758 THE COLLIERY GUARDIAN. October 11, 1918. Notes on General Operation. The systematic overhauling of induction motors will not alone ensure freedom from breakdowns. Many troubles can be detected in their early stages by sound as well as by sight. If the bearings of a motor are wearing, or have possibly moved, thus bringing the rotor from the centre of the stator, it is unnecessary for those who take an interest in the healthy sound of a normal motor to have to resort to' fine measurements of the air gap to pronounce the rotor to be out of centre. Once a machine has been properly set, any move- ment from that position, however slight, can be detected by the difference in sound as the stator Switch is closed. When a rotor is badly out of centre—and this does not mean anywhere near touching the stator—there is an unpleasant jar when the machine is switched on. Careful attention to these sounds will indicate when it is necessary to adjust the air gap, without waiting for the rotor to begin to rub the stator. An intelligent driver will tell in a moment if a machine is normal, and co-operation between the driver and those responsible for the upkeep of a machine certainly tends toward the aim in view. Then close attention to the readings of the. ammeter is also a guide to developing trouble. Fig. 6. Pi Fig. 5. and bring splash out Few colliery electricians have escaped a loose rotor connection but unfortunately in many cases its con- ' nection with the ammeter readings has not been noticed, and except for the sparks which eventually develop, the trouble might go bn indefinitely without being discovered. A loose rotor connection always shows itself on the machine ammeter by the regular oscillations of the pointer. In the case of slow-speed motors, with a large number of poles, these oscillations may be very rapid, and only a close examination can detect them, whilst a high-speed machine might show about one oscillation per second. These oscillations appear to depend on the slip and the number of poles possessed by the stator. For instance, a 4-pole 1,500 R.P.M. machine, running on a 50-cycle circuit with a slip of 1 per cent., would show oscillations e'qual to 4 x 15 = 60 per minute. Attention to these oscillations will enable a loose rotor connection to be re-soldered before it has had time to burn itself out and char the surrounding insulation. It should be noted that, on machines fitted with a short-circuiting for the slip-rings, the bad connection might be in the short-circuiting connection itself, and this should be always checked before spending much time in looking for a loose connection in the rotor windings. Further, the same oscillations may some- times be traced to a bad connection in a controller which obviously is a part of the rotor circuit and has the same effect as though it were in the rotor itself. Protection of Motors. It is not the object of these remarks to deal with protective switchgear for motors, but in so far as switchgear may be considered directly concerned with the maintenance of motors, this must be just referred to. If a steam engine is overloaded, usually nothing happens beyond its being brought to a standstill. This, however, is not the case with a motor, and the efficient protection of such apparatus is as essential to prolonging its life as is the general overhauling and inspections. If automatic switchgear is installed, the trips should be set low enough to afford ample protection to the motor, and the setting should be checked periodically in order to make certain that the locking screws have not come free and allowed the setting to alter. To protect motors with fuses cannot be recom- mended, although the method is largely used. This, however, is no proof of its being good practice, be- cause many equipments are bought without any regard to details so long as the price is reasonable. The danger of a fuse blowing and leaving a motor running on two phases is sufficient to condemn fuses. Many motors have either been burnt out at the time, or have been so badly charred that it has eventually contributed to their premature breakdown. It is obvious that the automatic circuit breaker does away with the possibility of the motor being left on two phases. Bearings. The possible useful life of a motor is greatly affected by its bearings. When bearings are continually throwing oil in large quantities, the inevitable result may be postponed by continual cleaning, but it cannot be averted, owing to the windings becoming so satu- rated that any external cleaning or application of varnish is of little avail. There are six things which cause oil throwing: (1) No grooves on the shaft at the ends of the bear- ings ; (2) excessive end play in bearings; (3) oil drains blocked in bearings; (4) quality of oil too thin; (5) excessive draught and no provision to prevent its passing through the bearings; (6) bearings too slack. Oil Grooves.