SSC JE 2015 Electrical question paper with solution

The only type of meter used in modern electronics instruments is the moving coil or d’Arsonval. This equipment is accurate and Robust and may be obtained with extremely sensitive movements.

To measure resistance a known EMF is applied in series with the meter and the resistor, and Ohm’s law is used to perform the calibration. A protective resistor is placed in series with the meter, so that if the unknown resistance should, in fact, be zero, the meter will read the only full scale, instead of passing a destructively large current. The value of the protective resistor is subtracted in each case from the total calculated resistance to obtain the unknown resistance.

The relation between sag or deflection and span length is

d = wL²/8T

Where d = deflection

L = span length

T = Tension

If the length is double and all the factors remain constant then

d₁ = 2²

d₁ = 4d

Hence if the length is doubled the sag of the line will become 4 times

A linear circuit is an electric circuit in which circuit parameters (Resistance, inductance, capacitance, waveform, frequency etc) are constant. In other words, a circuit whose parameters are not changed with respect to Current and Voltage is called Linear Circuit.

Lamp efficiency is given as

η = lumen/ VIcosΦ

= 2860/200 x 1

=  14.3 lumens/W

In electromagnetism, permeability is the measure of the ability of a material to support the formation of a magnetic field within itself. Hence, it is the degree of magnetization that a material obtains in response to an applied magnetic field.

Reluctance in the magnetic circuit

R = l/µA

Where l = length in meter

µ = Permeability of the material —————1

Also, Reluctance in magnetic circuit is the ratio of m.m.f and flux and the SI unit is amp.turn/Weber……..2

From equation 1 permeability of the material is given as

µ = l/aR

= meter/(meter² x amp.turns/Weber)

= wb/At × m

PF = active power/apparent power. So from question

0.6=300/apparent power.

Apparent power=500 Kva.

At unity power factor, active power=apparent power. So active power=500kw. So you will get an additional 200kw at unity power factor

Leakage Flux

The flux that does not pass through the air gap or useful part of the magnetic circuit is called as leakage flux since air is the bad magnetic insulator, therefore, it is a common phenomenon in the transformer.

The shaded-pole motor is the original type of AC single-phase induction motor. A shaded-pole motor is a small squirrel-cage motor in which the auxiliary winding is composed of a copper ring or bar surrounding a portion of each pole. This auxiliary single-turn winding is called a shading coil. Since the phase angle between the shaded and unshaded sections is small, shaded pole motors produce only a small starting torque relative to torque at full speed.

The shaded pole motor has the lowest starting torque as compared to all single-phase induction motors.

Asymmetrical fault gives rise to symmetrical fault currents that are displaced by 120° each other. The symmetrical fault is also called the balanced fault. This fault occurs when all three phases are simultaneously short-circuited. Two kinds of symmetrical faults include line to line (L-L-L) and line to line to ground (L-L-L-G).

At no load practical transformer, primary copper loss I₀²R is very small and this loss may be neglected. Hence, the primary no-load input power of a practical transformer is equal to the iron loss.

No-load input power, W₀ = Iron loss