# SSC JE electrical 2019 question paper with solution SET-1

Ques.61. A magnetic circuit is applied with a current that changes at a rate of 5 A/sec. The circuit has an inductance of 2H, then the self-induced EMF is

1. −10 V
2. −4 V
3. −2.5 V
4. 0.4 V

Explanation:-

Given

Inductance L = 2 H

Rate of change of current di/dt = 5 A/sec

Self induced EMF = − (Rate of change of current × Inductance) = −L(di/dt)

= −(5 × 2) = −10V

Ques.62. The annual load factor of a generation station is equal to

1. (No. of units actually supplied per year) ⁄ (Max. possible demand × 8760)
2. (No. of units actually supplied per year) ⁄ (Max. possible demand)
3. (No. of units actually supplied per month) ⁄ (Max. possible demand × 8760)
4. (No. of units actually supplied per year) ⁄ (Max. possible demand)

Answer.1. (No. of units actually supplied per year) ⁄ (Max. possible demand × 8760)

Explanation:-

Load factor is defined as the average load divided by the peak load in a specified time period say one month or one year

The annual Load factor is the ratio of total energy production in a year to the total maximum demand over a year.

[latex display=”true”]{\text{Annual load Factor = }}\dfrac{{{\text{Annual Energy Production}}}}{{{\text{Annual Maximum Demand}} \times 8760}}$Note:- In one year there are 8760 Hours. Ques.63. When the supply to the excitation fails, the synchronous generator acts like a/an 1. Synchronous generator 2. Induction generator 3. Synchronous motor 4. Induction motor Answer.2. Induction generator Explanation:- When a generator with sufficient active load loses its excitation due to some reasons, it exhibits the following characteristics: 1. Overspeeding, loss of synchronism, operation as an asynchronous machine (Induction Generator); it obtains its excitation from the power system in the form of reactive power. The induction generator draws lagging (inductive) reactive power from the grid during operation. 2. The parts of the stator and rotor are subject to unnecessary overheating because the machine is allowed to keep running for a long time while at a high slip speed. 3. The generator terminal voltage changes in accordance with the variation of the reactive current taken from the power system. 4. The loss of excitation causes excessive heating of the stator core during normal system conditions of the generator or while operating at full load. 5. There will be a high current induced in the rotor of the generator owing to a loss of synchronism. The high current may cause mechanical damage that results in overheating of the stator windings. 6. The level at which reactive power is being drawn from the power system may reduce the power system voltages and thereby affect the performance of the generators. 7. The increase in the reactive power flow in the power system causes voltage reduction. This will affect the power system stability. 8. A loss of excitation results in the loss of reactive power from a generator; this creates a substantial amount of reactive power to be obtained from the power system. 9. It can cause severe machine damage due to large stator currents. Ques.64. Which of the following voltage range is used in medium transmission lines? 1. More than 200 kV 2. 20 kV to 100 kV 3. 50 kV to 200 kV 4. Less than 20 kV Answer.2. 20 kV to 100 kV Explanation:- Medium transmission line:- When the length of an overhead transmission line is between 100 km and 250 kin with an operating voltage ranging from 20 kV to 100 kV, it is considered as a medium transmission line. In medium lines, the series impedance and shunt admittance (pure capacitance) lumped at a few pre-determined locations are considered for calculation. These lines can be analyzed by using load end capacitance, nominal-T, and nominal-π methods. Short transmission line:- When the length of an overhead transmission line is less than 80 km with an operating voltage upto 20 kV, it is considered a short transmission line. Due to the smaller length and low operating voltage, the charging current is low. So, the effect of capacitance on the performance of short transmission lines is extremely small and therefore, can be neglected. Long transmission line:- Lengths of more than 250 km are classified as long transmission lines; with an operating voltage of above 100 kV. Ques.65. The laws involved in the expression e = −dφ/dt are 1. Faraday’s and Lenz’s laws 2. Coulomb’s and Faraday’s laws 3. Faraday’s and Ampere’s laws 4. Coulomb’s and Ampere’s laws Answer.1. Faraday’s and Lenz’s laws Explanation:- Faraday’s 1st laws of electromagnetic induction tell us about the condition under which an e.m.f. is induced in a conductor or coil a when the magnetic flux linking a conductor or coil changes. Faraday’s 2nd laws of electromagnetic induction give the magnitude of the induced e.m.f in a conductor or coil and may be stated as: The magnitude of the e.mf induced in a conductor or coil is directly proportional to the rate of change of magnetic flux linkages. Suppose a coil has N turns and magnetic flux linking the coil increases (i.e. changes) from φ1 Wb to φ2 Wb in t seconds. Now, magnetic flux linkages mean the product of magnetic flux and the number of turns of the coil. N = e dφ/dt Lenz Law:- Lenz’s law states: the direction of the induced e.m.f. is such as to oppose the change producing it. Therefore, the magnitude and direction of induced e.m.f. should be written as : N = −e dφ/dt Ques.66. The output of the logic circuit given below represents the gate. 1. NAND 2. OR 3. NOR 4. AND Answer.1. Nand Explanation:- In the given diagram all are NOR Gate . The final output is shown in the figure. At stage 1 the output will be [latex]\overline A \& \overline B$

At stage 2 the output will be [latex]\overline {\overline A + \overline B } = A.B$And the final output will be [latex]\overline {A.B}$

Hence for input A & B the output is [latex]\overline {AB}\$ in case of Nand gate.

Ques.67. A synchronous motor runs at 600rpm, which of the following case is true?

1. P  = 12, f = 60 Hz
2. P = 10, f = 60 Hz
3. P= 12, f = 50 Hz
4. P = 8, f = 50 Hz

Answer.1. P  = 12, f = 60 Hz

Explanation:-

Synchronous speed NS = 120f/P

When

P = 12 & F = 60 then Ns = ?

NS = 120f/P = (120 × 60)/12 = 600 RPM

Ques.68. For a three-layer stranded wire, the number of strands in the first, second, and third layer respectively are:

1. 1, 7 and 14
2. 1, 4 and 8
3. 1, 6 and 12
4. 1, 5 and 10

Explanation:-

The total number of the conductor in stranded cable is given by

N = 3x2 − 3x + 1

Where

x = no. of layer

For layer 1

N1 = 3(1)2 − 3(1) + 1

N1 = 1

For layer 2

N2 = 3(2)2 − 3(2) + 1

N2 = 7

In layer 2 no. of conductor = N2 − N1 = 7 − 1 = 6

For layer 3

N3 = 3(3)2 − 3(3) + 1

N3 = 19

In layer 3 no. of conductor = N3 − N2 = 19 − 7 = 12

Ques.69. The power factor of an A.C. circuit is equal to

1. R/XL
2. Z/R
3. R/XC
4. R/Z

Explanation:-

In electrical engineering, the power factor of an AC electrical power system is defined as the ratio of the real power flowing to the load to the apparent power in the circuit ” R/Z”.

Ques.70. _____ is NOT a component in a thermal power plant

1. Economizer
2. Reactor
3. Boiler
4. Chimney