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
A compressed air audit reveals that 30% of generated air is lost to leaks. If the compressor runs 8,760 hr/yr at 75 kW average input, and electricity costs $0.09/kWh, what is the annual cost of leaks?
Answer: $11,800
Leak cost = 0.30 × 75 kW × 8,760 hr × $0.09/kWh ≈ $17,766 — closest answer is $17,800; correction: 0.30 × 75 × 8760 × 0.09 = $17,766 ≈ $17,800.
What is the function of a demand expander (pressure/flow controller) installed at the inlet to a compressed air distribution system?
Answer: Stabilize distribution pressure and prevent pressure spikes from reaching end uses
A demand expander decouples the supply-side receiver from distribution, stabilizing downstream pressure and preventing artificial demand from pressure fluctuations.
Which motor protection feature is most critical for preventing damage from voltage unbalance in a 3-phase motor?
Answer: Voltage unbalance protection relay or negative-sequence relay
Negative-sequence or voltage unbalance protection relays detect unbalanced voltages that cause excessive heating in motor windings and trip the motor before damage occurs.
In a rotary screw compressor, what is the role of the oil separator?
Answer: Remove lubricating oil from the compressed air before it leaves the compressor
The oil separator removes injected lubricating oil from the compressed air discharge stream, returning oil to the sump and delivering cleaner air.
When sizing a VFD for a motor retrofit, what minimum rating should the VFD have relative to the motor's nameplate horsepower?
Answer: Equal to motor nameplate hp
VFDs are typically sized to match the motor's nameplate horsepower (or current rating), with derating applied only for specific high-torque or ambient conditions.
An energy audit of a compressed air system shows the system pressure is maintained at 120 psi, but all end uses require only 90 psi. What is the recommended action?
Answer: Lower the compressor discharge pressure setpoint to 90–95 psi
Lowering the compressor discharge pressure setpoint to the minimum required (90–95 psi) directly reduces compressor work and energy consumption.
What is 'artificial demand' in a compressed air system?
Answer: Extra air consumed by end uses when system pressure is higher than actually needed
Artificial demand is the additional compressed air consumed by leaks and unregulated end uses when system pressure exceeds what applications actually require.