Certified Energy Auditor Certification Motors, Drives, and Compressed Air 5

Question 1

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?

Accepted Answer

$11,800

Answer

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.

Question 2

What is the function of a demand expander (pressure/flow controller) installed at the inlet to a compressed air distribution system?

Accepted Answer

Stabilize distribution pressure and prevent pressure spikes from reaching end uses

Answer

A demand expander decouples the supply-side receiver from distribution, stabilizing downstream pressure and preventing artificial demand from pressure fluctuations.

Question 3

Which motor protection feature is most critical for preventing damage from voltage unbalance in a 3-phase motor?

Accepted Answer

Voltage unbalance protection relay or negative-sequence relay

Answer

Negative-sequence or voltage unbalance protection relays detect unbalanced voltages that cause excessive heating in motor windings and trip the motor before damage occurs.

Question 4

In a rotary screw compressor, what is the role of the oil separator?

Accepted Answer

Remove lubricating oil from the compressed air before it leaves the compressor

Answer

The oil separator removes injected lubricating oil from the compressed air discharge stream, returning oil to the sump and delivering cleaner air.

Question 5

When sizing a VFD for a motor retrofit, what minimum rating should the VFD have relative to the motor's nameplate horsepower?

Accepted Answer

Equal to motor nameplate hp

Answer

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.

Certified Energy Auditor Certification Practice Test

Certified Energy Auditor Certification Practice Test

Certified Energy Auditor Certification Motors, Drives, and Compressed Air 5