CEM Certified Energy Manager Motors and HVAC 2

Question 1

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

Accepted Answer

52.3 kVA

Answer

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?

Accepted Answer

Chilled water temperature reset

Answer

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?

Accepted Answer

Reduce inrush current during motor startup

Answer

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?

Accepted Answer

49%

Answer

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?

Accepted Answer

Difference between condensing and evaporating pressures

Answer

The compressor pressure ratio is the condensing pressure divided by the evaporating pressure; a larger difference increases compression work.

CEM - Certified Energy Manager Practice Test

CEM - Certified Energy Manager Practice Test

CEM Certified Energy Manager Motors and HVAC 2