Motors, Drives, and Compressed Air Flashcards
7 cards from real Certified Energy Auditor Certification practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Motors, Drives, and Compressed Air flashcards as text
What is the approximate rule of thumb for energy savings in a compressed air system for every 2 psi reduction in system pressure?
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.
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?
Answer: $29,200
Input power = (100 × 0.75 × 0.746) / 0.92 = 60.8 kW; cost = 60.8 × 6,000 × $0.08 ≈ $29,200.
Which type of positive-displacement compressor is best suited for applications requiring oil-free air at high pressure with low flow rates?
Answer: Reciprocating (piston) compressor
Reciprocating compressors can achieve high pressures with small flows and are available in oil-free configurations for clean air applications.
An energy auditor finds a compressed air system where the compressor unloads frequently. What does this most likely indicate?
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.
What is the primary advantage of a variable-speed drive (VSD) compressor over a fixed-speed compressor with inlet valve modulation?
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.
The NEMA design classification of a motor most directly determines which performance characteristic?
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.
In motor rewinding analysis, at what point does NEMA typically recommend motor replacement over rewinding from an energy efficiency standpoint?
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.