CEM Electrical Power Systems & Motors 5

Question 1

What is demand factor in electrical system design?

Accepted Answer

The ratio of peak demand to total connected load

Answer

Demand factor is the ratio of the maximum demand of a system to the total connected load, used to size transformers and distribution equipment efficiently.

Question 2

A centrifugal fan currently draws 50 kW. If the fan speed is reduced by 20% using a VFD, approximately how much power will it consume?

Accepted Answer

25.6 kW

Answer

Fan power follows the affinity laws: power scales with the cube of speed, so (0.80)³ × 50 kW = 0.512 × 50 = 25.6 kW.

Question 3

Which motor insulation class is rated for a maximum winding temperature of 155°C?

Accepted Answer

Class F

Answer

NEMA Class F insulation is rated for a maximum total temperature of 155°C (105°C rise above a 40°C ambient), commonly used in modern motors.

Question 4

What is the main purpose of performing a load factor analysis on a motor system?

Accepted Answer

To identify oversized motors operating inefficiently at low load

Answer

Load factor analysis reveals motors operating well below nameplate load (typically <50%), where efficiency is poor, flagging candidates for downsizing or VFD installation.

Question 5

In a facility power system, what does 'load diversity' mean?

Accepted Answer

The probability that all loads will reach peak demand simultaneously is low

Answer

Load diversity recognizes that individual loads peak at different times, so the system peak demand is less than the sum of individual peaks.

(CEM) Certified Emergency Manager Practice Test

(CEM) Certified Emergency Manager Practice Test

CEM Electrical Power Systems & Motors 5