Accelerated Life tests for Electric drives in Aircrafts

Final Report Summary - ALEA (Accelerated Life tests for Electric drives in Aircrafts)

Executive Summary:

Electro Mechanical Actuators (EMA) are safety-critical components of an aerospace system, where an undetected actuator failure can lead to serious consequences. EMA fault diagnosis poses an interesting research problem as it is composed of electrical, electronic and mechanical subsystems, which results in intricate failure modes and effects.

In the framework of the All Electric Aircraft (AEA) and More Electric Aircraft (MEA), electric machines and drive will gain more and more impact on the aircraft weight. By allowing a more insightful knowledge of the electric machine insulation degradation mechanisms, the design procedure would be more efficient. Moreover, by avoiding over engineering in the machine design, it will be possible to reduce the size and weight of the electric drive without sacrificing safety and reliability.

ALEA project focuses at achieving complete and accurate life time models for electrical drives and, to this aim, a test rig was developed. The test rig is able to validate aging models by applying multiple age accelerating stresses and to conduct on-line tests of the machine drive under different operating conditions (mission profiles).

In order to spread the knowledge and results generated during the ALEA project the relevant results were submitted and presented at international conference with high reputation inside the scientific community, such IEEE IECON, ECCE and VDE VERLAG GmbH PCIM.

Project Context

Electro Mechanical Actuators (EMA) are safety-critical components of an aerospace system, where an undetected actuator failure can lead to serious consequences. EMA fault diagnosis poses an interesting research problem as it is composed of electrical, electronic and mechanical subsystems, which results in intricate failure modes and effects.

Among the different causes of failures in EMA, studies revealed that approximately 30% of the failures are related to electrical faults and that the majority of electrical fault happens in stator or rotor windings as a consequence of insulation degradation. The breakdown of the stator insulation system is usually a slowly developing process, which at first leads to deterioration of the inter-turn insulation and finally leads to phase-to-phase or phase-to-ground insulation respectively. A lifetime model, capable of taking into account also data other than the diagnostic measurements alone, can combine information from past use as well as expected future to perform a better estimation of the remaining EMA lifetime. The target of the lifetime estimation is to maximize the EMA useful life but also to still keep enough risk tolerance to prevent major failures from happening in the near future.

In this context, for many years the mainstream method was the quantitative reliability prediction based on empirical data and various handbooks on reliability, released by both the military and the industry. A different approach is the Physics Of Failure (PoF): a methodology based on root-cause failure mechanism analysis and the impact of materials, defects, and stresses on product reliability. The integration of different models, related to different failure mechanisms, into a unique model, taking into account all the physical processes and stress factors involving in a given component fault, is not an easy task. The approach is even more difficult at system level, as a variable number of components could be involved.

Project Objectives

ALEA project aims at laying foundations for the development of the ageing super-model for electric drives in aerospace applications. The initial model is a lifetime estimation supermodel for electric machine insulation. The project involves the development and construction of a specialized test rig for the future development and validation of lifetime degradation model as well as other electric drive lifetime models. The test rig will be capable of experimentally investigate the effects of four major sources of stress in winding insulation (thermal, ambient, electrical and mechanical), both as single effects and in combination.

The test rig is aimed at characterizing electric motors with mechanical rating of 40kW at 10kRPM with constant power up to 20kRPM.

The test rig will be capable of replicating mission profiles encountered in aerospace applications: test environment at various temperatures and pressures within a thermal vacuum chamber, where test temperature range of -40 to +180°C and a pressure range from 1000 mBar down to 30 mBar, equivalent to altitudes from sea level to 50,000 ft, can be replicated. A peculiar characteristic of the ALEA test rig is the development of a custom motor under test (MUT) inverter for the variable dv/dt operation of the MUT in order to assess the impact on insulation degradation of wide bandgap devices adoption in high speed drives.

Project Results:

ALEA project objective is the development of an ageing super-model for electric drives in aerospace applications. The S & T results/foregrounds of the project are:

• A lifetime estimation supermodel for electric machine insulation degradation
• The development and construction of a specialized test rig capable of replicating various mission profiles and combined aging stresses
• The development of a custom MUT converter capable of applying variable dV/dt stresses typical of the wide bandgap devices operation

A detailed description of each foreground item is given in the following subsections.

Insulation testing and modelling

The realisation of a lifetime model that takes in consideration the all stress factors involved in aircraft systems is not a trivial task. However, knowing the operating condition is possible to calculate the inputs of the simulative aging model by means different physical based algorithms, reducing the variables in a restricted number. In particular, the developed Electrical Stress estimate Block permits to extrapolate the level of electrical stress at which the MUT is subject during operation, starting from multiple information regarding electrical layout, carrier frequency, commutation time, etc. The information can be then used as a input of a simulative aging model that has to be developed, tested and integrated with the results collected from a test campaign conducted on different MUTs with the realized test rig.

Test rig development

The test rig is aimed at characterising electric motors with mechanical rating of 40kW at 10kRPM with constant power up to 20kRPM.

