In the 115V AC/400 Hz system, the transformer-rectifier unit (TRU) produces 28V DC for DC buses and charging. Which statement is true about this function?

• A. The TRU steps down and rectifies AC to DC for the DC buses and charging
• B. The TRU increases AC voltage for main distribution
• C. The TRU stores energy for later use
• D. The TRU converts DC back to AC for emergency loads

Answer: (A)

In this power system, the transformer-rectifier unit is used to convert high-voltage AC into the 28V DC that feeds the DC buses and charges the battery. It does this by first stepping down the 115V AC at 400 Hz with a transformer, then rectifying the stepped-down AC into DC using diodes or controlled rectifier elements. This provides a stable 28V DC supply for distribution and keeps the aircraft battery charged.

It’s not designed to raise AC voltage, store energy, or convert DC back to AC for emergency loads. Those functions are handled by other components: boosting or distribution of AC voltage is not the TRU’s role, energy storage is in the battery system, and inverters handle converting DC to AC when needed for AC emergency loads.