CEM Electrical Power Systems & Motors 5 — Questions and Answers
Question 1: What is demand factor in electrical system design?
- The ratio of peak demand to total connected load (Correct answer)
- The ratio of actual energy used to peak power
- The percentage of time a motor runs at full load
- The power factor at peak demand
Correct answer: The ratio of peak demand to total connected load
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?
- 40 kW
- 32 kW
- 25.6 kW (Correct answer)
- 45 kW
Correct answer: 25.6 kW
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?
- Class A
- Class B
- Class F (Correct answer)
- Class H
Correct answer: Class F
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?
- To determine the correct VFD carrier frequency
- To identify oversized motors operating inefficiently at low load (Correct answer)
- To calculate the transformer turns ratio
- To measure harmonic distortion at the motor terminals
Correct answer: To identify oversized motors operating inefficiently at low load
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?
- Using motors of different voltage ratings
- The probability that all loads will reach peak demand simultaneously is low (Correct answer)
- Installing generators of different fuel types
- Switching between delta and wye connections
Correct answer: The probability that all loads will reach peak demand simultaneously is low
Load diversity recognizes that individual loads peak at different times, so the system peak demand is less than the sum of individual peaks.
Question 6: What effect do harmonics from VFDs have on upstream power transformers?
- Harmonics improve transformer efficiency by reducing no-load losses
- Harmonics increase transformer heating, requiring derating or K-factor rated units (Correct answer)
- Harmonics eliminate copper losses in the transformer windings
- Harmonics have no effect if the transformer is delta-connected on the primary
Correct answer: Harmonics increase transformer heating, requiring derating or K-factor rated units
Harmonic currents increase eddy current and stray losses in transformers, causing excess heating that requires transformer derating or use of K-factor rated designs.
Question 7: Which measurement best indicates whether a motor is a good candidate for VFD installation?
- High nameplate horsepower rating
- Frequent starts and stops or significant variation in load over time (Correct answer)
- Old motor insulation class (Class A or B)
- High ambient operating temperature
Correct answer: Frequent starts and stops or significant variation in load over time
Motors with variable loads or frequent speed changes show the greatest energy savings potential from VFD control due to the cubic relationship of power to speed.
CEM Electrical Power Systems & Motors 5
What is demand factor in electrical system design?