1.3 AC fundamentals

License Preparation NepalNEC ELECTRONICS

Full marks: 29 · Questions: 29

01Generation of AC

Which, among the following, is the correct expression for alternating emf generated in a conductor moving in a magnetic field?

e = 2Blv sin(θ)

e = 2B^2lv sin(θ)

e = Blv sin(θ)

e = 4Blv sin(θ)

02Generation of AC

Calculate the maximum emf generated when conductor velocity is 10 m/s, conductor length is 3 m, and magnetic field density is 5 T:

150V

100V

300V

03Generation of AC

What should the angle theta (θ) between conductor motion and magnetic field be in order to generate maximum sinusoidal emf?

0°

90°

180°

45°

04Generation of AC

In an A.C. generator, an increase in the number of turns in the armature coil:

Increases emf

Decreases emf

Makes the emf zero

Maintains the emf at a constant value

05AC Values

The r.m.s. value of an alternating current is given by the steady (D.C.) current which when flowing through a given circuit for a given time produces:

more heat than produced by A.C. when flowing through the same circuit

the same heat as produced by A.C. when flowing through the same circuit

less heat than produced by A.C. flowing through the same circuit

none of the above

06AC Values

The power consumed in an AC circuit element will be least when the phase difference between current and voltage is:

180°

90°

60°

0°

07AC Values

The r.m.s. value and mean (average) value are identical in the case of a:

triangular wave

sine wave

square wave

half wave rectified sine wave

08AC Values

For the same peak value, which of the following alternating waveforms has the highest r.m.s. value?

square wave

half wave rectified sine wave

triangular wave

sine wave

09AC Values

For the same peak value, which of the following waveforms has the least mean (average) value over a cycle?

half wave rectified sine wave

triangular wave

sine wave

square wave

10AC Values

The peak value of a sine wave voltage is 200 V. Its average value over a half-cycle is:

127.4 V

141.4 V

282.8 V

200 V

11AC Values

The nominal domestic AC supply voltage is 220V. This figure represents the:

mean value

r.m.s. value

peak value

average value

12AC Values

The instantaneous value of a sinusoidal AC current is equal to its r.m.s. value at an electrical angle of:

90°

60°

45°

30°

13AC Values

The ratio of the effective (RMS) value to the average value of an alternating quantity is called the:

form factor

peak factor

average factor

Q-factor

14AC Values

The RMS value of a sine wave current is 100 A. Its peak value is approximately:

70.7 A

141 A

150 A

282.8 A

15R/L/C Circuit and AC Power

In a series RLC circuit, what is the phase difference between the current through the capacitor and the current through the resistor?

0°

90°

180°

360°

16R/L/C Circuit and AC Power

In a series RLC circuit, what is the phase difference between the circuit current and the voltage across the capacitor?

0°

90°

180°

360°

17R/L/C Circuit and AC Power

Which among the following is the correct expression for electrical impedance (Z) in terms of admittance (Y)?

Z = Y

Z = 1/Y

Z = Y/2

Z = 1/(2Y)

18R/L/C Circuit and AC Power

In a pure resistive AC circuit:

Current lags behind the voltage by 90°

Current leads the voltage by 90°

Current can lead or lag the voltage by 90°

Current is in phase with the voltage

19R/L/C Circuit and AC Power

For a purely resistive AC circuit, which of the following statements is correct?

Power factor is zero

Power consumed is zero

Heat produced is zero

Power factor is unity

20R/L/C Circuit and AC Power

A 90 Ω resistor, an inductor with 30 Ω of inductive reactance, and a capacitor with 50 Ω of capacitive reactance are connected in series across a 12 V AC source. What is the current through the resistor?

9 mA

90 mA

13 mA

130 mA

21R/L/C Circuit and AC Power

How many distinct types of electrical power are defined in an alternating current (AC) circuit?

22R/L/C Circuit and AC Power

Which of the following equations correctly expresses the power factor (cos Φ) of an AC circuit?

sin Φ = True power / Apparent power

cos Φ = True power / Apparent power

sin Φ = Apparent power / True power

cos Φ = Apparent power / True power

23R/L/C Circuit and AC Power

What is the power factor in a purely inductive or purely capacitive AC circuit?

-1

Infinity

24R/L/C Circuit and AC Power

The unit of Power factor in AC circuits is:

watt

unit less

Ampere

VAR

25R/L/C Circuit and AC Power

Active power and apparent power in AC systems are respectively represented by which units?

kW and kVAR

kVAR and kVA

kVA and kVAR

kW and kVA

26Three-Phase System

The total electric power in a balanced three-phase AC circuit is given by:

P = 3 * V_ph * I_ph * cos(Φ)

P = √3 * V_L * I_L * cos(Φ)

Both 1 and 2

None of the above

27Three-Phase System

In a balanced three-phase AC circuit, the instantaneous sum of all three generated phase voltages is equal to:

Infinite (∞)

Zero (0)

One (1)

None of the above

28Three-Phase System

In a star-connected (Y) balanced three-phase AC circuit, which relationship holds true?

Phase voltage is equal to line voltage and phase current is three times the line current

Phase voltage is square root three times less than line voltage and phase current is equal to line current

Phase voltage is equal to line voltage and line current is equal to phase current

None of the above

29Three-Phase System

In a delta-connected (Δ) balanced three-phase AC circuit, which relationship holds true?

Line current is equal to phase current

Line voltage is equal to phase voltage

None of the above

Line voltage and line current are zero