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Colliery Engineering

October, 1948

Review of developments

The most noteworthy developments during the past 25 years, set out approximately in their chronological order, were

Turbo-Electric System. — This consisted of the introduction, for Ward-Leonard equalised winders, of a high speed geared steam turbine to drive the d.c. generator(s), that is, to take the place of the a.c. motor normally employed. Thus, a colliery with its own boiler plant could combine the benefits of modem high-efficiency turbo drive with the advantages of electric winding. The turbine is connected to the generator/flywheel shaft through a reduction gearbox, and the effect of the flywheel enables the steam demand to be limited (by a fixed valve setting except when actuated by the overspeed governor) to approximately 15 per cent. above the average demand over the winding cycle, thus relieving the boiler plant of the objectionable peak demands made by reciprocating engines, and effecting great economies in steam and fuel consumption. Installations of this type are to be seen at Harworth colliery and Bickershaw colliery — see Fig. 5. The turbo-electric system also found some application on the Witwatersrand at new gold mine sinkings where the mining regulations of the South African Union required that the winding plant, if used for sinking, must be independent of outside power supplies. Incidentally, this requirement has since been rendered superfluous by the adoption of electrical detonation for the blasting operations, which permits the sinking platform to be raised to a safe distance before the charges are fired.

In all of the turbo-electric installations the ac. motor was retained on the generator set so as to provide an alternative to the turbo drive and enable the plant to operate in the orthodox electrical manner from the local or public power supply. It is also interesting to note that these turbo-electric sets were, it is believed, the only equalised plants installed on the Witwatersrand during the period under review.

Automatic Brake Governor. — This development arose primarily from the need of greater restraint of the braking power on double-drum winders as commonly employed at the metalliferous mines abroad, where the regulations governing braking power are very stringent in regard to each drum individually, so that when both brakes are in operation, i.e., for balanced winding, when the braking requirements are fairly easy, the sudden application of the brakes can have violent effects on drums, ropes and conveyances, with risks of injury to personnel. The role of the automatic brake governor was to prevent the brakes from being applied at any time with greater force than was necessary to produce, under any conditions of balance, more than a moderate safe rate of deceleration — usually fixed at 10-12 ft. per sec. per sec. Even this rate of deceleration is considerably greater than that encountered on modem single-drum colliery electric winders hence the need for brake governing did not arise in these cases and there are, therefore, no brake governors to be seen in operation in this country.

Automatic Winding Control System. — This development was fostered chiefly by the overseas metalliferous mines for the skip winding of ore. From supervisory "cam" control of Ward-Leonard winders it was a comparatively simple step to semi-automatic or fully automatic control, though the need for exact decking with varying skip or cage loads presented some tricky problems. The first installations with automatic control were at the mines of the North and South Broken Hill Companies, and the Zinc Corporation, in Australia, and many of the plants since supplied for the copper mines of Northern Rhodesia and the gold mines of the Witwatersrand are similarly equipped; so is the skip-winding plant at Bickershaw colliery, but to the best of the author's knowledge none of the plants is being operated in a fully automatic manner, pushbutton initiation of each wind usually being preferred. The changeover from manual to automatic control is an extremely simple operation. The driver has merely to throw over the change-control lever while the winder is standing, with conveyances at the landings, and he can then walk away and leave the winding operations in the hands of the operators at the underground levels where the pressing of a pushbutton is all that remains to be done in between one wind and the next.

Automatic control was developed also for a.c. winder drives, an interesting example, believed to be the only one, being at Lynemouth colliery.

The more complicated and costly nature of the automatic-control equipment for a.c. winding appears likely to leave the d.c. variable-voltage system in possession of the field, as the latter is inherently suited to the reproduction of an exact speed-time cycle by purely electrical control. Possibly, however, the developments in dynamic braking and light-current servo technique will result in the emergence of a simpler and less costly method of automatic control for a.c. winders.

