor will also change the

  1. Speed of the motor
  2. Load angle
  3. Supply voltage
  4. Power factor

Ans.4. Power factor

Explanation:-

When the DC excitation i.e. field current of the synchronous motor is changed keeping the load constant, the synchronous motor reacts by changing its power factor of operation.

Consider a synchronous motor operating at a certain load. The corresponding load angle is δ. At the start, consider normal behavior of the synchronous motor, where excitation is adjusted to get Eb = V i.e. induced e.m.f. is equal to an applied voltage. Such excitation is called Normal Excitation of the motor. The motor is drawing certain current la from the supply and the power input to the motor is say Pin.

Pin = √3 VL IL cosφ

Under Excitation:- When the excitation is adjusted in such a way that the magnitude of induced EMF is less than the applied voltage (Eb < V) the excitation a called Under excitation. The p.f. cosφ decreases and becomes more and more lagging in nature.

Over Excitation:- The excitation to the field winding for which the induced e.m.f. becomes greater than the applied voltage (Eb > V), the excitation a called over-excitation. In this case, the power factor is leading in nature.

 

Ques.4. Four relations have been given. Select the correct relation?

  1. MMF = Flux/Reluctance
  2. Flux = MMF/Permeance.
  3. MMF = Flux/Permeance
  4. MMF = Flux/Reluctance

Ans.3. MMF = Flux/Permeance

Explanation:-

The amount of flux produced by the magnet indicates the strength of the magnet. The more the magnetizing force (MMF), the more is the flux produced. The more the opposition to the flux path (i.e., reluctance or magnetic resistance) less is the flux produced. This relationship is expressed as

Flux = MMF/ Reluctance

Reluctance is the opposition offered by the material in the flux path to the establishment of the flux. The reluctance in a magnetic circuit is similar to the resistance in an electric circuit. Reluctance is the inverse of permeance.

MMF = Flux/Permeance

 

Ques.5. In Boolean algebra (A. Ā) + A

  1. 0
  2. 1
  3. A
  4. Ā

Ans.3. A

Explanation:-

According to Boolean algebra law

A. Ā = A

A + A = A

Therefore in the given question

(A. Ā) + A = A + A = A

 

Ques.6. Both the main winding and auxiliary winding of a single-phase induction motor is placed on the ______?

  1. Both stator and rotor
  2. Stator
  3. Rotor
  4. Neither stator nor rotor

Answer.2. Stator

Explanation:- 

A single-phase induction motor with only one winding on the stator cannot produce any starting torque. Hence, some extra arrangement is required to start the motor. In the running condition, the motor is capable of developing the torque with only one winding on the stator.

The simplest method of starting is to provide an auxiliary winding on the stator in addition to the main winding. The two windings are placed in the stator with their axes displaced by 90 electrical degrees in space.

 

Ques.7. Which of the following power plants employs the Rankine cycle?

  1. Nuclear power plant
  2. Solar power plant
  3. Thermal power plant
  4. Hydro-electric power plant

Answer.3. Thermal power plant

Explanation:- 

A thermal power station is a power station in which heat energy is converted to electric power.

  • As shown in the figure the plant generates electricity by first boiling liquid water to produce steam. The source of the heat may be either fossil fuel energy or nuclear energy.
  • The expansion of the water during the transformation from the liquid to the gaseous state creates a pressure, which is used to drive a turbine.
  • The turning of the turbine powers the generator, which produces electricity.
  • The steam from the turbine is then condensed back to liquid water and recycled to the boiler. Condensation is achieved by running cooling water through a long coiled pipe exposed to the steam.
  • Heat is transferred through the pipe from the hot steam to the cooling water until the steam condenses. The whole process is known as a Rankine cycle.

Rankine-cycle

The Rankine cycle is a model used to predict the performance of steam turbine systems. It was also used to study the performance of reciprocating steam engines.  The heat is supplied externally to a closed loop, which usually uses water as the working fluid. It is named after William John Macquorn Rankine, a Scottish polymath and Glasgow University professor.

 

Ques.8. The torque angle of a 3-phase synchronous motor depends on its _____

  1. Excitation
  2. Speed
  3. Load
  4. Supply voltage

Answer.3. Load

Explanation:- 

The rotor of a synchronous motor rotates in synchronism with the rotating flux of the stator. The increases in shaft load cause the rotor magnets to change their angular position with respect to the rotating flux. The torque angle is defined as

T = K sinδ

The angle δ is called torque angle, power angle, or load angle.

A synchronous motor operates at the same average speed for all values of the load from no load to peak load. When the load on a synchronous motor is increased, the motor slows down just enough to allow the rotor to change its angular position in relation to the rotating flux of the stator and then goes back to synchronous speed.

Similarly, when the load is removed. it accelerates just enough to cause the rotor to decrease its angle of lag in relation to the rotating flux, and then goes back to synchronous speed.

Torque Angle Characteristics of Synchronous Motor

When the load is increased, the δ also increases and the rotor falls back more and more with respect to resultant, weakening the magnetic locking. At δ = 90° the rotor comes out of synchronism and the corresponding torque pulling the rotor out of synchronism is called pull-out torque. The corresponding power is called pull-out power.

Torque Angle Characteristics of synchronous motor

The motor comes to stop if the load is increased beyond the point when δ = 90°. The torque-angle characteristics for the synchronous machine are shown in Fig. for both motoring as well as generating action.

The δ is positive if the rotor field lags behind the resultant and is negative if the rotor field advances the resultant. The δ is positive in motoring action and negative in the generating action.

As the load on the synchronous motor increase, there is no change in its speed. But what gets affected is the load angle δ i.e. the angle by which the rotor axis retards with respect to the stator axis.

Hence as load increases, δ increases but speed remains synchronous.

 

Ques.9.Permeance is analogous to an electrical circuit?

  1. Conductance
  2. Permittivity
  3. Conductivity
  4. Resistance

Answer.1. Conductance

Explanation:- 

The permeance of the magnetic circuit is defined as the reciprocal of the reluctance.

Permeance = 1/Reluctance

It is defined as the property of the magnetic circuit due to which it allows the flow of the magnetic flux through it. Permeance is analogous to conductance in an electric circuit.

 

Ques.10. 1 kcal is approximately equal to

  1. 420 watt-hour
  2. 4200 watt-sec
  3. 4.2 joules
  4. 420 joules

Answer.2. 4200 watt-sec

Explanation:- 

1 kcal = 4200 J

Watt = Joule/S

joule =  watt/sec

4200 joule = 4200 watt-sec

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