The temperature of all induction motors will rise above that of the atmosphere while running under load. As long as the hand can be held continuously on the machine there is no danger, but as soon as the heat can be borne but for a few seconds, and particularly if the odor of burning oil is noticeable, the danger point has been reached. It will seldom be necessary to do more than supply the bearings with an abundance of fresh clean lubricant, care being taken that the oil or grease reaches the bearing surface; sometimes it may be necessary to remove excessive belt tension. If relief is not afforded in this way, a heavy oil should be poured directly on the journal if possible. If necessary, part or all of the load should be removed but the rotor should be kept in motion enough to prevent the bearing from becoming set or frozen. When an induction motor is overloaded beyond its limit, it will stop or pull-out. Should a motor stop when it is not overloaded, and an examination of the bearings and air gap shows that the motor and stator are not rubbing, the stoppage may be due to abnormally low voltage in the supply circuit. The torque exerted by induction motors decreases as the square of the voltage; hence, a comparatively small drop in the voltage produces a large decrease in torque and the motor may come to a standstill if it happens to be carrying a heavy load at the time the voltage drops. To secure the best results from an induction motor, full voltage should be maintained and it is better to have the voltage too high than too low provided excessive heating does not result therefrom. TRANSFORMERS Transformers used for raising the voltage are known as step-up transformers; those used for lowering the pressure are known as step-down transformers. C The The transformer consists of a laminated iron core upon which two coils of wire are wound; these coils are entirely distinct, having no connection with each other. One of these coils, called the primary, is connected to the mains; the other coil, called the secondary, is connected to the circuit to which current is delivered. Fig. 1 shows the arrangement of coils and core for a common type of transformer. secondary coil is wound in two parts S and S', and the primary coil, also in two parts P and P', is placed over the secondary. C is the core, built up of thin iron plates. Fig. 2 shows a weather-proof cast-iron case for this transformer. When a current is sent through the primary it sets up a magnetism in the core which rapidly alternates with the changes in the current. This changing magnetism FIG. 1 FIG. 2 sets up in the secondary an alternating electromotive force, which depends on the number of turns in the secondary coil. If the secondary turns are greater than the primary, the secondary electromotive force will be higher than that of the primary. The relation between the primary electromotive force and secondary electromotive force is given by the following: Secondary E. M. F. = primary E. M. F.X secondary turns primary turns _primary E. M. F. primary turns secondary turns primary turns The ratio is known as the ratio of transformation of the secondary turns transformer. For example, if a transformer had 1,200 primary turns and 60 secondary turns, its ratio of transformation would be 20 to 1, and the secondary voltage would be one-twentieth that of the primary. Transformers are made for a number of different ratios of transformation, the more common ones being 10 to 1 or 20 to 1. Of course, a transformer never gives out quite as much power from the secondary as it takes in from the primary mains, because there is always some loss in the iron core and in the wire making up the coils. The efficiency of trans- all loads, when furnished with a constant primary pressure. Fig. 3 shows transformers connected on a single-phase circuit, Fig. 4 shows the connection for a two-phase circuit, and Fig. 5 shows one method of connection for a three-phase circuit. ELECTRIC SIGNALING BATTERIES Batteries are used for various purposes in connection with mining work, principally for the operation of bells and signals. The Leclanché cell is one that is widely used for bell and telephone work. It is made in two or three different forms, one of the most common being shown in (a) of the accompanying illustration. The zinc element of this battery is in the form of a rod Z, and weighs about 3 oz. The other electrode is a carbon plate placed in a porous cup and surrounded with black oxide of manganese, mixed with crushed coke or carbon. The electrolyte used in the battery is a saturated solution of sal ammoniac. The electromotive force of this cell is about 1.48 volts when the cell is in good condition. In another form of the cell, known as the Gonda type, black oxide of manganese is pressed into the form of bricks and clamped against each side of a carbon plate_by means of rubber bands. The Leclanché type of cell will do good work if it is only used intermittently in circuits where the insulation is good and where there is no leakage causing the cell to give out current continuously. If current is taken from it for any length of time, it soon runs down, but will recuperate if allowed to stand. The Dry cells are essentially the same as a Leclanché liquid cell, but the electrolyte is limited to the amount that can be retained in some absorbent material, such as paper, that is placed inside a zinc can which forms one electrode. other electrode is a carbon rod in the center. The space between is filled with crushed coke and peroxide of manganese and the