SSC JE 2010 Electrical question paper with Explained Solution | MES Electrical

Ques 1. The value of voltage across the diode in the figure given below is

0 Volt

4 Volt

8 Volt✔

Depend upon the value of R

There will be no current in the circuit as the diode is reverse biased and diode in reversed biased offer infinite resistance. You can think of a reverse-biased diode as an open circuit.

So, the whole voltage will across the diode i.e 8V

Ques 2. The temperature coefficient of an intrinsic semiconductor is

Zero

Positive

Negative✔

Like this of Metal

Properties of intrinsic semiconductors

They have four valence electrons

Their properties lie between that of conductors and insulators.

Their conduction property can be varied by varying the temperature.

They have a negative temperature coefficient of resistance.

Their properties can be varied by adding impurities to the semiconductor.

The resistivity of a semiconductor is greater than a conductor and less than an insulator.

Ques 3. A 4-pole, 1200 rpm DC lap wound generator has 1520 conductors. If the flux per pole is 0.01 weber, the emf of generator is

608 V

304 V✔

152 V

76 V

EMF Of generator is given by

E = PΦZN/60A

Φ = flux per pole in Weber

P = number of poles

Z = total number of armature conductor

N = rotation speed of the armature in revolution per minute (r.p.m)

A = number of parallel paths (Number of parallel paths in lap winding, A = P)

E = (4 x 0.01 x 1520 x 1200)/(60 x 4)

= 304 V

Ques 4. In a 3-phase induction, motor starting torque will be maximum when

R_{2} = 1/X_{2}

R_{2} = X_{2}✔

R_{2} = X^{2}_{2}

R_{2} = √X_{2}

The equation of torque is

When slip s = R_{2} / X_{2}, the torque will be maximum and this slip is called maximum slip Sm and it is defined as the ratio of rotor resistance to that of rotor reactance.

If supply voltage V is kept constant, then flux ɸ and E_{2} both remain constant. Hence,

At starting S = 1, so the maximum starting torque occurs when rotor resistance is equal to rotor reactance. So it can be proved that maximum starting torque is obtained when rotor resistance is equal to standstill rotor reactance. i.e. R_{2}^{2} + X_{2}^{2} =2R_{2}^{2 }.

Ques 5. The ratio of resistance of a 100 W, 220 V lamp to that of a 100 W, 110 V lamp will be at respective voltages

4✔

2

1/2

1/4

For 100 W, 220 V

R_{1} = V^{2}/P = 220^{2}/100 = 484

For 100 W, 110 V

R_{2} = V^{2}/P = 110^{2}/100 = 121

Ratio of resistance R1 and R2

R_{1}/R_{2} = 484/121 = 4

Ques 6. Two sinusoidal equations are given as

E1 = A sin(ωt + π/4)

E2 = B sin(ωt – π/6)

The phase difference between the two quantities is:

75°✔

60°

105°

15°

Phase difference Φ = Φ1 – Φ2

= π/4 – (-π/6) = 5π/12 = 5 x 180°/12 = 75°

Ques 7. The moderator used in fast breeder reactor is

Heavy water

Graphite

Ordinary water

None of the above✔

A breeder reactor is a nuclear reactor that produces more fissionable material than it consumes to generate energy. This special type of reactor is designed to extend the nuclear fuel supply for electric power generation. All fast neutron reactor designs use liquid metal as the primary coolant, to transfer heat from the core to steam used to power the electricity generating turbines.

A fast reactor needs no moderator to slow down the neutrons at all, taking advantage of the fast neutrons producing a greater number of neutrons per fission than slow neutrons. For this reason, ordinary liquid water, being a moderator as well as a neutron absorber, is an undesirable primary coolant for fast reactors.

Ques 8. The ratio of the puncture voltage to the flashover voltage of an insulator is

Equal to one

Zero

Lower than One

Greater than One✔

Insulators are rated by three voltages:

Working voltage (or rated voltage)

Puncture voltage

Flashover voltage.

The working voltage rating is the voltage at which an insulator is designed to bear the steady state voltage stress. If the line voltage is V_{LL}, the working voltage will be V_{LL}/√3.

The flashover voltage is the voltage at which flashover occurs through the air surrounding the insulator or the breakdown of the gaseous medium is called flashover.

The puncture voltage is the voltage at which the insulator breaks through between conductor and pin. It destroys the insulator or Breakdown of the solid insulator is called Puncture voltage. This rating is determined by applying the voltage while insulator emerges in oil. This is done because before the puncture, there will be flashover. Flashover voltage is less than puncture voltage and higher than the working voltage of insulators.

The ratio of puncture strength to flashover voltage is called the “safety factor” of the part or of the insulator against puncture. This ratio should be high enough (>1) to provide sufficient protection for the insulator from puncture by the transients.

Ques 9. Bucholtz relay cannot be used on

5000 kV transformer

1000 kV transformer

Three phase transformer

Air cooled transformer✔

The Buchholz relay is a gas operated relay used for the protection of oil immersed transformers against all the types of internal faults. It is named after its inventor, Buchholz. It is used for the protection of a transformer from the faults occurring inside the transformer, such as impulse breakdown of the insulating oil, insulation failure of turns etc.

Buchholz relay can be used only in oil immersed transformer having a conservative tank. Due to economic considerations, Buchholz relay is not provided for the transformers having below 500 KVA.

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Ques 10. An ammeter is obtained by shunting a 30Ω galvanometer with 30Ω resistance. What should an additional shunt be connected across it to double the range?

15Ω✔

10Ω

5Ω

30Ω

The value of the shunt resistance required to convert the galvanometer into an ammeter

Where Ig = the current for full-scale deflection in the galvanometer