In 1961 No. 10800 was bought by
Brush Traction and rebuilt for experiments in
AC power transmission. Brush named the locomotive
Hawk. DC transmission in diesel-electric locomotives was reaching its limits, at least for single-generator locomotives. The brushgear and
commutator of a DC generator became prone to flashover for engine powers greater than around 2,700 bhp. The most straightforward solution, rather than redesigning the DC generator, was to adopt an AC
alternator instead, which could use a much simpler slipring connection to its field rotor rather than a commutator. Although the advantages of AC transmission were obvious, the best means to implement suitable
traction motors was not. Should these be the familiar DC motors (requiring AC–DC rectification)? Or should AC traction motors be developed, and if so, how would their speed be controlled? High-power solid-state electronics was in an early state at this time, although Brush were keen to become leaders in the field. A more powerful engine would be required to give a realistic trial, and Brush already had a suitable one on hand, as a spare from the development of
Falcon. This was a
Maybach MD655 of 1,400 bhp. The Maybach was also a high-speed
diesel engine, running at 1,500 rpm rather than the Paxman's 1,250 rpm. The Paxman was itself considered a high-speed engine, running significantly faster than the
English Electric or
Sulzer medium-speed (< 1,000 rpm) engines which had been adopted for high-powered British Railways locomotives. The new 100 Hz AC generator was rated at , a conversion efficiency of 91%. The output was at 1,325V / 600A. New AC traction motors were used, with
squirrel cage motors supplied by
thyristor variable-frequency drives. Brush's experience with AC transmission, gained with
Hawk, would be put to use a few years later with the development of the 4,000 bhp single-engined AC transmission
HS4000 Kestrel.
Hawk was used until 1968, after which its control equipment was becoming obsolete and was superseded by the direct experience from
Kestrel. During the
1972 miners' strike the engine and generator were removed as a standby generator for the Brush Falcon works. After this, parts began to be stripped until it was finally scrapped around 1976. ==References==