Future aircraft will use a large amount of electrical power. Not only are extra systems needed for e.g. in-flight entertainment, communication and passenger comfort, but more importantly there is a trend to replace aircraft systems powered by oil or hot engine air (bleed air) by electrical systems. This means that aircraft electrical generators must be scaled up accordingly. How can this be done safely and reliably, without making the equipment too heavy?
One solution is running the generators at a high speed, which results in better power density, but also puts a lot more stress on the rotor and on rotor-mounted electrical diodes. The environment wasn’t very friendly to start with, with high temperatures and high mechanical forces, and the extra stress is making it worse. How can we be sure that the generator doesn’t fail prematurely due to diode fatigue?
In the DiDi-FaCT project, NLR performed long-term thermo-mechanical stress tests on about 100 samples of multiple diode types, to determine their extreme limits. NLR developed a fully automated dedicated fatigue test bench, which used a mechanical press to mechanically load the power diode multiple times a second, while keeping it at a constant temperature. The diodes were tested electrically to assess their health. To simulate the life cycle of a plane, this cycle of stress was repeated many times. The test continued until the diode broke, or a maximum of 100,000 cycles, the equivalent of 50 years of use.
In addition to mechanical cycling at a fixed temperature, NLR also developed a test bench for thermal cycling. Under a constant mechanical load, the diode was cooled to extremely cold conditions using liquid nitrogen, and then electrically actuated, heating it to higher temperatures. After an electric test to assess the diode’s health, the cycle was repeated.
The test results were used to create a predictive fatigue model of the diode. The model allows designers that apply power electronic components in harsh environments to optimise the construction of the packaging so that it has the required long-term reliability. This is an essential step towards obtaining large weight and efficiency improvements in generators and other equipment, which in turn leads to reduced fuel consumption and emissions of the aircraft.
The main conclusions of the project were:
1. The fatigue behaviour of the diodes differs fundamentally from ‘classic’ materials
2. The upper applied pressure of the mechanical cycles determines the life of the diode.
3. Contrary to expectation, the lower pressure of the mechanical cycles and the test temperature have no influence.
4. Above a certain threshold level of the upper pressure, there is a large scatter in lifetimes – some samples fail instantly while others can last a long time. Below the threshold no devices were seen to fail before the end of the test.
