Some recent developments in electric motors, with special reference to the use of new insulation materials

General trends in the design of a.c. motors are discussed, and attention is drawn to the increasing popularity of the t.e.f.c. motor. Particular mention is made of the problems of internal cooling, and a successful design is described. The method of eliminating condensation inside the machine is outlined. The problem of low-current direct-start motors is touched upon and the alternative designs of rotors discussed. General trends in d.c. motor design indicates a situation similar to the above, and a typical t.e.c.a.c. machine is mentioned. The present position on motor standardization in England is recorded. Improvements in machine insulation are dealt with for both Class A and Class B machines. Varnish treatments are discussed and attention drawn to the rarely mentioned feature of moisture absorption by the windings due to the permeability of varnish films. The improvements in magnetic sheet steel are noted, and reference is made to the possible use of the new cold-rolled directional steels. Ball and roller bearings are discussed with particular reference to oil or grease lubrication. Pump drives raise special problems in alignment and end location of rotors. Some a.c. motor troubles are mentioned.