Historically, Transformer Rectifiers Units (TRUs) have been unregulated devices used to provide 28-Volt Direct Current (vdc) power on 400 Hertz, 115-Volt Alternating Current (vac) aircraft. By design, a TRU produces an unregulated voltage output that varies with current (load) required by utilization equipment on the 28-Volt Buss. This TRU voltage output variation typically ranges from 30-31 volts (at no load) down to 24 volts (at 100%load) under stable input conditions. AC transients, in combination with sudden or heavy DC loads, can cause a typical TRU voltage output to fall into the 17-18 volt range or even lower.
Modern aircraft are experiencing three trends which are impacted by older TRU characteristics. The first is that airframes are being used for longer periods of time; scheduled use is being increased to avoid the budget constraints of replacing costly airframes. The second, related trend is that these longer-life airframes are being updated/modified with avionics, mission equipment, and/or ordinance which in many cases is microprocessor-based (digital) and therefore DC-powered. When older TRUs and/or existing aircraft electrical systems were originally developed, they did not consider these new loads. In many cases this new equipment is highly sensitive to low voltages and/or DC power quality “noise”. When new equipment is installed on older aircraft, both higher DC loads and nuisance trips/power quality issues in general can be a problem. The US Navy aircraft fleet is experiencing an increasingly common problem of nuisance tripping due to poor voltage quality from older, and overloaded, TRUs. The third trend is related to DC buss growth which results in poor charging of aircraft batteries that “float” on the DC buss.
Smiths Industries Aerospace - Leland Electrosystems has designed and developed a family of Regulated Transformer Rectifier Units (R/TRUs™) which greatly enhance the quality of DC power in aircraft applications. Naval Air Systems Command, Electrical Power Systems Division (EPSD) began a Qualification Test Program on a 200 ampere R/TRU™ in FY 98/99. The objectives of the USN program were to qualify a device which would:
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Provide constant DC voltage output over 0-100% load,
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Weigh no more than existing MIL-C-7115 200 amp units,
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Install in the existing MIL-C-7115 envelope.
Program objectives were to replace four (4) existing TRUs in US Navy inventory with one (1) common design that would meet both MIL-C-7115 electrical requirements (especially overload current requirements) and the worst case environmental requirements existing aircraft applications.
This paper discusses the approach taken to establish requirements for the R/TRU™, the development approach, R/TRU™ basic design, and qualification test results. The paper also discusses the aircraft that the 200-Ampere R/TRU™ is compatible with, and the status of Navy Flight Evaluation.