GROL Power Sources and Distribution 2 — Questions and Answers
Question 1: What is the power factor of a purely resistive AC circuit?
- 0 (no real power consumed)
- 0.5 (half of apparent power)
- 0.707 (unity of reactive component)
- 1.0 (voltage and current in phase) (Correct answer)
Correct answer: 1.0 (voltage and current in phase)
In a purely resistive circuit, voltage and current are perfectly in phase, so the power factor equals 1.0.
Question 2: A transformer with a primary-to-secondary turns ratio of 1:2 connected to 120V AC will produce what secondary voltage?
- 240V, stepping up the voltage (Correct answer)
- 60V, stepping down the voltage
- 120V, maintaining the same voltage
- 480V, doubling the ratio twice
Correct answer: 240V, stepping up the voltage
A 1:2 turns ratio means the secondary has twice as many turns, doubling the voltage from 120V to 240V.
Question 3: What happens to capacitive reactance (Xc) as the frequency of the applied AC signal increases?
- Xc increases proportionally with frequency
- Xc remains constant regardless of frequency
- Xc decreases as frequency increases (Correct answer)
- Xc becomes purely resistive at high frequencies
Correct answer: Xc decreases as frequency increases
Capacitive reactance Xc = 1/(2πfC), so reactance decreases as frequency increases.
Question 4: In a balanced three-phase power system, what is the phase angle difference between each of the three voltage waveforms?
- 60 degrees
- 120 degrees (Correct answer)
- 180 degrees
- 90 degrees
Correct answer: 120 degrees
In a three-phase system, the three voltage waveforms are equally spaced 120 degrees apart in the electrical cycle.
Question 5: What is the primary role of the neutral conductor in a single-phase 120/240V AC distribution system?
- To provide a path exclusively for ground fault currents
- To carry return current and establish a stable voltage reference point (Correct answer)
- To protect equipment from lightning-induced surges
- To regulate power factor for connected loads
Correct answer: To carry return current and establish a stable voltage reference point
The neutral conductor carries return current from loads and is bonded to ground to provide a stable voltage reference.
Question 6: In a three-phase delta-connected system, what is the relationship between line current and phase current?
- Line current equals phase current
- Line current equals phase current divided by √3
- Line current equals phase current multiplied by √3 (Correct answer)
- Line current equals twice the phase current
Correct answer: Line current equals phase current multiplied by √3
In a delta connection, each line is shared by two winding phases, so line current equals phase current multiplied by √3 (approximately 1.732).
Question 7: In a three-phase wye-connected system, what is the relationship between line voltage and phase voltage?
- Line voltage equals phase voltage
- Line voltage equals phase voltage divided by √3
- Line voltage equals phase voltage multiplied by √3 (Correct answer)
- Line voltage equals twice the phase voltage
Correct answer: Line voltage equals phase voltage multiplied by √3
In a wye connection, line voltage is the vector sum of two phase voltages and equals phase voltage multiplied by √3 (approximately 1.732).
GROL Power Sources and Distribution 2
What is the power factor of a purely resistive AC circuit?