Abstract:
As a consequence of the advances in power electronics and magnetic materials, new machines with improved characteristics and sophisticated controllers have appeared in the marketplace to meet the demand for ever increasing consumer specifications. Among these new machine designs, Brushless DC (BLDC) machines are particularly attractive with superior performance in comparison to conventional machines and are, therefore, now considered to be the preferable drive for numerous high performance industrial applications. This Thesis presents a novel design for a BLDC machine. The novelty of this machine is brought about by the adoption of an 'inside-out' construction with an 'ironless' stator structure and, hence, the name Brushless Ironless DC (BLILDC) machine. The machine uses a plurality of ceramic magnets, as the field excitation, assembled in a thin cylindrical rotary drum which is external to a stator core. A three phase stator winding is skein wound and firmly glued onto the plastic stator core in a thin slotless configuration. The machine is simple to manufacture and low in cost, and with an external rotor structure it is robust and mechanically safe even at high rotor speeds as no special precautions are necessary for retaining the magnets. Two magnetic circuit models are proposed for the magnetostatic field analysis in this BLILDC machine. One model assumes ideal conditions in the machine whereas the other is applicable for a practical machine. Both models use the concept of magnetic charges to represent the permanent magnets and the two dimensional magnetic field distribution in the machine is obtained by solving Laplace's equation for the magnetic scalar potential. Whilst an analytical solution is presented for the magnetic field distribution in an idealized machine, a numerical technique is adopted to obtain that in a practical machine. In order to validate the accuracy of the proposed models, particularly the second, a theoretical magnetic field distribution is compared with that measured, directly from a flux meter as well as from flux estimates obtained with search coils, in a prototype machine. BLILDC machines are preferably operated in a single-ended switching configuration with three transistor switches. This switching topology is an incentive for the proposed sensorless driving technique for the machine as the commutating instants of the transistor switches can be derived by observing the unenergized terminal voltage waveforms. A computer model is used for the prediction Qf the drive performance, and the good agreement between simulated and measured results confirms its validity. In principle, BLILDC drives are robust and easy to manufacture, and need no extra transducers for rotor position detection in their simple three transistor switching arrangement. The prototype drive used is capable of producing a peak efficiency of 60-65% and has characteristics similar to that of a conventional DC commutator motor. Thus its speed control is relatively simple and can be achieved by varying the DC supply voltage. Consequent upon these features, the drive is preferably used for low power applications such as fans, blowers, conveyers and small pump controls where its reliability, low cost, quiet operation and maintenance free advantages are attractive.