Large electric hoisting plants

The development of large electrically-driven hoisting plants has gone on simultaneously with that of balancing systems designed to equalize the input, thus relieving the generating stations of excessive peak loads. It will be readily understood that success from a commercial standpoint could hardly be hoped for with large plants requiring a maximum of 2000 or 3000 horse power unless the average load had some reasonable relation to the maximum, excepting, of course, those cases where the generating stations were of such capacity that the peak loads did not interfere with the operation or regulation of the plant generally. Such cases are, however, seldom met with, a notable instance being the mines at Johannesburg, which are to be supplied with power by the Victoria Falls Co. There it is intended to use three-phase motors up to 2000 h.p. maximum for hoisting, but owing to the large amount of power required for other purposes, the peak loads due to these motors will not materially affect the generating stations, which will have a capacity of about 100,000 kw. In the majority of cases where electric hoists are used the mines have either their own generating plants or purchase power under such conditions that the cost is materially affected by the load factor, and for economical working it is essential that the average load should be as high as possible. When a mine generates its own power it will usually be found that the proportion of the hoisting load to the rest of the requirements is very large, especially with deep mines, and the peak loads necessitate special provision being made to prevent them from interfering with the operation of the other part of the plant. The success of electric hoisting plants in Europe when motors up to 3500 h.p. have been used, superseding steam hoists, has demonstrated that by carefully studying the subject there is no difficulty in securing very satisfactory results, not only from an operating, but also from a commercial standpoint. When considering large hoisting plants with the idea of driving them electrically it is essential that the load characteristics should be carefully studied, to determine not only the correct size of the machine, but also the system to be adopted. With steam-driven hoists this is not absolutely essential, for since the worst conditions are provided for, they cannot very well be overloaded; but as the output of electric motors is usually limited by the heating, rather than by the maximum safe load, the output at all points of the hoisting period must be known in order to design intelligently the electrical part of the equipment, including also any equalizing system which may be adopted. In this paper it is proposed to describe briefly some methods used by the author for the determination of load diagrams and also the characteristics of balancing systems and their economy.