INDUSTRIAL MOTORS AND CONTROL GEAR
The larger motors for heavy drives, particularly in steelworks and collieries, forged rapidly ahead. In 1933 a system of variable automatic excitation was developed for synchronous induction motors; part of the excitation is controlled automatically by the load current, enabling a motor of high efficiency to deal with heavy overloads without falling out of step. The first of these' VAX' motors was applied to a rolling mill where the peaks were as high as twice full load. In the same year d.c. equipments having a maximum operating peak of 25,000 hp were installed at the Corby steelworks, and an induction motor 18^ feet in diameter and having a peak rating of 3940 hp at 70-5 r.p.m. was supplied to a rolling mill in Russia. In 1938 a 7000/19,300-hp d.c. reversing mill motor was installed in a new steel plant at Ebbw Vale; it was the largest single-unit mill motor that had been made in this country.

For the control of d.c. rolling mill motors G. A. Juhlin and E. P. Hill introduced a load-limiting excitation scheme in 1936. This simplified the control gear by reducing the number and size of the contactors required and enabled rolling mills to be rapidly reversed without causing excessive currents in the driving motors. About sixty of these equipments have been supplied for rolling mill and other applications.

In winding engines the Company had for many years secured by far the largest part of the business placed in this country and indeed in most parts of the world. One outstanding installation—at the Simmer and Jack mines on the Rand—ranked in 1935 as the largest multi-compartment single shaft winder in the world. It consisted of three 3620/6600-hp units installed in one hoist hauling from a depth of 6400 feet. Each unit had a bicylindroconical drum, 35 feet in diameter, driven by twin motors through pinions and a common gear-wheel.

With this unrivalled experience in winding equipment, generally as main contractor for both the electrical and mechanical parts, the Company was in a good position to originate improvements. From 1930 onwards a series ot innovations from Trafford Park has influenced electric winding practice everywhere.

Braking problems, particularly those encountered in metalliferous mines abroad, were attacked from several angles. On double-drum winders the regulations governing braking power on each drum are very stringent, and with balanced winding the sudden application of both brakes can have a violent effect on drums, ropes and cages. To avoid this and the consequent risk of injury to the men being wound, the Company designed an automatic brake governor, which under any conditions of balance prevents the brakes from being applied with more force than is necessary to produce a safe rate of slowing down. This governor is now used generally throughout the industry.