Some design and operation aspects of axial flux permanent magnet synchronous machines, wound with concentrated coils, are presented. Due to their high number of poles, compactness, and excellent waveform quality and efficiency, these machines show satisfactory operation at low speeds, both as direct drive generators and as motors. In this paper, after a general analysis of the model and design features of this kind of machine, the attention is focused on wind power generation: The main sizing equations are defined, and the most relevant figures of merit are examined by means of a suitable parametric analysis. Some experimental results obtained by testing a three-phase, 50-kW, and 70-rpm prototype are presented and discussed, validating the modeling theory and the design procedure.

This study deals with the analytical modelling of the steady-state operation of a three-phase diode rectifier whose dc bus is connected to a constant voltage source. Such a system exhibits several operation modes that are fully investigated. Analytical expressions of the boundaries of each mode as well as the average dc and the rms ac currents are deduced as a function of the ac and dc parameters, including voltage drops across diodes. They allow us to evaluate the power delivered to the dc bus as well as the efficiency of the conversion in each mode of operation. These results can also be extended to practical rectifiers with bulk dc capacitors and allow for performing parametric analyses and optimization in the design of drives or permanent magnet synchronous generators directly connected to a diode rectifier. The theoretical results are validated by experimental tests

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The operation of a doubly fed induction generator (DFIG) connected to a dc net is considered in this study. The dc net is connected to the stator of the DFIG by a diode rectifier and simultaneously feeds the rotor-side inverter, which is used to control the machine. When the dc net operates at constant voltage, this solution preserves the possibility to work with a low flux at low speed, and, at the same time, reduces the cost of the power electronics. In fact, the only needed electronic converter is the rotor inverter designed for a fraction of the machine power. After introducing a simple control technique, the study proposes an analytic model for the steady-state operation and discusses the design specifications of the DFIG for given values of the dc voltage and of the power of the wind turbine. The most appropriate stator and rotor rated powers, rated stator voltage and turns ratio of the DFIG are deduced. Simulations and experimental results are presented in order to support the theory

Abstract-This paper deals with a new conversion topology for DFIGs suitable for wind energy conversion systems integrated in micro-grids. It consists of a DFIG which is fed by a PWM converter on the rotor and with the stator connected to a dc grid through a diode rectifier. In this configuration, the stator diode rectifier and the rotor-side inverter share the same dc-bus, so that the conventional grid-side inverter is avoided. Since only a diode rectifier designed for the full power and a reduced power inverter are required, this layout allows a cheap and effective integration of the DFIG with other generating and storage systems connected to the same dc-bus. A simple control technique suitable to regulate the power delivered to the dc grid is proposed. The scheme is based on the regulation of the amplitude of a suited fraction of the rotor flux linkage: the optimal value of this fraction is theoretically deduced in order to minimize the DFIG derating due to the current harmonics. The effectiveness of the proposed control is proven by simulations and experimental tests.