controlled by means of a centrifugal governor constructed to strengthen the field by reducing the opposing turns in the event of an increase of load causing a reduction in the speed, or by increasing the opposing turns in the event of the speed of rotation being increased. This method of control is somewhat complicated by the large number of windings required to obtain a steady regulation, and it is in consequence little used, except for cases where it is desired to reverse the direction of rotation. The usual method FIG. 203.-Excess potential cutout of varying the strength of the field is to shunt the field windings by means of a variable resistance. For motors of a greater output than 50 or 60 horsepower it is generally necessary to alter the lead of the brushes simultaneously with the variation of the field. This is done by means of a small strap, the ends of which are fastened to the opposite points of the brush regulator. This strap passes over a small pulley on a shaft, which also carries the arm of a rheostat shunting the field winding of the motor. This shaft is rotated in one direction or the other by a double ratchet and pawl movement, which is in turn controlled by a centrifugal governor. The type of switch used for controlling the motors is similar to that used for the generators. A modification is required, however, for starting large motors, as the self-induction of the windings of the motor is liable to cause considerable sparking at the switch when a large motor is connected in series with the main. To overcome this difficulty the switch is provided with an auxiliary device for breaking the arc. It is obvious that no fuses or excess current cutouts are required in connection with this system, as it is impossible to obtain an excessive current under any conditions. It is, however, necessary to provide against abnormal rises of pressure due to an open circuit in any part of the system, and for this purpose short-circuiting cutouts constructed on the principle illustrated in fig. 203 are used. One of these cutouts is connected in parallel with each motor, or across any loop of the system in which it is anticipated an open circuit may occur. Under normal conditions the current flowing through the solenoid will be insufficient to lift its core. If, however, an abnormal rise of pressure occurs, the core will be lifted, thereby releasing the catch, and allowing the switch to shortcircuit the defective loop. Valtellina Electric Railway.—Of the several power transmission schemes inspected by the Institution of Electrical Engineers during their trip to Northern Italy, the undertaking that excited greatest interest, probably on account of its novelty, was the electrification of that portion of the Italian State Railway between Lecco and Sondrio, and the branch line to Chiavenna, a total distance of 67 miles. This is the first practical application of the Ganz high-tension three-phase system to railway work. Power is transmitted from one generating station at a pressure of 20,000 volts, and transformed down to 3000 volts, at which pressure it is collected from the trolley wires and carried directly to the windings of the motors, without the intervention of transformers. The power is obtained from turbines, driven by water from the river Adda, brought by a canal from a point three miles above the power station, which is situated at Morbegno, 15 miles from Sondrio. This canal is 13 feet wide at the bottom, increasing in width to 14 feet 4 inches at the water level The average gradient is 1 per 1000. The water is conducted from the canal to the turbines through steel flumes 8 feet in diameter. These run down the hillside at an angle of 45 degrees inclination to the horizontal. Packed gland expansion joints are provided near the upper end of the flumes, where the water pressure is light. The minimum water flow is stated to be 880 cubic feet per second, and as the available fall from the top level in the flume to the tail-race is 88 feet, the theoretical output of the fall is 9100 horse-power. It is estimated that over 7000 horse-power will be developed by the turbines from this water consumption. During the summer months, owing to the melting of the snow on the mountains, the water flow is considerably greater, while the summer railway traffic is at least twice as heavy as that to be served during the winter months. Each flume serves two turbines, arranged with right and left-handed intakes. The speed of each turbine is controlled by a centrifugal governor which varies, by means of a relay, the angle of the guide blades, which are placed round the rotating blades carried by the main shaft. The relay consists of a cylinder provided with a piston which is thrust backwards and forwards by oil, at a pressure of about 140 lbs. per square inch, admitted to one or the other end of the cylinder by means of a slide valve controlled by the centrifugal governor referred to above. The pressure of the oil is maintained by a pump on the end of the turbine shaft filling an ordinary hydraulic accumulator. The three-phase alternators are separately excited by small generators on the end of each turbine shaft, the fields of these exciters being connected to a small auxiliary turbine-driven dynamo. A rheostat is inserted in series with each field, by which the E.M.F. of each generator may be regulated from the switchboard. As an