SSC JE Electrical Previous Year Question Paper 2018-SET1|SSC JE 2018

Eddy current loss: In the transformer, we provide alternating current in the primary which produces alternating flux in the core, this flux links with the secondary of the transformer and induces emf in the secondary.

It may be possible that flux also links with some other conducting parts of transformer such as ferromagnetic core or iron body and induces local emf in these parts of transformer which will cause a circulating current to flow in these parts causing heat loss. these currents are called eddy currents and this loss is called eddy current loss.

Eddy current losses in the transformer are given by

Eddy current losses = K_e × B_m² × f² × V ×t²

Where
K_e = Eddy current constant 
t = thickness of the core
V = Volume of material 
B_m = Maximum flux density
f = frequency 

Hence from the above expression, it is clear that the eddy current losses do not depend on the susceptibility of the material.

Energy consumption or power consumption refers to the electrical energy per unit of time is given as

E = Power × Time = P × T

(i) 5 fans of 70 watts working 16 hours per day

Energy consumption = power × Time = 5 × 70 × 16 = 5600 watt-hr

(ii) One washing machine of 2000 watt operating for 1 hours

Energy consumption = 2000 × 1 = 2000 watt-hr

Energy consumption in the month of June i.e for 30 days

(Energy consumption of Fan + Energy consumption of Washing Machine) × 30

 (5600 + 2000) × 30 = 228000 watt-hr or 228 KWH

Voltmeter sensitivity (ohm per volt ratings)

The sensitivity of a voltmeter is given in ohms per volt. It is determined by dividing the sum of the resistance of the meter (R_m) plus the series resistance (R_s), by the full-scale reading in volts. In equation form, sensitivity is expressed as follows:

Sensitivity (S) = (R_m + R_s) ⁄ V_fld

Now as per the given question

 

R_m = 400 KΩ

R_s = 100 KΩ

V_fld = 250 V

S = (400 + 100) ⁄ 250

S = (500 × 10³) ⁄ 250

S = 2000 ohm/volt

System voltage i.e Nominal voltage = 11000 V

V₁/V₂ = N₁/N₂

98/V₂ = 104

Primary = Turn ratio × Secondary voltage = 98 × 104 = 10192 V

Percentage of error is the Potential error

= (Nominal voltage − Primary voltage)/Primary voltage

(11000 − 10192)/10192

= 7.92%

The power triangle or Impedance triangle of the AC circuit is shown Below

Cosφ = Base ⁄ Hypotenuse = Active Power  ⁄ Apparent Power

Active Power = Apparent power × Cosφ

Sinφ = Perpendicular ⁄ Hypotenuse = Reactive Power  ⁄ Apparent Power

Reactive Power = Apparent power × Sinφ

or

Reactive Power = (Active Power × Sinφ) ⁄ Cosφ

cosφ = 0.8

Sin²φ = 1 − cos²φ

Sin²φ = 1 − (0.8)²

Sinφ = 0.6

Reactive Power =(40 × 0.6) ⁄ 0.8

Reactive Power = 30 VAR

Reading of Wattmeter 1 W₁ = 200 W

Reading of Wattmeter 2 W₂ = 1200 W

The power factor of the two wattmeters is given as

$\begin{array}{l}\tan \Phi = \sqrt 3 \left( {\left| {\dfrac{{{W_1} – {W_2}}}{{{W_1} + {W_2}}}} \right|} \right)\\\\\tan \Phi = \sqrt 3 \left( {\left| {\dfrac{{200 – 1200}}{{200 + 1200}}} \right|} \right)\\\\\tan \Phi = \sqrt 3 \left| {\dfrac{{ – 1000}}{{1400}}} \right|\\\\\tan \Phi = \dfrac{{\sqrt 3 }}{{1.4}}\\\\\tan \Phi = 1.237\\\\\Phi = {\tan ^{ – 1}}1.237\\\\\Phi = {51^ \circ }\end{array}$

The principle on which a Permanent Magnet Moving Coil (PMMC) instrument operates is that torque is exerted on a current-carrying coil placed in the field of a permanent magnet.

PMMC type of instrument can be operated in direct current only. In alternating current, the instrument does not operate because in the positive half the pointer experiences a force in one direction, and in the negative half, the pointer experiences the force in the opposite direction. Due to the inertia of the pointer, it remains in its zero position.

A multimeter is a very useful electronic instrument that can be used for the measurement of three quantities, namely voltage, current, and resistance. This instrument can also be used for both dc and ac voltages and currents. Multimeters are available in both analog and digital forms. Although analog multimeters are being replaced by digital multimeters.

The magnetic susceptibility expresses the responsiveness of a material to the applied magnetic field and can vary with the external applied magnetic field.

The symbol for magnetic susceptibility is χ and the formula is

χ = M/H

Where M = Magnetization = 0.084 Amp/M

H = Magnetic field intensity = ?

∴ Magnetic field Intensity H is

H = M/χ

H = 0.084 ⁄ 0.0012

H = 70 A/m

The amount of flux produced by the magnet indicates the strength of the magnet. The more the magnetizing force (MMF), the more the flux produced. The more the opposition to the flux path (i.e., reluctance or magnetic resistance) less is the flux produced. This relationship is expressed as

Flux = M.M.F ⁄ Reluctance

Given

M.M.F = 48 Amp-turns

Reluctance = 4 Amp-turns/Weber

Flux = 48 ⁄ 4 = 12 Weber