Electrical Theory & Principles Flashcards

In a balanced three-phase wye (Y) connected system, the line voltage is related to the phase voltage by:

Answer: VL = Vφ × √3

In a wye system, line voltage equals phase voltage multiplied by √3 (approximately 1.732).

In a balanced three-phase delta (Δ) connected system, the line current is related to the phase current by:

Answer: IL = Iφ × √3

In a delta system, line current equals phase current multiplied by √3 because each line feeds two phases.

True power in a three-phase balanced circuit is calculated as:

Answer: Both B and C are correct

Three-phase true power can be calculated as either 3 × Vφ × Iφ × PF or √3 × VL × IL × PF — both yield the same result.

What is the primary cause of eddy current losses in transformer cores?

Answer: AC magnetic flux inducing circulating currents within the core material

Alternating magnetic flux induces voltages in the iron core, driving circulating (eddy) currents that produce heat.

The synchronous speed of a 4-pole induction motor operating on a 60Hz system is:

Answer: 1,800 RPM

Ns = (120 × f) / P = (120 × 60) / 4 = 1,800 RPM.

Slip in an induction motor is defined as the difference between:

Answer: Synchronous speed and rotor speed

Slip = (Ns - Nr) / Ns × 100%, representing the rotor's lag behind the rotating magnetic field.

Which grounding concept states that all metallic equipment enclosures must be bonded together to ensure a low-impedance fault current path?

Answer: Equipment grounding

Equipment grounding bonds all conductive non-current-carrying metal parts to ensure fault current quickly trips overcurrent protection.