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Electrical Control Systems Flashcards

7 cards from real SACA practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.

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  1. In a motor control circuit, what is the purpose of an overload relay's normally closed (NC) contact wired in series with the contactor coil?

    Answer: It de-energizes the contactor coil when excessive current trips the relay

    The NC contact of an overload relay opens on a thermal or electronic trip, breaking the coil circuit and dropping out the contactor to protect the motor.

  2. Which IEC standard primarily governs the design and testing of low-voltage switchgear and control gear assemblies (MCCs and panel boards)?

    Answer: IEC 60947-1

    IEC 60947-1 establishes general rules for low-voltage switchgear and controlgear, including ratings, performance, and test requirements.

  3. A 480 V, 3-phase motor draws 28 A at full load. The NEC requires motor branch-circuit conductors to be sized at a minimum of what percentage of full-load current?

    Answer: 125%

    NEC 430.22 requires motor branch-circuit conductors to be rated at least 125% of the motor's full-load current.

  4. What distinguishes a soft starter from a variable frequency drive (VFD) in motor control applications?

    Answer: A soft starter only controls voltage during starting and stopping; a VFD continuously varies both voltage and frequency

    A soft starter reduces voltage during start and stop transitions but runs the motor at line frequency; a VFD varies both voltage and frequency to control speed continuously.

  5. In an electrical control panel, a 'hold-in' or 'seal-in' circuit is achieved by wiring which component in parallel with the start pushbutton?

    Answer: An auxiliary normally open (NO) contact of the contactor

    An auxiliary NO contact of the contactor is wired in parallel with the start button so that once the contactor energizes, it seals itself in without requiring continuous pressure on the start button.

  6. When testing insulation resistance of a motor winding with a megohmmeter, a reading below which value at 1,000 V DC is generally considered marginal for a 480 V motor?

    Answer: 1 MΩ

    IEEE 43 recommends a minimum insulation resistance of 1 MΩ for motors rated below 1 kV; readings below this indicate degraded insulation.

  7. Which type of relay operates based on the ratio of two electrical quantities and is most commonly used for protection of power transformers and large motors?

    Answer: Differential relay

    A differential relay compares current entering and leaving a protected zone; an imbalance indicates an internal fault, making it ideal for transformer and large motor protection.