The test rig comprises four main components: a thermal vacuum chamber to apply environmental stress factors; a reconfigurable test bench that can be embedded in the test chamber; a data acquisition and control system to monitor an acquire relevant operating parameters and a custom inverter to drive the motor under test (MUT) while applying different levels of stress.

The test rig will be capable of replicating mission profiles encounterd in aerospace applications: test environment at various temperatures and pressures within a thermal vacuum chamber, where test temperature range of -40 to +180°C and a pressure range from 1000 mBar down to 30 mBar, equivalent to altitudes from sea level to 50,000 ft, can be replicated.

The ALEA test rig control and DAQ interface is developed in LabView 15.1; the functionalities are:

• Re-create variable length test procedure (mission profiles) , with different load curves depending on different triggers (e.strength. time, temperature, pressure, or a combination of thereof)
• Communicate with the MUT and brake inverters to follow the speed curve / torque demands for a given mission profile
• Acquire and monitor the analog signals (temperature, torque) to check the proper functioning of the system
• Manage and organize the acquired data for later postprocessing

Custom MUT converter

The peculiar part of the realized MUT converter is the gate driver circuit logic, as it permits to vary the dv/dt ratio of voltage supplying the MUT, thus varying the enables of the electrical stress at which the motor is subject.

The realised gate driver architecture consists of 2 two different 'enablable' ICs connected in parallel and having different value of external gate resistance Rg. By means of the enable signals it is possible to choose which gate driver will trigger the power switching, making them working singularly of in conjunction. Thus it is possible to choose up to three different value of Rg and therefore three different dv/dt ratio of the MUT converter output voltage.

Project foreground dissemination

In order to spread the knowledge and results generated during the ALEA project the relevant results were submitted to international conference or journal with high reputation inside the scientific community, such IEEE, IES or IAS journals and conferences.

Potential Impact:

In the framework of the All Electric Aircraft (AEA) and More Electric Aircraft (MEA), electric machines and drive will gain more and more impact on the aircraft weight. By allowing a more insightful knowledge of the electric machine insulation degradation mechanisms, the design procedure would be more efficient. Moreover, by avoiding over engineering in the machine design, it will be possible to reduce the size and weight of the electric drive without sacrificing safety and reliability.

ALEA project aims at achieving complete and accurate life time models for electrical drives and the realisation of a test bed that is able to validate these models by applying multiple age accelerating stresses and to realize the on-line tests of the machine drive under different operating conditions.

Regarding the short-term impacts, the initial technology review will thoroughly analyse the state of the art regarding lifetime models and reliability of drives. Such studies, which today have never been conducted in too much detail or applied to generic life models (applicable for a wide range of electrical machines) represent a considerable improvement in terms of progress beyond the state of the art. Designers and manufacturers of electrical machines would benefit greatly from such models.

The results from this investigation are predicted to give also a higher level understanding of these phenomena and will also result in the current design, manufacturing and assembly procedures to be improved.

The main output in terms of lifetime modelling of electrical drives will be the creation of an accurate combined life model. This will result in considerable improvement on the understanding of the mechanisms of failures of electrical drives. This is projected to have a huge impact on the design of electrical drives. The application of comprehensive lifetime model to the design procedures of electrical drives will improve reliability of electrical drives, which for the aerospace sector has always been a main limiting factor.

In fact, this project will enable highly improved design methodologies that will reduce the risk of insulation degradation. One expected outcome will be the improvement of geometrical positioning of impregnation compounds in order to reduce insulation degradation including degradation due to PDs. The project will also have impact on the industrial process for realising motors. As a matter a fact, the capability of motors to avoid PD effects is greatly affected by the industrial process of automated winding. Such improvements in reliability will result in a considerable help for European motor and drive manufacturers to improve their products and thus become more competitive world-wide.

Having more reliable and higher performing motor drives could aid European institutions in the race against the ever-increasing Asian market.

The procedures to realise the test bed, the data acquisition software and the post-processing software that was developed during the project will be helpful to further push the limits of testing of electrical drives. Moreover, the test of actual motors and the data acquired will constitute a database that will improve the existing knowledge of life time models.

The ability to modify the actual voltage waveform characteristics of the converter driving the motor under test, in conjunction with the other test facilities, constitutes a great improvement on the existing testing methodology, that usually imply off-line accelerated testing of the twisted pairs. The ability of the test set-up to look at different technologies of motors is also an asset that will improve the understanding of failure mechanisms related to specific motor technologies.

The long-term impacts of the project are related to the new power devices that are being researched and tested by the inverters’ manufacturers. In fact, the constant pursuit of the maximum efficiency and power density, in addition to the increasing requirements of the control system, have forced the designer to increase the power devices’ switching frequencies. In this framework, the machines are subjected to extremely high voltage stresses, whose effects in real operating conditions have not been assessed yet. As a matter of fact, the impact of the work from this proposal will be critical for motor manufacturers that can optimize their design workflow depending on the expected lifetime of the machine.

In order to spread the knowledge and results generated during the ALEA project the relevant results were submitted to international conference or journal with high reputation inside the scientific community, such IEEE, IES or IAS journals and conferences.