Electrical Systems and Motors Flashcards

Which motor starting method reduces voltage applied to the motor during startup by approximately 58% of line voltage, reducing starting torque to about 33% of full voltage starting torque?

Answer: Wye-delta start

Wye-delta starting connects windings in wye (reducing voltage by 1/√3 ≈ 58%) during starting, then switches to delta; torque is proportional to voltage squared, so torque = (0.577)² ≈ 33%.

In an industrial facility, 'demand ratchet' clauses in utility tariffs typically penalize customers how?

Answer: By billing demand based on a percentage of the highest demand in the past 11-12 months

Demand ratchet clauses bill the customer based on a percentage (often 75-100%) of the highest peak demand recorded in the previous 11-12 months, even if current demand is lower.

What is the primary energy efficiency advantage of an EC (Electronically Commutated) motor compared to a PSC (Permanent Split Capacitor) motor in HVAC fan applications?

Answer: EC motors use permanent magnets and variable speed control for significantly higher part-load efficiency

EC motors combine permanent magnet rotors with built-in variable speed electronics, achieving efficiencies of 70-85% across a wide load range versus PSC motors at 25-50% efficiency.

A facility's power factor is 0.78 lagging. The utility applies a power factor penalty below 0.90. To avoid the penalty, the facility installs capacitors. This is an example of correcting what type of reactive power?

Answer: Lagging reactive power from inductive loads

Inductive loads (motors, transformers) create lagging reactive power (kVAR); capacitors supply leading reactive power to offset it, raising the displacement power factor.

What is the function of a 'soft starter' and how does it differ from a VFD in steady-state operation?

Answer: A soft starter reduces voltage only during start/stop; a VFD continuously controls speed

Soft starters use SCRs to ramp voltage during starting and stopping only; once the motor reaches full speed, it bypasses the SCRs and runs at line voltage with no speed control.

Which IEEE standard defines voltage distortion limits and current distortion limits for harmonic control in electrical power systems?

Answer: IEEE 519

IEEE 519 establishes recommended harmonic voltage and current distortion limits at the point of common coupling (PCC) between the utility and customer.

A 100 kW motor load operates at 70% load factor for 4,000 hours/year and 30% load factor for 2,000 hours/year. What is the annual energy consumption?

Answer: 340,000 kWh

Energy = (100×0.70×4,000) + (100×0.30×2,000) = 280,000 + 60,000 = 340,000 kWh.