As the depth of the mine becomes greater, it is anticipated the load will be increased materially. The motor we have found to be satisfactory and amply large, but in line with the discussion of some of the papers yesterday, we prefer to be on the safe side in specifying motors of sufficient capacity to take care of the heavy loads which are obtained in pulling up around the curves when the loaded cars become derailed. In M. A. Whiting (communicated after adjournment): answering the criticism of the high slip (and consequent low efficiency) at full speed, Mr. Cheney stated that where the period of full-speed running is long it will be advantageous to short-circuit the motor secondary at the brushes by means of switches. It is extremely doubtful whether any simple and wholly reliable method can be obtained whereby contactors (or other forms of switches) can be used to short-circuit the rheostat automatically when the liquid rises to its maximum level. Provided that some means can be found for accomplishing this, there still remains another point to consider, viz: the fluctuation of load when the rheostat is short-circuited. In the installation under discussion (see Fig. 13 in the paper), the maximum hoisting speed is 335 rev. per min., representing 10 per cent slip at a load of approximately 135 h.p. (27 per cent rated load). For a motor of this size, of normal design, the full-load slip with brushes short-circuited may be assumed to be approximately 2 per cent. Now with the motor running at 10 per cent slip, to short-circuit the motor secondary at the brushes will cause an instantaneous peak of about 250 per cent of rated torque and rated current, i. e., the fluctuations of torque and current will be of a magnitude equal to approximately 200 per cent of rated load. On the other hand, if this equipment were called on to deliver rated load at full speed (i.e., with minimum resistance of the liquid rheostat), the slip would be about 35 per cent, and to short-circuit the liquid rheoscat under this condition would obviously be out of the question. It might be considered possible to short-circuit the liquid rheostat in several steps by means of an ordinary rheostat to be thrown in multiple and cut out in steps. Such an arrangement, however, necessitating several contactors, accelerating relays and a device for interlocking with the liquid rheostat, all in addition to the liquid rheostat itself, would be entirely too complicated to merit serious consideration. The only proper remedy for the high slip and consequent poor full speed efficiency revealed by Fig. 13 is, therefore, to use a liquid rheostat with a much lower minimum resistance. Liquid rheostats have been built (at lower costs than required for equivalent secondary control equipments using contactors), in which the slip introduced by the rheostat is not more than 4 per cent at full load. Louis C. Marburg (communicated after adjournment): This paper is of particular interest on account of its description of a type of control still unusual in this country. The writer wishes to join most emphatically with Mr. Cheney in his statement that liquid rheostats have not found in America the attention they deserve. It is a fact known by all those that observe developments in various countries, that invariably it takes a number of years before improvements made in one country are adopted in other countries. However, the liquid rheostat has surely had more than its due share of waiting in this country before it has found even the slightest favor. Let us remember that the locomotives of the well-known Lecco-Collico-Chiavenna threephase line in Italy, which was in operation as far back as 1901, use liquid rheostats in regular operation and that innumerable control equipments of this type have been installed in Europe during the last ten years. Among the most interesting examples were two large liquid type controllers installed at a mine near Essen, Germany, some years ago, for use in connection with two large Ilgner motorgenerator sets. When the writer visited the plant in question for the first time, large and extremely expensive control equipments of the metallic resistance type were trying to take care of the large induction motors. They were entire failures and upon his next visit the writer found them replaced by liquid rheostats and everybody was happy. In advocating these equipments in this country the writer has found rather general opposition. When the hoisting equipment described by Mr. Cheney was constructed, the writer was connected with the company that built this hoist and was in charge of electric hoisting equipments. To convince the customer regarding the merits of liquid rheostats which to the writer, in view of European experience, appeared the only feasible control, was not so difficult. With his own company, however, the writer encountered a general disbelief that the equipment would ever be a success and there were many to prophesy certain disaster. It is only this attitude, which was in line with a dislike of the liquid rheostat still general among engineers, that makes it worth while to call attention to the successful operation of the equipment. American Institute of Electrical Engineers, Copyright 1912. By A. I. E. E. NOTES ON THE USE OF ALTERNATING CURRENT IN UNLOADING COAL BY W. N. RYERSON AND J. B. CRANE The receipts of coal in Duluth-Superior Harbor increased from 2,600,000 tons* in 1900 to 8,300,000 tons in 1910. This coal comes principally from Pennsylvania and West Virginia and is brought by rail to Lake Erie ports where it is loaded into boats for transportation to Duluth and Superior. Of the receipts in 1910, two million tons were anthracite and the remainder bituminous coal. At Duluth-Superior Harbor the coal is unloaded from the boats and stored for future demand or loaded directly into cars for shipment to various points in Minnesota, North Dakota, South Dakota and Montana. The storage capacity of all the docks at this port increased from 1,000,000 tons in 1900 to over 5,000,000 tons in 1910. In 1906 there were two docks equipped for the use of electrical energy, both using direct current, one of them purchasing current from the local lighting company and the other owning and operating its own generating plant. In 1911, eleven of the twenty-one coal docks were equipped for the use of electrical energy, and nine of these are using alternating current directly on the hoisting apparatus, while another has installed a synchronous converter in order to supplement its existing directcurrent generating equipment by the use of purchased power. In 1909, twenty-six per cent of the coal received was handled by electrical energy, in 1910, forty per cent, and in 1911, it was estimated that sixty per cent of the total coal received would be handled by the use of electricity. Before the introduction of electrical energy, the largest dock *One long ton = 1.016 metric tons. had a storage capacity of 250,000 tons, whereas two of the newer docks have storage capacities of 1,000,000 tons each and another is projected of this same capacity but with provision for an ultimate storage of 2,000,000 tons. The coal handling machinery as at present installed is divided into three types: bridge tramway, cable car, and man trolley. Bridge Tramway. Figs. 1 and 2 give a general idea of the equipment on one of these docks. The installation consists of moving bridges, locomotive cranes, box car loaders, and screening towers. The boats are moored to the unloading side of the dock. In case the unscreened coal is to be shipped out immediately the bucket takes the coal from the boat and loads it into the cars at the opposite end of the dock. In case the coal is to be screened the bucket carries it to the rear end of the dock and dumps it into the screening towers. Moving buckets carry the screenings on to the screenings pile at the rear of the dock. The screened coal is loaded into cars by gravity. Coal for storage is dropped directly upon the storage pile. Twenty-five-cycle, three-phase, 13,000-volt power is delivered to the terminals of a transformer house. Three 500-kw. threephase transformers reduce the pressure to 440 volts for distribution about the dock. This distribution is accomplished in a novel manner. Posts about four ft. (1.2 m.) high, as shown in Fig. 2, are spaced at intervals along both ends of the dock. Three contacts about 15 in. (38 cm.) apart are placed vertically on these posts and the current is transferred to the moving machinery by means of shoes, which span two posts at a time. The cables for supplying current to the contacts are carried in troughs about one ft. (30 cm.) above the ground and protected by means of metal covers, which can be easily slipped off for the purpose of making repairs to the cables. The bridges have an extreme length of 506 ft. (154.2 m.). The buckets are controlled from cabs at either end of the bridge. They operate by means of cables running over sheaves from the cabin on top of the bridge. One 225-h.p., three-phase, 440-volt wound-rotor motor drives the hoist for closing and hoisting the bucket. For moving the bridge two 75-h.p. motors are used. The buckets are all of the clam-shell type and on three of the four bridges on this dock weigh seven tons and hold three tons of coal. The fourth bridge, installed in the spring of 1911, has a bucket weighing six tons and hoists four tons of coal. On this |