Thermo-mechanical fatigue of electric machine windings

Due to the intermittent operation of electric machinery used in electric and hybrid-electric vehicles, thermo-mechanical fatigue is an important concern. This small research project, a collaboration between KTH and Scania, has the overall goal to determine whether thermo-mechanical wear is an important aging factor and to set up models for how this aging can be modelled.

To be able to predict the expected life time of a component is important. In order to do that the following must be known:

1) The ageing factors in the actual operation of the component.
2) The ageing process of each factor.For Scania, 1) is found through either from physical tests or from simulations. The ageing factors depend of course on the type of component. For electric machines, the main factors are:
a) Ageing caused by temperature (of the winding and insulation system).
b) Ageing caused by thermal cycling.
Factor a) is well known and is modelled by a material degradation by temperature and is considered to follow Arrhenius equation which is well established in the electrical engineering industry.

For thermo-mechanical ageing/wear on electric machines there is the IEC TS 60034-18-34 publication which aims for form-wound high voltage machines. Application to low voltage machines (<1000 V) is seldom seen in literature.

An electric machine in a heavy vehicle powertrain has a duty that is strongly intermittent. This means that the thermal cycling is more pronounced compared to normal industry grade machines. It also means that the ageing due to thermal cycling could be more important and therefore it is necessary to investigate this.

The overall goal of this work is to:
-Determine whether thermo-mechanical wear is an ageing factor or not.-Set up a model describing how ageing is ruled by for example thermal cycle amplitude, cycle rate or other parameters.

The scope of the work will be:
-Literature study on previous work.
-Adapt existing methods in structural strength theory and apply them to the varying thermal load of the electric machine. The load cycles in structural mechanics can be assumed to be very similar to the thermal load cycles in electric machines and therefore the same models should apply.
-Simulation with a thermo-mechanical model of a core-winding object. This will help understanding what the important parameters are and will thus give input to both evaluation of test set-up and results.
-Analyze measured data from vehicle.

A laboratory set-up is being planned at Scania and depending on the timing of the completion of this set-up the scope of work may also include:
-Revision of existing test methods.
-Design of a suitable test object.
-Analysis of test data.

The work will be conducted both at KTH and at Scania in Södertälje.

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