Though we have now reached roughly the half-century mark in the history and development of electrically driven mine-winding plants, it was not until about 1920 that electric winding began fully to emerge from the semi-novelty stage, and this review is confined chiefly to the developments in practice and technique which have occurred in the past 25 years. Most of the developments were fostered by the demand of the metalliferous-mining industries in the Dominions, though the influence of the more varied and exacting needs of the home coal mines was never unimportant. By about 1920, a number of sizeable electric winding plants had been installed in this country and about twice the number manufactured for installation abroad — mainly in South Africa — the numbers being, at a very rough estimate, 50 and 150 respectively. Among these were some quite impressive installations, as shown in Figs. 1 & 2.

In the early 1920's, electric winding began seriously to challenge steam winding in this country, but had to fight grimly for some years owing to the depressed state of the coal industry and the extremely low values which were placed on the inferior grades of coal that were burned under the colliery boilers. When this fuel was valued at nil to 2s. 6d. per ton, it was extremely difficult to make a case on economic grounds for electric winding at main coal-drawing shafts, though it was comparatively easy to justify it for auxiliary shafts with their own isolated boiler plants ; hence the preponderating number of electric winders to be found at auxiliary shafts, with drives of comparatively small power. By the early 1930's, with the advent of the Grid and an increase in the boiler-coal valuation to something like 7s. 6d. per ton, the electric winder began rapidly to forge ahead, until, today, only the most exceptional circumstances would bring steam winding into serious consideration. In the early 1930's, the metalliferous-mining industries in the Dominions entered into a period of great expansion and it was mainly from this "boom" that electric winding development derived its greatest impetus and commercial reward. The great preponderance of export orders — notably for the South African gold mines and the Northern Rhodesian copper mines — has continued, and, in fact, grown in the last year or two to rather embarrassing proportions, such that the requirements of the home coal mines on any large scale can be met only at the cost of some retarding of export production.

Reviewing the position to date, it is estimated that at the mines of Great Britain approximately 200 electric winding plants are now operating, the average power of the driving motors being about 450 h.p. In the Dominions, there are between five and six hundred electric winding plants of British manufacture, the average driving power of which is about 900 h.p. ; hence, the most impressive installations of British-manufactured winding plants are to be found abroad. Our home installations may give the impression that we lag behind the Germans in this, as we may do in other branches of our mining technique, but, in actual fact, though British electric winding technique has developed on somewhat different lines from the Continental, it is in no important respect inferior and is in certain respects superior to the Continental practice of which we have heard so much in recent days.

Considering, now, the prospects for electric winding at the coalmines of Great Britain, it will be clear from the foregoing rough statistics that the real expansion period is only now about to begin. The great majority of our coalmining shafts are still served by steam winding engines, and the next 25 years will doubtless see a big proportion of these replaced by electric winders. We may also predict that, almost without exception, the winding plants at the new coalmines now being planned in considerable numbers will be electrically driven.

Many of the existing main and auxiliary shafts will continue to operate much as before for an indefinite number of years and electrification of the winding plant would be warranted in most cases. A ten years' expectation of life is, the author considers, sufficient to justify the electrification of any isolated auxiliary shaft winders that are fed from their own boiler plants fifteen years' expectation would justify electrification in cases where steam is drawn from a central boiler plant but exhausted to atmosphere, and a twenty years' expectation would warrant, in most cases, the shutting down of low or mixed-pressure turbo plant (utilising exhaust steam from the winding engine) and complete electrification on public supply. Incidentally, the rather decrepit state of many of the existing winding engines and their inadaptability to modem braking and overwind-prevention standards may be even more potent factors than the expectation of working life in determining replacement by new electric winders.

It is difficult to estimate closely the contribution which the complete electrification of winding plants in Great Britain would make to economisation of fuel, but, seeing that most of the new and replanned mines will be equipped with highly efficient winders handling large payloads at moderate rope speeds, one may justifiably anticipate an annual saving of four to five million tons, much of it of marketable grades and the remainder capable of being upgraded in cleaning plants to become a marketable product. Electrification of the winding plant is a major step in the direction of complete electrification of the collieries, the consummation of which might well release eight to nine million tons of coal per annum for the market.

