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Computer-Aided Design of Heavily Saturated Dc Motors

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Antonio Savini and Giancarlo Fasola

Abstract - The paper deals with problems of computeraided

design of a DC motor used as starter on vehicles.

A two-dimensional finite element analysis of the magnetic

field in the presence of heavy saturation is shown

to be the fundamental tool for design considerations,

enabling very useful suggestions for optimizing the

motor performance to be drawn. The results, in particular,

show the influence, upon the performance and

weigth of the motor, of the size of stator and rotor

yoke and the shape of poles.


The necessity of optimizing weigth, size and cost

of automotive electrical machines, and especially of

starters, in recent years has stimulated the research

for a better utilization of materials and, at the same

time, has underlined the need for more accurate design

calculations. General methods of magnetic field computation,

both analytical and numerical ones, have proved

to be insufficient for such machines, mainly because

of the particular problems involved by high saturation.

In such machines, in fact, flux density may reach values

in excess of 2 T, thereby causing a considerable

amount of flux to circulate in air. Thus the necessity

has been felt of developing an accurate and reliable

numerical simulation of the magnetic field as a tool to

establish criteria suitable for the design of these

particular machines.

discussed with regard to a DC motor, used as starter

on vehicles, having the following nameplate data: voltage

12 V; power 1 kW; poles 4; weigth 5.1 kg.

In the paper problems of computer-aided design are


Design procedures of electrical machines, to large

extent, still today are substantially based on analytical

calculations of the magnetic field. This necessarily

implies reducing the field to a one-dimensional

field, thereby neglecting fringing effects, influence

of slots and teeth as well as presence of saturation.

Such an approximation is rather gross and unsatisfactory

in many cases, particularly in the case of automotive

DC machines where the number of slots is small, the

size of slots is large and the effect of saturation is


Fortunately,during the last fifteen years numerical

methods have been developed 111, 121, making it

possible to perform accurate analysis of magnetic field

by taking into account most of the above mentioned effects.

A numerical solution of the actual three-dimensional

field in an electrical machine is still presently

a very cumbersome problem to tackle because of the

difficulty in defining input data and interpreting output

results, and, above all, because of the required

The authors are with Istituto di Elettrotecnica, UniversitP,

1-27100 Pavia, Italy, and F. I. Magneti Marelli,

1-20099 Sesto S. Giovanni, Italy, respectively.

computer memory and the large computing time necessary.

However, in the case that the axial length of the machine

is not small compared with the radial size, a

two-dimensional simulation in the cross-section of the

machine is sufficient to provide reasonably accurate analysis

of the magnetic field.

Due to the complexity of electrical machine structure,

the simulation is usually carried out by means of

the finite element method. Based on this method, a number

of general-purpose computer programs are now available.

Their application in computer-aided design is

not critical, except when heavy saturation is involved.

In fact, for design purposes, in the presence of highly

nonlinear problems, it is necessary to be provided with

computer programs easy to handle and able to offer reliable,

rather accurate and fairly quick solutions, at

the same time,

In the case under investigation the analysis of

the magnetic field is performed by a finite element

program especially devised for strongly nonlinear problems

/ 3 / . Main features oZ such a program are as follows.

To be able to study various design configurations

of the motor an input procedure is used which furnishes




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