Simplified analytic study of the HBC fuse ability to withstand induction motor start cycles

One of the main causes of unexpected and incorrect operation of the high breaking capacity medium voltage fuses protecting induction motors is the aging originated in the start cycles. Up to now the main source of information to keep in mind the start cycle consists in graphic or charts given by the fuse makers for a reduced number of start typical conditions, smaller times to 6 s, 15 s and not bigger than 60 s, with start numbers from 2 to 32 per hour. If the cycle in the study differs from those mentioned, the user is forced to make extrapolations without too much knowledge on the consequences of the same. The under study phenomenon of aging takes place for the well known process of mechanical fatigue, originated by the compression and traction stresses, due to the increases and descents of temperature of the fuse elements during the start cycles. A simple analytic method was developed based on the start and off-start currents and the duration of the diverse cycles that it allows, in order to predict the maximum and minimum temperature that will be reached. Such temperatures by means of their comparison with temperature corresponding to the factor K of the Standard IEC 644 and their extension to other conditions different from the one normalized, allows the calculation in order to guarantee the fuse operation fret of aging. The method was experimentally tested on fusible of 125 A, 2.3 kV with very good results. Besides, was used to verify its application to cases mentioned in the specific references. Their practical employment is direct from the information usually given by the fuse maker, that is to say having the characteristic curve current-time and the K mentioned factor. The method can be extended to other applications of fuses with recurrent load, as it is the case of other protection of power semiconductors.