Certified Energy Auditor Motors, Drives, and Compressed Air Questions and Answers — 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?
- 30%
- 51%
- 66% (Correct answer)
- 91%
Correct answer: 66%
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?
- Replacing the primary air compressor with a new VSD model.
- Increasing the size of the primary air receiver tank.
- Implementing a comprehensive ultrasonic leak detection and repair program. (Correct answer)
- Lowering the system pressure by 5 psig.
Correct answer: Implementing a comprehensive ultrasonic leak detection and repair program.
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:
- is specifically designed to be operated with a Variable Frequency Drive (VFD).
- has a service factor of 1.25 or higher for handling overloads.
- meets or exceeds a set of minimum full-load efficiency levels established by NEMA. (Correct answer)
- is interchangeable with any other motor of the same horsepower rating.
Correct answer: meets or exceeds a set of minimum full-load efficiency levels established by NEMA.
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:
- inappropriate use of compressed air. (Correct answer)
- artificial demand.
- a lack of system storage.
- compressor short-cycling.
Correct answer: inappropriate use of compressed air.
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:
- Higher than 95.0% because the motor is not stressed.
- Exactly 95.0% as per the nameplate rating.
- Likely significantly lower than 95.0%. (Correct answer)
- Impossible to determine without power factor measurements.
Correct answer: Likely significantly lower than 95.0%.
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.
Question 6: Which of the following is the primary energy-saving mechanism when a throttling valve on a centrifugal pump is replaced with a Variable Frequency Drive (VFD) to control flow?
- The VFD corrects the motor's power factor, reducing reactive power charges.
- The VFD allows the motor to run at a slower, more efficient speed, eliminating the energy wasted across the valve. (Correct answer)
- The VFD reduces the motor's inrush current during startup, saving demand charges.
- The VFD's internal electronics operate more efficiently than the motor itself.
Correct answer: The VFD allows the motor to run at a slower, more efficient speed, eliminating the energy wasted across the valve.
A throttling valve controls flow by creating a large pressure drop, which wastes the energy used to create that pressure in the first place. A VFD saves energy by slowing down the pump to match the system's flow requirements, which drastically reduces power consumption according to the Affinity Laws. This eliminates the energy that was previously dissipated as heat and pressure drop across the valve.
Certified Energy Auditor Motors, Drives, and Compressed Air Questions and Answers
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?