NETA - InterNational Electrical Testing Association Certified Technicians AC/DC Theory and Circuits Questions and Answers 1 — Questions and Answers
Question 1: A technician is analyzing a series AC circuit operating at 60 Hz. The circuit contains a resistor (R) of 8 ohms, an inductor with an inductive reactance (XL) of 10 ohms, and a capacitor with a capacitive reactance (XC) of 4 ohms. What is the total impedance (Z) of this circuit?
- 14.0 ohms
- 10.0 ohms (Correct answer)
- 22.0 ohms
- 6.0 ohms
Correct answer: 10.0 ohms
The total impedance (Z) in a series RLC circuit is calculated using the formula: Z = √[R² + (XL - XC)²]. In this case, Z = √[8² + (10 - 4)²] = √[64 + 6²] = √[64 + 36] = √100 = 10.0 ohms.
Question 2: In an AC power system, which of the following best describes the relationship between true power (P), reactive power (Q), and apparent power (S)?
- Apparent power is the algebraic sum of true power and reactive power.
- True power is the vector sum of apparent power and reactive power.
- Reactive power is always greater than apparent power.
- Apparent power is the vector sum of true power and reactive power. (Correct answer)
Correct answer: Apparent power is the vector sum of true power and reactive power.
The relationship between true power (measured in Watts), reactive power (measured in VAR), and apparent power (measured in VA) is represented by the power triangle. Apparent power (S) is the hypotenuse, representing the vector sum of true power (P) and reactive power (Q), expressed by the formula S² = P² + Q².
Question 3: A technician closes a switch to apply a DC voltage source to a circuit containing only a resistor and a large inductor in series. Immediately at the instant the switch is closed (t=0), how does the inductor behave?
- As a short circuit, with maximum current flow.
- As a closed switch with no voltage drop.
- As an open circuit, with maximum voltage drop. (Correct answer)
- As a resistor equal to its DC resistance.
Correct answer: As an open circuit, with maximum voltage drop.
Due to its property of opposing a change in current, an inductor will initially behave as an open circuit the moment a DC voltage is applied. At this instant (t=0), current has not yet begun to flow, so the current is zero, and the full source voltage is dropped across the inductor.
Question 4: A field service report notes that a three-phase transformer is consistently running hotter than its nameplate rating would suggest, despite the load current being within limits. Which of the following is the most likely cause for the overheating?
- Low system frequency
- Excessive capacitive loading
- High levels of harmonic distortion (Correct answer)
- A leading power factor
Correct answer: High levels of harmonic distortion
Harmonic currents, which are multiples of the fundamental frequency, can cause significant additional heating in the windings and core of a transformer due to increased eddy current and hysteresis losses. This leads to overheating even when the fundamental load current is not excessive.
Question 5: A technician uses a power quality analyzer on an AC circuit and measures the true power as 4.0 kW and the apparent power as 5.0 kVA. What is the power factor of this circuit?
- 0.80 (Correct answer)
- 1.25
- 0.75
- 0.90
Correct answer: 0.80
Power factor (PF) is defined as the ratio of true power (P) to apparent power (S). The formula is PF = P / S. In this scenario, PF = 4.0 kW / 5.0 kVA = 0.80.
Question 6: According to Kirchhoff's Current Law (KCL), what is the current (I_total) entering a junction if three branches leaving the junction have currents of 2 A, 5 A, and 3.5 A?
- 7.0 A
- Cannot be determined without voltage
- 10.5 A (Correct answer)
- 5.5 A
Correct answer: 10.5 A
Kirchhoff's Current Law (KCL) states that the algebraic sum of currents entering a node (or junction) must equal the sum of currents leaving that node. Therefore, I_total = 2 A + 5 A + 3.5 A = 10.5 A.
NETA - InterNational Electrical Testing Association Certified Technicians AC/DC Theory and Circuits Questions and Answers 1
A technician is analyzing a series AC circuit operating at 60 Hz.
The circuit contains a resistor (R) of 8 ohms, an inductor with an inductive reactance (XL) of 10 ohms, and a capacitor with a capacitive reactance (XC) of 4 ohms.
What is the total impedance (Z) of this circuit?