Electrical Power Systems & Motors Flashcards

What is the purpose of IEEE 519 in industrial power systems?

Answer: To set harmonic current and voltage distortion limits at the point of common coupling

IEEE 519 establishes recommended limits for harmonic currents injected into the utility and harmonic voltage distortion at the point of common coupling (PCC).

What happens to motor efficiency when it is operated significantly below its rated load (e.g., at 25% load)?

Answer: Efficiency drops sharply because fixed losses become a larger fraction of input

Fixed losses (core, friction, windage) remain constant, so at very low loads they dominate input power, significantly degrading efficiency.

In a wye (star) connected 3-phase system with 480 V line-to-line voltage, what is the phase voltage?

Answer: 277 V

Phase voltage in a wye system = Line voltage / √3 = 480 / 1.732 ≈ 277 V.

What is the primary advantage of a synchronous motor over an induction motor for large industrial drives?

Answer: Ability to operate at leading power factor, providing reactive power compensation

Synchronous motors can be over-excited to operate at leading power factor, effectively acting as a capacitor bank and improving facility power factor.

A 100 kW motor has a power factor of 0.80. What is the apparent power (kVA) drawn from the utility?

Answer: 125 kVA

Apparent power (kVA) = Real power (kW) / Power factor = 100 / 0.80 = 125 kVA.

Which loss component in a transformer increases with the square of the load current?

Answer: Copper (I²R) loss

Copper losses (I²R) vary with the square of load current, making them the dominant variable loss at high loading conditions.

What is the recommended approach when performing a simple payback calculation for a motor replacement project?

Answer: Measure actual load and operating hours; use measured efficiency at that load point

Accurate payback analysis requires field-measured load levels and actual operating hours because motors rarely run at nameplate conditions.