EE MCQ

SSC JE Electrical 2019 with solution SET-1
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The laws involved in the expression e = −dφ/dt are

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

Related Question

SSC JE Electrical 2019 with solution SET-1
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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

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

SSC JE Electrical 2019 with solution SET-1
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Which of the following voltage range is used in medium transmission lines?

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.

SSC JE Electrical 2019 with solution SET-1
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The ratio of conductor voltage and the voltage across the disc nearest to the conductor multiplied by the number discs are

The non -uniformity in the voltage distribution over a string of suspension insulators is expressed in terms of a parameter called “string efficiency“.

The voltage across the unit nearer to the conductor is more than the voltage in the unit nearer to the support. This unequal potential distribution over the string is expressed in terms of string efficiency.

The efficiency of a string is defined as the ratio of voltage across the whole string to the product of the number of discs and the voltage across the disc nearest to the line conductor. Mathematically, it can be expressed as,

Where n = Number of the unit in the string

The more the value of the string efficiency, the more uniform is the voltage distribution across the string. For the ideal case, the string efficiency is 100% and the voltage across each disc of the string is equal. Practically various methods are used to obtain string efficiency as high as possible.

SSC JE Electrical 2019 with solution SET-1
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Hunting occurs in a/an motor

Sudden changes of load on synchronous motors may sometimes set up oscillations that are superimposed upon the normal rotation, resulting in periodic variations of a very low frequency in speed. This effect is known as hunting or phase-swinging.

SSC JE Electrical 2019 with solution SET-1
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P = ρQHg is the water power equation. Head ‘H’ is measured in meter ‘g’ gravity constant is measured in meter/second square, then the measuring unit of ‘Q’, the flow rate of water is:

P = ρ QHg is the water power equation.

Where

H = heead = meter

g = gravity =m/s2

ρ =density = (kg/m3)

P = power = watt = N-m/sec

Q = water flow rate = m3/sec