Design for Manufacture of a Brushless Synchronous Servomotor

This thesis presents research into the design of brushless permanent magnet (BPM) interior rotor synchronous servomotor for both performance and manufacturability. The investigation has been supported by experimental evidence gathered with the aid of two prototype servomotors, having the same frame sizes and stator stack lengths, designed by the author and manufactured by the sponsoring company, Control Techniques Dynamics (CTD) Ltd. One servomotor, upon which the research is focussed, has the relatively new segmented stator structure containing concentrated windings. The other servomotor has a conventional structure with a solid stator containing distributed windings and this has provided the means of assessing the degree of improvement attainable with the new structure. It has been established that the new structure enables more cost-effective manufacture than attainable with the conventional structure. A significant contribution of the research programme is the special notch for retention of the surface mounted permanent magnets in the rotor. This innovation enables a more uniform and smaller air gap, which greatly improves the dynamic and thermal performance for a given frame size and stator stack length, thereby advancing the state of the art. This, together with the greatly reduced stator winding overhang in the new structure, enables a physically smaller motor for a given application, thereby reducing active material usage. Also the design methodology has focused on reducing energy and eliminating waste in the manufacturing process. Regarding the thermal aspects, only natural cooling has been considered. Conduction, convection and radiation heat transfer in the servomotor has been investigated theoretically, by simulation and by experiments to identify where design improvements can be made. The most critical area identified is the paper wall insulation and comprehensive experiments have been carried out to identify the commercially available material with the highest thermal conductivity to maximise the removal of heat from the stator windings. An array of strategically located thermocouples was used to obtain temperature distributions. The performance indicators for comparison of the new and conventional servomotors are cogging torque, iron loss and dynamic torque. The cogging torque proved to
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increase with the permanent magnet retaining feature for the new servomotor and becomes unpredictable but this disadvantage was offset by material saving and its maximum torque being approximately 40% greater than that of the conventional servomotor. The iron losses have been measured at speeds ranging from 500 r/m to 6000 r/m and at surface housing reference temperatures approximately 810C. At maximum reference winding temperature ranging from 1000C and 1250C, the dynamic torque rating performances of the new and conventional servomotor prototypes have been compared, the torque/speed characteristics being generated from a near stall speed of 50 r/m to the rated 3000 r/m. The contributions of this research programme may be summarised as follows: 1. Above all this thesis provides valuable design steps for permanent magnet synchronous servomotors using three world leading software packages (OPERA_2D, SPEED and Motor-CAD) for machine design and will therefore be a useful reference for electrical machine designers, especially beginners. 2. Material saving and simplified BPM synchronous servomotor manufacturing process for segmented stator, concentrated winding configuration. 3. Novel permanent magnet retaining method permitting more uniform and smaller air gap giving performance enhancement. 4. Systematic finite element analysis (FEA) based optimisation is applied to implement a new design principle for BPM synchronous servomotors regarding the stator parameters. 5. A simplified frequency dependent equation for open-circuit iron loss calculation. 6. Experimental and theoretical investigation of thermal impact of stator wall paper insulation entailing comparison of several different materials. 7. Direct benefit to industry through adoption of the new servomotor design by the company who supported the research