FFX Next Generation Frigate
In 1984, during the electric drive full scale update effort, NAVSEA initiated exploratory design studies on a 1052-class frigate replacement. Small Water plane Area Twin Hull (SWATH) designs had long been known for exceptional stability in heavy seas. The FFX study carried monohull and SWATH variants, in this case 25 khp/shaft, and dictated that a common baseline machinery plant be used in both platforms. An electric propulsion system which more or less represented a composite of the best features of the near term systems produced under the full scale update was adopted for this purpose.
The SWATH lower hull space limitations dictated use of a high speed ac motor and gear. Subsequent to a series of technology tradeoff studies, propulsion derived ship service (PDSS) was added to form an integrated electric system. Also, a much more stringent acoustic requirement was placed on this propulsion system than had been encountered in any of the previous surface ship electric drive efforts.
The FFX effort was eventually canceled (the combination of requirements resulted in excessively large ship designs), but not before it substantially influenced the technical requirements and procurement specifications for the advanced development of an Integrated Electric Drive (IED) system. The IED contract, which was awarded to General Electric Co. in November,1988, was directed at the 25 khp/shaft SWATH platform complete with acoustic requirements. A single machine was to serve as both generator and high speed, geared motor for the SWATH and as generator for a monohull application., while an option was included for development of a direct drive 50 khp motor as the need arose.
The generator/motor was a four-pole, dual 3-phase, synchronous machine with direct liquidcooled stator and air-cooled rotor. The generator application drove the machine rating of 22.187 MVA at 3600 rpm. The solid state" multi-mode" frequency converter (MMFC) was only rated for single engine cruise condition. At full power the generator and motor could be operated synchronously. The MMFC was a load commutated dc-link converter with SCR based rectifier and inverter bridges. An alternative soft switched pulse width modulated inverter was also designed and partially built, but not completed due to funding and program re-direction. The integrated electric aspect was provided via a PDSS generator system driven from a power-take-off gear.
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