Chris Gerada

Multiphase Power Converter Drive for Fault-Tolerant Machine Development in Aerospace Applications

This paper describes an experimental tool to evaluate and support the development of fault-tolerant machines designed for aerospace motor drives. Aerospace applications involve essentially safety-critical systems which should be able to overcome hardware or software faults and therefore need to be fault tolerant. A way of achieving this is to introduce variable degrees of redundancy into the system by duplicating one or all of the operations within the system itself. Looking at motor drives, multiphase machines, such as multiphase brushless dc machines, are considered to be good candidates in the design of fault-tolerant aerospace motor drives. This paper introduces a multiphase two-level inverter using a flexible and reliable field-programmable gate-array/digital-signal-processor controller for data acquisition, motor control, and fault monitoring to study the fault tolerance of such systems.

Integrated PM Machine Design for an Aircraft EMA

This paper looks at the requirements and challenges of designing a permanent-magnet (PM) motor for a directly driven electromechanical actuator for aerospace applications. Having a directly driven system, the intermediate gearbox is eliminated, bringing advantages in terms of lower component count and reduced jamming probability. The design of a low-speed high pole number PM motor will be investigated as a potential solution. The main goals of the design are a high level of actuator integration in order to minimize weight and volume, fault tolerance, and high reliability. The design will be tailored to the requirements of a typical midspoiler actuation system for a large civil aircraft.

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

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.

A Thermal Improvement Technique for the Phase Windings of Electrical Machines

In electrical machines, a higher torque/force density can usually be achieved by increasing the current density in the windings. However, the resulting increase in copper losses leads to higher temperatures in the coils, particularly in the center of the slots where the thermal resistance to the ambient/cooling surfaces is highest. In this paper, a novel, simple technique is presented in which a higher thermal conductivity path between the center of the slot and the cooling arrangement is created, thus increasing the heat flow away from the slot center. A lumped-parameter thermal model is presented and used along with finite-element analysis to investigate the effectiveness of the proposed technique. The lumped-parameter model is also used for optimizing the high conductivity path for maximum air-gap shear stress and to obtain a compromise between the reduced slot area and the improved temperature distribution. Experimental validation is then presented to compare the predicted results with the measured results on a purposely built instrumented setup.

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

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.

Design of a Five-Phase Brushless DC Motor for a Safety Critical Aerospace Application

This paper describes a five-phase brushless dc (BLDC) motor designed for an electrohydrostatic actuation system (EHA) suited to the thin and optimized wings. The foundation of the design is a motor with fault tolerance and high reliability, compact structure, and low weight. The motor power rating is 12 kW at 12 000 rpm, and a “wet” form of construction is used where hydraulic oil is present in the motor to reduce the number of oil seals of the EHA for enhanced reliability and lifetime. The losses and thermal behavior are evaluated for an optimized design. Fault tolerance for BLDC motors is discussed. A five-phase motor has been manufactured, and test results are presented to validate the design.

A Single Sided Matrix Converter Drive for a Brushless DC Motor in Aerospace Applications

This paper describes a brushless dc (BLDC) drive with a single sided matrix converter (SSMC) for an electrohydrostatic actuation system in aerospace application. The use of an SSMC with a BLDC motor is novel and is used to achieve operation without a microprocessor. A simple hysteresis current control strategy is implemented to control motor torque. The multiphase SSMC provides high reliability and fault tolerance with the penalty of more power devices. A five-phase SSMC prototype is built. The experiment results are presented to verify the drive performance.

Modeling of Different Winding Configurations for Fault-Tolerant Permanent Magnet Machines to Restrain Interturn Short-Circuit Current

This paper describes an analytical model to evaluate the short-circuit (SC) current resulting from an interturn fault by computing the self and mutual inductances under SC fault condition. Two different concentrated winding configurations, i.e., horizontally and vertically placed conductors in the slot of a fault-tolerant permanent magnet synchronous machine are considered. By computing the associated slot-leakage and air-gap fluxes, the self inductance of both healthy and faulty windings as well as the mutual inductance between them, the SC current can be determined for any position and number of shorted turns. The proposed model is verified with finite-element analysis and validated experimentally. It will be shown that the magnitude of an interturn SC current depends on both the number of shorted turns and their position in the slot. The measured SC inductance shows that a new proposed concentrated vertical winding configuration can inherently limit the SC current and reduce its dependence on the position within the slot.

Winding turn-to-turn faults in permanent magnet synchronous machine drives

This paper examines the effect of different numbers of shorted turns in one coil of the stator winding upon the operation of vector-controlled permanent magnet synchronous machine drives. A simple thermal model of the coil is used to examine the duration the fault can be allowed to exist before there is further degradation of the insulation. Simulation results by DMRM incorporate saturation and slotting effects. Short circuit current is difficult to limit due to the uncontrolled permanent magnet produced field. Permanent magnets are also prone to demagnetization due to the compound effect of the demagnetising MMF produced by the high short circuit current together with the armature reaction MMF. Shorting small numbers of turns gives very little time before further failures could occur. Shorts of large numbers of turns change machine flux levels, saturation and torque constant. Part functionality following a winding fault might aid controlled, drive shutdown or keep up operation in intermittent, short duration, load cycles.

Automatic Design of Synchronous Reluctance Motors Focusing on Barrier Shape Optimization

The automated design of synchronous reluctance (SyR) motors based on multiobjective genetic optimization and finite-element analysis is considered in this paper. Three types of barrier shapes are considered, all described by an effective limited set of input variables. The three solutions are investigated to establish which of the geometries can give the best torque output and also which one represents the best compromise between output performance and computational time. The analysis presented in this paper shows that SyR motors designed automatically can give a good performance and can be designed in a reasonable time, and it is also shown that not all design degrees of freedom are useful in terms of motor performance. Two prototypes of automatically designed machines have been fabricated and experimentally compared with a third prototype designed according to state-of-the-art design principles.