Disc-geometry homopolar synchronous machine

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Abstract
The paper presents the results of an experimental and theoretical investigation of a disc-geometry homopolar synchronous machine with field excitation on the primary side. The unlaminated mild-steel rotor contained no windings and was brushless. The prototype machine produced approximately 75 kW of mechanical output at 3000 rev/min, with a product of power factor and efficiency greater than 0.7. The construction of the stator core was unusual and incorporated both laminated and unlaminated portions. The magnetic circuit was also arranged to minimise the axial force between the stator and rotor. A novel rotor design which achieved a reduced quadrature-axis reactance is shown experimentally to be superior to the conventional homopolar rotor. A three-dimensional magnetostatic computer program ‘GFUN’ is shown to predict closely the flux densities prevailing in the machine. The quadrature-axis reactances calculated from these results also agreed well with measurements. An analytical method of calculating the axial force in the machine is validated by comparison with experimental data. The method uses the principle of Maxwell's stresses, and is based on the two-axis model of the machine. Prediction of the steady-state synchronous performance of the machine is also shown to be possible, using the two-axis model, provided that the assumptions implicit in that model are valid for the operating conditions of the machine. However, it was found that the output of the machine could be improved by a substantial amount by producing a degree of saturation in the magnetic circuit by means of the field winding. Under these conditions, the output of the machine exceeded predictions.

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