Pseudo Direct Drive Research Articles

Pseudo-Direct-Drive Electrical Machine for a Floating Marine Turbine

The paper presents an investigation into the application of a magnetically geared Pseudo Direct Drive (PDD) electrical machine in a floating tethered 6kW, 100rpm marine current turbine. The turbine is designed to harness the energy from ocean currents, in regions with relatively high-speed water flows at depths that are a long distance from the seabed and cannot be economically exploited by conventional fixed turbines. The marine current turbine is a robust, easy to service and cost-effective solution for regions with variable or fixed sea depth of more than 60 metres. The paper presents the electromagnetic design and analysis of the PDD generator, and predicted performance such as efficiency, which is compared with dynamometer measurements on a prototype PDD generator. Furthermore, test results on a demonstrator marine turbine in a towing tank are also presented.

Magnetically Geared Pseudo Direct Drive for Safety Critical Applications

This paper presents an investigation into the electromagnetic design and performance of a fault-tolerant (FT) magnetically geared pseudo-direct-drive (PDD) electrical machine for primary flight control surface electromechanical actuation. It is shown that a large number of combinations of high-speed rotor pole pairs, pole-piece numbers, and stator slot numbers exist, for which a duplex three-phase FT configuration can be realized. Furthermore, in addition to facilitating a lower mass solution, it is also shown that a PDD presents a significantly lower inertia referred to the screw when compared to direct-drive or mechanically geared motor solutions. The findings are validated on a prototype PDD, which has been designed and built to meet the requirements of a primary control surface electromechanical actuator.

Pseudo direct drive simulation and analysis

PurposeThe purpose of this paper is to simulate a pseudo direct drive (PDD) by using permanent magnet arrays.Design/methodology/approachA 2D finite element model of the PDD was built. The static magnetic torque on air-gaps was obtained by Coulomb’s virtual work method using Ansys Maxwell software. To simulate the relative movement between input rotor and output rotor, two movement bands were applied.FindingsThe PDD’s torque relation was proved. The PDD simulated presents low cogging torque.Practical implicationsThe manufacturing steps and materials applied in a construction of a coaxial magnetic gear, PDD’s main component, are presented.Originality/valueThe value of this paper is to present the numerical techniques applied to simulate a PDD and the manufacturing steps and materials applied in a construction of a coaxial magnetic gear.

“Pseudo” Direct Drive Electrical Machines With Alternative Winding Configurations

The integration of a magnetic gear and a brushless permanent magnet machine to realize a “pseudo” direct drive (PDD) with alternative winding configurations is investigated. It is shown topologies which facilitate the winding process and achieve a more robust stator construction exist. Comparisons with conventional PDD topologies which are essentially equipped with concentric windings have shown that similar efficiencies can be achieved, albeit at the expense of reduced torque densities. Furthermore, analysis of the magnetic forces exhibited by a pole piece has shown that both the average and dynamic forces are significantly affected by the rated load condition, exacerbating the radial forces.

Rotor Position Estimation of a Pseudo Direct-Drive PM Machine Using Extended Kalman Filter

The paper describes an improved method to control a pseudo direct-drive (PDD) permanent magnet machine with only one sensor on the low-speed rotor. Due to the magnetic coupling between the two rotors, the PDD machine exhibits low stiffness and nonlinear torque transmission characteristics, and hence, the position of the high-speed rotor (HSR) cannot be determined using a simple gear ratio relationship. An extended Kalman filter is proposed to accurately estimate the position of the HSR which is used to provide electronic commutation for the drive. The technique has been implemented on a prototype PDD subjected to various speed and load torque profiles.

Coil Excited Pseudo Direct Drive Electrical Machines

A pseudo direct drive (PDD) equipped with a coil-excited high-speed (HS) rotor is presented. An analytical model which predicts the flux density in the air spaces and the permanent magnets (PMs) of the PDD is presented, and it is shown that good agreement exists between the analytical and finite-element predictions. Furthermore, the model is employed to investigate the effects of the key design parameters on the performance of a coil-excited PDD for a 10-MW wind turbine application, and an optimized design is proposed. It is shown that shear stress in excess of 100 kPa can be achieved, and that compared with a PM-excited PDD, a reduction in PM mass can be realized. It is also shown that the efficiency over the operating range of the wind turbine can be maximized by adopting an appropriate control strategy of the HS rotor excitation currents.

