Final answer:
It is true that a no load tap changer on an AC transformer can be adjusted while the transformer is energized to change the secondary voltage, which works according to the transformer equation where the secondary to primary voltage ratio is equal to the ratio of the number of turns in their coils.
Step-by-step explanation:
To address the question: it is True that to change the value of secondary voltage, a no load tap changer on the transformer can be adjusted while the transformer is energized. Tap changers allow for voltage adjustments without de-energizing the transformer by altering the number of turns in the secondary coil, which in turn changes the voltage ratio according to the transformer equation. The transformer equation itself indicates that the ratio of the secondary voltage to the primary voltage in a transformer is equal to the ratio of the number of turns in their coils. Remember that transformers work on AC (Alternating Current) and are based on Faraday's law of induction. In a step-up transformer, the secondary voltage is higher than the primary voltage, whereas in a step-down transformer, the secondary voltage is lower than the primary voltage.
Regarding transforming DC voltages, as transformers require a changing (AC) current, they cannot be used directly to change DC voltages because there is no voltage induced in the secondary if there is no change in primary voltage.