Essay About General Methods Of Magnetic Field Computation And Design Of A Dc Motor

Computer-Aided Design of Heavily Saturated Dc Motors
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COMPUTER-AIDED DESIGN OF HEAVILY SATURATED DC MOTORS
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.
INTRODUCTION
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
FIELD ANALYSIS
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
dramatic.
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
adjustable