The most serious problem encountered. in the application of electric drive to large mine hoists is that of electrical control. It is necessary, in order to secure continued successful operation of an electric hoist, to furnish a control system which is absolutely reliable at all times. The design must be such that all parts of the apparatus will withstand the most severe, and even unreasonable, conditions of service without giving trouble and without repeated attention and repairs. It should be

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... so designed that it is impossible for the operator to damage the controller or any part of the machinery by a wrong movement of the operating handle or by misjudgment of conditions. This may be said of electrical control in general, but it applies with particular force to mine hoists, which are usually located remote from supply centers, are operated by men who arz unfamiliar, as a general rule, with electricity, and where delays in operation mean tremendous losses to the mining companies. In the electrical control of large induction motor-driven hoists, of which there are, to date, very few in this country, conditions are unusually exacting. The embodiment of all the desirable features of control for this class of service is a problem of no small moment, and is deserving of the best efforts and attention of engineering talent, for the advantages of the use of large induction motor-driven mine hoists are many and their use will become more general with the advent of suitable means of control. It is the purpose of this paper to give a description of the general layout of a mine, an electrically driven mine hoist, a more complete description of a novel control system embodying 685 686 CHENEY: MINE HOIST CONTROL [April 27 a liquid rheostat employed for the the control of the hoist motor, and to outline some of the tests made upon the entire outfit during actual operation. An electrical mine hoist (see Figs. 1 and 2) was installed at the No. 3 iron mine of the Woodward Iron Company at Woodward, near Birmingham, Ala., and was ready for operation in December, 1909. It is of theunbalanced type, and consists of a single drum 8 ft. (2.4 m.) in diameter and 40 in. (1.01 m.)long, with winding space for 2500 ft. (762 m.) of 1-4-in. (3.8 cm.) wire rope. The drum is provided with a bancd brake which is automatically applied by a weight and released by air pressure under the control of the operator. It is driven by a 500-h.p., threephase, 25-cycle, 375-rev. per min. wound-rotor induction motor through a flexible coupling, triple reduction gear and an airoperated friction clutch of the Lane type. The hoist is designed for a maximum rope speed of 750 ft. (228.6 in.) per minute, and a maximum rope pull of 25,000 lb. (11,339 kg.) Provision is also made for hand operation of the clutch and band brake from the operator's platform, to provide means for operation in case of emergency or failure of the air supply. An important feature of the hoist consists of means for automatically applying the brake in case the supply of currrent to the hoist motor fails. This consists of an alternating-current solenoid, energized from the supply circuit through a potential transformer, so arranged that when the solenoid circuit fails the core of the magnet drops and actuates an air valve on the brake cylinder, allowing the brake to be set as usual by gravity. The plan and profile of the mine where this hoist is installed, as developed at the time of installation, is illustrated in Fig. 3 . It will be noted that a uniform grade of approximately 7' deg., and 676 ft. (206 m.) in length, reaches from the surface to the knuckle, and that the slope beyond the knuckle varies, but is approximately 25 to 30 deg., although local faults may increase this grade materially at points. Branch headings are designated as " first left," " first right," " second left," " second right," etc. These headings may eventually be developed to a length of from one-quarter to one-half of a mile, (402 m. to 804 m.), or until met by similar headings from an adjacent mine. The track of the mine is 42-in. (1.07 m.) gage, and standard mine cars weighing 2500 lb. (1134 kg.), empty, are used. As many as five cars are hitched in train and each may be loaded