—Oil throwing grooves are turned in the shaft at both ends of the coupling bearing and one end of the slip-ring bearing. They are similar to those shown in fig. 6. When the oil creeps along the shaft, it succeeds in getting into the first groove, but when it ascends the other side and gets on to the sharp corner A, fig. 6, it is thrown off and falls inside the carcass of the bearing. Two or more grooves are usually provided, so that there are two chances of the oil being thrown off before getting to the outer end of the shaft or towards the windings. End Play.—Some motors are provided with far too much end play, with the result that when the rotor floats from one end to the other, the shaft collar strikes the end of the bearing and causes the oil which is entrapped to splash outwards. When a motor is under load there is usually no tendency for the rotor to float, as the magnetic field keeps it in position. However, when motors are direct-coupled to an ordinary rough-cast helical gear, the uneven teeth will often cause side movements in spite of the magnetic pull. Where oil splashing is traced to this cause, it should be dealt with at once. There is such a variety of ways in which this problem is likely to present itself that it is difficult to offer any suggestions without taking up a great deal of space. Blocked Oil Drains.—These are a fruit- ful cause of oil leakage. After the oil has passed along the grooves surrounding the shaft, it enters a groove which runs around the end of the bearing. At the bottom part of this groove a hole is provided to enable the oil to drain into the oil well. It frequently happens that a piece of waste or other matter finds its way into the groove and stops the drain hole, with the result that the groove gradually fills up. When bearings are in two halves, the oil fills up the groove to the joint between the two halves and leaks out from under the cap of the bearing. Thin Oil.—If the oil used in a high- speed motor is very thin, the oil rings on the bearings run at an abnormal speed up large quantities of oil, causing it to of the top of oil wells. This can usually be remedied by using a class of oil a shade heavier. Care, however, is necessary in making a change as suggested, and the bearings should be closely watched until they have quite settled down to the new oil. Excessive Draught.—This trouble is often very diffi- cult to deal with, but some success has been obtained by fixing a leather washer around the inside end of the bearing so that it forms a close fit to the shaft. This prevents the draught circulating right through the bearing. ‘ Slack Bearings.—When a bearing has become worn, it is liable to get an abnormal supply of oil, the oil brought up by the ring having then two paths open, viz., through the grooves of the bearing and also by running back off the.shaft to the oil well. In a normal bearing some of the oil doubtless follows each of these paths, but when there is excessive slack, the whole or a greater part of the oil is trapped and carried through the bearing and delivered in such quantities at each end that it is difficult to drain it away quickly enough, especially if the drain holes are rather small. Suitable Oil for Bearings. It is often found that plants are supplied with a certain quality of dynamo oil, which is used for all purposes indiscriminately. It is hardly necessary to point out that such a practice is entirely wrong. The class of oil which would be suitable for slow-speed, heavy motor would be altogether unsuitable for a high-speed machine. In the case of heavy, slow-speed motors a fairly heavy oil is best, as it provides the required film be- tween the shaft and the bearing, which is so essential. The weight of a heavy rotor shaft on a film of light oil is sufficient to break down the film and capse the bearings to wear rapidly and become hot. On the other hand, a high-speed machine requires a liberal supply of fairly light oil to afford rapid circulation through the bearings and so carry off the heat. A heavy oil used on such a machine will generally result in hot bearings and rapid wear. Again, in the case of a high-speed motor, even of considerable size, the whirling motion at speed re- lieves the bearings of much of the weight; but it must be borne in mind that at starting and stopping there is a danger of considerable friction in the bearings if the oil is too thin. Testing. One of the best methods for giving a continuous check on motors and all other apparatus is to take weekly tests with a megger. Such determinations enable one to look back over the record of any piece of apparatus and detect any falling off in the insula- tion tests as recorded from week to week. A. lowering of the insulation tests should be closely watched over a period of weeks, provided that the fall does not occur too rapidly, and meanwhile steps should be taken to ascertain the cause. In conclusion, it may be added that systematic examination of all apparatus should be made in each shift, and air gaps checked by means of suitable gauges provided for the purpose. The Late Mr. Alfred Hewlett.