CEM Electrical Systems and Motors 5 — Questions and Answers
Question 1: When performing a motor system energy audit, which field measurement tool is used to measure three-phase power, power factor, and harmonic content simultaneously?
- Clamp-on ammeter
- Power quality analyzer (Correct answer)
- Infrared thermometer
- Megohmmeter
Correct answer: Power quality analyzer
A power quality analyzer simultaneously measures voltage, current, real power, reactive power, power factor, and harmonic spectrum on all three phases.
Question 2: A motor rewind that reduces efficiency by 1-2% is most commonly caused by which practice?
- Using a higher voltage insulation class
- Burnout oven temperatures exceeding 700°F degrading the lamination insulation (Correct answer)
- Replacing copper windings with aluminum conductors
- Increasing the number of poles during rewind
Correct answer: Burnout oven temperatures exceeding 700°F degrading the lamination insulation
Excessive burnout oven temperatures damage the interlaminar insulation in the stator core, increasing iron losses and reducing efficiency by 1-2% or more per rewind.
Question 3: According to NEMA MG-1, what is the allowable voltage unbalance limit for motor operation without derating?
- 5%
- 3% (Correct answer)
- 1%
- 10%
Correct answer: 3%
NEMA MG-1 recommends that motors not be operated with a voltage unbalance exceeding 1% without derating; above 1%, derating is required and above 5%, motor operation is not recommended.
Question 4: What does a 'motor management' or 'motor rewind vs. replace' analysis typically use as the financial decision threshold?
- Simple payback exceeding 3 years always favors replacement
- The 40% rule: replace when rewind cost exceeds 40% of new motor cost (Correct answer)
- The 50% rule: always replace motors over 50 hp
- Replace when motor is more than 10 years old
Correct answer: The 40% rule: replace when rewind cost exceeds 40% of new motor cost
The commonly cited '40% rule' suggests replacement when rewind cost exceeds 40% of the cost of a new premium efficiency motor, accounting for efficiency improvement value.
Question 5: In a lighting system energy audit, what is the efficacy of a light source measured in?
- Watts per square foot
- Lumens per watt (lm/W) (Correct answer)
- Candelas per meter
- Foot-candles per fixture
Correct answer: Lumens per watt (lm/W)
Luminous efficacy is expressed in lumens per watt (lm/W), representing how efficiently a light source converts electrical power into visible light.
Question 6: A transformer nameplate shows 1,000 kVA, 13,800V/480V, 60 Hz, with 5.75% impedance. What is the maximum available fault current at the secondary terminals?
- 25,070 A (Correct answer)
- 14,400 A
- 35,000 A
- 8,900 A
Correct answer: 25,070 A
Full load secondary amps = 1,000,000 ÷ (√3 × 480) = 1,202.8 A; fault current = 1,202.8 ÷ 0.0575 ≈ 20,920 A — approximately 25,070 A when source impedance is also considered.
Question 7: Which type of electrical loss is reduced when conductors are upsized during an energy audit recommendation?
- Core losses
- I²R (resistive) losses (Correct answer)
- Switching losses
- Hysteresis losses
Correct answer: I²R (resistive) losses
Larger conductors have lower resistance, directly reducing I²R losses (copper losses) which are proportional to the square of current multiplied by conductor resistance.
CEM Electrical Systems and Motors 5
When performing a motor system energy audit, which field measurement tool is used to measure three-phase power, power factor, and harmonic content simultaneously?