EE MCQ

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SSC JE Electrical 2019 with solution SET-2
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If the AC voltage and current are given by the following expressions?

v = 200sin (314t — 60°)

i = 50sin (720t), then what is the phase relation between them?

v = 200sin (314t — 60°)

i = 50sin (720t), then what is the phase relation between them?

When two alternating quantities of the same frequency have different zero points, they are said to have a phase difference. The angle between zero points is the angle of phase difference φ.

In the above-given question, the frequency of voltage and current is different so we can’t calculate the phase difference between them

SSC JE Electrical 2019 with solution SET-2
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In the two-wattmeter method of three-phase power measurement of a balanced load, if the reading of one meter is −200W, then the power factor of the load is

The reading of two wattmeters can be expressed as

W1 = VLILcos(30 + φ)

W2 = VLILcos(30 − φ)

(i) When PF is unity ( φ = 0°)

W1 = VLILcos30°

W2 = VLILcos30°

Both wattmeters read equal and positive reading i.e upscale reading

(ii) When PF is 0.5 (φ = 60°)

W1 = VLILcos90° = 0

W2 = VLILcos30°

Hence total power is measured by wattmeter W2 alone

(iii) When PF is less than 0.5 but greater than 0 i.e ( 90° > φ > 60°)

W1 = Negative

W2 = positive (since cos(−φ) = cosφ)

The wattmeter W2 reads positive (i.e.upscale) because for the given conditions (i.e. ( 90° > φ > 60°), the phase angle between voltage and current will be less than 90°. However, in wattmeter W1, the phase angle between voltage and current shall be more than 90° and hence the wattmeter gives negative (i.e. downscale) reading.

SSC JE Electrical 2019 with solution SET-2
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If ‘V’ is the voltage phasor and ‘I’ is the current phasor, then VI represents

Apparent Power (S): It is defined as the product of r.m.s value of voltage (V) and current (1). It is denoted by S.**S = V/I Volt Ampere**

SSC JE Electrical 2019 with solution SET-2
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Oxygen is a/an ____ gas?

Atoms or ions with unpaired electrons are attracted to a magnetic field; the more unpaired electrons, the greater is the attraction. Such substances are called **paramagnetic**. Molecular oxygen is a paramagnetic gas by virtue of its two unpaired electrons.

SSC JE Electrical 2019 with solution SET-2
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For a three-phase induction motor slip at No Load ÷ slip at Full Load

In terms of slip, the actual speed of the motor (N) can be expressed as

N = Ns(1 − s)

At start, the motor is at rest and hence its speed N is zero.

s = 1 ( start)

This is the maximum value of slip s possible for the induction motor which occurs at start. While s = 0 gives us N = N. which is not possible for an induction motor. So slip of the induction motor cannot be zero under any circumstances.

Practically motor operates in the slip range of 0.01 to 0.05 i.e. 1 % to 5 %. The slip corresponding to the full load speed of the motor is called full load slip.