Avionic Instruments 3PH303-1C Phase Lock Box

The 3PH‑303‑1C—also known simply as the 3PH‑303 Phase Lock Box—is a three‑phase wye output module manufactured by Avionic Instruments.

It is used to lock the outputs of multiple inverters so that they operate in a synchronized three‑phase wye configuration, typically at aircraft power frequencies such as 398 Hz or 400 Hz.

When multiple inverters are used (e.g. three inverters in parallel), the box ensures one inverter operates as the master and the others act as slaves, maintaining proper phasing across all phases.