AFPM Motor/Generator Flywheel for Electric Power Stabilization

Abstract: The study of a motor/generator for flywheel energy storage, meant to smooth the power transients in a vehicular power train, is expounded in this licentiate thesis. The proposed motor/generator has two sets of windings in the stator, rated at different power and voltage levels, and a significant moment of inertia to store energy in the form of rotational kinetic energy. The need of a machine with such specific requirements is described in Section 1. The power drive presented has components operated at two different voltage levels and requires certain kinetic energy storage capability to smooth the operation under transients in power. The key component that linked both voltage levels in the same driveline and smoothes the operation is a flywheel energy storage system operated simultaneously by two sets of windings. Flywheel energy storage systems are operated at high rotational speed to obtain high energy density. The losses in electrical machines increase rapidly with the rotational speed and unconventional solutions are required to achieve high efficiency performances. Main loss mechanisms are described in Section 2. The geometry selected for the motor/generator, in the described specific application, is the Axial-Flux, Permanent-Magnet coreless topology due to the potentially high efficiency and scalability. Machines with two windings in the stator with different voltage and power rates but mutual induction coupling are not described in literature. Section 3 presents an equivalent circuit and the equations that describe the electric properties of the motor/generator studied. Section 4 presents the analysis of an electrical machine. The dispersion of the magnetic flux in coreless machines advises against the accuracy of 2-D FEM methods. Therefore a 3-D FEM method has been developed specifically for coreless machines. The mechanical analysis of the machine rotor is also presented. The mechanical limitations of high speed machines impose also power rate restrictions due to the lack of space in the stator. A study to determine the power limits in high speed coreless machines is also presented. The calculation method and the loss mechanism models presented have been validated in small scale prototypes. The tests performed are presented in Section 5. A scale prototype of a machine with two voltage levels in the stator has been tested as part of a full system. The low voltage side has been driven by a low voltage DC/AC power converter while the voltage induced in the high voltage side has been rectified and connected to a variable resistance load. The results show a steady deliver of energy from the low voltage side and abrupt power transients in the power delivered in the high voltage side, showing a promising response of this novel power drive system.