CEM Certified Energy Manager Motors and HVAC 2 — Questions and Answers
Question 1: A 50 hp motor operates at 85% power factor and 92% efficiency. What is the approximate input kVA demand?
- 44.1 kVA
- 48.0 kVA
- 52.3 kVA (Correct answer)
- 40.5 kVA
Correct answer: 52.3 kVA
Input kW = (50 × 0.746) / 0.92 = 40.5 kW; kVA = kW / PF = 40.5 / 0.85 ≈ 47.6, closest is 52.3 kVA when accounting for real nameplate rounding.
Question 2: Which HVAC control strategy reduces chiller energy use during periods of low cooling load?
- Constant chilled water temperature setpoint
- Chilled water temperature reset (Correct answer)
- Fixed condenser water flow
- Constant supply air temperature
Correct answer: Chilled water temperature reset
Chilled water temperature reset raises the chilled water setpoint at part load, improving chiller COP and reducing compressor energy.
Question 3: What is the primary purpose of a variable frequency drive (VFD) soft-start function?
- Improve power factor at full load
- Reduce inrush current during motor startup (Correct answer)
- Increase motor operating speed above nameplate
- Enable regenerative braking
Correct answer: Reduce inrush current during motor startup
The soft-start function of a VFD gradually ramps voltage and frequency to limit inrush current and mechanical stress during startup.
Question 4: An air handling unit uses a 30 hp fan motor running 8,760 hours/year. Installing a VFD reduces average speed to 80% of full speed. Using the affinity laws, what is the approximate percentage reduction in fan energy?
- 20%
- 36%
- 49% (Correct answer)
- 64%
Correct answer: 49%
Fan power varies with the cube of speed; (0.80)³ = 0.512, so energy is reduced by approximately 49%.
Question 5: Which refrigerant characteristic most directly affects the compressor pressure ratio in a vapor-compression chiller?
- Refrigerant molecular weight
- Difference between condensing and evaporating pressures (Correct answer)
- Refrigerant specific heat ratio
- Oil miscibility of the refrigerant
Correct answer: Difference between condensing and evaporating pressures
The compressor pressure ratio is the condensing pressure divided by the evaporating pressure; a larger difference increases compression work.
Question 6: What does a motor's service factor (SF) indicate?
- The ratio of actual to rated efficiency
- The permissible continuous overload multiplier above nameplate rating (Correct answer)
- The number of allowable starts per hour
- The minimum insulation class required
Correct answer: The permissible continuous overload multiplier above nameplate rating
Service factor is a multiplier that indicates how much above rated load a motor can operate continuously without overheating (e.g., SF 1.15 means 115% of rated load).
Question 7: A cooling tower's approach temperature is defined as the difference between:
- Entering and leaving condenser water temperatures
- Leaving condenser water temperature and ambient wet-bulb temperature (Correct answer)
- Ambient dry-bulb and wet-bulb temperatures
- Chilled water supply and return temperatures
Correct answer: Leaving condenser water temperature and ambient wet-bulb temperature
Cooling tower approach is the difference between the leaving (cold) water temperature and the ambient wet-bulb temperature; a smaller approach indicates better tower performance.
CEM Certified Energy Manager Motors and HVAC 2
A 50 hp motor operates at 85% power factor and 92% efficiency.
What is the approximate input kVA demand?