ISCET AC Circuits and Analysis 5 — Questions and Answers
Question 1: In a series RLC circuit, if XL > XC, the circuit is said to be:
- Capacitive and current leads voltage
- Resistive only
- Inductive and current lags voltage (Correct answer)
- At resonance
Correct answer: Inductive and current lags voltage
When XL > XC, the net reactance is inductive, so the circuit behaves inductively and current lags behind voltage.
Question 2: The average (DC equivalent) value of a full-wave rectified sine wave with peak value Vpeak is:
- 0.636 × Vpeak (Correct answer)
- 0.707 × Vpeak
- 0.5 × Vpeak
- 1.414 × Vpeak
Correct answer: 0.636 × Vpeak
The average value of a full-wave rectified sine is Vavg = (2/π) × Vpeak ≈ 0.636 × Vpeak.
Question 3: Impedance matching between a source and load in an AC system is important primarily to:
- Minimize current flow
- Maximize power transfer from source to load (Correct answer)
- Increase the resonant frequency
- Reduce capacitive reactance
Correct answer: Maximize power transfer from source to load
Maximum power is transferred when the load impedance equals the complex conjugate of the source impedance.
Question 4: An oscilloscope shows a sine wave with a period of 4 ms. What is its frequency?
- 25 Hz
- 400 Hz
- 250 Hz (Correct answer)
- 4 kHz
Correct answer: 250 Hz
Frequency f = 1/T = 1/0.004 s = 250 Hz.
Question 5: In a 60 Hz AC power system, an inductor of 100 mH has an inductive reactance of approximately:
- 6 Ω
- 37.7 Ω (Correct answer)
- 60 Ω
- 376.8 Ω
Correct answer: 37.7 Ω
XL = 2πfL = 2π × 60 × 0.1 ≈ 37.7 Ω.
Question 6: What is the primary purpose of power factor correction in industrial AC systems?
- To increase the resonant frequency of the system
- To reduce reactive power demand and improve efficiency (Correct answer)
- To raise the supply voltage level
- To increase the impedance seen by the source
Correct answer: To reduce reactive power demand and improve efficiency
Power factor correction (typically using capacitor banks) reduces reactive power, lowering apparent power demand and reducing I²R losses in the distribution system.
Question 7: When two AC voltages are 'out of phase by 180°', combining them in series results in:
- Double the voltage
- Zero net voltage (Correct answer)
- A phase shift of 90°
- An increase in frequency
Correct answer: Zero net voltage
Two equal-magnitude voltages that are exactly 180° out of phase cancel each other, producing zero net voltage.
ISCET AC Circuits and Analysis 5
In a series RLC circuit, if XL > XC, the circuit is said to be: