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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 the typical full-load efficiency range of a NEMA Premium 100 HP, 4-pole induction motor?

    Answer: 95–96%

    NEMA Premium 100 HP motors typically achieve full-load efficiencies in the 95–96% range, significantly above standard or energy-efficient tiers.

  2. A facility has a 500 kVA transformer operating at 60% load. Approximately what percentage of transformer losses are core (no-load) losses versus copper (load) losses?

    Answer: Core losses dominate because they are constant regardless of load

    Core (no-load) losses are constant and exist whenever the transformer is energized, so at low loads they form a larger fraction of total losses.

  3. What is the primary energy benefit of using soft starters on large centrifugal pumps?

    Answer: They reduce mechanical stress and inrush current during starting, extending equipment life

    Soft starters ramp voltage gradually, reducing inrush current (often 6–8× FLA) and mechanical shock, lowering maintenance costs and improving reliability.

  4. Which IEEE standard is the accepted method for measuring 3-phase induction motor efficiency in the US?

    Answer: IEEE 112 Method B

    IEEE 112 Method B is the US standard for measuring motor efficiency using input/output power testing with specified loss segregation.

  5. When rewinding a failed motor instead of replacing it, what is the main energy efficiency concern?

    Answer: Poor rewind practices can reduce motor efficiency by 1–2% or more

    If rewind temperature and process controls are inadequate, core losses increase and efficiency drops by 1–2%, costing more in energy over the motor's life.

  6. A 25 kW motor runs 6,000 hours/year at 75% load with 92% efficiency. What is the approximate annual energy consumption?

    Answer: 122,283 kWh

    Input power = (25 kW × 0.75) / 0.92 = 20.38 kW; annual energy = 20.38 × 6,000 = 122,283 kWh.

  7. Which type of motor is best suited for constant-speed, high-starting-torque applications such as compressors and conveyors?

    Answer: NEMA Design C

    NEMA Design C motors have high starting torque (200–250% of full-load torque) with normal starting current, suitable for hard-to-start constant-speed loads.