What type of current do transformers primarily operate with?

Transformers primarily operate with alternating current (AC) because they rely on the principle of electromagnetic induction, which is effective with AC. The operation of a transformer involves inducing a voltage in a secondary coil through a changing magnetic field created by the alternating current in the primary coil. This changing current produces a varying magnetic flux, which is essential for transforming voltage levels between the primary and secondary windings.

With direct current (DC), the magnetic field produced is constant and does not change over time. This stability means that a transformer cannot function to step up or step down voltage because there is no changing magnetic flux to induce a current in the secondary coil. High-frequency current can be used in specific applications related to transformers, like in radio-frequency transformers, but standard transformers in power applications typically use AC at standard mains frequencies (50 Hz or 60 Hz). Static current is not applicable in this context, as it does not refer to a type of current that can be transformed. Therefore, the most accurate choice for the type of current transformers operate with is indeed alternating current.