whole interior is saturated with the solution and the top sealed to prevent its evaporation. Dry cells are extensively used in place of wet Leclanché cells because they are as good and so much cheaper that it is economical to throw them away when exhausted and buy new ones instead of spending the time and money required to replenish the wet Leclanché cells. The internal resistance of cells not over 1 yr. old nor entirely exhausted varies from .1 to .8 ohm and the electromotive force from 1.3 to 1.5 volts. In cases where the insulation is apt to be poor, as it often is in mines, it is best to use a battery that will stand a continuous delivery of current and that will at the same time operate all right on intermittent work or on work where the circuit is open most of the time. For work of this kind, cells of the Edison or Gordon type are excellent. View (b) shows the Edison cell. The elements consist of two zinc plates Z hung on each side of a plate of compressed cupric oxide C. The electrolyte is a saturated solution of caustic potash, which is kept covered with a layer of heavy paraffin oil, to prevent the action of the air on the solution. The voltage of the cell is only .7 volt, but its internal resistance is very low and its current capacity correspondingly large. The electrolyte used in the Gordon cell is also caustic-potash solution, and the two cells are much the same, so far as their general characteristics are concerned. The preceding table gives data relating to a number of different types of cell. BELL WIRING The simple bell circuit is shown in Fig. 1, where p is the push button, b the bell, and c the cells of the battery connected up in series. When two or more bells are to be rung from one push button, they may be connected in series, as shown in Fig. 2, or arranged in parallel across the battery wires, as at a and b, Fig. 3. The battery B is indicated in each diagram by short parallel lines, this being the conventional method. In the parallel arrangement, the bells are independent of each other, and the failure of one to ring will not affect the others; but in the series grouping, all but one bell must be changed to a singlestroke action, so that each impulse of current will produce only one movement of the hammer. The current is then interrupted by the vibrator in the remaining bell, the result being that each bell will ring with full power. The only change necessary to produce this effect is to cut out the circuit-breaker on all but one bell by connecting the ends of the magnet wires directly to the bell terminals. When it is desired to ring a bell from one of two places some distance apart, the wires may be run as shown in Fig. 4. The pushes p and p' are located at the required points, and the battery and bell are put in series with each other. A single wire may be used to ring signal bells at each end of a line, the connections being given in Fig. 5. Two batteries are required, B and B', and a key and bell at each station. FIG. 5 The keys k and k' are of the double-contact type, making connections normally between bell b or b' and line wire L When one key k is depressed, a current from one battery B flows along the wire through the upper contact of the other key k' to a bell b' and back through the ground plates G' and G. When a bell is intended for use as an alarm apparatus, a constant-ringing attachment may be introduced, which closes the bell circuit through an extra ο όνοσο This wire as soon as the trip at a door or window is disturbed. In the diagram, Fig. 6, the main circuit, when the push pis depressed, is through the automatic drop d by way of the terminals a and b to the bell and battery. current releases a pivoted arm which, on falling, completes the circuit between b and c, establishing a new path for the current by way of e, independent of the push p. The bell will then ring until the drop d is restored by some one or the battery becomes exhausted. For operating electric bells, any good type of open-circuit battery may be used; dry and Leclanché cells are largely used for this purpose. FIG. 6 Annunciator System.-The wiring diagram for a single annunciator system is shown in Fig. 7. The pushes 1, 2, 3, etc., are located in various places, one side being connected to the battery wire b, and the other to the leading wire in communication with the annunciator drop corresponding to that place. A battery B of two or three Leclanché cells is placed in any convenient location. The size of wire used throughout may be No. 18 annunciator wire. Telephones are also used for signaling and communicating purposes. The mine telephone system performs two functions: It expedites the work, thereby lowering the cost of production, and it enhances the safety of the mine workers and mine property. Its value was realized first by some of the leading mining companies and several states have enacted mine laws requiring its use. The principles involved in a mine telephone system are identical with ordinary telephone practice, with such changes in details as are necessary to meet conditions existing in mines. The telephone case in mine work must be damp-proof and dust-proof, and wiring must be of such a nature as to resist both dampness and the corroding influence of drops of acidulated mine water. It must also be suspended so as to be protected from injury due to falls of roof, cars, or the carelessness of employes. FIG. 7 It has been found that a first-class, long-distance, bridging telephone is the best type to use; bridging telephones are so called because they are bridged |