INDUSTRIAL MOTORS AND CONTROL GEAR
Electric motors in all sizes were finding a growing market, the largest machines being for rolling mills and mine winders. Motors having peak outputs of 25,000 hp were supplied to a Moss End steelworks in 1923 (similar motors were made for the Corby plant ten years later). Many large motors were included in a complete electrification scheme carded out for a new steelworks near Scunthorpe; these were ordered in 1919, but owing to the general depression in steel and shipbuilding they were not started up until 1928, when the plant became one of the most modern in the world.

Among electric winders an equipment of 1928 at the City Deep mine in South Africa was rated at 5000/12,500 hp and raised a net load of 9| tons from a depth of nearly a mile at a speed of 40 m.p.h. This winder was the largest in the world and has remained so, though it will be outshone by a skip winding equipment now on order for Mosley Common colliery, Lancashire.

The introduction of the inverted type of synchronous induction motor in 1920 provided a design in which the active material could be used to the greatest advantage. In this machine the rotor is connected to the mains (limiting it to low voltages), and the stator carrying the secondary winding is connected to the exciter.

Motors up to 300 hp began to appear in new designs soon after the war. An early innovation was to improve the cooling of d.c. motors by fitting a large fan and sectionalizing the field coils to give more surface; the old round brackets and sleeve bearings were retained. At about the same time ball and roller bearings began to come into use, though slowly at first, and—an important event—the doublewound squirrel cage rotor was developed commercially in 1924, thus providing a type of high torque motor new to this country.

In 1926 R. Johnson formed an engineering development section of the motor department and put D. B. Hoseason in charge. This heralded major changes in design. The existing line of a.c. motors up to 150 hp was replaced by a new type having ball bearings and square endshields, which allowed other types of enclosure to be provided by just changing the endshield covers. Shortly afterwards some of the d.c. machines were similarly treated. By 1928, besides a complete range of motors for general purposes, special types were available for ship ventilation (requiring particularly quiet operation), for steelworks auxiliaries, for lifts (tandem type motors), for high starting torque drives, and for use in explosive atmospheres.