Hydraulic Slip-Coupling. — This represented a successful adaptation of the Vulcan-Sinclair hydraulic coupling to flywheel-equalisation service. It enables the rheostatic (usually liquid) slip regulator in the rotor circuit of the a.c. converter set to be dispensed with, and, with the hydraulic slip coupling mounted between the driving motor and the rest of the set, provides an easily controllable means of slipping the flywheel and generator while maintaining constant speed on the a.c. motor. Thus the latter can be of the synchronous or synchronous-induction type capable of producing leading wattless current and thereby improving the system power factor. In terms of electric units consumed for the winding service the hydraulic type of slip regulator is slightly less efficient than its electrical equivalent, owing to the additional friction, windage, and oil-circulation losses incurred by the hydraulic coupling, but the more precise and consistent performance of the latter, coupled with the facilities for power-factor improvement, weighed heavily in its favour. Later improvements in the rheostatic type of slip regulator, by introducing a highly torque-sensitive servo-control mechanism to actuate the moving electrodes, have removed its disadvantages from the purely slip-regulating aspect. There are, as yet, no hydraulic-coupling-equalised winder installations of any impressive size installed in this country, but the majority of the large equalised winders installed in Northern Rhodesia and Australia in recent years are equipped with hydraulic slip couplings.

Dynamic Braking of a.c. Winders. — By making available a reliable and easily controlled method of applying electric braking on ac. winders this development constituted probably the most valuable advance of the era. Regenerative braking of a.c. motors is possible only at hyper-synchronous speeds, whereas reverse-current braking is coarse, very costly in energy consumption, and introduces serious heat-dissipation problems. Consequently, the lowering of loads into the mine was usually done under control of the mechanical brakes, and on the high-powered deep-shaft winders at the metalliferous mines abroad the problem was so acute that costly dynamometer-type hydraulic brakes were being installed to relieve the mechanical brakes of these duties — a state of affairs which, it was felt, could and must be remedied. The outcome was the Metro-Vick system of controlled dynamic braking, the first installation of which was carried out in 1937 on the service winder at the new Comrie colliery. The sequel of this development is now to be seen in the incorporation of dynamic-braking features on practically every large ac. winder manufactured in Britain, and the extension of a.c. drives to winders of much higher power — up to 3,000 h.p. or so normal rating — which would not previously have been contemplated.

With dynamic braking available, the control of ac. winders is greatly facilitated and is brought within comparable limits of the easy and precise control that was previously the prerogative of the Ward-Leonard system.

Mechanical Braking — Improved Control. — The traditional British (and American) method of actuating mechanical brakes on winding engines is by a single power engine energised by compressed air, steam or pressure oil, the braking force being provided by deadweights attached to the brake arm, and the releasing force by the brake engine, which, when energised, holds the weights in the raised position. The brake engine was invariably of the vertical type and full release of the pressure medium from the underside of the piston permitted the weights to descend rapidly with approximately gravitational acceleration. The momentum thus gathered was expended on the brake arm in the form of a more or less sudden blow which imposed a momentary braking force greatly exceeding the normal value due to the static suspension of the weights. Thus, very objectionable and sometimes dangerous shocks could be caused to the winding drum, rope, conveyances and personnel, and in order to reduce this risk many brake engines of the compressed-air or steam type are provided with a cataract cylinder or dashpot charged with soapy water or oil to retard the descent of the deadweights. Brake engines of the pressure oil type were similarly damped by controlled restriction of the oil discharge from the main cylinder, but failure to adjust or maintain these damping devices in proper operation could have serious consequences. Moreover, at the best, such devices can only be made effective by sacrificing speed in applying the brakes, which constitutes a most serious handicap when attempting to comply with the more stringent C.M.A. Overwind Prevention Regulations introduced in 1937.

With the object of removing some of the above-mentioned deficiencies, the compound brake system was developed and successfully applied. This system provides comparatively shockless and sufficiently rapid application of a moderate braking force by means of tension springs under independent control, and at the same time retains the well-known advantages of deadweights as the main source of braking power. One pressure-oil engine controls the spring braking and another the reinforcement by weight braking, both engines responding to a common control lever in such a way that "inertia-less" spring braking is always first applied (giving up to 50 per cent. of full braking power) and may then be augmented leisurely by the dead-weights, acting always, be it noted, through the springs. In the remote event of partial or complete failure of the nest of springs, the full braking power remains available by virtue of a simple mechanical by-pass arrangement.

