March 23, 1917. THE COLLIERY GUARDIAN. 581 ruary 24, 1917, at mines which worked on the average 5*98 days per week, compared with 2,658 workpeople in January at mines which worked 6’02 days, and with 2,551 workpeople in February, 1916, at mines which worked 5’97 days per week. Pig Iron.—Employment continued good, and was better than a year ago. District. No. of furnaces included in the returns in blast at end of Inc. (+) or dec. (-) in Feb. 1917 on a Month Year Feb. Jan. Feb. England Wales: 1917. 1917. 1916. ago. ago. Cleveland 75 ... 76 ... 69 ... - 1... + 6 Cumberland & Lancs. 30 ... 32 ... 29 ... - 2... + 1 S. and S. W. Yorks ... 13 ... 12 ... 11 ... + 1... + 2 Derby & Nottingham... Leicester, Lincoln and 28 ... 27 ... 27 ... + 1... + 1 Northampton 28 ... 28 ... 27 — + 1 Staffs and Worcester... 30 ... 29 ... 29 +i::: + 1 S. Wales & Monmouth 10 ... 12 ... 11 ... - 2... - 1 Other districts 5 5 ... 5 ... —... — 219 ... 221 ... 208 ... - 2... + 11 Scotland 71 ... 71 ... 57 ... — ... + 14 Total 290 ... 292 ... 265 ... - 2... + 25 Iron and Steel Works.—Employment at iron and steel works continued very good. It showed a little change from the previous month, but was better than a year ago. Tinplate.—The number of mills working at the end of February showed a decrease of 15 compared with the previous month, and of 145 compared with February 1916. Complaints as to shortage of labour continue to be reported. Much short time was worked owing to the serious curtailment in the supply of steel bars. Steel and Galvanised Sheets.—The number of mills working at the end of February showed a decrease of five as compared with the previous month, and of 31 compared with February 1916. Short time was again worked at the mills on account of the shortage of steel bars. Tubes.—Employment with tube makers was good at Wednesbury and very good at Birmingham. COMPARISON OF ELECTRIC WINDING SYSTEMS.* By S. E. T. Ewing. Sufficient experience is now available' with both alter- nating-current and Ward-Leonard winding plants to make it evident that either type can economically and safely carry out all the present requirements of main winding engines, and that definite advantages appertain to each type, making each commercially and technically preferable under given circumstances. The principal operations which main winding engines are required to perform are : Shaft sinking; winding from one level; winding from several levels; raising and lowering men; lowering timber, pipes, drills, and general machinery and supplies; dead slow winding for shaft and rope exam- ination, repairs, etc. All these operations are normally carried on in balance, i.e., with both drums clutched to the driving .shaft. Operations which usually necessitate out of balance running, i.e., with one drum unclutched, are : Taking off, turning end for end, or renewing main and tail ropes; adjustments for stretch of iropes; altera- tions of rope lengths for drawing from different levels, etc. Shaft Sinking. The general practice hitherto in South Africa has been to instal temporary steam winders for this purpose, changing over later to permanent steam or electric plant. Apparently only one deep level shaft has been sunk with electric plant, viz., the Cinderella Deep South Shaft. The obvious danger inherent in electric winding in relation to shaft sinking is the possible failure of power after the lighting up signal for blasting has been given. At the Cinderella, where direct three-phase motors were used, the difficulty was got over by the use of a synchronous machine coupled to a flywheel floating on the winder busbars, which was put into commission at blasting time. Ward-Leonard plant, with the Ilgner flywheel arrangement, would, of course, equally well meet this difficulty. Further, the adoption of electric blasting would remove the difficulty entirely. Main winding, at the present purchased price of power on the Rand being admittedly far more economical than steam winding, the economic advantage of sinking with electricity is patent, provided the load factor of the plant does not fall too low to secure an advantageous price for power. The standard seven compartment shafts on the Far East Rand are generally sunk with two engines. The work goes on day and night, and in nearly every case an air compressor has to be provided for, so that a load factor quite comparable with that of any ordinary winding plant during the crushing stage may be expected. Winding from One Level. Relative Potver Consumption.—The economy of the Ward-Leonard system under this condition is greater in so far as the acceleration losses are smaller, and, further, if and when the speed-time curve for the wind involves braking in order to secure a rapid retardation near the surface, this braking is performed regeneratively. On the other hand, the economy of the alternating current plant is greater in so far as it has no motor generator losses. Hence the relative economy of an alternating current and a Ward-Leonard plant operating under the same mechanical conditions is a function of the frequency of winding. If the electrical efficiencies of the two plants are known, it is easy to determine the relative power consumption for a given frequency of winding. The important point to bear in mind is that while the maxi- * From a paper read before the South African Institution of Electrical Engineers. mum frequency of winding is limited and defined by the general conditions, the average may be much lower, and it is therefore essential, if a proper comparison is to be -arrived at, to estimate this average correctly. It is obvious that the lower the frequency of winding, the greater the gain to the alternating current, and con- versely, the higher the frequency the greater gain to the Ward-Leonard. For example, it has been found that the consumption per hour is equal for the two engines dis- cussed at a frequency of 12-5 trips per hour, the maxi- mum frequency being- 30, and the average 25. Relative Over-all Economy.