CEM Certified Energy Manager Motors and HVAC 3 — Questions and Answers
Question 1: Which motor protection device guards against single-phasing in a three-phase motor?
- Ground fault relay
- Thermal overload relay with phase-loss sensitivity (Correct answer)
- Power factor correction capacitor
- Soft-start controller
Correct answer: Thermal overload relay with phase-loss sensitivity
A thermal overload relay with phase-loss sensitivity detects the current imbalance caused by single-phasing and trips before winding damage occurs.
Question 2: In a chilled water system, what is the benefit of increasing chilled water delta-T from 10°F to 14°F at the same cooling load?
- Higher chiller COP
- Reduced chilled water flow rate and pump energy (Correct answer)
- Increased cooling tower capacity
- Lower condenser water temperature
Correct answer: Reduced chilled water flow rate and pump energy
A higher chilled water delta-T allows the same cooling load to be met with less flow, reducing pump energy consumption.
Question 3: What is the NEMA efficiency designation for a motor that meets or exceeds NEMA Premium efficiency levels?
- NEMA MG1 Table 12-11
- NEMA MG1 Table 12-12 (Correct answer)
- EPACT compliant
- IE2 class
Correct answer: NEMA MG1 Table 12-12
NEMA Premium efficiency motors meet or exceed the efficiency levels defined in NEMA MG1 Table 12-12.
Question 4: An economizer on an air handling unit uses outdoor air for free cooling when:
- Outdoor enthalpy exceeds return air enthalpy
- Outdoor dry-bulb or enthalpy is below the return air condition (Correct answer)
- Relative humidity is above 70%
- The chiller COP drops below 3.0
Correct answer: Outdoor dry-bulb or enthalpy is below the return air condition
An economizer brings in outdoor air when it is cooler (or has lower enthalpy) than return air, allowing mechanical cooling to be reduced or eliminated.
Question 5: Which parameter is used to evaluate the part-load efficiency of a chiller under real-world mixed operating conditions?
- COP at full load
- Integrated Part Load Value (IPLV) (Correct answer)
- Energy Efficiency Ratio (EER)
- Seasonal Energy Efficiency Ratio (SEER)
Correct answer: Integrated Part Load Value (IPLV)
IPLV (Integrated Part Load Value) is a weighted average efficiency metric that accounts for chiller performance at 25%, 50%, 75%, and 100% load.
Question 6: What is the primary energy penalty associated with operating a motor at loads well below its rated capacity?
- Increased bearing wear
- Reduced motor efficiency and power factor (Correct answer)
- Higher ambient noise levels
- Increased starting current frequency
Correct answer: Reduced motor efficiency and power factor
Motors operating significantly below rated load have reduced efficiency and power factor, increasing energy losses per unit of useful work.
Question 7: In a heat pump, the coefficient of performance (COP) in heating mode is related to COP in cooling mode by:
- COP_heating = COP_cooling − 1
- COP_heating = COP_cooling + 1 (Correct answer)
- COP_heating = 1 / COP_cooling
- COP_heating = COP_cooling × efficiency
Correct answer: COP_heating = COP_cooling + 1
COP_heating = COP_cooling + 1 because the heating output equals the cooling capacity plus the compressor work input.
CEM Certified Energy Manager Motors and HVAC 3
Which motor protection device guards against single-phasing in a three-phase motor?