A typical installation with compound brakes (and control desk) is shown in Fig. 6, and an impressive demonstration of the benefits of this system can be seen at Treeton colliery on the 2,650/5,300 h.p. Ward-Leonard equalised winder at the No. 1 shaft.

General Refinements. — Under this heading the following improvements are worthy of mention.

l. The cleaning-up of the mechanical and electrical interconnections between the various auxiliaries, and the simplification of interlocking arrangements, control levers, trip gear and resetting features. A comparison of the earlier prototypes with the modern installations is very striking in these respects.

2. The introduction of servo-mechanisms to provide the effort for actuating large controllers and regulators, thus paving the way for another important refinement, namely, the control desk — see Fig. 7 — with its miniature operating levers and armchair facilities for easy control of power, braking, tripping and resetting, in welcome contrast to the crude energy-wasting decentralised arrangements which characterise the older installations.

3. The adoption of precision indicating, recording and overwind-prevention instruments and devices, such as the electric tachometer and winding recorder, the Lilly electromechanical overwind and overspeed preventer, and other supervisory controls which, in combination with the improved braking technique, enable modem electric winding plants readily to conform to the high standard of safety now demanded by the C.M.A. regulations.

Future trends

The general trends, as the author sees them, are in the following directions

1. Increasing power of a.c. winder drives — up to 3,000 h.p. or more — employing twin geared motors for the larger plants. This is a natural corollary of the successful development of controlled dynamic braking.

2. Decreasing tendency to install load-equalising plant, in view of the increased power capacity of the electric supply systems and much greater diversity of load.

3. Pursuit of higher winding efficiency, with reduced power consumption and greater precision of control of driving torque, braking and winding speed — to the extent, preferably, of being semi-automatic and leaving the driver with only initiatory and supervisory functions to perform.

4. Corresponding improvements in protective devices and in measuring and recording instruments. In this connection, more sensitive and precise overwind and overspeed prevention devices are in the course of development, utilising mercury tubes with acceleration-response characteristics in place of the orthodox mechanical governors, and with all ancillary switches also of the mercury type.

5. Reduction in the number of rotating machines with their high friction and windage losses and objectionable noise.

6. Freedom of choice of speed of driving motors, even to the extent of eliminating reduction gears and direct coupling the motors to the drumshaft.

There is now little doubt that the answer to desiderata 3, 5 and 6 will be found in electronic control, utilising invertible grid-controlled variable-voltage mercury-arc rectifiers to supply the winder motors with direct current as for Ward-Leonard control. The motor will thus be the only rotating unit (as in the case of the simpler ac. system) and the promptness and accuracy of its response to the master control lever will be greater than is attainable with a rotating converter set, even when the latter is equipped with Amplidyne methods of excitation and voltage control, and other elaborations. Moreover, the power consumption per ton raised will be, in the generality of cases, at least 10 per cent. less than with the present control systems, either a.c. or d.c., so that, in the more favourable cases, winding efficiencies — net raising work/total power consumed — will be around 70 per cent. Reduced to a coal consumption basis, this means that the colliery winding of the future will be five or six times more efficiently performed than by the average steam-driven plants of today.

As regards the purely mechanical problems and trends, there is, in the author's opinion, considerable leeway to make up. The construction of winding drums can and will be improved by more scientific design and the use of modem fabrication technique to secure maximum strength and rigidity with minimum weight of metal. Further attention will be given to the design and arrangement of mechanical brakes ; in fact, for Koepe-pulley winders, brakes will have to be given very careful consideration with the object of eliminating the grabbing propensities of some of the present designs, and the compound brake system would appear to be invaluable in this connection.

Improved designs of pressure oil accumulators may be expected, the ballasted-ram type being now outmoded. A horizontally mounted cylinder with automatic compressor to provide pressure for the oil accumulator will obviate the objectionable hydraulic shocks imposed by heavy rams and weights.

It is early to say what impact the Continental Koepe winding practice will have on the future British installations, but almost certainly a big proportion of the new plants will follow the established British practice and employ rope-coiling drums, with a pronounced tendency to the double-drum arrangement, one drum having a clutch to facilitate major adjustments of the ropes for serving different levels, and minor adjustments for exact decking.

Drawings and Photographs accompanying the article