Novel stator rotor means more compact, lightweight genset

One of the UK’s oldest names in the drives business, Mawdsley’s of Gloucester, has developed an innovative, diesel-driven battlefield generating set, which weighs less than half that of conventional models. And although the unit has been designed for a wide variety of military uses, it looks almost certain to have plenty of industrial applications too. The key breakthrough with Mawdsley’s new design is that it generates power at a reduced weight and space than other existing units and so has a lower noise profile and higher fuel efficiency. And, when not operating at full load, the engine speed can be decreased, further reducing fuel consumption and noise levels as well as cutting maintenance costs through reduced engine wear. Dr David Kite, business development manager at Mawdsley’s, commented on the new design: “At the heart of the new design is a novel stator rotor assembly, just 200mm in diameter and less than 100mm thick. The dual wound stator produces outputs for both 230 volt AC and 28 volt DC supplies while the permanent magnet rotor eliminates the need for brushes or an excitation supply.” Kite explained to Eureka that the stator output is converted by an inverter/regulator of a battlefield-proven design, with load sensing feedback to the engine controller to control engine speed while maintaining fully regulated AC and DC outputs. “This lightweight configuration means that the whole electro-mechanical assembly weighs little more than a third of that of a conventional generator with AVR control, and it can be driven by petrol or diesel.” The company secured a contract in January this year - worth £15m over the next three years - to supply the MOD with 1,600 4.5kW gensets. The units will replace two older versions, one of which is petrol-driven and so does not conform to NATO’s Battlefield Single Fuel Policy, which aims to prevent fuel logistical problems occurring. The generators are being fitted to a number of battlefield support vehicles, including mounting of some sets on army communication vehicles. The British Army invited manufacturers to tender for the battlefield power generation contract. The Army was using generators that were either mounted in service trailers or integrally mounted in the communication vehicles. The communications equipment that the generators supplied required 230V, 50Hz AC or 28V DC supplies and were using two generators to meet this need. The unit generating a DC output is capable of producing 4.5kW and weighs approximately 380kg. The AC generator produces 3.5kW and weighs approximately 270kg. In order to improve logistical considerations, the Army wanted a single generator replacement unit that would be capable of producing all voltage and power needs, would offer a substantial weight reduction and would be dimensionally compatible with the current gensets. This meant the units would have to weigh less than 125kg (so that they could be carried by two soldiers) and fit in an envelope of outer dimensions 870 x 580 x 570mm and be interchangeable with the existing units. Other requirements were that the gensets must be diesel driven, take a supply from a standard army jerry can and conform to the single battlefield fuel policy, ie. diesel or avtur (kerosene). Because of these requirements, Mawdsleys could not use a conventional form of genset that uses an automatic voltage regulator-controlled field wound synchronous generator to produce an AC output. Instead, the new genset uses a diesel-driven permanent magnet (PM) generator with two windings, one supplying an inverter to generate a 230V AC output and the other a DC full-wave regulator to produce the DC output. As Kite explained: "A high speed air-cooled diesel engine is used as the prime mover. The engine drives a bearing-free PM generator, which is mounted in a cantilevered fashion. The stator is mounted off the engine block and the rotor, which is mounted outside the stator, is attached to the engine shaft." But Mawdsleys needed specialist help with the rotor/stator design from a US sub-contractor based in California, Goodman Ball. This company had a proven stator rotor design and had been manufacturing these for US Coastguard and US Marine applications. Mawdsleys met Goodman in June 2001 and both companies made a joint presentation (competitive evaluation trial) to the MOD in September 2001. Mawdsleys, who acted as the prime contractor for the project, also used another sub-contractor, Harrington Generators based in Derby, who were responsible for the mechanical interfaces between generating set and the communications vehicle. According to Kite, the benefits of this design are that it ensures cooler running, better maintainability and improved reliability. The PM generator produces two sets of high frequency multi-phase AC outputs which are used for two purposes: first, to produce a fully filtered 28V DC voltage that can be controlled through the use of a PWM (pulse width modulator) action to produce a DC output with characteristics to Defence Standard 61-5 Part 2. And second, to produce a fully filtered separate single phase 230V AC voltage through the use of a PWM action to produce an AC output with characteristics to the same Defence Standard. Kite added that, in this application, due to space restrictions, the AC inverter and DC regulator were combined in a single package and use a common heatsink. In order to minimise mutual interference, the units also use a digital input to select the operating mode, ie. AC or DC. Kite explained to Eureka that conventional generators produce an AC output whose frequency is proportional to engine speed. For example, in a four-pole generator 1,500 rpm produces 50Hz and 1,800 rpm produces 60Hz. In Mawdsleys PM generator, because of the use of the solid state inverter, the output frequency is independent of speed. Instead, the engine is used primarily as a source of power. Kite said that, as a consequence, the genset has two-speed operation. "Full speed gives maximum output power, the reduced speed gives reduced power but better fuel consumption , reduced mechanical wear and thus reduced maintenance costs, as well as producing less noise." Nigel Shepherd, Mawdsley’s technical sales director, said the new model also has a place in other markets, not just military: “It is often the case that the wider industrial market reaps the benefit of developments sponsored by the military. A demilitarised version of the new genset is planned, without the robust battlefield survivability of the MOD model, which will offer significant advantages of greater portability and competitiveness against most of the units currently available.” Mawdsleys Design Pointers * PM technology means the genset can produce a 50Hz output regardless of engine speed and means that the speed of the engine is now dictated by the mechanical output power required * PM permits a much more efficient generator than conventional technology, ie. >90% mechanical to electrical energy conversion efficiency compared with >80% for a wound field synchronous genset of the same power * Dual speed operation means that the engine can run at a lower speed for most of its operational life resulting in longer periods between major overhauls and thus reduced service costs. This is because higher engine speeds means increased engine wear