—At the last monthly meet- ing of the board of management of Lancashire and Chesire Miners’ Permanent Relief Society, held at Wigan, reference was made by members of the board to the loss sustained by the death of Mr. Alfred Hewlett. A reso- lution eulogised his services as founder and first presi- dent of the society, and concluded by expressing sympathy with the relatives. EXPLOSIBILITY OF COAL DUST AND AIR MIXTURES.* By J. K. Clement and J. N. Lawrence. During the course of the investigations at the ex- perimental mine of the Bureau, of Mines it was found that dusts of too low inflammability to be ignited in the laboratory apparatus previously were nevertheless capable of propagating violent explosions in the mine. The laboratory method now described is capable of measuring the inflammability of the least inflam- mable dusts that will propagate an explosion in the mine. A relation has been established between the results of tests on a large scale at the experimental mine and the results of laboratory tests, so that it is now possible to determine from laboratory tests alone whether or not a given dust may give rise to or propa- gate explosions. Normal air has been used in previous laboratory experiments with dusts. Recent work at the experi- mental mine has shown that dusts which in normal mine air are not capable of propagating an explosion may, in the presence of a small percentage of natural gas, readily propagate an explosion. The laboratory method has therefore been so modified that dusts may be tested in the presence of various percentages of combustible gas, and the effect of small amounts of combustible gas on the inflammability of dust has been investigated. The igniting agent may be the flame from a blown-out shot, the flame of an oil lamp, or the surface of an incandescent solid body. In the apparatus used to measure the inflammability in the experiments herein described, a heated platinum tube serves as the igniter. The igniter is placed at the centre of a spherical vessel, and a cloud of dust is blown past it through an opening in the bottom of the vessel. The pressure produced by the combustion of the dust is taken as a measure of its inflammability. Apparatus. The apparatus is shown in the drawing. The essen- tial features are explosion globe a, the igniting tube 5, the pressure-recording device c, the lifting device d, and auxiliary devices for measuring the tempera- ture of the igniting tube and for throwing the dust into suspension. device To oxygen tank; To millivolt- meter Inflammability Apparatus. The explosion globe, capacity 1,400 c.c., is fitted with tubulures at its top and bottom. The lower tubulure is provided with a rubber gasket, and the upper tubulure presses against a rubber gasket cemented to the head of the apparatus. The joints at the top and bottom of the globe are made gas-tight by screwing down the thumb nuts e. The glass dust injector f is the receptacle for the coal dust, and is provided with & cap of 14-mesh copper gauze, not shown in the figure. The dust is raised in a cloud by a puff of oxygen. The oxygen is drawn from a steel cylinder, and the puff is controlled by means of the manometer g, the reservoir h, and the releasing device i. The rapidity with which the oxygen forces the dust out of the in- jection funnel is regulated by a capillary tube 2 mm. in diameter and 6 cm. long, inserted between the releasing device and the injection funnel. The igiting tube consists of a hardened “ lavite ” tube on which is wound a heating coil of 0-23 mm. platinum wire 40 turns to the inch, enclosed in a platinum tube made of 0T mm: sheet platinum. On the surface of the “ lavite ” tube is cut a suitable spiral groove in which the platinum wire is wound. The wire is insulated from the platinum tube by a layer of alundum cement which is baked on. The igniting tube is held in place by platinum leads of 0-4 mm. diameter, which are fused to the ends of the heating coil and secured in the binding posts at the* lower extremities of the steel leads j. The temperature of the platinum tube is measured by a platinum and platinum-rhodium thermocouple and a Siemens and Halske millivoltmeter. The ends of the platinum and platinum-rhodinum wires, instead of being fused together in the usual manner to form the hot junction, are attached separately to the platinum tube; the platinum-rhodium wire at the middle of the tube and on the front side; the platinum wire about 3mm. to one side of the platinum-rhodium wire. The point of contact of the platinum-rhodium wire with the platinum tube is therefore the hot junction of the couple. To reduce to a minimum the loss of heat from the tube by conduction along the wires, the ends are drawn down to a diameter of * From United States Bureau of Mines Technical Paper 141.