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Transformers Flashcards

7 cards from real EETC 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. A transformer's no-load test (open-circuit test) is performed to determine:

    Answer: Core losses and magnetizing current

    The open-circuit test is conducted at rated voltage with the secondary open to measure core (iron) losses and the magnetizing branch parameters.

  2. What is the effect of connecting transformers with different voltage ratios in parallel?

    Answer: Circulating currents flow between the transformers even at no load

    Mismatched voltage ratios create a voltage difference that drives circulating currents through the low-impedance winding loops, causing overheating.

  3. Hysteresis loss in a transformer core is minimized by using core material with:

    Answer: A narrow B-H hysteresis loop (high permeability, low coercive force)

    A narrow B-H loop means less energy is lost per magnetization cycle; silicon steel is used because it has this property.

  4. Which transformer connection is most susceptible to third-harmonic voltage distortion on the secondary side?

    Answer: Wye-wye (Y-Y) with ungrounded neutrals

    An ungrounded Y-Y transformer cannot circulate third-harmonic currents, so they appear as third-harmonic voltages on both windings.

  5. The regulation of a transformer is defined as the change in secondary terminal voltage from:

    Answer: Full load to no load, expressed as a percentage of full-load voltage

    Voltage regulation = (V_no-load − V_full-load) / V_full-load × 100%, and a lower value indicates better performance.

  6. A step-up autotransformer has a common winding of 120V and a series winding of 80V. The input is 120V. What is the approximate output voltage?

    Answer: 200V

    In a step-up autotransformer, Vout = V_common + V_series = 120V + 80V = 200V.

  7. What does the polarity marking 'H1' and 'X1' on a single-phase distribution transformer indicate?

    Answer: These terminals have the same instantaneous voltage polarity

    ANSI/IEEE standards define H1 and X1 as 'like' polarity terminals — when H1 is positive, X1 is also positive.