The analysis and design of brushless D.C. motors.
In modem computer systems electrical drives are used to rotate hard discs. These
motors require special torque-speed characteristics. The speed of the hard disc must
be constant in order for the unit to function correctly. One type of drive, which is used
for hard discs is the brushless-DC-motor. This machine has permanent magnet
excitation and a polyphase armature winding. It therefore takes the form of a
synchronous machine, and in order to be brushless the armature winding is stationary
and the excitation rotates. To run at adjustable speed a variable frequency supply is
required. In the brushless D.C form the inverter output is phase locked to the rotor
position, it is therefore electro-magnetically similar to the D.C machine where fixed
brushes determine the frequency and phase of the current in the armature winding.
In order to perform the phase lock action signals measuring the rotor position are
needed. In one form of machine Hall effect probes detect the position of the rotor
poles, in a second given that the rotor is in motion the induced back emf is used.
Experimentally it is convenient to replace these magnet systems with a phototransistor
and rotating shutter system and this was the approach used in this thesis.
The objective of this research is to investigate different aspects of this motor. The
generation, measurement, and placement of signals is described and illustrated, and
the design and construction of an inverter supply circuit is described.
Both 2D and 3D finite element analysis is used in order to find the machine
parameters as well as cogging torque analysis, using the concept of permanent magnet
magnetisation characteristics. It is shown that the cogging can be reduced for certain
types of magnetisation. The finite element analysis is taken further by connecting the
drive circuit needed to run the machine into the finite element mesh, the machine
parameters are defined using this method. The close agreement between the
simulation and practical results indicates that the complete modelling can be achieved
using this method.The mathematical models of both Star, and Delta connected brushless DC machines
are presented, and a complete model of the machine inverter system is built for both
cases. The work is accomplished by the use of the Matlab analogue simulation
toolbox, Simulink, and the simulation program calculates the electrical performances
of both machines under steady state. The results obtained are compared to the
experimental data, and a comparison between both machines is presented.
A brushless DC motor using an insulated iron core material is presented, and analysed
using 2D finite element analysis. The newly constructed machine is also tested and
compared to the conventional version.