March 12, 1915. THE COLLIERY GUARDIAN. 543 commutating poles and a compound winding. The speed is automatically varied by controlling the rheostat ■according to the air pressure in the delivery pipe, the arm of the field rheostat being connected to the piston of -a small regulator having a 10 in. stroke and worked by compressed air. A pilot valve controls the position of the piston; this is actuated by a weighted diaphragm which is subjected to the air pressure in the delivery pipe of the compressor. For deliveries of air between half and full load output, the air pressure is maintained practically constant by varying the speed; for outputs less than half-load a mechanical unloading device is automatically brought into operation - if the air pressure exceeds 701b. per sq. in. when the compressor is running at minimum speed. The rapidity of the speed variation is controlled by throttling the exhaust of the regulator. This adjust- ment makes it impossible for the motor to draw heavy accelerating current, or for the speed to vary within wide limits for small variations in the air pressure. The motor is started through series resistance by switches of the contactor type with current limit con- i =’---------------------------------------------------------------------- Foot brakes volts. A? 8 [7,- -Al .. 2ooh.p. motor, z85 np.m. _ Fig. 26.—Single-reduction Haulage. 25oh.p. motor, 300 np.m. 27' 6' [j 3,000 volts Incoming* supply . Switch iF^ipillar Fig. 27.—Double-reduction Haulage. trol. The maximum temperature rise of the motors ■and the rotary converter is 35 degs. Cent. Rotary converter.. Compressor motor Efficiency. Full Three-quarter Half load. load. load. Per cent. Per cent. Per cent. .. 93 ... 92 ... 90 .. 90 ... 88 ... 85 The figures for the compressor are as follow :— Rating in Cubic feet cubic feet of free of free air per air per minute. minute per b.h.p. 5,000 ......................... 6’2 3,750 .......................... 6T 2,500 .......................... 5’9 At Abercwmboi the ' compressor is of the Beiliss enclosed forced lubrication type. Its capacity is 2,500 cu. ft. of free air per minute at a pressure of 701b. per sq. in., and it is driven by a three-phase induction motor. The plant runs at *a constant speed, the air pressure being governed by an unloading device. The stand-by compressor at this point is driven by a 140 b.h.p. induction motor. The larger compressor is of a similar type to the smaller compressor in use at the Britannia Pit. Motor—Rating .......... 410 b.h.p. Speed.............. 243 r.p.m. Efficiency ........Fullload90’5p c., half load 88p.c. Power factor ........ „ 78 „ „ 62 „ Compressor—Duty........ 2,500 cub. ft. of free air per min. Pressure .......... 701b. per sq. in. Cub. ft. of free air per min.perb.h p..... Full load 6’25, half load 5’2, one- tenth load 3’55 (/) Haulages.—Owing to the intermittent nature of haulage loads, the necessity for high torque at starting, and the tendency of all haulages to be used together towards the end of the shift, this load is one of the most costly to supply unless a large number of haulages are worked from the same power house. With steam or compressed air haulage the speed of the journey on gradients decreases with increasing load. With induc- tion motor drive, however, the speed is nearly constant, consequently the power demand is proportionately increased on gradients, thus decreasing the already poor ia 6 o » a 3 4 5 e 7 s 9 io Feet load factor with this class of load. Owing to the posi- tion of haulages it is undesirable to employ continuous current motors, which involve with an alternating current supply a special converting plant. The usual form of drive is single reduction gearing for the larger powers when room can be safely provided, and double reduction gear for the smaller haulages or when space is limited. The advantages of the single reduction drive are the decreased wear and tear and the more silent operation, the disadvantages being the increase in the size of the motor and in the width of the haulage room due to placing the motor on one side of the haulage bed. There is also the risk of the motor and the drum shaft getting out of alignment owing to movement in the workings. Two typical main and tail haulages as used by the Powell Duffryn Company are shown in figs. 26 and 27. Fig. 26 shows a single reduction gear, the motor giving 250 b.h.p. at 285 revolutions per minute, the speed of the haulage drum being 36 revolutions per minute, and the length of the rope, which is of -Jin. diameter, being 1,900 yds. The bed carrying the motor is rigidly fixed to the haulage frame. The motor shaft is connected to the drum shaft through a flexible coupling. Cut steel gearing is employed, the ratio being 8 to 1. The drum clutches are lever operated, and the post brakes are hand and foot actuated. The switch pillar and controller are mounted on the operating platform. The haulage driver has complete control over the engine, and a clear view of both drums, without leaving the operating point. This is essential, as the precise position of the journey on the haulage road is indicated by the amount of rope on the winding drum. Fig. 27 indicates a typical double reduction main and. tail haulage. The motor is rated at 250 b.h.p. at 290 revolutions per minute, the speed of the haulage drum being 30 revolutions per minute, and the length of the iin. rope