Apparatus for protecting the armature of an electric motor against excessive temperatures comprises a resiliently compressible and expansible pad in the armature circuit and composed of thermally sensitive, non-conductive material containing a plurality of conductive particles engageable with one another when the pad is in a compressed condition to establish a conductive path through the pad. The pad normally is maintained in a compressed, conductive condition by resilient force applying means which is of such resilience as to permit expansion of the pad in response to an increase in its temperature and render the pad non-conductive.