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Electrical Power Systems & Motors 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 Electrical Power Systems & Motors flashcards as text

  1. What is demand factor in electrical system design?

    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.

  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?

    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.

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

    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.

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

    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.

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

    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.

  6. What effect do harmonics from VFDs have on upstream power transformers?

    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.

  7. Which measurement best indicates whether a motor is a good candidate for VFD installation?

    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.