A.C. Circuits Interview Question - Day 1

[Q:-01] What are the basic parameters of an a.c. circuit and what are their units?

Ans :

      Resistance (R), inductance (L) and capacitance(C) are the basic parameters of an a.c circuit.

      The unit of resistance is 'ohm', the unit of inductance is 'henry' and the unit of capacitance to 'farad'.

[Q:-02] How does the ohm's law stand in A.C. circuits ?

Ans :

      According to ohm's law in a.c. circuit, I = V / Z

Where I is the current in the circuit, V is the applied voltage to the circuit and Z is the total impedance of the circuit.

[Q:-03] What is called a purely resistive circuit ? •

Ans :

      The circuit, which does not possess any inductance or capacitance but possessing resistance only, is called a purely resistive circuit.

[Q:-04] What is the vector relationship between the current and voltage in purely resistive circuit ?

Ans :

      In purely resistive circuit the current vector and the voltage vector are actually coincident i.e. both are in phase.

[Q:-05] What is called purely inductive circuit ?

Ans :

      The circuit, which does not possess any resistance or capacitance but possessing inductance only, is called a purely inductive circuit.

[Q:-06] What Is the vector relationship between the current and voltage in purely inductive circuit ?

Ans :

      In purely inductive circuit the current vector lags behind the voltage vector by a quarter of a cycle or 90° electrical.

[Q:-07] What is the basic difference between a purely inductive load and an inductive load?

Ans :

      A purely inductive load is that load which has a pure inductance as only parameter of the circuit having no resistance whereas an inductive load consists of an inductance in series with a resistance.

[Q:-08] Is it possible to have practically a purely inductive load?

Ans:

      As the inductive element would have certainly some material resistance along with the inductance of the load practically it is not possible to have a purely inductive load.

[Q:-09] What is called a purely capacitive circuit ?

Ans :

      The circuit, which does not possess any resistance or inductance but possessing capacitance only, is called a purely capacitive circuit.

[Q:-10] What is the vector relationship between the current and voltage 'in purely capacitive circuit ? 

Ans :

      In purely capacitive circuit the current vector leads the voltage vector by a quarter of a cycle or 90° electrical.

[Q:-11] What is the impedance of a series R-L circuit ?

Ans :

(Impedance)² = (Resistance)² + (Inductive Reactance)²

or Z² = R²+ X_L²

or Z = √( R²+ X_L² ) = √((R) ²+(2Πfl) ²)

[Q:-12] What will be the current in a circuit having a resistance ?

Ans :

I = E / Z  = E  / √ (R² + X²) = E / √ ((R) ² + (2Πfl)²)

[Q:-13] What is the impedance of a series R-C, circuit

Ans :

(Impedance)² = (Resistance)² + (Capacitive Reactance)²

Or ,  Z² = R² + X_C²

Z = √( R² + X_C² ) = √((R)² + (1/2Πfc)²)

[Q:-14] What will be the current in a circuit having a resistance and a capacitance in series ?

Ans :

I = E / Z = E / √(R² + X_C²) = E / √((R)2 + (1/2Πfc)²)

[Q:-15] What is the impedance of a series R-L-C circuit ?

Ans :

(Impedance)₂ = (Resistance)² + (Net Reactance)²

Or , Z² = R² + X²

Or , Z² = R² + ( X_L ~ X_C)

Or , Z = √( R² + ( X_L ~ X_C))

[Q:-16] What will be the current in a circuit having a resistance, an inductance and a capacitance in series ?

Ans :

I = E / Z = E / √( R² + ( X_L - X_C))  Or , E / √( R² + ( X_L - X_C)) 

[Q:-17] What are the voltage drops in resistance, inductive reactance and capacitive reactance in series circuit ?

Ans :

V_R = I X R, VL = I X X_L and Vc = I X X_C

[Q:-18] What is the angular displacement between voltage drop in inductive reactance and capacitive reactance ?

Ans :

180° electrical.

[Q:-19] What phase difference will indicate if an alternating voltage of 240 volts impressed on a circuit causes a current of I = (55 - j 55) amps to flow ?

Ans :

A phase difference of 45°.

[Q:-20] What is the condition for maximum current In an a.c. series circuit ?

Ans :

      In an a.c. series circuit comprising R, L & C, when X_L = X_C i.e. under resonance condition the value of current will be maximum.

[Q:-21] What are the methods for finding the total current In a.c. parallel circuit ?

Ans :

There are three methods —

(i) Vector method,

(ii) Admittance method , and

(iii) Complex algebra.

[Q:-22] How is the total current calculated by vector method in a.c. parallel circuit ?

Ans :

      Suppose there are three parallel circuits having current I₁, I₂ & I₃ respectively and power factor cos₁, cos₂ and cos₃ respectively.

Therefore, total active component of current,

Icos = I₁cos₁ + I₂cos₂ + I₃cos₃

and total reactive component of current,

I sin = I₁sin₁ + 1₂sin₂ + I₃sin₃

So total current, I = √((Icos)² + (I sin)²)

[Q:-23] What Is the formula for finding out the impedance of a.c. parallel circuit ?

Ans :

In R - L parallel circuit -

= 1 / Z² = 1 / R² + 1 / X_L²

In R - L - C parallel circuit—

1 / Z² = 1 / R² + 1 / X² ,

Or , 1 / Z² = 1 / R² + ( 1 / X_L – 1 / X_C )² 

Or , 1 / Z² = 1 / R² + ( 1/ X_C – 1/ X_L )²

[Q:-24] What is the power factor in a.c. parallel circuit ?

Ans :

Power factor =  

I_R (current in resistance component) /I (total current)

= ( V / R ) / ( V / Z ) = Z / R

[Q:-25] How is the total current calculated by admittance method In a.c. parallel circuits ?

Ans :

Suppose there are three circuits having impedances Z₁ ,Z₂ and Z₃ respectively.

∴ Conductance , G₁ = R₁ / Z₁² , G₂ = R₂ / Z₂² , G₃ = R₃ / Z₃²

And susceptances B1 = X₁ / Z₁² , B₂ = X₂ / Z₂² , B₃ = X₃ / Z₃²

Total conductance, G_T= G₁+ G₂+ G₃

Total susceptance, B_T = B₁ + B₂ + B₃

∴ Admittance , Y = √((G_T)² + (B_T)²)

∴ Total current , I = V.Y

.*. Admittance, Y = 1(GT )2 + PT )2 Total current, 1 = V.Y.