being 1,400 yds. The motor is carried on the haulage frame, and is connected to the first motion shaft through a flexible coupling. The motor has cut steel pinion wheel and spur wheel, ratio 3 to 1, the drum shaft spur and pinion having double helical cast steel teeth. The original controllers were of the oil immersed drum type, with contactors controlling the metallic resistances. These were not only costly, but occupied considerable space. Where water is available all larger controllers are now of the liquid type. The drum type controllers were mounted either vertically or horizon- tally in a cast iron tank filled with oil, the contacts for varying the resistance in the rotor circuit being mounted on this drum. The resistances of the metallic grid type were at first -air insulated, and then oil immersed in cast iron tanks. In the earlier types of controllers the con- tact fingers gave considerable trouble through burning and breaking. This difficulty has now been surmounted and rarely appears on controllers of good design. The drum shaft is mechanically connected with the stator reversing switch. The liquid controllers consist of a tank or tanks, in the bottom of which -are mounted con- tacts connected to the rotor slip rings. Corresponding movable contacts are -carried on vertical rods actuated by a lever or by a hand wheel, the resistance in circuit being proportional to the distance apart of the fixed and moving electrodes in the electrolyte. The stator reversing switches are fixed in the framework carrying the tank, and are mechanically connected to the lever or hand wheel operating the electrodes. The liquid is cooled by pipes through which water is circulated. The chief advantages of the liquid type of controller are the finer adjustment of the speed, especially at starting when taking up slack on the rope and shackles, and its simplicity of design, resulting in fewer working parts and greater ease of inspection. The principal dis- advantage is the necessity for a supply of cooling water. The latter difficulty can be overcome where no water is available by fitting a system of radiator cooling pipes and a small motor-driven centrifugal pump. While this method works satisfactorily, it adds considerably to the first cost. Owing to the heavy duty imposed on reversing switches of both types of controllers, a special design is called for, as the frequency of operation under condi- tions of maximum load is much higher than that required from any other switch. To prevent overheat- ing and formation of carbon deposit, it has been found necessary to design these switches on generous lines with an increased area of contact and a big head of oil as compared with standard switches of the same rating. (g) Small Surface Motors.—The smaller motors up to 30 b.h.p. used for driving the various auxiliaries on the surface do not call for special notice. The motors are of the standard protected three-phase type wound for 500 volts. The majority of them have slip ring rotors with liquid resistances for starting, this type of rotor being preferable to the squirrel cage type on account of the reduced switchgear maintenance. Normally these motors are handled by unskilled labour, and in consequence the switchgear should be as simple as possible. (To be continued.) Grimsby Coal Exports. — Returns for the week ending March 5, show that the coal exported from Grimsby con- sisted of the following :—Foreign : To Dieppe, 103 tons; Esbjerg, 726; and Skein, 746. Coastwise: To London, 1,150 tons—total, 1,575 tons foreign, and 1,150 tons coast- wise; against 12,587 tons foreign during the corresponding period of last year. Coal for German Liners.—In the Commercial Court on the 4th inst., Mr. Justice Sankey gave judgment in the action brought by Messrs. G. T. Harper and Son, of Southampton, against Messrs. Keller, Bryant and Company, for the sum of £2,347 8s. 9d., the price of coal sold and delivered by plaintiffs to defendants at various dates in July 1914, for use on the steamer “ Kaiser Wilhelm II.,” one of the Norddeutscher Lloyd steamers, then lying at Southampton. Giving judgment, his lordship said there was no dispute as to whether the coal had been supplied or as to the price, but the defendants said that throughout the transaction they acted solely as agents of the Norddeutscher Lloyd Company, and were not personally liable. Plaintiffs took two points. First, that there was a direct agreement between them and the defendants; and, second, that the defendants were agents for foreign principals, and so were personally liable. Under the circumstances, he was satisfied that the contract was not made by the plaintiffs with the defendants direct, and therefore the first contention of the plaintiffs failed. It followed also, it seemed to him, that though the defendants were agents for foreign principals, so that a presumption arose that they (the defendants) were liable, there was here a contrary intention, as plainly appeared from the construction of the contract . and the surrounding circumstances. There was ample evidence, in his opinion, that .the defendants had acted throughout merely as agents for carrying out the contract between the N.D.L. Company and the plaintiffs. He was of opinion that the plaintiffs knew that the defendants were so acting, and therefore the claim of the plaintiffs failed, and there must be judgment for the defendants.