変圧整流器（TRU）

概要

The Transformer Rectifier Unit (TRU) is the essential power conversion bridge between the aircraft's primary 115V AC distribution system and the 28V DC buses that power avionics, engine controls, fuel management systems, and many other critical functions. Every commercial transport aircraft operating on a conventional AC electrical architecture requires multiple TRUs to sustain its DC loads. Unlike the sophisticated power electronics found in modern ground vehicles and industrial applications, aircraft TRUs are fundamentally passive devices: they use transformer windings to step down voltage and diode bridges to convert alternating current to direct current, with no active switching components in their primary conversion path.

The 28V DC standard for aircraft systems dates to the 1940s, when it was established as a compromise between wire gauge practicality, insulation requirements, and the voltage limits of early avionic equipment. It has persisted through seven decades of aviation development because the cost of redesigning the entire ecosystem of DC-powered equipment, from engine starters to galley equipment controllers to flight data recorders, would be prohibitive. Some modern aircraft supplement 28V DC with higher-voltage DC buses, notably the Boeing 787's 270V DC distribution for its large motor loads, but 28V DC remains the standard for avionics and control system power on all commercial types.

動作原理

A TRU contains two primary elements: a three-phase transformer and a rectifier assembly. The transformer accepts three-phase 115V AC from the main AC bus and produces a lower AC voltage at its secondary winding. Multiple secondary windings at slightly different phase angles are a common design feature, producing a twelve-pulse or eighteen-pulse rectifier output that significantly reduces ripple voltage compared with a simple six-pulse bridge. Higher pulse count reduces the size of filtering components needed to produce smooth DC and reduces harmonic distortion injected back into the AC bus.

The rectifier assembly consists of high-current silicon diodes arranged in a bridge configuration. As the AC input cycles, different diode pairs conduct in sequence, clipping the alternating waveform to produce a unidirectional output that fluctuates above a minimum voltage. Smoothing inductors and capacitors in the output filter reduce this ripple to a small percentage of the nominal 28V output. An output voltage adjustment trimmer allows the output to be set precisely to 28V, compensating for diode forward voltage drops. The TRU is a passive device with no active control of output voltage; output voltage varies slightly with load and input voltage, requiring the upstream AC voltage regulation to be accurate to maintain DC output within the required 26 to 30V band.

主要コンポーネント

• Three-Phase Input Transformer: Laminated iron-core transformer accepting 115V AC three-phase input, with primary taps permitting adjustment for different input voltage ranges and secondary windings configured for multi-pulse rectification.
• Silicon Diode Rectifier Bridge: High-current diode assembly, typically twelve-pulse configuration using two six-pulse bridges phase-shifted by 30 degrees to reduce output ripple and AC harmonics.
• Output Filter: Series inductor and shunt capacitor network smoothing rectified output, sized to keep ripple below the 500 mV specification of most aircraft DC buses.
• Thermal Management: Heat sink fins or forced-air cooling managing the heat generated by diode conduction losses, typically one to three watts per diode at rated current.
• Current Shunt: Precision low-resistance shunt in the output circuit enabling accurate current measurement for cockpit indication and overload protection.

航空機への適用

The Boeing 737-800 carries three TRUs: TR1 and TR2 supply the left and right main DC buses from the AC main buses respectively, while TR3 serves as a backup, connecting to the AC transfer bus and supplying the DC bus when a main TRU is inoperative. The Airbus A320-200 uses three TRUs (TR1, TR2, and an essential TR) in a similar arrangement, plus an additional static inverter that can convert battery DC to single-phase AC for the AC essential bus in emergencies. The Boeing 777-300ER uses four TRUs matched to its four-generator AC system. The Boeing 787 supplements conventional TRUs with an Auto Transformer Rectifier Unit (ATRU) design that handles higher power levels from the 115V variable-frequency AC buses to supply the 28V DC buses, with additional 270V DC buses supplied by separate rectifier units for the high-power motor loads.

利点と制限事項

TRUs are among the most reliable components in the aircraft electrical system. With no moving parts, sealed construction, and inherently passive operation, mean time between failures measured in tens of thousands of hours is typical. Their simplicity makes replacement straightforward: most TRUs are single line-replaceable units requiring no calibration after installation. The multi-pulse transformer design reduces harmonic distortion to acceptable levels without active filtering, simplifying certification. The primary limitation is fixed conversion ratio: the TRU output tracks input voltage proportionally, so AC bus voltage deviations translate directly to DC output deviations. On variable-frequency systems, the transformer core must be sized for minimum expected frequency to avoid magnetic saturation, increasing weight compared with a fixed-frequency equivalent. TRUs also dissipate conversion losses as heat, typically 2 to 4% of rated power, requiring adequate cooling airflow from the electronics bay environmental system.