Certified Energy Auditor Certification Motors, Drives, and Compressed Air 3 — Questions and Answers
Question 1: What is the approximate rule of thumb for energy savings in a compressed air system for every 2 psi reduction in system pressure?
- 0.5% energy savings
- 1% energy savings (Correct answer)
- 3% energy savings
- 5% energy savings
Correct answer: 1% energy savings
A commonly accepted rule of thumb is that reducing compressed air pressure by 2 psi saves approximately 1% in compressor energy.
Question 2: A 100 hp motor operates at 75% load for 6,000 hours/year. At $0.08/kWh and 92% efficiency, what is the approximate annual energy cost?
- $29,200 (Correct answer)
- $36,600
- $39,100
- $49,000
Correct answer: $29,200
Input power = (100 × 0.75 × 0.746) / 0.92 = 60.8 kW; cost = 60.8 × 6,000 × $0.08 ≈ $29,200.
Question 3: Which type of positive-displacement compressor is best suited for applications requiring oil-free air at high pressure with low flow rates?
- Reciprocating (piston) compressor (Correct answer)
- Rotary screw compressor
- Scroll compressor
- Centrifugal compressor
Correct answer: Reciprocating (piston) compressor
Reciprocating compressors can achieve high pressures with small flows and are available in oil-free configurations for clean air applications.
Question 4: An energy auditor finds a compressed air system where the compressor unloads frequently. What does this most likely indicate?
- The compressor is undersized for the load
- System demand is significantly less than compressor capacity (Correct answer)
- The pressure relief valve is defective
- Inlet air filters are clogged
Correct answer: System demand is significantly less than compressor capacity
Frequent unloading indicates the compressor capacity exceeds actual system demand, causing it to cycle between load and unload states.
Question 5: What is the primary advantage of a variable-speed drive (VSD) compressor over a fixed-speed compressor with inlet valve modulation?
- Higher maximum discharge pressure
- Lower energy consumption at part-load conditions (Correct answer)
- Reduced maintenance intervals
- Higher volumetric efficiency at full load
Correct answer: Lower energy consumption at part-load conditions
VSD compressors match motor speed to actual demand, consuming significantly less energy at part-load than modulated fixed-speed units.
Question 6: The NEMA design classification of a motor most directly determines which performance characteristic?
- Motor efficiency class
- Speed-torque and starting current characteristics (Correct answer)
- Insulation temperature rating
- Enclosure type and cooling method
Correct answer: Speed-torque and starting current characteristics
NEMA design letters (A, B, C, D) define the speed-torque curve shape and locked-rotor current, matching motors to load types.
Question 7: In motor rewinding analysis, at what point does NEMA typically recommend motor replacement over rewinding from an energy efficiency standpoint?
- Motors below 1 hp
- Motors below 5 hp
- Motors below 25 hp with efficiency losses greater than 1% (Correct answer)
- Motors below 40 hp whenever rewind cost exceeds 50% of replacement
Correct answer: Motors below 25 hp with efficiency losses greater than 1%
NEMA and DOE guidelines suggest replacing rather than rewinding motors below 25 hp when rewinding would degrade efficiency by more than 1 percentage point.
Certified Energy Auditor Certification Motors, Drives, and Compressed Air 3
What is the approximate rule of thumb for energy savings in a compressed air system for every 2 psi reduction in system pressure?