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December, 1941

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New developments of particular interest are incorporated an electric winder which has just been put into commission at an important colliery in the North of England. By the courtesy of Mr. J. H. B. Forster, the Managing Director of the colliery company concerned, we were permitted to inspect the new plant in operation, and are able to publish the following account. This installation, which is one of the largest of its kind in the country, was designed and erected by Metropolitan-Vickers Electrical Co. Ltd., the mechanical parts having been supplied by Markham & Co. Ltd., and a second similar installation for the other coal-drawing shaft of the same colliery is on order.

The new plant is of the Ward-Leonard type, but the usual faceplate controller has been superseded by a new form of servomotor apparatus which gives automatic control of acceleration and retardation at any point in the wind besides affording the driver the benefit of finger-tip control from a small and light hand lever. Metropolitan-Vickers' latest system of compound braking is also incorporated, and the control of the winder is centred in a neat desk which is now a standard. feature of Metrovick winders. Interest also attaches to the Ward-Leonard motor-generator set, which is driven by a type of synchronous motor the excitation of which is automatically adjusted to the load.

At the colliery in question there are three shafts, of which two are used for coal drawing and are 20 ft. in diameter. The shaft just equipped draws from the Main Coal seam at a depth of 1,158 ft., but the winder is designed so that it can draw eventually from the Hutton seam at 1,411 ft. The new plant has a duty of 400 tons per hour from the Main Coal seam, and this is achieved by a winding cycle which involves the raising of a net load of 6 tons in 438 sec., a further 10 sec. being allowed for decking. The cages have three decks with four tubs per deck, and simultaneous decking is carried out by Fletcher hydraulic decking equipment, drop cages being employed for both full and empty tubs. Locked-coil winding ropes of 7 in. circumference are employed, having a breaking load on test of 294 tons. A tail rope is used, of 8 in. circumference and of stranded construction, there being six strands arranged round a hemp core. Guide ropes are provided at each corner, and two rubbing ropes between cages. The total static load on the winding rope capel, comprising cage, tail rope and twelve full tubs, is 32 tons. An oak-lagged parallel drum is employed, of 20 ft. diameter and 11 ft. wide. It has a maximum speed of 38.8 r.p.m., the corresponding rope speed being 40.6 ft. per sec.

Motor-generator set

Power for the winder is derived, from the public mains, but as the supply system is a large one there has been no need to include a flywheel in the motor-generator set. There is, however, an agreement to limit the input to the motor-generator to 3,000 h.p., and the duty cycle reproduced in Fig. 1 shows how the initial peak on the horsepower-time curve has been shaped down to comply with this requirement. For normal working this is achieved by accurate profiling of the accelerating cams, whereas. for abnormal loads the servo-operated controller limits the acceleration of the winder so that the specified power consumption is not exceeded.

Views of the motor-generator, taken from opposite ends, are given in Figs. 2 and 3. The driving motor, which is mounted centrally, is a 1,000 r.p.m., 1,750 h.p. synchronous induction motor of the Metrovick "VAX" type in which the excitation is adjusted to the load, In a synchronous induction motor with fixed excitation, the value of the excitation must be sufficient to ensure that the motor will not pull out of step when subjected to the maximum overload it is likely to encounter. This means that at all other loads the excitation is greater than necessary, and the efficiency of the machine suffers accordingly; in fact, it is usually necessary wastefully to increase the air gap to obtain an excitation value sufficient to meet any expected overload. In the "VAX" motor the exciter has two field windings; one is the usual shunt field, but the other .is a series field which is fed through a current transformer and a full-wave metal rectifier from the three-phase supply to the motor. The diagram of connections is given in Fig. 6. The shunt field provides a certain minimum excitation at no load, but the series field provides an excitation which varies automatically with the load on the machine. This method of excitation not only enables the motor to be designed with a smaller air gap and consequently a higher efficiency, but this efficiency is much better maintained at light loads. The power factor is also maintained constant over a wide range of load. Furthermore, the overload capacity is higher, as the excitation continues to increase with the load. In Fig. 3 the exciter for the synchronous induction motor is in the foreground, and a preset diverter resistance can be seen mounted on the side of the exciter. To the left of Fig. 2 can be seen the switch cubicles controlling the synchronous motor. It is started as a plain induction motor by closing the oil switch and operating a liquid rheostat, located in the basement, the changeover to synchronous running being effected by the switch on the right-hand cubicle, which brings the exciter into circuit. The liquid rheostat can be seen in the background in Fig. 4, and its operating handwheel is to the right of the switch cubicles in Fig. 2.

