Certified Energy Auditor Certification Motors, Drives, and Compressed Air 2 — Questions and Answers
Question 1: A facility replaces a standard efficiency motor (90% efficient) with a premium efficiency motor (95% efficient) rated at 50 hp. Approximately how many kW does the premium motor save at full load?
- 1.97 kW (Correct answer)
- 3.93 kW
- 5.85 kW
- 7.46 kW
Correct answer: 1.97 kW
Power saved = 50 hp × 0.746 kW/hp × (1/0.90 − 1/0.95) ≈ 1.97 kW.
Question 2: Which motor nameplate parameter is most directly used to calculate full-load amperage for a single-phase motor?
- Rated horsepower only
- Voltage, horsepower, and power factor (Correct answer)
- RPM and frame size
- Service factor and insulation class
Correct answer: Voltage, horsepower, and power factor
FLA = (hp × 746) / (V × PF × efficiency) — voltage, horsepower, and power factor are all required.
Question 3: A VFD reduces motor speed from 60 Hz to 48 Hz. By approximately what percentage does the motor's power consumption decrease (affinity laws)?
- 20%
- 36%
- 49%
- 51% (Correct answer)
Correct answer: 51%
Power ∝ (speed)³; (48/60)³ = 0.512, so power drops by ~51%.
Question 4: In a compressed air system, what is the primary purpose of an aftercooler?
- Increase discharge pressure
- Remove moisture from compressed air before storage (Correct answer)
- Lubricate compressor cylinders
- Reduce intake air temperature to boost flow
Correct answer: Remove moisture from compressed air before storage
Aftercoolers cool hot discharge air, condensing moisture before it enters the receiver and distribution system.
Question 5: Which compressed air leak detection method is most practical for finding leaks during normal production operations?
- Soap bubble testing
- Pressure decay testing
- Ultrasonic leak detection (Correct answer)
- Helium tracer gas
Correct answer: Ultrasonic leak detection
Ultrasonic detectors can locate leaks safely in noisy, pressurized systems during normal operation without shutdown.
Question 6: A motor's service factor is listed as 1.15. What does this indicate?
- The motor can run continuously at 115% of rated load without damage (Correct answer)
- The motor efficiency is 115% at rated conditions
- The motor has a 15% safety margin only for short-duration surges
- The motor insulation class is rated 15°C above standard
Correct answer: The motor can run continuously at 115% of rated load without damage
A service factor of 1.15 means the motor can operate at 115% of its nameplate horsepower on a continuous basis.
Question 7: When evaluating a multi-stage centrifugal compressor, which performance curve best describes the relationship between pressure and flow at constant speed?
- Rising curve — pressure increases with flow
- Flat curve — pressure is independent of flow
- Drooping curve — pressure decreases as flow increases (Correct answer)
- Parabolic curve — pressure is proportional to flow squared
Correct answer: Drooping curve — pressure decreases as flow increases
Centrifugal compressors have a drooping (falling) pressure-flow curve where discharge pressure decreases as flow rate increases.
