EE MCQ

Related Question

SSC JE Electrical 2019 with solution SET-2
######
Four relations have been given. Select the correct relation?

The amount of flux produced by the magnet indicates the strength of the magnet. The more the magnetizing force (MMF), the more is 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 = MMF/ Reluctance

Reluctance is the opposition offered by the material in the flux path to the establishment of the flux. The reluctance in a magnetic circuit is similar to the resistance in an electric circuit. Reluctance is the inverse of permeance.

MMF = Flux/Permeance

SSC JE Electrical 2019 with solution SET-2
######
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
######
If a circular conductor carries a current of ‘I’ ampere having radius ‘r’ meter, then the magnetizing force at the center of the coil is given by

Magnetic Field Strength (H) gives the quantitative measure of strongness or weakness of the magnetic field.

H = B/μo

Where

B = Magnetic Flux Density

μo = Vacuum Permeability

The magnetic Field strength at the center of circular loop carrying current I is given by

B = μoI/2r

B/μo = I/2r

H = I/2r At/m

Where r = Radius