DMRC JE Electrical Question Paper With Explained Solution – 2016

Ques 21. Two capacitor C1 and C2 have C1 =20 µF and C2 = 30µF, are connected in parallel across a 100V source. The net capacitance of the circuit is?

  1. 50 µF
  2. 60 µF
  3. 25 µF
  4. 100 µF

Since the two capacitance are connected in parallel therefore there Net capacitance will be equal to

C = C1 + C2

C = 20 + 30 = 50μF

 

Ques 22. Power-factor of industrial arc and induction furnace is:

  1. High Lagging
  2. High Leading
  3. Low Lagging
  4. Very Low Lagging

Arc lamps

Arc lamps and electric discharge lamps operate at low lagging power factor. An electric arc is essentially unstable and requires the use of ballast or choke to make it stable. It is due to the requirement of the magnetizing current by the choke that all devices employing electric arc have low power factor.

Induction and Arc Furnace:

Induction heating furnace such as arc furnace and induction furnace usually operates on the principle of striking of an arc which requires very high magnetizing current and operates on very lagging power factor

 

Ques 23. A 100mA meter has an accuracy of ± 2%. Its accuracy while reading 50mA will be:

  1. ± 1%
  2. ± 2%
  3. ± 4%
  4. ± 20%

Limiting Error =  accuracy * full-scale reading

= ± 2 × 100 = ± 200%

Now Accuracy for 50 mA will be

Accuracy = Limiting Error / Scale reading

= 200 ÷ 50 = ± 4%

 

Ques 24. A series inductor is normally added in a thyristor circuit for achieving protection against:

  1. High current
  2. High voltage
  3. High di/dt
  4. High dv/dt

Overcurrents may damage the semiconductor devices, due to the junction temperature exceeding the rated value. Thus, there is a need for overcurrent protection of semiconductor devices. Overcurrent Protection is also called as di/dt protection.

di/dt Protection
  • When a forward voltage is applied to a thyristor, and it is turned ON by means of a gate current, conduction of anode current across the junction commences in the immediate neighborhood of the gate connection and spreads from there across the whole area of the junction.
  • Modern thyristors are so designed that the conduction area spreads as rapidly as possible. Nevertheless, if the rate of rising of anode current di/dt is very high, then, a local hot spot will be formed in the neighborhood of the gate, due to the high current density in that part of the junction that has commenced to conduct.
  • This localized heating may permanently damage the device.
  • To limit di/dt to safe limits, a small inductance L is connected in series with the device as it prevents the high rate of change of current through it.

 

Ques 25. Clamp-on ammeter is used for measurement of:

  1. Large alternating currents
  2. Small direct currents
  3. Small alternating currents
  4. Large direct currents

Clamp-on ammeters are much easier to use than inline-ammeters. They are designed with a movable jaw that can be opened, this allows measurement of the current in a conductor without the need to make physical contact with it, or to disconnect it for insertion through the probe.

When current is flowing through a conductor, the magnetic field is set up around that conductor. The more current flowing through the conductor, the strong the magnetic field will be. The clamp-on ammeter picks up the strength of the magnetic field and converts the magnetic field’s strength into a proportional value of Current. The clamp-on ammeter is used to measure the value of large alternating current.

 

Ques 26. Overhead transmission lines are provided with earth wires:

  1. To protect the transmission line from the direct lightning strike
  2. To protect the transmission line insulation from the indirect lightning strike
  3. To balance the line currents
  4. To provide the path for neutral current

Which of these statement/s is/are correct?

  1. 1 and 3
  2. 1 and 4
  3. 1 only
  4. 2 only

Earth wire is also called as the guard wire and it mainly protects lines from lightning.
If in case lightning struck then it carries the excessive current inrush to ground.
Its radius is much smaller than the actual transmission wire because as the resistance is inversely proportional to an area of cross section and as the cross-section decreases, the resistance increases.
The increased resistance of earth wire is able to withstand high inrush of current caused due to lightning and it will safely guide this inrush into the ground.
It is placed above all the conductors mainly because if lightning struck then it will strike at the uppermost point in the line configuration and protect the actual conductors.

 

Ques 27. An ammeter has a current range of 0–5 A, and its internal resistance is 0.2Ω. In order to change the range to 0–25 A, what should be the value of resistance added and how it would connect with the meter (i.e. series/parallel)?

  1. (0.05Ω/series)
  2. (0.05Ω /parallel)
  3. (0.20Ω/parallel)
  4. (0.20Ω/series)

In ammeter designs, external resistors added to extend the usable range of the movement are connected in parallel with the movement rather than in series as is the case for voltmeters. This is because we want to divide the measurer current, not the measured voltage, going to the movement and because current divider circuits are always formed by parallel Resistance.

Current in the shunt

Ish = Ifl − Ifs

= 25 − 5 = 20 A

As the two resistances are in parallel, therefore, the voltage drop across them is the same

Vsh = Vf

Ish × Rsh = Ifl × RI

20 × Rsh = 5 × 0.2

Rsh = 0.05 Ω/Parallel

 

Ques 28. Power consumed by a balanced 3-phase, 3-wire load is measured by two wattmeter method. The first wattmeter reads twice that of the second. Then what will be the load impedance angle in radian?

  1. (π/6)
  2. (π/3)
  3. (π/2)
  4. (π/4)

Let the reading of the second wattmeter is W
Hence the reading of the first wattmeter will be 2W

The power factor of the wattmeter is

tanΦ = √3(W1 – W2)(W1 + W2) = √3 (2W – W)(2W + W)

tanΦ = (√3W)(3W)

Φ = tan-1(√3W)/(3W)

Φ = tan-1(1/√3)

Φ = 30°

Φ = (30 × π)/180 = π/6

 

Ques 29. The X/R ratio for distribution lines is:

  1. Less than unity
  2. More than unity
  3. Equal to unity
  4. None of these is necessary

  • Every electrical circuit contains resistance (R) and inductive reactance (X) and they are electrically in series. Their combined effect is called Impedance.
  • X depends on the equivalent distance between phases (which increases at higher voltages) and the GMR of the conductor. so you can have (X/R) ratio as high as 10 -15 for a 400 kV line.

 

Ques 30. Corona loss is minimum in:

  1. Monopolar HVDC line
  2. Bipolar HVDC line
  3. Both in Monopolar& Bipolar HVDC lines
  4. EHV AC lines

Since Corona loss increase with the frequency, in fact, it is known be proportional to (f + 25), The frequency in DC voltage is 0. Therefore for a given conductor diameter and applied voltage, there is much lower corona loss and hence less radio interference with DC.

Corona losses on DC lines is an important factor and should be considered while calculating the line losses for designing the HVDC lines. Losses in HVDC monopolar lines are less when compared to those in HVDC bipolar lines.

The reason behind it the Monopolar link work with the negative polarity and the bipolar link work with the positive and negative polarity. The corona effect is considerably less with negative polarity as compared with positive polarity.

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