The calculation of equaliser currents in a 4.1 MW lap wound DC machine using finite element analysis

The design of equaliser windings for large lap wound DC machines is generally of an empirical nature. This paper presents an analytical technique, using finite element analysis, for the calculation of equaliser currents in lap windings. Each path in a simplex lap winding is influenced by a different pair of poles. It is practically impossible to make the fluxes of all poles identical in large DC machines. The result of uneven pole strength is that the voltages induced in the parallel paths are unequal. Since the paths are in parallel, circulating currents flow in the winding; when the machine is loaded the tendency will be to overload certain of the brushes. To mitigate this effect, permanent equaliser connections are usually made to the back of the winding to join points whose EMF should at every instant be the same. The circulating currents then superimpose a partially correcting magnetic field. Since equaliser currents cannot be easily measured during operation, this investigation was undertaken to see how finite element analysis could be used to determine the equaliser current sensitivity to changes in the structure. The analytical technique was used to determine the equaliser currents in a large lap wound DC generator. The machine modelled had been rebuilt after a major breakdown when problems occurred with equaliser windings burning out.