Low-Cost Redesign Criteria for the Efficiency Improvement of Universal Motors used in Domestic Appliances

The great diffusion of small motors makes interesting their efficiency increase, but it requires to consider the great commercial competition, the constraints of a large scale production and the needs of low cost, simplicity, reliability and safety. A very popular small machine is the universal motor (UM), largely adopted in a lot of domestic appliances. A typical and important example is the UM used in high speed vacuum cleaners, produced in several millions per year, with ratings over one kilowatt, at 30000 – 45000 rpm. Even if the manufacturers made great technological efforts to improve their performances, an energetic approach to their design criteria has not been adequately considered yet. The paper analyses some low cost efficiency improvement opportunities, considering the “UM - vacuum cleaner” system, under the following constraints: unvaried lamination geometry and stack length, so that the punching tools and the frame sizes can be preserved; torque-speed curve displacement, to be matched with the turbine curve; modification of field and armature winding data (total copper section unvaried); brushes axis displacement, to achieve an acceptable commutation behaviour. After some motor modelling elements, different redesign strategies will be analysed and compared. Some redesign examples will be discussed, together with the corresponding test results, obtained with expressly constructed UM prototypes.