Electrokinetica The Electro-mechanical Museum

22kW searchlight generator

Power for carbon-arcs

Identification plate

Anti-aircraft searchlights used during World War Two were based on high-intensity carbon-arc light sources. The 90cm and 150 cm types both required a heavy DC supply of up to 220 amps at 100 volts, for which the Air Ministry in Britain specified a standard design of diesel-engined generating set rated at 22kW, built in large numbers by R. A. Lister of Dursley. The prime mover was a four-cylinder indirect-injection engine of 5.7 litres, from the 'JP' range, uprated to 40 horsepower at 1100 rpm. At the other end of the bedframe was a 220A 100V dynamo usually from Mawdsleys although units by both Reyrolle and Crompton made occasional appearances. The set was mounted on a Taskers single-axle drawbar trailer for towing behind the vehicle carrying the searchlight (or 'Projector, AA' to give it its proper name.) The equipment of the generator was spartan; no electric starter (hand-start only), gravity fuel feed from an overhead tank, manual voltage control. What it had in its favour was robustness; there was little to go wrong and the high build quality of both the engine and dynamo ensured that they gave good service throughout the war and beyond. Every searchlight crew included a generator operator called 'No.9' whose job it was to ensure the current kept flowing.

Power for amusements

Once made redundant by the Air Ministry, the 22kW sets were popular with travelling funfairs for powering rides and lighting, for which 100-110V DC had already been standardised during the heyday of the steam-driven Showmans' road locomotive. Many sets saw continuous service until the engines reached the end of their useful lives, at which time the owners often replaced the Lister engines with Gardner units salvaged from lorries. By the end of the 20th century, most searchlight sets had been re-engined at least once, with the dynamos still going strong by the time their owners updated their rides to AC power. Even today, a few Gardner-powered ex-Lister sets can be seen working on fairs powering traditional rides that have been kept in original configuration by their proprietors. What you will no longer find are genuine Lister-powered searchlight sets still generating on the fairground. They have become very rare beasts indeed, now that even the most dedicated traditionalists have switched over to Gardners.

Our searchlight set

Built in 1941, plant No. 4894 probably saw plenty of active service during the war. In peacetime our set is thought to have belonged to Joyland of Yarmouth but does not seem to have had much use on the fairground as it lacks the characteristic modifications made by most showmen; much of its time must have been spent in a standby role. When Lucien took delivery of it, everything was complete and original apart from the missing drawers of tools and spare parts and the lack of a canvas cover. Unusually it has a Reyrolle dynamo; Mawdsleys were the preferred supplier as they were local to Lister in Dursley and had strong business connections with them. At the height of production it is likely that Mawdsleys were busy on other projects and could not meet the demand.

General view

General view

Wiring repairs

Wiring repairs

DC switchboard

DC switchboard

Quick repairs

On arrival our set was seized and could not be persuaded to turn with the handle. Diesel was poured into the bores and left to stand for a week. Then the valvegear and ancillaries were freed off while the water, oil and sludge were drained from the sump. The pistons were eventually freed with a few sharp knocks. There was little evidence of water having got into the upper cylinder area, although plenty had collected in the crankcase. A big-end was released and inspected for signs of lubrication failure or lining breakup but was in good order. The whole thing was cleaned up and put back together and all the ancillaries readied for a quick run. A mouse nest was removed from the dynamo and some perished wiring was disconnected, after which the insulation resistance was found to be excellent. The fuel system was full of evil gloop which had to be scraped off and blown out with compressed air, but eventually after the obligatory fiddling with the injection pump to get the last bubbles out, we were ready for a test.

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