Design considerations for a PM-BLDC machine for flywheel energy storage applications

This paper gives the guidelines for the design of BLDC generator and selection of a bi-directional converter topology for optimal energy harvesting from a Flywheel Energy Storage (FES) system. A typical FES system consists of a rectifier, a bi-directional converter (BDC), an electrical machine (BLDC generator), and a flywheel. The BDC is operated as a boost converter for maintaining a constant dc bus voltage while energy is being harvested from the flywheel. Internal resistance (R_s) of the source significantly affects the boost converter gain and harvestable energy. It is important to model this source resistance to design an effective energy harvesting system. Dependency of this source resistance on machine parameters needs to be studied and modeled. An extensive analysis of the BLDC machine has been carried out using FEM/FFT to understand this phenomenon. This has helped in predicting R_s and choosing appropriate converter topology for optimizing the harvestable energy. Those losses in a BLDC generator, which reflect as series source resistance to the BDC, have been quantified. An FES system of 5.0kW rating with a backup time of 60sec has been built and evaluated. The results obtained from this set up have been analyzed to validate the theoretical analysis of the overall system.