A transformer is used to

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Transformers change voltage through magnetic induction between primary and secondary windings. The secondary voltage is determined by the turns ratio of the windings: more turns on the secondary produce a higher voltage, while fewer turns produce a lower voltage. This is why transformers are used to move voltage up for long-distance transmission and then step it down for safe local use. A transformer does not convert AC to DC, regulate frequency, or store energy; those functions are handled by rectifiers, frequency regulators, and energy storage devices, respectively.