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.

The aim of this work is to investigate the methods of representation and calculation of the stability of multi-machine power-systems. Two aspects of the problem are particularly considered: the modelling of rotating machinery and the solution method for the ordinary differential equations. The results obtained from site tests are compared with results obtained using a range of machine representations. The accuracy of the models is thus judged, some refinements made and other deficiencies indicated. Methods of improving the efficiency of the trapezoidal method and Zakian's method for the solution of the system differential equations are investigated. For comparative purposes the Backward Euler and Runge-Kutta- Gill methods are also studied, and other means of obtaining a step by step solution discussed. It is concluded that at present the trapezoidal method is the most satisfactory but it has limited potential for further development.

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The thesis begins with a general introduction to switched-mode power conversion. The main application areas of the technique are outlined and a summary is presented of current research work. The following chapters then focus on one recent development in the field, the Cuk converter and integrated magnetics. First, the steady-state performance limitations of the Cuk converter are explored through a comparison with the basic switched-mode converter topologies. The circuit is seen to possess several attractive properties including input and output current waveforms which are continuous rather than pulsating. However, the switching losses are shown to be high. A regenerative snubber is presented which reduces the transistor turn-off loss whilst preserving the high-quality input and output current waveforms of the converter. The integrated magnetics technique is discussed in the following chapter. This is the technique whereby converter magnetic components are coupled to eliminate input and output ripple currents and therefore improve steady-state performance. The possible problems are highlighted of attempting to couple transformers and inductors on a single core and an alternative coupling technique is outlined which uses an auxiliary electric circuit. The introduction of magnetic coupling in the Cuk converter to eliminate input and output ripple currents and improve steady-state performance results in more complicated dynamic characteristics and a more complex control problem. The state-space averaging technique is used in chapter 4 with the aid of a control engineering design package, CLADP, to analyse the problem. The impact of component coupling on the converter dynamic characteristics is explained and possible control strategies are suggested and verified. Finally, the Cuk converter with component coupling is assessed as a large-signal waveshaper. Several inverter configurations are examined in which a switched-mode converter operating under closed-loop control is used as a waveshaper to synthesise high-quality sinusoidal output currents. The implementation of the schemes using a Cuk converter with component coupling is described.

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