Showing papers by "Chris Gerada published in 2011"

Design Considerations for a Fault-Tolerant Flux-Switching Permanent-Magnet Machine

Abstract: In safety critical aerospace applications, fault-tolerant drives can help reach the necessary system reliability levels without replicating the entire drive system and thus minimizing the overall system weight. Machine selection and design for fault tolerance has to be considered at an early stage to ensure optimal performance at a system level. This paper looks at the fault-tolerant properties of permanent-magnet flux-switching machines (PMFSMs) and proposes a new configuration able to fulfill the fault-tolerant requirements. PMFSMs have the distinct property of having a robust rotor construction with the permanent magnets embedded in the stator while having their operational characteristics similar to those of synchronous permanent-magnet machines. While these machines have numerous inherent advantages for achieving a high power density, in their basic form, they are not tolerant to short-circuit winding failures. This paper will look at a novel stator structure able to achieve a 1-p.u. winding inductance and will subsequently look at design iterations to maximize the torque density.

Design Aspects of High-Speed High-Power-Density Laminated-Rotor Induction Machines

Abstract: This paper deals with the considerations associated with the design of high-speed high-power-density laminated-rotor induction machines (IMs). The considerations discussed are described by the design of an actual 10-kW machine, which runs at speeds of up to 75 kr/min with a rated power density of 28 MW/m3 for an electrically assisted turbocharger. Using a developed multidomain design environment which puts equal weight on the electromagnetic, mechanical, and thermal aspects, the rotor split ratio, electric and magnetic loadings, lamination material, rotor-bar material, and rotor-bar shape are identified as important and sensitive parameters in the design of high-speed IMs. Finally, general guidelines for designing high-speed high-power-density IMs are presented.

Winding condition monitoring schemefor a permanent magnet machine using high-frequency injection

Abstract: This study presents a new technique for condition monitoring of motor windings in variable speed drive systems. The technique employs high-frequency injection to monitor the machine's incremental inductances. These inductances can provide an indication of changes in the magnetic signature owing to the presence of electrical faults. The Space modulation profiling (SMP) technique is used to create a look-up table that defines the healthy incremental inductance signature of the drive as a function of load and rotor position. A similar signature is obtained online; this is compared with the SMP look-up table to derive an error signal which can be used to trigger warnings or alarms as faults develop. In this study, the emphasis is on the detection of stator winding faults in a permanent magnet AC machine, including incipient and short-circuit faults. The result is a monitoring scheme with improved sensitivity with respect to monitoring schemes based on the passive measurement of terminal quantities. Experimental results show that the monitoring scheme has a fast response and high sensitivity which make it suitable for predicting faults at an early stage of development, before they degenerate into a serious fault which requires drive shutdown.

Feasibility and electromagnetic design of direct drive wheel actuator for green taxiing

Abstract: This paper considers the feasibility of equipping the main landing gears with electric motors for the aircraft traction during the taxi phase. Those electromechanical wheel actuators make possible a “Green Taxi” operation by considerably reducing the on-ground carbon emission. Moreover, this will enable important fuel saving for short distance flights with high frequency of landing and take-off. In this work, direct drive wheel actuator is considered for energy efficiency and mechanical reliability. Two possible locations of the actuator are examined and the weights of the corresponding electric machines are compared. The most weight efficient location is then selected. A high torque density permanent magnet machine is then designed to fit in this envelope and to satisfy peak torque, weight and flux weakening capability requirements. The design procedure as well as several technologies adopted to maximize the torque density are presented.

Development of a new fault-tolerant induction motor control strategy using an enhanced equivalent circuit model

Abstract: Variable speed induction motor drives operating with an open circuit phase fault generally have a high harmonic current content and impose a large torque ripple on the load. These effects result from the unbalanced nature of the faulted machine, and under these conditions, the effects of machine and converter non-linearities become more pronounced. It is important to include these non-linearities when modelling drive systems which may operate under fault conditions, and developing fault ride-through control algorithms. This paper proposes a modelling method for induction motor drives which includes machine saturation and space harmonics effects as well as inverter non-linearities, which does not require significant computation times. Results from an experimental test rig are presented to validate the model. The second part of the paper describes how the model was used to develop a new fault ride through control strategy for a voltage-fed vector controlled drive. Experimental results demonstrate the improved drive behaviour with an open-circuit winding fault when controlled with the new strategy.

Electrical machines for high speed applications with a wide constant-power region requirement

Abstract: This paper discusses the issues associated with the design of high speed machines with a wide constant-power region requirement. Using described multi-physics design environments, which put equal weight on the electromagnetic, thermal and mechanical considerations, the suitability and power density achievable using Induction Machines (IM) and Permanent Magnet Synchronous Machines (PMSM) are compared.

