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Air Ministry Mark II 20kVA Mobile Generating Set - Background

Britain needs radar

Trailer exterior

Trailer exterior

General view of plant

General view of plant

Most of the generating sets supplied to the War Department by Lister were built at their Dursley factory. The Air Ministry Mk.2 20kVA set shown here is somewhat different, because Lister supplied only the engine and bed. The electrical equipment was added by Metropolitan-Vickers, consisting of a 20kVA 3-phase alternator and comprehensive switchboard including Tonum automatic carbon pile regulator. The principal application of this model was to power Type 14 Search Radar units and similar, for which the switchboard was specifically arranged. A Leyland Hippo lorry would carry the generator as part of the vehicle complement making up the radar convoy. These sets were designed to survive under adverse conditions; Hand-starting capability made operation independent of troublesome batteries; the radiator was generously proportioned to enable continuous running at full power in high ambient temperatures; a variable compression system enabled reliable starting in the cold. The generator windings were insulated to a high specification and protected from dirt ingress by an air filter. The electrical output is rather unusual; although the alternator has a three phase star-connected winding the phase-neutral voltage is only 132V, giving a phase-phase voltage of 230V for delta applications, therefore the star point is not brought out. The output of the set can be delivered into two equal 230V single phase loads sharing a common terminal, in a format neither strictly single or two-phase as a phase difference of 120 degrees exists between them. A transformer was provided amongst the switchgear to step up part of the output power to conventional 230 / 400 volt three phase configuration.

Air Ministry Mk.2 20kVA set No. 2284

Air Ministry ID plate

Air Ministry ID plate

This example of the 20kVA set was built in 1945 according to the date code on the engine spec. plate. It may have seen a number of years’ service with the Air Ministry, powering radar somewhere far afield. Within a few years, however, it was to provide a rather different public service back home...

Britain needs X-Rays

X-Ray machine switch

X-Ray machine switch

Trailer signwriting

Trailer signwriting

Starting in 1944, a nationwide program was undertaken in the UK to screen the population for tuberculosis. Cases of the disease had been steadily decreasing in number before the war but the trend had reversed and started to pose a significant threat again. Chest X-rays were required to identify infection, and a special miniature radiographic technique using roll film was adopted for speed and economy. The newly-formed National Health Service set up a fleet of mobile X-ray stations called ‘Mass Radiography Units’ to tour the country over the following years and examine as many of the population as possible, so that treatment could be offered promptly before the disease progressed. Each unit could radiograph literally hundreds of people per day making the scheme an important step towards eradication of TB, and one especially effective in the critical areas of high population density such as industrial towns. Mobile power was needed for the X-ray machines, and that’s where our Air Ministry generating set resurfaced once surplus to requirements for radar purposes. The Ministry of Works engineering department had the set modified slightly to provide the correct electrical switching for the X-ray machine and general power requirements of the radiography unit. It was installed in its present 5-ton drawbar trailer and towed from town to town behind a Leyland Beaver truck which housed the combined X-ray theatre and darkroom. We have yet to confirm the number of units in the fleet and establish whether they were all alike. The front of the trailer bears the number ‘63’ in metal figures, which may have been its fleet number. This set has seen 4600 hours use in its 62 years. One has to guess how many of those hours were spent looking for aeroplanes and how many looking at lungs!.

Life after Mass Radiography

The trailer was thought to have been parked up at a hospital on standby duty, and subsequently operated briefly by a showman, prior to being taken into working conservation by stonemason Malcolm Morriss, who needed 3-phase power where no supply existed. Although the set was still just about in running order, Malcolm discovered some mechanical issues requiring attention including a faulty shaft coupling causing stress on the main bearings, which he removed to allow the engine to be run and worked on. The electrics were also in rather worn condition, with missing and broken switchboard parts and some burnt wiring. Due to the lack of documentation, it was not until Malcolm started investigating that the absence of 400V output became apparent. As things turned out, his requirement for three-phase power was resolved during a move to larger premises and the Lister was never put into service.

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