EE MCQ

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.

Related Question

SSC JE Electrical 2019 with solution SET-2
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Four statements are given below. Identify the correct statement?

**Basic logic gates**: Basic logic gates are the building blocks of digital circuits that perform logical operations such as AND, OR, and NOT, on the binary inputs. Basic gates are mainly used in the fabrication of digital systems and are called integrated circuits (ICs).

**Derived logic gates**: Derived logic gates arc those logic gates that perform logical operations on the binary inputs with the help of two or more basic logic gates. A combination of AND, OR, and NOT are called derived gates. NAND, NOR, XOR, and XNOR are such a combination of AND, OR, and NOT gates.**Exclusive OR (XOR) Gate**

A gate that identifies the odd number of 1’s in the combination of the inputs is called the XOR gate. It is a functional combination of NOT, AND, and OR gates.

SSC JE Electrical 2019 with solution SET-2
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IL = Load Current, then the relation between these currents for a DC series generator is

When the field winding is connected in series with the armature winding while supplying the load then the generator is called a series generator.

Current Relations

As all armature, field, and load are in series they carry the same current.

IL = Ise = Ia

SSC JE Electrical 2019 with solution SET-2
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Which of the following heating element has the lowest operating temperature?

The melting point of Eureka is 1221 to 1300 °C

The melting point of Kanthal is 1,425 °C

The melting point of Nichrome is 1400 °C

The melting point of Graphite is 3,600°C

Hence Eureka has the least range of temperature.

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.