Certified Energy Auditor Motors, Drives, and Compressed Air Questions and Answers

Question 1

A centrifugal fan's speed is reduced from 100% to 70% using a Variable Frequency Drive (VFD). According to the Affinity Laws, what is the approximate reduction in power consumption?

Accepted Answer

66%

Answer

The Affinity Laws state that power consumption is proportional to the cube of the speed change. The new power is (0.70)^3 = 0.343, or 34.3% of the original power. The reduction in power is therefore 100% - 34.3% = 65.7%, which is approximately 66%.

Question 2

An energy auditor identifies a compressed air system with a significant number of audible leaks. The plant operates 24/7. Which of the following actions will likely provide the most rapid and cost-effective reduction in energy consumption?

Accepted Answer

Implementing a comprehensive ultrasonic leak detection and repair program.

Answer

Fixing leaks is typically the most cost-effective first step in improving a compressed air system's efficiency. Leaks represent a direct waste of energy, and repairing them provides immediate savings with a high return on investment. While lowering pressure and installing a VSD compressor are also valuable measures, they are generally more capital-intensive or have a smaller initial impact than a robust leak repair program, especially when many leaks are present.

Question 3

The NEMA Premium® efficiency designation on an electric motor indicates that the motor:

Accepted Answer

meets or exceeds a set of minimum full-load efficiency levels established by NEMA.

Answer

The NEMA Premium® program defines specific minimum full-load efficiency levels for motors that are higher than standard-efficiency (EPAct) motors. This allows purchasers to easily identify and specify motors that will reduce electrical consumption and operating costs. While some NEMA Premium motors may be inverter-duty rated, the designation itself is purely about energy efficiency.

Question 4

An energy audit of a factory reveals that 100 psig compressed air is being used through open-ended hoses to blow dust and debris off workbenches. A more energy-efficient alternative would be to use a low-pressure blower. This situation is a classic example of:

Accepted Answer

inappropriate use of compressed air.

Answer

Using high-pressure, energy-intensive compressed air for tasks that can be accomplished with a low-pressure, more efficient energy source (like a blower) is termed 'inappropriate use'. Compressed air is one of the most expensive utilities in a plant, and it should be reserved for applications that specifically require its unique properties.

Question 5

An energy auditor observes a 100 hp motor operating at a consistent 40% of its rated load. The motor's nameplate efficiency is 95.0% at full load. The auditor should conclude that the motor's actual operating efficiency is:

Accepted Answer

Likely significantly lower than 95.0%.

Answer

Motor efficiency is not constant across all load points. Most standard motors reach peak efficiency around 75% of full load and efficiency drops off significantly, often dramatically, below 50% load. Therefore, a motor running at 40% load will be operating at an efficiency that is noticeably lower than its nameplate full-load rating.

Certified Energy Auditor Certification Practice Test

Certified Energy Auditor Certification Practice Test

Certified Energy Auditor Motors, Drives, and Compressed Air Questions and Answers