PC General Theory and Calculations 3 — Questions and Answers
Question 1: A 120V, 60Hz circuit has an inductor with 10Ω inductive reactance. What is the inductance (L) in millihenries?
- 26.5 mH (Correct answer)
- 37.7 mH
- 1.67 mH
- 16.7 mH
Correct answer: 26.5 mH
XL = 2πfL → L = XL / (2πf) = 10 / (2π × 60) ≈ 26.5 mH.
Question 2: When two 100 µF capacitors are connected in series, what is the total capacitance?
- 200 µF
- 50 µF (Correct answer)
- 100 µF
- 25 µF
Correct answer: 50 µF
For equal capacitors in series, C_T = C/n = 100/2 = 50 µF.
Question 3: A 480V, 3-phase balanced load consumes 50 kW at 0.8 PF lagging. What is the line current?
- 60.1 A
- 75.1 A (Correct answer)
- 86.6 A
- 43.4 A
Correct answer: 75.1 A
I = P / (√3 × VL × PF) = 50,000 / (1.732 × 480 × 0.8) ≈ 75.1 A.
Question 4: What is the capacitive reactance of a 100 µF capacitor at 60 Hz?
- 26.5 Ω (Correct answer)
- 37.7 Ω
- 0.038 Ω
- 6.28 Ω
Correct answer: 26.5 Ω
XC = 1 / (2πfC) = 1 / (2π × 60 × 0.0001) ≈ 26.5 Ω.
Question 5: In a 3-phase wye (Y) system, how does line voltage relate to phase voltage?
- V_line = V_phase
- V_line = V_phase / √3
- V_line = V_phase × √3 (Correct answer)
- V_line = V_phase × 3
Correct answer: V_line = V_phase × √3
In a wye system, line voltage is √3 (≈1.732) times the phase voltage.
Question 6: A circuit has 8Ω resistance and 6Ω inductive reactance in series. What is the impedance?
- 14 Ω
- 2 Ω
- 10 Ω (Correct answer)
- 48 Ω
Correct answer: 10 Ω
Z = √(R² + XL²) = √(64 + 36) = √100 = 10 Ω.
Question 7: What is the unit of electrical conductance, and how does it relate to resistance?
- Henry; G = R × L
- Farad; G = 1/(R × C)
- Siemens; G = 1/R (Correct answer)
- Ohm; G = V/I
Correct answer: Siemens; G = 1/R
Conductance (G) is measured in Siemens and is the reciprocal of resistance: G = 1/R.
PC General Theory and Calculations 3
A 120V, 60Hz circuit has an inductor with 10Ω inductive reactance.
What is the inductance (L) in millihenries?