—Assuming that in any given case the estimated frequency of winding is suffi- ciently high to show a clear gain for the Ward-Leonard in power consumption per ton hoisted, it then becomes necessary to consider whether the capitalised value of the power saved is sufficient to cover the additional capital cost of the Ward-Leonard equipment, which would presumably in all cases be in the neighbourhood of 50 per cent, in excess of the alternating current equip- ment. Provided that the premises with regard to the average frequency of winding are correct, and also that correct allowance is made for the possibility and con- venience, or the reverse, of shutting down the motor generator during idle spells, this investigation should present no difficulties. In this latter connection, it may be pointed out that in general a winder is operated steadily for a spell of perhaps seven or eight hours, with an idle spell of four to five hours, twice daily. Relative Maintenance Costs. — Experience has shown that the maintenance cost of both types of plant is quite small relatively to that of a steam plant. Other things being equal, the Ward-Leonard plant, with its two large sets of brush gear and commutators, entails more main- tenance than the plain induction motor of the alternating current plant. On the other hand, a certain amount of attention is required by the forward and reversing switches and liquid controller of the latter. Relative Safety and Ease of Handling. — From the point of view of safety in winding, neither type offers any marked advantage over the other. The usual over- winding, over-speed, and no-volt trips which are gener- ally arranged to open the main control switch and apply the brakes, can be applied with equal facility to either type. As regards relative ease of handling for winding, this point depends mainly on whether the conditions do or do not require a reversal of the direction of torque on the drum -shaft. The simplest condition, and the one which is most favourable to ease of handling of alter- nating current control, is the case where uniformly balanced running is secured by means of a tail rope, and at the same time the energy of the moving system is recovered by shutting off power and allowing the engine to “ run out,” timing the moment of shutting off so that the skip comes to rest in the tip with a minimum appli- cation of the brakes. Several large alternating current winders are running under these conditions, and the ease of control leaves nothing to be desired. The contrary condition is that in which no tail rope is used in a deep level shaft, and consequently the weight of down-going empty skip and rope exceeds, at and below a certain point in the shaft, the weight of the full up-coming skip and rope. ’ To tip the skip then necessitates a reversal of torque on the drum shaft, or the application of the brakes. . The brakes merely waste energy, and in the case of the alternating current plant a reversal of torque means a corresponding dissipation of energy in the con- troller. On the other hand, with a large Ward-Leonard control, the negative work is absorbed regeneratively and with perfect ease of control. Winding from Several Levels. The most important point arising from this condition of winding, apart from the consideration already given, relates to the necessity for frequent clutching and unclutching of drums. The difficulties arising there- from apply equally to alternating current and the Ward- Leonard plant, and will be separately considered later. Raising and Lowering Men. Under ordinary conditions it is not practicable to assure an up-coming load of men to balance a down- going load. Hence in cases where a large number of men have to be handled, the Ward-Leonard system, on account of its regenerative properties, shows a distinct gain in economy. In a particular case, where approxi- mately 900 men form the complement on one shift, a single Ward-Leonard winder using three-deck cages holding 60 men, and lowering the shift in one hour to 3,800 ft., the net input of energy over the hour is practi- cally nil, the regenerated energy sufficing to wipe out the motor generator and winding motor losses. Raising and lowering men present no particular difficulty to the alter- nating current plant, but where a large complement is handled the Ward-Leonard plant has a clear economic advantage. Lowering Timber, Pipes, etc. The regenerative property of the Ward-Leonard again offers an advantage under these conditions, where, as in large mines, the quantity of timber, pipes, rails, and drills lowered is very considerable, and is always in excess of the weights of drills raised. One, if not the greatest, source of trouble with the earlier alternating current plants arose out of this matter of lowering. It is, of course, perfectly feasible to lower at synchronous speed, and also to apply reverse current when braking, but designers appear to some extent to have overlooked the fact that under the latter conditions a large amount of additional energy has to be dissipated at the con- troller. Overheating and flashing over at the latter resulted. No doubt, however, this difficulty need not be anticipated in present designs, as the conditions are more thoroughly understood. In short, the alternating current plant is essentially less economical than the Ward-Leonard when lowering at less than the 'synchro- nous speed, and in practice only a small portion, if any, of a lowering trip can