Foucault Currents

—Stray currents which are liable to be set up in the core of an armature, because the iron of the core cuts the flux in the same manner that the windings do. To prevent these local currents, usually known as eddy currents, the armature is built up of laminations consisting of thin stampings of steel. When the construction of the armature core and conductors does not fulfil the nece ary conditions required for the prevention of eddy currents, such as the laminations not being sufficiently insulated or numerous enough, a great heating of the whole of the armature results, which may even extend to the bearings. There is no remedy for this defect other than the purchase of a new armature, or the entire reconstruction of the old. The fault may be detected by exciting the field magnets and running the machine on open cireuit, with the brushes raised off the commutator for some time, when the armature will be found to be exce ively heated.