Fault-tolerant, matrix converter, permanent magnet synchronous motor drive for open-circuit failures

Abstract: A fault-tolerant matrix converter drive topology, which can be effectively used to drive a permanent magnet synchronous motor for research in critical applications, with particularly emphasis on aerospace applications, is presented. The proposed matrix converter configuration, a four output-phase matrix converter, uses a normally redundant output phase connected between the input supply and the neutral point of the motor in order to provide fault-tolerant capability in the presence of faults. This proposed fault-tolerant matrix converter can deal with open-circuit faults caused by failures in either the power converter or machine windings. Without using any additional connecting devices and without modifying the modulation technique, the satisfactory system performance under faulty operating conditions can be achieved using the developed control strategy. The space vector representation of the direct space vector modulation technique is adaptable so that the technique used for a conventional three-phase matrix converter can be applied to the four-phase matrix converter even in the fault-tolerant mode. The verification of the proposed fault-tolerant matrix converter drive is confirmed using extensive simulation and experimental results.

Rotor losses in fault-tolerant permanent magnet synchronous machines

Abstract: The necessary reliability of safety-critical aerospace drive systems is often partly achieved by using fault-tolerant (FT) electrical machines. There are numerous published literatures on the design of FT machines as well as on control algorithms used to maintain drive operation with an incurred fault. This study is set to look at the rotor losses in three- and five-phase surface mount permanent magnet machines when operating in faulty mode in order to highlight the influence of the post-fault control strategy and winding configuration. Although the work presented in this study is mainly focused on FT control methodologies targeted at mitigating phase open circuit faults, the implications of short-circuit faults is also considered and discussed.

Analytical modeling of a vertically distributed winding configuration for Fault Tolerant Permanent Magnet Machines to suppress inter-turn short circuit current limiting

Abstract: This paper presents an analytical model to evaluate the inter-turn short-circuit current in a Fault Tolerant Permanent Magnet Synchronous Machine (FT-PMSM) for different winding configurations. By evaluating slot-leakage and air-gap fluxes, the inductance of both healthy and faulty parts of the phase winding, as well as the mutual inductance between them, can be determined for any position and number of the shorted turns. Using these inductances, the short-circuit current is determined. The proposed model is verified with finite-element analysis and validated experimentally. It will be shown that the magnitude of an inter-turn short-circuit current depends on both the number of shorted turns and their position in the slot. A new concentrated winding configuration with vertical plate conductors in the slot is proposed. The measured short-circuit inductance shows that this winding can inherently limit the short-circuit current and reduce its dependence on the position within the slot.

Weight optimisation of a surface mount permanent magnet synchronous motor using genetic algorithms and a combined electromagnetic-thermal co-simulation environment

Abstract: This paper proposes a new approach for the design optimisation of a naturally ventilated, vertically mounted surface mount permanent magnet synchronous motor (PMSM). This approach is based on a parallel computing (electromagnetic and thermal) environment and non-linear constrained optimisation problem solving using genetic algorithms (GA). The optimisation is simultaneously applied to both the electromagnetic and thermal design parameters rather than optimising them sequentially. It will be shown that the new design approach allows a weight reduction of more that 4% compared to the approach where the electromagnetic and thermal designs are done separately based on state-of-the art typical parameters values.

Considerations for the design of a tubular motor for an aerospace application

Abstract: This paper deals with the design of high force density tubular linear permanent magnet motors for aerospace applications. The main requirements of aerospace applications such as force density and fault tolerance are considered in a general review of recent high force density linear machines, highlighting the merits and disadvantages of each motor. Analytical considerations, including two dimensional finite element analyses are used to compare the performance of two main tubular machine topologies, namely a single air-gap tubular motor with inset permanent magnets and its double air-gap counterpart. An experimental validation is presented comparing the predicted analytical results with the measured results on a built motor prototype.

A Brushless DC motor design for an aircraft electro-hydraulic actuation system

Abstract: This paper describes a 5-phase Brushless DC motor designed for an electro-hydrostatic actuation system. The foundation of the design is a motor with compact structure, low weight, fault tolerance and high reliability. The motor power rating is 12kW at 12,000 rpm and a “wet” form of construction is used where hydraulic oil is present in the motor in order to reduce the number of oil seals of the EHA for enhanced reliability and lifetime. The thermal behavior is evaluated for an optimized design. Fault tolerance for Brushless DC motors will be discussed. A 5-phase motor has been manufactured and test results are presented to validate the design.

Performance evaluation of induction motor efficiency and in-service losses measurement using standard test methods

Abstract: This paper investigates consequences of the differences between the actual, practical operating conditions of in-service induction motors and the ideal operating conditions that apply for the Standard Type Test, following the IEEE 112-B method. In these tests, the significance of each factor influencing motor efficiency, including operating temperatures, temperature rise, voltage supply magnitude, voltage balance and different load conditions is assessed upon the motor performance. The preliminary analysis shows the ambient temperature has a noticeable influence on motor losses, especially on the conductor losses and thus efficiency. The actual part load efficiency is higher than catalogue part load efficiency figure when the motor is running at partial load continuously in the field. The voltage magnitude alters the balance of the five loss mechanisms for a particular load; one can adjust the supply voltage to improve the motor efficiency for a continuous constant load operation. The increase in losses under unbalanced voltages leads to the necessity of derating the motor. The most significant factor which influences the efficiency of motor is the determination of operational loading.

