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Certified Energy Manager Motors and HVAC Flashcards

7 cards from real CEM practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.

Read the first 7 Certified Energy Manager Motors and HVAC flashcards as text

  1. A 50 hp motor operates at 85% power factor and 92% efficiency. What is the approximate input kVA demand?

    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.

  2. Which HVAC control strategy reduces chiller energy use during periods of low cooling load?

    Answer: Chilled water temperature reset

    Chilled water temperature reset raises the chilled water setpoint at part load, improving chiller COP and reducing compressor energy.

  3. What is the primary purpose of a variable frequency drive (VFD) soft-start function?

    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.

  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?

    Answer: 49%

    Fan power varies with the cube of speed; (0.80)³ = 0.512, so energy is reduced by approximately 49%.

  5. Which refrigerant characteristic most directly affects the compressor pressure ratio in a vapor-compression chiller?

    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.

  6. What does a motor's service factor (SF) indicate?

    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).

  7. A cooling tower's approach temperature is defined as the difference between:

    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.