Electrical and Electronic Knowledge Flashcards

In a 3-phase wye-connected system, the phase-to-neutral voltage is 277 V. What is the line-to-line voltage?

Answer: 480 V

Line voltage in a wye system equals phase voltage times the square root of 3 (277 x 1.732 ≈ 480 V).

What is the approximate forward voltage drop across a conducting silicon diode?

Answer: 0.7 V

Silicon diodes typically drop about 0.6 to 0.7 V when forward biased.

A 10 kΩ resistor is in series with a 100 µF capacitor. What is the RC time constant?

Answer: 1 second

τ = R x C = 10,000 Ω x 0.0001 F = 1 second.

A megohmmeter (megger) is primarily used to test which property of a motor or cable?

Answer: Insulation resistance

A megger applies high DC voltage to measure insulation resistance in megohms.

A transformer with a 4:1 turns ratio has 480 V applied to its primary. What is the secondary voltage?

Answer: 120 V

Voltage divides by the turns ratio in a step-down transformer: 480 / 4 = 120 V.

What happens to a normally open (NO) relay contact when the relay coil is energized?

Answer: It closes

NO contacts are open at rest and close when the coil is energized.

Power factor in an AC circuit is defined as the ratio of:

Answer: True power to apparent power

PF = true power (W) / apparent power (VA), equal to the cosine of the phase angle.