Analysis of two-part rotor, axial flux permanent magnet machines

Abstract: This paper presents a study of the effects obtained when a portion of permanent magnet material is replaced by a soft magnetic composite piece over the rotor poles of an axial flux machine. This technical solution can be adopted to increase the machine inductance in order to limit short circuit currents or to increase the motor saliency and implement sensorless control schemes based on high frequency signal injection. Some design rules for the design of the soft magnetic poles have been derived in order to obtain the desired inductance and/or saliency. The analysis has been validated with finite element analisys and numerical simulations using Matlab/Simulink environment. Some preliminary experimental results have also been reported.

Permanent magnet motor design optimisation for sensorless control

Abstract: This paper looks at a novel optimisation approach to the design of surface mounted permanent magnet (SMPM) machines with self-sensing capabilities. A methodology will be presented which will look at the use of genetic algorithms (GA) to contemporarily maximise the output torque and the self sensing properties of such machines. A GA optimisation environment has been grafted with a finite element analysis (FEA) environment to enable the designer to account for both geometrical and saturation saliencies for an effective determination of the machine's self sensing characteristics. Satisfactory results were obtained in terms of torque maximization and self sensing capability. In addition sensitivity of the major geometrical parameters of the machine will be discussed in terms torque density and the self-sensing.

Hybrid sensorless control of axial flux permanent magnet motor drives, including zero speed

Abstract: Sensorless control of an axial flux permanent magnet motor drive is proposed and tested. The motor is not purposely designed for sensorless control and shows a very small inherent saliency. This significantly affects the saliency-based position estimation in the low speed region. Other non-idealities, such as the non-sinusoidal back-EMF waveforms and possible misalignment between stator and rotor make the control more challenging. A robust sensorless control scheme is proposed, able to deal with these non-idealities with a rather simple implementation. The position estimation is based on a closed loop hybrid observer of the permanent magnet flux linkage. Experimental results report torque and speed sensorless control.

Two part stator for an electrical machine

Abstract: A stator comprises an inner stator component 10 comprising an annular base portion 12 and a plurality of teeth 14 extending radially outwards and defining slots for stator windings 24. An outer stator component 20, which may form a back iron, is arranged to surround the inner stator component 10. The stator is assembled by first inserting the stator windings into the slots in the inner stator component, and then inserting the inner stator component with the stator windings in place into the outer stator component. The teeth of the inner stator may engage slots in the outer stator to prevent relative rotation between the stator parts. The windings may be preformed and comprise concentrated or distributed windings. The teeth may have parallel or divergent sides and may extend at an angle to the radius and may be straight or curved.

Modeling of Flux Reversal Machines for direct drive applications

Abstract: Flux Reversal Machines can be easy to manufacture, are reliable and well suited for direct-drive applications. They also have the advantage that they can be controlled as standard synchronous PM machines. As for most of doubly salient machines, the machine design is relatively complicated due to the highly non-linear magnetic behavior that makes it difficult to device a general optimal design procedure. A simple hybrid approach, partly analytical and partly finite element based is proposed to address the main FRM design parameters influencing its performance. Based on the modeling approach, a prototype is designed and it is currently under construction, as documented in the paper.

Detection of inter-coil short circuits in the stator winding of a PM machine by using saliency tracking schemes

Abstract: This paper demonstrates the feasibility of using saliency tracking schemes (normally associated with sensorless control of drives) for the detection of inter-coil short circuits in the stator windings of a PM machine. This is obtained through a co-simulating of Matlab/Simulink and a Finite Element Model (FEM). Vector control and sensorless control schemes based on saliency tracking were implemented using Simulink, while FEM was used to implement a detailed model of the machine. This facilitates the evaluation of several winding faults while keeping a realistic drive representation. Two different schemes for saliency tracking were evaluated: The first is based on High Frequency injection -which was demonstrated by these authors to be a good alternative for winding condition monitoring-. The second approach is based on PWM transient excitation which is becoming very popular in sensorless drives applications. Even though both schemes use very different demodulation techniques, they share the advantage of having a good resolution for the detection of small changes in the machine saliency, which is beneficial for condition monitoring applications. This paper evaluates and compares both schemes in order to determine their suitability for the implementation of condition monitoring schemes.

A fault-tolerant control scheme for a dual flux-switching permanent magnet motor drive

Abstract: A novel fault tolerant flux-switching permanent magnet synchronous machine (FS-PMSM) drive topology is presented able to operate during open and short circuit winding and converter faults. The scheme is based on a dual winding motor supplied from two separate vector controlled, voltage sourced inverter drives. The windings are arranged in a way so as to form two independent and isolated sets. Simulation and experimental work will detail the drive's performance during both healthy and faulty scenarios and will show the drive robustness to operate with faults.

Evaluation of the machine non-linearities as an aid to develop self-commissioning in sensorless drives

Abstract: This paper investigates the minimum requirements to implement compensation schemes for saliency based sensorless control schemes that will work successfully irrespective of the method or machine type. The paper focuses on analyzing the saliency disturbance components due to the machine only, by simulating a machine with a FEM model, and coupling this to a simulation of a full sensorless control which uses ideal power electronic components. The initial results show common saliency harmonics which can therefore simplify the setting up of the filters used to clean up position estimates in sensorless drives.