SSC JE 2012 Electrical question paper with solution

Ques 51. In a thyrite lightning arrester the resistance

  1. Decrease linearly with the applied voltage
  2. Is high at low current and low at high current
  3. Is low at low current and high at high current
  4. Increase linearly with the applied voltage

Thyrite is used widely in lightning arresters. It does not follow Ohm’s law, for each time the voltage is doubled the current increases 12.6 times. This means the resistance decreases as the current and voltage increase

 

Ques 52. A consumer is offered the following rate of tariff. He has to pay a fixed charge of Rs. 1,000 per month and a running charge of Rs. 4.50 per unit consumed. If the consumer runs a motor load of 1kW at 0.85 power factor lagging on an average of 15 hours per day, his annual bill is

  1. 25637. 50
  2. 36637.50
  3. 40985.29
  4. 32941.88

Fixed charge for consumer = 12 x 100 = 1200 Rs

Running Charge = 1 x 4.5 x 15 x 365 = 24637.5 Rs

Annual bill = Running charge + fixed charge

= 24637.5 + 1200 = 36637.50

 

Ques 53. Glare is caused due to

  1. Excessive lighting contrast in the field of vision
  2. Excessive luminance
  3. Both 1 and 3
  4. None of the above

Glare is a visual sensation caused by excessive and uncontrolled brightness.Glare is caused by a significant ratio of luminance between the task (that which is being looked at) and the glare source.

 

Ques 54. Which of the following method is suitable for heating of conducting medium?

  1. Induction heating
  2. Radiant heating
  3. Eddy current heating
  4. Both 1 and 3

Induction heating is the process of heating an electrically conducting object (usually a metal) by electromagnetic induction, through heat generated in the object by eddy currents. An important feature of the induction heating process is that the heat is generated inside the object itself via heat conduction.

Eddy current heating

This type of heating is also one of the forms of induction heating. Due to a high-frequency current ( 10 kHz to 400 kHz),  the conversion of electromechanical energy into heat takes place in the charge itself.

 

Ques 55. Furnaces used for cremation use are

  1. Dielectric heating
  2. Arc heating
  3. Resistance heating
  4. Induction heating

A resistance furnace is an electric furnace in which heat is generated by the conductor that offers resistance to the passage of a current through them. Temperature up to 3000°C can be obtained in the furnace chamber hence they are preferred for cremation.

 

Ques 56. In a fluorescent tube circuit, a high voltage surge is produced by

  1. Heater
  2. Electrode
  3. Starter
  4. Choke

There are mainly two functions of the choke coil

  1.  Limit the current.
  2. Produce high voltage across tube light.

Produces High voltage Across Tubelight

  • Choke is nothing but the coil/ballast ( inductor) which is used to induce the high voltage across it. as we know that inductor has the property to induce high voltage for a brief period of time, this high voltage is required to ionize the gases in the starter.

 

Ques 57. In AC operated electromechanical contractors, the problem of chattering is eliminated by

  1. Increasing the conductor cross-section
  2. Using aluminum instead of copper as the conductor material
  3. Laminating the electromagnet’s core placing copper shading band on the pole face of electromagnets
  4. Placing copper shading band on pole face of electromagnets

The main reasons for an Electromechanical contactor or coil to ‘chatter’ is under voltage, an incorrect device (DC coil or no shading ring) or wrong/low frequency.

To eliminate this undesirable feature a shading coil is added. A shading coil usually takes the form of a single turn of heavy copper wire, or a copper ring, placed around one half of the end of the pole piece. With suitable adjustments of these copper shading bands, the maximum phase difference, Φ = 90°, is obtained between the fluxes electromagnets.

.

 

Ques 58. Magnetic blowouts coils are generally used in

  1. Vacuum circuit breaker
  2. Air breaker circuit breaker
  3. Air Blast circuit breaker
  4. Oil circuit breaker

Magnetic blow-outs have been used in contactors, circuit breakers and controllers they are particularly used for low voltage alternating current. They extinguish the arc by means of a magnetic blast. It is limited to circuits up to 11 kV since air circuit breaker is used in low voltage, therefore, they are preferred for Air breaker circuit breaker.

 

Ques 59. The pitch factor for a full pitch winding of a synchronous machine is

  1. 0.5
  2. 0.9
  3. 0.0
  4. 1.0

Pitch factor is the ratio between the winding pitch and pole pitch

Pitch factor= Winding pitch/Pole pitch

In the case of full pitch winding, the winding pitch is equal to pole pitch. in other words, for a full pitch winding the pitch factor is equal to unity and the winding is known as “full pitch winding”.

 

Ques 60.  An AC meter of resistance Rm and reactance Xm has been connected in series with a resistance Rs. A shunt of impedance (Rsh + jXsh ) is applied in parallel to the existing combination of meter and Rs. The current division across the two branches will be independent of frequency when

numerical 60 1

 

4

Voltage in two parallel branches are same

V = Im(Rm + Rs + jXm)

V = Ish(Rsh + jXsh)

Im/Ish = (Rsh + jXsh)/(Rm + Rs + jXm)

This ratio should be independent of frequency. Performing rationalization and putting the imaginary part equal to zero as

9

For frequency Independency, the Imaginary part is zero

10

i.e., the shunt time constant should be equal to the meter time constant.

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