Tweet

---

December, 1953

---

For some time the economic advantages likely to arise from using a.c. instead of Ward-Leonard drives for automatic winding have been recognised, and the National Coal Board is now recommending that all new winders shall be capable of being made automatic. Until recently, however, the only high speed automatic a.c. winder in operation was one designed by the Metropolitan-Vickers Electrical Co. Ltd. as early as 1934 and installed in the following year by the Ashington Coal Co. Ltd. at Lynemouth No. 1 shaft in the Newcastle-upon-Tyne district. Although this winder was later transferred to the nearby Linton colliery in the No. 3 area of the Northern (N. & C.) Division as described in COLLIERY ENGINEERING of February, 1951, the full technical potentialities of an automatic a.c. winder controlled on the closed-loop principle thus remained unknown.

Early this year Metropolitan-Vickers successfully commissioned a 1,225-h.p./6.6-kV automatic a.c. winder at Lynemouth No. 2 shaft (see Fig. 1). In addition to automatic control, the winder is equipped for manual control, either from the engine house or from bank, and it is also provided with dynamic braking on the Metropolitan-Vickers "transductor-controlled system" with d.c. supply from a grid-controlled mercury arc rectifier as indicated in Fig. 2. This is the first winder employing closed-loop speed control to be manufactured and installed in the United Kingdom since the war, and it is the first automatic winder in the world to employ dynamic braking. The following notes give a brief examination of the scheme used and the resulting performance.

Manual control

Local manual control is provided for use when carrying out special maintenance work, and not for normal winding operations. It employs an orthodox arrangement of the electrical-control lever and mechanical-brake lever, and the driver has a direct view of the drums and depth indicator — all typical features of a normal winder. The driver's electrical control lever operates directly an oil servo controlling the liquid controller, and his brake lever operates directly the brake-control valve. Single-slot lever gear is employed so that the driver is not called upon to move his electrical-control lever in any direction except forward and reverse, irrespective of whether power or braking is required.

Remote manual control is provided for normal "men" and "service" winding. An electrical control lever and a mechanical brake lever are installed at the bank position, as shown in Fig. 3, and these operate through remote-control servos to the winder house. A remote-operated miniature depth indicator, also at bank, enables decking operations to be carried out.

In this case the driver's electrical-control lever is coupled to the slider of a potentiometer, which is supplied with a constant d.c. voltage from a stabilised source. The voltage selected by the driver is fed into the moving-coil element of an electro-hydraulic servo, causing the output ram of the servo to take up a position dependent on the voltage. The output ram is coupled mechanically to the main oil servo control valve operating the liquid controller, so that every movement of the ram causes a corresponding movement of the oil servo. Hence for every position of the remote electrical-control lever there is a corresponding position of the liquid controller, and the driver at bank can thus control the torque of the winder.

For remote operation of the mechanical brakes the remote brake lever is coupled through gears to a power Selsyn, which in turn is connected electrically to a receiver Selsyn situated in the winder house. The receiver Selsyn controls the position of the control valve of the brake engine, and thus, for every movement of the remote brake lever there is a corresponding movement of the mechanical brake in the winder house.

Automatic control

The primary function of the Lynemouth winder — the raising of coal in five-ton mine cars — is done automatically in response to pushbuttons operated by the banksman, who, in addition to starting the winder, operates the car-tippling arrangement. One of the control cubicles is shown in Fig. 4.

Automatic winding is carried out on the closed-loop system, in which the required speed signal (derived from a potentiometer actuated by unidirectional cam gear) is electrically compared with the actual speed signal (derived from a tacho-generator driven by the winder) the "error signal" resulting from the difference in speed is used to operate the liquid controller. To do this the error signal is amplified and fed to the electro-hydraulic servo, which in turn actuates the controller. The torque applied to the winder depends on the magnitude of the speed error, and the selection of power or braking torque is determined by the polarity of the error. A torque-limiting circuit prevents the winder from being subjected to more than twice full load torque.

Test results

Although the winder is intended to operate automatically on raising duty only, the extensive tests conducted during commissioning were carried out with various raising and lowering loads. During the whole of these tests the winder operated automatically, including the final landing on keps. A comparison of winder speed and motor current for the extreme conditions of raising and lowering a full load of five tons is shown in Fig. 5.

In order to simplify the scheme, acceleration control is not fitted, and when on automatic operation the winder is allowed to accelerate freely, whether raising or lowering, under the influence of the available torque; thus it is governed only by the peak torque permitted by the control system. This method allows the winder motor to develop the peak torque during all accelerating periods and thus reduces the time of winding when a load less than the maximum is being raised or any load is being lowered. As the curves show the resulting accelerations are not excessive.

For raising loads the motor is run at sub-synchronous speeds as on a normal winder, but for the lowering wind it was considered that a more economical arrangement would obtain if the motor was allowed to run above synchronism, i.e., regenerating. Accordingly, the control system is arranged so that when lowering a load the motor operates regeneratively during the full-speed period. Whether raising or lowering, the winder is retarded by dynamic braking, and the lower curve in Fig. 6 shows that the rise in speed when transferring from regeneration to dynamic braking is kept to less than 3 per cent.

Because this winder is intended primarily for automatic raising duty a more detailed comparison of the effect of raising various loads on the retardation period is of interest. From Fig. 6, which shows rotor current, stator current and winder speed, it will be seen that the speed-time curve is very consistent for all loads this indicates the high accuracy obtained with the speed control system.

A series of tests was also conducted at an artificial bank (without using keps), applying the full braking effort to stop the cage once the minimum creep speed had been attained. It was found that the stopping accuracy was within +- 1½in. for all raising loads.

From the results quoted it is clear that the accuracy and precision obtainable on an a.c. winder with dynamic braking and closed-loop control make it a serious rival to the Ward-Leonard winder for automatic operation.

It may be added that the N.C.B. has on order from Metropolitan-Vickers four more automatic a.c. winders for use in this Area. These comprise a 1,225-h.p. equipment, similar to that described above, for Lynemouth No. 1 shaft (where it will replace the original M-V automatic winder, which is being transferred to Ellington colliery) and three 450-h.p. winders, two for Linton and one for Ellington colliery. These winders are all under construction or erection.

Drawings and Photographs accompanying the article