Basic Electrical Engineering

Alternating Voltage or Current Viva Questions

Alternating Voltage or Current Viva Questions

Alternating Voltage or Current Viva Questions, Sinusoidal Voltage or Current Viva Questions, Alternating Voltage Viva Questions, Alternating Current Viva Questions, Electronics Viva Questions

Short Answer Type Questions with Answers

Q.1. What is the difference between AC and DC?

Ans. A current is called alternating if it periodically changes its direction and magnitude whereas direct currents are steady and flow in one direction only.

Q.2. Why sinusoidal wave shape is insisted for voltages and currents while generating, transmitting, and utilizing ac electric power?

Ans. Sinusoidal wave shape is insisted for voltages and currents while generating, transmitting, and utilizing ac electrical power because it has the following advantages.

  • Sinusoidal voltages and currents produce minimum disturbance in the electrical circuits during operation.
  • Sinusoidal voltages and currents cause less interference (noise) to nearby communication circuits (telephone lines etc.).
  • Sinusoidal voltages and currents result in low iron as well as low copper losses in transformers, and AC rotating machines for a given output. So ac machines with sinusoidal voltages and currents operate at higher efficiency.

Q.3. What do you understand by ω?

Ans. Each cycle of a sinusoidal wave spans 2π radians. Hence. if this quantity is divided by the time period, the angular velocity of the sinusoidal wave is obtained. It is denoted by e and is expressed in radians per second.

Q.4. Why the RMS value of an alternating current or voltage is used to denote its amplitude?

Ans. RMS value of an alternating current or voltage is used to denote its amplitude because it is related to the power developed in resistance by the alternating current or voltage.

Q.5. What is the RMS value of an alternating current?

Ans. The effective or RMS value of an alternating current is given by that steady current which when flows through a given resistance for a given time produces the same amount of heat as when the alternating current is flowing through the same resistance for the same time duration.

Q.6. Do wave shapes other than sine waves have effective value? Explain.

Ans. Yes. all the wave shapes including the sine wave have effective value since, in each half cycle of the wave, work is being done. Actually, the effective value of an alternating wave (sinusoidal or non-sinusoidal) is defined as below:

The RMS or effective value of an alternating current or voltage is given by that steady current or voltage which when flows or applied to a given resistance for a given time produces the same amount of heat as when the alternating current or voltage is flowing or applied to the same resistance for the same time.

Q.7. Differentiate between form factor and peak factor.

Ans. Form factor is defined as the ratio of the effective value to the average or mean value of the periodic wave while peak factor is defined as the ratio of peak or maximum value to the effective or RMS value of the periodic wave.

 i.e. K =  RMS value while K — Peak value ‘ Average value P RMS value

Q.8. What is the significance of the form factor?

Ans. Form factor is a means of relating the mean value with the effective or RMS value of alternating quantity and it is useful in the determination of effective or RMS values of the alternating quantities whose mean or average values over half a period can be determined conveniently.

Q.9. What is the significance of the peak factor?

Ans. Knowledge of the peak factor of an alternating voltage is very essential in connection with determining the dielectric strength since the dielectric stress developed in an insulating material is proportional to the peak value of the voltage applied to it.

Q.10. What is the difference between a vector and a phasor?

Ans. Vector is a quantity that has magnitude as well as direction. A sinusoidal quantity (voltage or current) is represented by a line of definite length rotating in a counter-clockwise direction with the same angular velocity as that of the sinusoidal quantity. Such a rotating line is called the phasor. Any sinusoidal quantity (current or voltage) at a given frequency is completely specified by its amplitude and phase angle, its similarity to a vector quantity is evident, since the amplitude may be considered as the magnitude and the phase angle as the direction of a vector. To account for the difference the term phasor has been adopted, instead of the term vector, for representing graphically the magnitude and phase of a sinusoidal frequency is current or voltage.

Q.11. What is the significance of the phasor representation of an alternating quantity?   

Ans. The phasor representation of an alternating quantity enables us to understand its magnitude and position with respect to a reference line.

Q.12. What do you mean by phase and phase difference?

Ans. The phase of an alternating quantity (voltage or current) at any instant is defined as the fractional part of a cycle through which the quantity has advanced while the may be defined as the angular displacement between the maximum positive values of the two phasors representing the two quantities having the same frequency.

Electrical Circuit Analysis Viva Questions

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