conveniently be done at synchronous speed. Dead Slow Winding for Shaft Examinations, etc. Again, no difficulty arises in practice in doing this class of work with either type of plant. On the score of ease of handling, the Ward-Leonard has the advan- tage, because a definite speed, however slow, corre- sponds to a definite position of the controller lever, whereas in the case of the alternating current the liquid controller is sensitive and subject to fluctuations in resistance at low speeds. On the score of economy, the Ward-Leonard is greatly superior, the input being prac- tically proportional to the speed, whereas in the case of the alternating current the input during slow winding amounts to nearly the same as would be required for full- speed winding over the same period. As ropes are examined at least monthly, and shafts at least weekly, and the operation in each case for a 4,000 ft. shaft occupies an hour or more, this latter becomes of some importance. . Working with One Drum Unclutched. Every winding engine should be capable of raising its full-rated skip or cage capacity when working single drum, i.e., unbalanced. A good many electric winders are now working with tail ropes, thereby reducing largely the maximum torque which the engine has to develop under ordinary working conditions. At the same time, the motor must be capable of exerting the full neces- sary torque under unbalanced conditions, both to be in accordance with the regulations for safety, and also in order to be able to perform various operations in connec- tion with the replacement of ropes, both main and tail. As these operations are comparatively infrequent, there is a natural tendency to cut down the size of the motor to a point where it can only perform this work under rather severe overload. conditions. When this is the case, the alternating current is apt to give trouble at its weakest point, the liquid controller, and the Ward- Leonard probably at the corresponding weak point, the commutator of the motor generator. Anything of this kind which renders probable the opening of the main control switch on overload is apt to be exceedingly dangerous, because at the moment that the power fails the engine is supporting its maximum unbalanced load; and an accident can only be averted by the holding power of the brake on one (the clutched-in) drum. In other words, when the plant is designed too fine, that is, without proper consideration of the importance of the occasional few minutes in a month1 when it is called on to exert its maximum torque, a short in the controller or a flash over on the generator commutator is likely to occur precisely at the most critical moment for the safety of the plant and the men concerned. It is obvious that this kind of under-estimate of the conditions cannot occur in the case of plants that run normally without tail ropes, as do several of the largest electric winders on the Rand. The point is one of considerable import- ance. The even turning movement of an electric winder renders the use of a tail rope (which is. apt to sway* with the uneven turning movement of a steam winder) a per- fectly simple proposition, and its use naturally reduces the peak load, with a corresponding reduction in the motor rating. On the other hand, it has to be borne in mind that the design of the plant must allow of sufficient torque being developed to raise the full unbalanced load without any signs of overstrain. In the case of the alternating current, owing to the large overload, torque possibilities of the induction motor compromise is not difficult, and resolves itself mainly into an extra large controller. In the case of the Ward-Leonard plant, it seems doubtful whether full security under maximum torque conditions is compatible with the considerable reduction in rating which1 the use of a tail rope renders possible under normal conditions. Problems in New Deep Shafts. The foregoing remarks apply to the conditions on a number of mines, where the depth is between 3,000 and 4,000 ft., and the tonnage hoisted is not greater than 160 tons per hour. At the present time, several deep level shafts are being sunk, and in the course of the next year or so no doubt several more will be begun under condi- tions which call for ire-consideration of the principles underlying present designs. The new deep level shafts will attain depths considerably over 4,000 ft. At such depths it is difficult to secure the minimum requisite factor of safety in hauling ropes, and therefore the pros and cons of two-stage winding are likely to come again under discussion. Further, the ■ additional power required tends to place the single large direct-connected induction motor out of court, owing to the excessive size and low-power factor. Mechanical reduction gear for these large motors is, as far as Rand practice is concerned, quite untried. Altogether, therefore, it may be assumed that a new and highly-interesting series of problems still lies before the designers of electric wind- ing plants, and that finality in size and design has by no means yet been rea’ched. The following summarises such of the foregoing remarks as refer to direct com- parison of the alternating current and Ward-Leonard systems under different conditions :— Conditions. 1. Shaft sinking..... 2. Rock winding from one level (a) With tail ropes (b) Without tail ropes 3. Rock winding from several levels 4. Raising & lowering men 5. Lowering supplies... 6. Dead slow winding A.C. Ward-Leonard. Equally effective on all points at approximately equal capital cost. Relative economy dependent on frequency winding. Lower capital cost Equal safety and ease of handling Lower capital") cost Lower capital Lo.XcpM cost Lower capital cost