Electric motors, fundamental for the propulsion of transport means, are becoming more and more crucial, due to the growing attention to energy efficiency. The design of these motors needs specialized competences in different engineering and physics ambits to optimize performances and reliability. Consequently, in recent years various dedicated software have been developed to aid engineers in their design and optimization.
How to design electric motors
In next years, one of the engineering challenges that expect us consists in constantly innovating electric motors, to increase their efficiency and to improve performances, with particular focus on the optimization of transport means’ autonomy. Due to the increase of the motors manufactured, a more sophisticated and tailored design is enabled, succeeding in satisfying the various use requirements in targeted manner. However, this kind of design needs a broad range of specialized competences in various sectors of engineering and of physics. It is necessary to understand in-depth the electromagnetism principles to assure an optimal operation of the motor, as well as to apply forefront fluid-dynamics knowledge to design efficacious cooling systems. Moreover, the construction science plays a crucial role in granting the structural reliability of the motor, ensuring it can withstand the mechanical stresses without affecting performances and duration in time.
In recent years, as response to the growing complexity of electric motors and to the need of assessing their performances in accurate and efficient way during the design phase, different dedicated software have been developed. These tools are designed to provide engineers with a virtual environment where it is possible to carry out multiphysics simulation, integrating electromagnetic, structural and fluid-dynamic analyses.
The multiphysics approach adopted by these software allows engineers to understand in complete and detailed manner the behaviour of electric motors under different operational conditions. The electromagnetic simulations permit to evaluate the distribution of the magnetic field inside the motor, to understand electromagnetic losses and to foresee electric performances. The structural analyses allow assessing the mechanical stress and the deformation of the material under load, identifying potential weak points in the motor structure. Fluid-dynamic simulations allow instead studying the cooling flow inside the motor, evaluating the efficacy of the cooling system and preventing the overheating.
Main software
Software houses have met the growing demand for tools dedicated to the design and analysis of electric motors by developing a set of specific solutions. Just to make an example, Ansys has developed Motor-CAD, Altair has introduced FluxMotor, while Siemens has presented E-Machine Design. These software are designed to provide engineers with a broad range of instruments to simulate, to analyse and to optimize electric motors, by integrating electromagnetic, thermal, structural and fluid-dynamic analyses.
Moreover, in the ambit of such simulations, it is possible to integrate the results obtained by more specific software, permitting a more in-depth and accurate assessment of motor performances. Therefore, it is advisable to use programmes of the same suite to assure a better integration of results and a higher coherence in the analysis.
In the ambit of open-source software as well, tools for the electric motor analysis are available, even if they are not so specialized and complete as their commercial counterparts. FEMM (Finite Element Method Magnetics), for instance, offers functions for the simulation of the magnetic field and also includes a simulator for the analysis of thermal exchanges. Elmer FEM is another example of open-source software, a multiphysics simulation environment that offers some skills for the analysis of electric motors, although it is not specifically dedicated to this purpose.
Unfortunately, at present, these software are not developed in a suite that includes other specific software, therefore the integration with CFD software such as OpenFOAM or FEM like Salomè is more difficult.
Driving cycle and duty cycle
A fundamental aspect that significantly distinguishes these software from more conventional ones is their capability of simulating precisely and quickly the operational cycles of electric motors. This is essential because motors, both those used in household appliances and in vehicles, do not work constantly at the highest power but they undergo instantaneous variations in the demand for power delivered and/or in the number of revolutions.
The motor of a washing machine, for instance, modulates its revolutions according to the kind of spin-drying demanded, with variations that can occur also inside the same washing cycle, according to its duty cycle. Likewise, a motor of a vehicle undergoes higher power demands during the acceleration, whereas it acts as a generator while braking or in other operational modalities.
It is fundamental to take such variations into account during the design phase, since there is a delay between the power request and the consequent temperature variation, caused by the thermal inertia. Not considering that can lead to some overheating that can cause some malfunctions or even fires. At the same time, oversizing fans or anyway keeping them in operation for a longer time than necessary can notably increase costs and consumptions.
In vehicles, specifically, standard routes have been defined that theoretically represent typical urban ad extra-urban paths. One of the most used is the WLTC (acronym of Worldwide Harmonized Light Vehicles Test Procedure) that is based on a series of predefined driving cycles, which include different phases of acceleration, deceleration, stops and constant speed, designed to simulate typical driving behaviours in various road conditions.
These driving cycles have been developed considering real traffic data and driving styles coming from different regions in the world, in order to grant a more accurate assessment of vehicles’ performances in terms of emissions and fuel consumption. The type 3, in particular, which must be used for most vehicles, is subdivided into four zones with different growing maximum speeds, from 55 km/h up to 130, to simulate the vehicle’s behaviour in urban and suburban ambit, out of town and on the highway.
This procedure is aimed at harmonizing vehicle approval tests among the various jurisdictions and to provide consumers with more accurate information about vehicles’ performances in terms of fuel and of emissions.
To carry out this kind of simulations, the software approximates the electric motor and the various components like a lumped parameter model, this allows a very quick analysis versus the use of numerical models. A lumped parameter model is a type of mathematical model used to describe the dynamic behaviour of a physical system, where the behaviour of the various components is approximated by parameters, each of which represents a specific part of the real system. Lumped parameters are some functions that characterize the element, like the thermal capacity in a thermal system.
The key idea of a lumped parameter model is simplifying the complex system by subdividing it into simpler elements, each of which is described by a limited number of parameters. This allows simplifying and speeding up the analysis, permitting to forecast its dynamic behaviour as response to determinate inputs or operation conditions.
In this way, it is possible to simulate various operation hours of the motor in few minutes. Besides, it is also possible to complicate the analysis, considering a part at lumped parameters and instead simulating directly with thermal or electromagnetic analyses either the most critical components or those exerting a higher influence on performances.
Acoustic simulations
Electric motors, even if generally more silent than internal combustion engines, can anyway emit sounds that are intrusive and sometimes unpleasant, because different from those we are used to. Therefore, it is essential to consider the acoustic impact of the motor during the design process.
Dedicated software tools allow examining in detailed manner all acoustic phenomena involved, including the effects of the vibrations induced by electromagnetic forces. In electric motors, such forces represent one of the primary causes of noise and can depend on various factors such as the air gap flux density, the number of poles, the number of slots in the stator, the type of winding and fault conditions.
Through the analysis of the behaviour of these electromagnetic forces, it is possible to identify the areas where noise increases occur and to develop strategies to mitigate them. This implies the optimization of magnetic, structural and motor cooling parameters. The key parameters to decrease the acoustic noise include the design of the structural motor housing, the strategic arrangement of the cooling fins and the minimization of the eccentricity of the rotating parts. Through a holistic approach to design, it is possible to improve significantly the acoustic characteristics of the electric motor, granting a more silent and comfortable operation.
Moreover, these software succeed in reproducing the generated noise, which can be then listened to and analysed directly by designers and by acoustic comfort experts.
In short
In the complex process of electric motor development, it is clear that an integrated and multiphysics approach is crucial to maximise their performances. The integration of all phenomenological aspects, from thermal to acoustic characteristics, represents a fundamental stage to guarantee an optimal operation of the system. The precocious adoption of such approach during design phases allows designers to evaluate with precision torque and speed curves of the motor in relation to the cooling choices adopted and at the same time to analyse and to characterize the level of noise generated according to the motor’s operating speed. Therefore, this methodology offers a sound basis for the development of highly efficient and performing motors, which will be the foundations of the next ecologic revolution.
(by Carlo Alberto Pasquinucci)