The motor-generator set includes two generators which together are rated at 1,700 kW R.M.S. rating, and are capable of a peak load of 3,920 kW. Separate excitation is provided by a direct-driven exciter, and to reduce the currents handled by the Ward-Leonard controller this exciter is itself fed from an exciter-exciter, the Ward-Leonard control being applied to the latter machine. The exciter-exciter also supplies current to the winder-motor fields and to all contactor coils and d.c. operated auxiliary gear. These machines are mounted in tandem at one end of the motor-generator set as seen in Fig. 2.

The winder

As will be seen from the general view given in Fig. 5, the winder is driven by two motors coupled to pinions on opposite sides of a common gearwheel. These motors have a rating of 2,500 h.p. R.M.S. and 5,000 h.p. peak and run at a maximum speed of 300 r.p.m.

Mechanical brakes

Curved suspended post-type brakes are provided on either side of the rope drum. The drum, together with its brake gear, is enclosed in a planished steel drumshield, and steps and platforms are provided inside for easy access to the brakes for lubrication and adjustment.

The brakes are operated on the compound-brake system recently introduced by Metropolitan-Vickers Electrical Co. Ltd. and described in our May 1940 issue. Fig. 9, reproduced from that issue, shows diagrammatically the principle of operation. In this system the initial brake application is made entirely by means of a spring, the use of a deadweight being confined to the latter half of the travel of the driver's brake lever. The use of a spring overcomes the inertia difficulties associated with a deadweight, and secures immediate but smooth brake application; on the other hand the retention of a deadweight ensures a positive and full brake application even if the spring should fail.

Referring to Fig. 9, two oil brake engines, a and b, are utilised, engine a being coupled direct to the brake lever c, and engine b to a separate arm carrying a deadweight d which is coupled to the brake lever c through a spring unit e. This unit contains a nest of springs f and a set of limit rods g which enable the deadweight d to apply the brake positively in the event of failure of the springs. Referring to the small diagrams on the right of Fig. 9, in the off position both brake engines are fully raised, and the spring unit is fully extended. As the driver's brake lever is moved over, engine a descends, allowing the spring unit to contract and engage the brake to half application. During the second half of the travel of the driver's lever, engine b descends, allowing the deadweight d to fall and extend the spring unit once more, increasing the braking to full application. In normal operation the winder can be effectively controlled by spring braking, leaving the deadweight available for emergency application.

The brake engines are depicted in Fig. 7, and in Fig. 8 can be seen the spring unit and deadweight. In this installation the arrangement differs somewhat from that outlined in Fig. 9, the deadweight being suspended direct from its brake engine by two downrods which are connected solidly to the bottom plate of the spring unit; the connection from the top plate of this unit to the arm of the braking gear can be clearly seen, as can the rods leading down from No. 1 brake engine to this arm. In the foreground in Fig. 7 can be seen a safety device which stops the winder electrically if the wear on the brake linings should exceed a predetermined amount as disclosed by the travel of the braking gear.

Duplicate electrically driven oil pumps, one of which is standby, supply oil to the accumulator feeding the brake engines.

During our inspection of the winder we had ample opportunity of observing its behaviour during normal coal drawing, and we were satisfied that the claims made in regard to smoothness of brake application and precision of control were fully substantiated.

(To be continued.)

Drawings and Photographs accompanying the article