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Electrical Systems and 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 Systems and Motors flashcards as text

  1. Which type of Variable Frequency Drive (VFD) topology is most common in industrial applications and converts AC to DC then back to variable-frequency AC?

    Answer: Voltage Source Inverter (VSI)

    Voltage Source Inverters (VSI) are the dominant VFD topology, using a diode rectifier, DC bus capacitor, and IGBT inverter to produce variable frequency output.

  2. Harmonics generated by VFDs and other nonlinear loads can cause which of the following problems in a facility's electrical system?

    Answer: Transformer heating and neutral conductor overloading

    Harmonics cause additional heating in transformers and can overload neutral conductors in 3-phase systems because triplen harmonics (3rd, 9th, etc.) add rather than cancel in the neutral.

  3. What is the purpose of a 'regenerative' VFD compared to a standard VFD?

    Answer: It returns braking energy back to the utility grid

    Regenerative VFDs use an active front end that can return energy from decelerating loads back to the grid, recovering energy that standard VFDs dissipate as heat in braking resistors.

  4. According to the affinity laws, if a pump motor speed is reduced from 1,750 RPM to 1,400 RPM (an 80% speed ratio), the power consumption changes to what percentage of original?

    Answer: 51.2%

    Power varies as the cube of speed ratio: (1,400/1,750)³ = (0.80)³ = 0.512 = 51.2% of original power.

  5. A motor's nameplate shows 'SF 1.15'. What does this service factor indicate?

    Answer: The motor can operate at 115% of rated load continuously without damage

    A service factor of 1.15 means the motor can continuously carry 115% of its rated horsepower under specified conditions without overheating.

  6. Which insulation class rating corresponds to a maximum temperature of 155°C and is commonly used in motors for high-ambient or overload applications?

    Answer: Class F

    Class F insulation has a maximum temperature rating of 155°C, while Class B is 130°C, Class A is 105°C, and Class H is 180°C.

  7. A demand charge is typically based on which measurement in a commercial or industrial facility's electric bill?

    Answer: Peak kW demand measured over a 15 or 30-minute interval

    Demand charges are based on the peak demand (kW) recorded during the billing period, measured over a specific interval (typically 15 or 30 minutes).