A high speed alternator for a small scale gas turbine CHP unit

With a gas supply in 17 million homes in Britain, there is scope for wide use of CHP in the home. Designs aimed at this market, using Stirling engines or thermoelectric generators, suffer from high initial costs and/or low efficiency. To circumvent this problem, a novel design has been developed using a turbocharger operating as a small gas turbine with the basic constituents of a gas fired central heating system. By coupling an alternator directly to the turbocharger shaft the necessity for gears is removed, reducing power losses and lubrication needs within the unit. This required the development of a high speed alternator with output power in the range 500 W to 1 kW, rotating at a shaft speed in the range 50000 to 150000 rpm. Several direct drive turboalternators have been built previously with small scale outputs in the range of 3 to 30 kW. Few have been designed to produce outputs below 1 kW. This paper describes the design and construction of a 500 W, ultra high speed alternator for a small scale, direct drive CHP unit. Experimental results of low speed testing are also given. The following aspects of the alternator are discussed: magnets, winding losses, generated voltage frequency, retainment shells, and output power.