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POWERTRAIN


behaviour at three different working points. The input data for all these simulations includes material properties, the electric properties (resistance and current supply) and the speed of the motor. After having reached steady- state for these simulations, the following output data could be extracted: torque, torque ripples, losses and efficiency. For the last simulation, 100kW at max speed test, a dedicated analysis was done for extracting losses in each region (iron losses in rotor and stator, eddy current losses in magnet, Joule losses in the coil). These are later used to feed the thermal simulation. The cooling is done through a water


jacket on the outside of the stator. Convection and radiation are accounted for through boundary conditions. The test case prescribes to stay at maximum speed for two hours. The goal is to check that there is no risk of overheating the coils. The losses determined from the previous test are used as input for this test. Finally, the temperature is determined as a function of time, to test the coil winding temperature at the last time step. When designing e-motors, it must be assured that the risk of magnet demagnetisation is minimised. Porsche uses a short-circuit test at the base point to address this issue. Based on such a simulation, a specific feature and procedure in the software is applied to compute the remnant flux density at the end of the computation. We can then extract a percentage of the magnet that is demagnetised. The challenge is to get the highest


value of current after short-circuiting. A parametric analysis has shown when to start the short-circuit.


Temperature versus time


MECHANICAL RESPONSES Mechanical stresses must be constrained to be kept below a specific level to assure mechanical integrity. The stress occurs mainly due to rotational forces at high speed. The starting point is a STEP file generated as a result of the Flux 2D load cases. Based on the geometry information in the file, an FE mesh is created, and all mechanical properties are automatically set in a batch process within Altair HyperMesh. The simulation to evaluate stresses is executed in Altair OptiStruct. The maximum stress values are finally extracted. At this point the focus lies on tensile stresses since they are considered more critical in comparison to compression stresses. The complete study with all


simulations was setup in Altair HyperStudy. Total run time to extract all responses for one single design was 29 minutes. A DoE with 358 runs was executed to cover the design space. The total run time was 17.45 hours running 15 jobs in parallel. Following the DoE, optimisation and design exploration could be performed. Such optimisations and design explorations can be executed on a subspace of simulation types and responses and on the complete thing. The optimisation problem can be formulated as a single or multi-objective optimisation problem. Using the DoE data, studies can be


Max current value versus time 28 www.engineerlive.com


executed on a subset of variables or on the complete problem. Studies can be performed concerning driving design variables, sensitivities and trade-offs between different design objectives and constraint settings.


ADDING THE POWER INVERTER In the final phase, how to improve the design of the motor is considered by adding the power electronics which improve the accuracy of the inputs driving the machine. The design process up to this point has assumed the inputs to the e-motor are idealised (i.e. purely first harmonic) sinusoidal inputs to the three phases. However, the actual system supplies input voltages based on modern power electronics and pulse width modulation (PWM) techniques to approximate the desired driving voltage from the control algorithms in the system. This particular system includes a two-level inverter along with a current and speed controller cascaded to drive the logic of the transistors with space vector pulse width modulation. PWM methods create higher order harmonic content of the electrical inputs which can degrade certain aspects of the performance off the e-motor, such as losses and torque ripple in the machine. The losses effect efficiency and the thermal behaviour, and the torque ripple cause speed pulsations and NVH problems, and thus simulating the inverter and dependent systems in an important aspect to getting toward an optimal design. Electromagnetic losses also contribute to the thermal behaviour, which is very important for design: consideration for critical components such as the coils and magnets are important to capture accurately and this helps to improve the accuracy for the cooling system design as well. In this way we can get a more accurate result that leads to more confidence that the e-motor has


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