Motors & Drives & Compressed Air Systems
What this domain tests
This domain combines motor efficiency, variable-speed drives, and compressed air. Expect affinity-law calculations, motor part-load behavior, and compressed-air specific power.
Core rules
- Motor efficiency peaks near 75% load; oversized motors run at poor load factor.
- VFD savings follow the cube law only when static head is negligible: at 60% speed, power ≈ 0.6³ = 21.6% of rated.
- Compressed air typically costs 18–22 kW per 100 cfm at 100 psig; leaks are the largest avoidable waste.
- Reducing pressure by 2 psi saves roughly 1% of compressor energy.
- Use heat recovery from compressors and match supply to actual demand with storage and controls.
Worked example
A 100-hp compressor delivers 500 cfm at 20 kW/100 cfm = 100 kW. If leakage is 20%, 20 kW is wasted. Running 6,000 h/yr, leak energy = 20 × 6,000 = 120,000 kWh. At $0.10/kWh that is $12,000/yr - usually fixed with a leak survey and repair program costing far less than new equipment.
Traps
Applying the cube law where static head dominates; ignoring VFD and motor losses; treating nameplate horsepower as input power; and forgetting that fixing leaks also reduces compressor load and run time.