additional safeguard, an emergency overnor inserts a resistance in the field of the exciter if the velocity of the turbine exceeds 170 revolutions per minute, the normal speed being 150 revolutions per minute. The switchboard consists of marble panels upon which are mounted the low-tension measuring instruments and the operating handles of the hightension switches. All high-tension apparatus and connections are carried on light iron frameworks in a spacious room at the back of the switchboard. A separate panel is provided for each generator, and there are two feeder panels. All high-tension switches are of the Schuckert horn break type; these switches are placed 20 feet above the floor level, and motion is communicated to them from the operating handles through an endless rope. There is only one three-phase feeder leaving the generating station, but this may be switched on to either set of 'bus bars through either of the two feeder panels referred to. No attempt has been made to duplicate the feeders. Arrangements are, however, made for dividing the high-tension line into sections, thus permitting any portion to be cut out for repairs and the supply maintained through the 3000-volt lines. The insulators used for supporting the high-tension feeders were all tested to withstand a pressure of 60,000 volts before erection. They are fixed on 8-inch iron brackets carried on the poles supporting the trolley wires. A distance of 2 feet is allowed between the three wires. The diameter of the 20,000-volt wires is 8 mm. in the sections adjoining the generating station, and 7 mm. towards the outer ends. These primary feeders are not run through the tunnels, as it was feared that the damp atmosphere would be liable to affect the insulation. Sub-stations are placed along the line at intervals of from five to seven miles. These sub-stations are divided into two parts. The incoming hightension wires, and all high-tension connections, are located in the back room, whereas the single stationary three-phase transformer is placed in the front room. The horn break switches used for both the high-tension and low-tension sides are fixed in the upper portion of the building, and are operated by rope gear from below, as in the main generating station. Overhead collection of the 3000 volt supply is made from two wires only, the third conductor of the three-phase system being the rails. The collectors consist of two rollers mounted upon one axle pole 65 inches in length. This pole is constructed of hard wood saturated with oil under pressure. Each contact roller consists of a copper cylinder, about 3 inches in diameter by 2 feet long, mounted to revolve upon ball bearings. The two rollers are separated by a 9-inch length of insulating material. The trolley arms are raised and lowered by compressed air. The valve controlling the air supply is so interlocked with the case containing the main switch that it is impossible for the drivers to open the latter without first lowering the trolley arm, and thus cutting off all supply. Air for operating the Westinghouse air brake blocks, the rheostats, trollies, etc., is compressed by an electrically driven two-stage air compressor. For this purpose an 8 K.W. three-phase motor is supplied with current through a small static transformer carried on the car. The pressure of the air supply is automatically maintained at 100 lbs. per square inch. One interesting feature in connection with the equipment of this installation is the coupling up of the motors in cascade. It is well known that, if a standing three-phase motor having a short. circuited rotor is connected across the full line pressure, the motor acts more or less as a transformer having a short-circuited secondary winding, and in consequence it takes a very heavy current. This heavy current does not produce a correspondingly powerful torque, because the induced current in the rotor is practically in quadrature with the primary current in the stator. The torque may, however, be greatly increased, and the starting current reduced, by inserting a non-inductive resistance in series with the windings on the rotor. In the Ganz cascade system, to obtain a big starting torque, the rotor windings of the primary motor are connected across the stator of a second motor, and a non-inductive variable resistance is connected in series with the rotor of this second motor. The connections between the starting switch motors and controller are shown diagrammatically in fig. 204. The high-tension three-phase currents collected from the rails and from. the trolley lines are conducted respectively through the wheels of the car A1, and through the trolley collectors A2 A3, the choking coils of the lightning arrestors B1 B2 and the fuses C1 C2, to the main high-tension switch D, and from this directly to the stator of the first motor E. The secondary currents induced in the rotor of this first motor are led to the stator of the second motor F through the lower connections of the controlling switch G. The rotor of this second motor is in turn connected through the upper section of the controller G to the variable non-inductive resistance H. This resistance consists of bundles of iron plates suspended in a vessel containing a solution of sodium carbonate; the height of this solution is controlled by the admission of air under pressure into the vessel through the valve I. |