General technique

During the period under review, the developments in design, construction and general technique of electric winding plants were neither fundamental nor spectacular. Prior to 1920 the ac. motor drive, through reduction gears in most cases but with direct coupled motors in others, had filled the majority of requirements, though there were many examples of d.c. motor drive, some of the smaller ones with series rheostatic control and most of the larger ones with variable voltage control on the Ward-Leonard system. Some of the direct coupled a.c. motor drives were quite impressive-looking installations. A few of the a.c. installations, all of which had orthodox slipring induction motors with rheostatic control, had a somewhat elaborate system of load equalisation in the form of separate machines independently connected to the power line to act as buffers against the peak power demands of the main winder motor. The auxiliary machines consisted of a rotary converter and a separate d.c. generator with a flywheel coupled to its shaft, the a.c. side of the rotary converter being connected through a transformer to the main power line, so that when the peak loads came on the winding motor the appreciable drop in the line voltage resulted in the rotary converter returning to the line energy which had in the interim been stored in the generator flywheel. This system was quite successful in its few applications, but never became popular and was not repeated during the period under review. Neither were there any further repetitions of direct coupled a.c. drives, as the manufacture and performance of helical reducing gears had by this time reached high standards, and enabled the high cost and poor power factor of the low-speed direct-coupled a.c. motor to be avoided.

Though there were no corresponding disadvantages in efficiency or performance attached to direct coupled motors of the d.c. type, but rather the reverse, the adoption of geared drives for d.c. motors became more general on account of the lower capital cost, and the majority of the large Ward-Leonard plants have twin geared motors meshing with a common spurwheel on the drum shaft. Recently there has been a tendency to swing hack to direct coupled drives, largely as a result of occasional gear-pitting troubles, but the introduction of a flexible coupling between the gearwheel shaft and the drumshaft may be an effective remedy.

There has also been a tendency away from the practice of equalising the winder peak demands by means of flywheels on the converter sets, a tendency that will continue with the growth of the capacity of the power-generating systems and the eventual discontinuance of private generation. Incidentally, practically the whole of the large electric winding plants on the Witwatersrand are non-equalised, as the huge capacity of the Victoria Falls power grid enables the heaviest and most fluctuating winding demands to be met without objectionable effects on the distribution system.

With odd exceptions, the whole of the electric winders installed in this country and the Dominions during the period under review fall into one of the following classes :–

In class 1 the variations are very few, the motor in almost every case being of the slipring induction type arranged for rotor rheostatic control. The controllers are usually of the liquid type, but in some cases metallic-grid resistances are employed in conjunction with contactor switches for the larger powers or a tramway-type controller for the smaller.

In class 2 the converter set almost invariably had a motor of the asynchronous induction type, but in odd cases synchronous motors have been installed for power factor improvement despite the restricted peak-torque capacity of this type. The d.c. generator is in all cases direct coupled to its driving motor, but many of the larger plants have twin generators, one coupled at each end of the motor.

In class 3 there are numerous variations in the arrangements of the converter set and methods of slip regulation, e.g. :–

In some of the earlier installations the flywheel was mounted at the end of the set to permit ready disconnection of the wheel and elimination of its friction and windage losses during offshift hours. In all cases the general practice has been to split the generator into two units when the rating exceeded about 1,500 kW, the flywheel then being mounted between the two generators to afford the best distribution of torsional stresses in the shaft. The employment of twin generators is doubly advantageous for the larger plants in that :–

Though the basic technique remained virtually unchanged during the period under review, marked advances were made on the operational side in the form of improvements of electrical and mechanical controls, braking methods, protective and safety technique and constructional practice. These, in combination with sensitive servo mechanisms and their electrical equivalents — viz., magnetic field-forcing and suppressing devices, such as "Bridge," "Amplidyne" or "Metadyne" exciters — rendered it only a simple step to semi-automatic or fully automatic control as exemplified in numerous installations, chiefly overseas. The terms "semi-automatic" and "automatic" require definition. With semi-automatic control the wind is initiated by pushbutton at surface or underground levels and thereafter automatically completed. With automatic control the manual operation of the pushbutton or other master switch is replaced by mechanical actuation resulting from a particular event in the sequence of operations, such as, in a skip winding plant, the closing of the door of the loading chute at the pit bottom. Incidentally, in perfecting control systems capable of taking full charge of the complete winding operation and "landing" the conveyances at the exact discharging or decking level (even with widely varying loads) quite independently of the human element, the British manufacturers may justly claim to have gone ahead of Continental practice. This applies not only to electric winders, as a system of automatic control for steam winding engines was also developed some years ago and successfully installed at Parsonage colliery on one of the largest plants in the country.

Drawings and Photographs accompanying the article