Article by Carlo Alberto Pasquinucci
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 software have been developed. 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.
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.
The duty cycle in washing machines
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.
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 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.
A lumped parameter model
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.
Conclusions
In the complex process of electric motor development, it is clear that an integrated and multiphysics approach is crucial to maximise the performances. 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.
