Certified Energy Auditor Certification Certified Energy Auditor Motors, Drives, and Compressed Air 1 — Questions and Answers
Question 1: According to the fan and pump affinity laws, if a centrifugal pump's rotational speed is reduced from 1,800 rpm to 1,440 rpm using a VFD, the shaft power consumption will change by approximately what factor?
- Power reduces to 80% of original
- Power reduces to 64% of original
- Power reduces to 51.2% of original (Correct answer)
- Power reduces to 40% of original
Correct answer: Power reduces to 51.2% of original
The affinity law for power states that power varies as the cube of the speed ratio. (1,440/1,800)³ = (0.8)³ = 0.512, so power drops to approximately 51.2% of the original — a savings of nearly 49%. This cubic relationship is why VFDs offer dramatic savings even at modest speed reductions.
Question 2: An auditor evaluates a failed 15 hp, 1,200 rpm motor that has been rewound twice previously. Which action is most consistent with best-practice energy auditing guidance?
- Rewind the motor again since it is below 25 hp
- Replace with a NEMA Premium efficiency motor because repeated rewinding progressively degrades efficiency (Correct answer)
- Install a soft starter on the rewound motor to offset efficiency losses
- Upsize to a 20 hp motor to reduce operating slip
Correct answer: Replace with a NEMA Premium efficiency motor because repeated rewinding progressively degrades efficiency
Industry guidance (including EASA/NEMA) indicates that repeated rewinding can reduce motor efficiency by 1–2 percentage points each time. For motors under 25 hp that have already been rewound, replacement with a NEMA Premium (IE3) unit is typically the cost-effective and energy-optimal recommendation.
Question 3: During a compressed air system audit with no production load, an auditor observes a 150 cfm-rated compressor cycling: loaded for 45 seconds, then unloaded for 105 seconds. Using the load/unload timing method, what is the best estimate of system leakage?
- 22.5 cfm
- 45 cfm (Correct answer)
- 67.5 cfm
- 105 cfm
Correct answer: 45 cfm
Leak rate = compressor capacity × (T_on / (T_on + T_off)) = 150 cfm × (45 / (45 + 105)) = 150 × 0.30 = 45 cfm. This method works because at no production load, the only reason the compressor loads is to replenish air lost through leaks.
Question 4: Which metric is the standard benchmark used to compare the overall energy efficiency of different compressed air systems or compressor packages?
- Volumetric efficiency (%)
- Pressure ratio (P_discharge / P_inlet)
- Specific power (kW per 100 cfm delivered) (Correct answer)
- Adiabatic efficiency (%)
Correct answer: Specific power (kW per 100 cfm delivered)
Specific power (kW/100 cfm) normalizes compressor energy consumption to a unit of useful output and accounts for both the compressor and ancillary equipment. It is the DOE and industry-preferred metric for benchmarking compressed air system efficiency across different sizes and types.
Question 5: A 4-pole induction motor is supplied by 60 Hz power. At full load, the rotor turns at 1,746 rpm. What is the motor's full-load slip?
- 1.5%
- 2.0%
- 3.0% (Correct answer)
- 5.0%
Correct answer: 3.0%
Synchronous speed = (120 × 60 Hz) / 4 poles = 1,800 rpm. Slip = (1,800 − 1,746) / 1,800 = 54/1,800 ≈ 3.0%. Higher slip indicates greater rotor losses; premium-efficiency motors typically exhibit lower slip at full load, improving both speed regulation and efficiency.
Question 6: How does elevated compressor inlet air temperature most directly affect rotary screw compressor energy consumption?
- It increases discharge pressure, requiring the compressor to unload more frequently
- It reduces air density, so the compressor must consume more energy to deliver the same mass flow rate (Correct answer)
- It increases volumetric efficiency by reducing condensate formation in the inlet
- It lowers the pressure ratio required, decreasing shaft power
Correct answer: It reduces air density, so the compressor must consume more energy to deliver the same mass flow rate
Air density decreases as temperature rises. Because compressors are volumetric machines, warmer inlet air contains less mass per unit volume. To deliver the required mass flow (and thus pressure), the compressor must work harder, increasing specific power. Locating compressor inlets near cooler outside air or shading them is a common low-cost recommendation.
Certified Energy Auditor Certification Certified Energy Auditor Motors, Drives, and Compressed Air 1
According to the fan and pump affinity laws, if a centrifugal pump's rotational speed is reduced from 1,800 rpm to 1,440 rpm using a VFD, the shaft power consumption will change by approximately what factor?