Certified Energy Auditor Certification Certified Energy Auditor Motors, Drives, and Compressed Air Questions and Answers 2 — Questions and Answers
Question 1: What is the primary benefit of installing a variable frequency drive (VFD) on a centrifugal fan motor?
- Energy savings through speed reduction following affinity laws (Correct answer)
- Increased motor starting torque
- Elimination of power factor correction needs
- Reduced motor insulation requirements
Correct answer: Energy savings through speed reduction following affinity laws
VFDs on centrifugal loads save significant energy because power consumption decreases with the cube of speed per the affinity laws.
Question 2: A compressed air system operates at 110 psig but end-use equipment only requires 80 psig. What is the most appropriate energy-saving recommendation?
- Install a dedicated low-pressure compressor or reduce system pressure with controls (Correct answer)
- Replace all pneumatic equipment with electric tools
- Add a larger receiver tank only
- Increase pipe diameter throughout the facility
Correct answer: Install a dedicated low-pressure compressor or reduce system pressure with controls
Operating at a lower discharge pressure or using a dedicated low-pressure supply eliminates the energy wasted compressing air beyond what is needed.
Question 3: Which motor efficiency standard, mandated by the Energy Independence and Security Act of 2007, applies to most general-purpose motors sold in the United States?
- NEMA Premium efficiency levels (Correct answer)
- NEMA Standard efficiency levels
- IEC IE1 efficiency levels
- EPACT minimum efficiency levels
Correct answer: NEMA Premium efficiency levels
EISA 2007 raised the minimum efficiency requirement for most general-purpose motors to NEMA Premium (IE3 equivalent) levels.
Question 4: During a compressed air audit, an auditor measures that the compressor is operating at 70% loaded and 30% unloaded time. Which control strategy would most improve part-load efficiency?
- Variable speed drive control (Correct answer)
- Load/unload with a large receiver
- Inlet modulation control
- Start/stop control with no receiver
Correct answer: Variable speed drive control
A VSD compressor adjusts motor speed to match demand, providing superior part-load efficiency compared to load/unload cycling.
Question 5: What is the approximate energy cost of compressed air leaks in a typical poorly maintained industrial system?
- 20-30% of total compressor energy (Correct answer)
- Less than 5% of total compressor energy
- Exactly 50% of total compressor energy
- Over 75% of total compressor energy
Correct answer: 20-30% of total compressor energy
Studies consistently show that compressed air leaks in poorly maintained systems waste approximately 20-30% of compressor output.
Question 6: When evaluating motor replacement, at what approximate load factor does rewinding a failed standard-efficiency motor become less cost-effective than replacing it with a premium-efficiency motor?
- Below 75% of rated load (Correct answer)
- Below 25% of rated load
- Above 100% of rated load
- At exactly 50% of rated load
Correct answer: Below 75% of rated load
Motors operating below approximately 75% load are strong candidates for right-sizing with a premium-efficiency replacement rather than rewinding to original specifications.
Certified Energy Auditor Certification Certified Energy Auditor Motors, Drives, and Compressed Air Questions and Answers 2
What is the primary benefit of installing a variable frequency drive (VFD) on a centrifugal fan motor?