This study presents the performance analysis and testing of a 250 kW medium‐speed brushless doubly‐fed induction generator (DFIG), and its associated power electronics and control systems. The experimental tests confirm the design, and show the system's steady‐state and dynamic performance and grid low‐voltage ride‐through capability. The medium‐speed brushless DFIG in combination with a simplified two‐stage gearbox promises a low‐cost low‐maintenance and reliable drivetrain for wind turbine applications.

This paper studies the unbalanced magnetic pull in the brushless doubly fed machine (BDFM) caused by both static and dynamic rotor eccentricities. Several parallel winding designs for the two stator windings are proposed, and the practicality of such designs is discussed with respect to direct coupling between the stator windings and with rotor undesirable harmonic fields. Once practical parallel winding designs are established, their effects on reducing deflection as a result of static and dynamic eccentricities are shown and compared with series wound stator. This study has been carried out on a prototype D400 250-kW BDFM.

The brushless doubly-fed machine (BDFM) has two stator windings with different pole numbers, supplied with different frequencies. Therefore, the distribution of magnetic fields in stator and rotor iron is complex. In addition, the stator flux density distribution is non-sinusoidal and has a DC offset at the natural speed. This makes the use of conventional hysteresis models utilised for sinusoidal fields impractical for the BDFM. In this paper a new hysteresis model is proposed for the BDFM stator iron based on the scalar Preisach model. The rotational characteristics of the magnetic fields in the BDFM are also considered and their effects in generating iron losses are assessed. 2-D time-stepping finite element (FE) models are developed for a prototype D160 BDFM to estimate iron losses and are validated by experiments.

In this work, an analytical study has been performed on the Brushless Doubly Fed Machine's (BDFM) vibration due to the interaction of its fundamental magnetic fields, exerting bending forces in the back iron. The effects of rotor eccentricity on exacerbating the machine's vibration have been considered by assessing the stator back iron displacement function in the presence of rotor eccentricity. Finite element analysis is carried out for a 250 kW BDFM built in frame size D400 to validate the analytical methods. The stator back iron displacement is determined for an ideally-constructed machine as well as when the rotor has static and dynamic eccentricity. In addition, the prototype BDFM was tested at different operating conditions in order to examine its noise and vibration levels. A set of measurements was conducted to assess the main vibration component frequencies developed by the machine at different rotor speeds. It is shown that the main vibration components are created by bending set-up in the back iron, rotor eccentricity, and the components with time and space harmonic natures. The results obtained from finite element analysis and experimentally agree with the analytical assessment of BDFM vibration.

This paper presents a method to estimate the BDFM equivalent circuit rotor current from rotor bar current measurements. Rotor currents are measured using a specially designed hardware that incorporates Rogowski coils and Bluetooth wireless transmission. The measurement of rotor currents enables the parameters in the BDFM's full equivalent circuit to be extracted unambiguously. In particular, stator and rotor leakage inductances can be estimated from experimental tests, which was not possible before from terminal measurement. The method is presented for a nested-loop rotor design and experimental measurements are shown for a prototype D180 frame BDFM.