Scaling of Pseudo Direct Drives for Wind Turbine Application

An analytical model for the prediction of the magnetic field distribution in the air gaps and permanent magnet (PM) regions of a pseudo direct drive (PDD) is presented in scale-invariant form for magnetizations only dependent on the circumferential coordinate. The scale-invariance of the model is then employed to discuss the scaling laws of the PDD and investigate the effects of scaling on the efficiency and the mass of the various active components, i.e., copper, PM, and lamination materials for large wind turbines with powers between 5 and 20 MW. It is shown that, with the exception of the copper winding, the masses per megawatt of the steel laminations and the PMs are increased when the power rating of the wind turbine is increased.

Analytical Modeling and Optimization of Pseudo-Direct Drive Permanent Magnet Machines for Large Wind Turbines

Analytical models for the prediction of the flux density distributions in the air gaps, permanent magnets (PMs), and iron regions of large pseudo-direct drives (PDDs) on no-load and on-load conditions are presented. Predictions from the analytical models are compared with those from 2-D finite-element analysis. It is shown that good agreements exist for the flux density distributions in the air, PM and iron regions, the transmitted torque and the torque ripple, as well as the iron loss in the stator core and the pole-pieces. Furthermore, design optimization studies are undertaken in order to determine the effects of the leading design parameters on the efficiency and masses of the active components of the PDD. It is shown that for a direct-drive 10 MW PDD, an efficiency of $\sim 99$ % can be achieved with a total active mass of less than 65 tons.

Slip Recovery and Prevention in Pseudo Direct Drive Permanent-Magnet Machines

A Pseudo Direct Drive (PDD) is a permanent magnet machine with an integrated magnetic gear. It has two magnetically coupled rotors. When subjected to a load torque higher than the pull-out torque, slip between the two rotors may occur. Although this may not cause any damage to the PDD or the load, it could, if left uncontrolled, result in undesirable consequences such as loss of power transfer, excessive speed on the high-speed rotor (HSR), incorrect commutation signal, and/or loss of load speed/position control. This paper proposes a technique for a PDD equipped with a position/speed sensor on each rotor to control the slip caused by external overload torques and to enable a swift recovery when they are removed. However, when the PDD is equipped with only one sensor on the HSR, it is impossible to determine the position/speed of the low-speed rotor using an observer after slip occurs, which precludes a swift and controlled recovery. Therefore, a novel technique which prevents the slip between the two rotors under overload conditions is also proposed. Both techniques have been successfully implemented and demonstrated on a prototype PDD drive.

Speed Control for a Pseudo Direct Drive Permanent-Magnet Machine With One Position Sensor on Low-Speed Rotor

This paper describes a novel approach to operate a pseudo direct drive (PDD) permanent-magnet machine with a single sensor on the low-speed rotor (LSR). Due to the magnetic coupling between its two rotors, the PDD machine exhibits torsional oscillation when conventional control techniques are employed. While full state feedback (SFBK) control can effectively suppress torsional oscillation, the need for a position sensor mounted on the high-speed rotor (HSR) for electronic commutation imposes great constraints on the PDD mechanical design. A novel technique has been developed in order to control the PDD machine using a single sensor mounted on the LSR, whereas the position and speed of the HSR required for electronic commutation and SFBK control are estimated with an observer. Both the SFBK controller and observer are optimally designed using the genetic algorithm against a defined set of criteria and constraints. The proposed control technique has been validated by experiments on a prototype PDD system.

Design and implementation of an observer‐based state feedback controller for a pseudo direct drive

A pseudo direct drive (PDD) permanent magnet (PM) machine, realised by a mechanical and magnetic integration of a PM machine and a magnetic gear can achieve torque densities in excess of 60 kNm/m 3 . Since the torque in the PDD machine is transmitted to the load through a relatively low-stiff magnetic gear, undesirable speed/position oscillations become significant. This study addresses the pertinent issues associated with PDD operation and control. A full-state feedback (SFBK) control strategy has been employed and the feedback gains are optimally tuned by genetic algorithm against a defined performance index and under torque, current and voltage limits. To realise this control strategy at low cost, a reduced order observer has been designed and implemented to reconstruct the system states required for the SFBK control. It has been shown that the proposed control strategy and tuning method are effective in suppressing torsional oscillation, and for improving PDD servo operation and control performance. Experimental results have demonstrated the effectiveness of the proposed control technique.