The method of regulating the Sprague electric railway motor is unique, and for the pur poses used has proven most economical. The field magnets are wound with three sets of coils of variable numbers of turns and resistances, each occupying the same space and of the same general dimensions, The coils are wound on vulcanized asbestos spools, and are practically made waterproof ; these are slipped over the cores of the field magnet, and the terminals attached to wises which go to the regulating switches on the ear. The method of winding, connecting, and the development of one-half of a controlling switch is shown in Fig. t4. When the contacts and the switch plates are in the position shown, all circuits in the machine are open: that is, although the trolley-wire or one branch of the multiple circuit is connected to one terminal of one of the field coils, tho other terminal and the terminals of the rest of the coils and the armature, as well as the con nection to the ground or other part of the circuit, are all open. As the cylinder is rotated beneath the contacts, the first movement throws all of the coils in series with each other and with the armature, and completes the circuit.
This interposes a comparatively high resistance in circuit, the machine having a very Large number of turns around the field magnet. As the switch continues to rotate, the coils are variously grouped without at any time breaking the circuit from the first position of three in series with each other to three in parallel circuit, changing the effective turns of wire in the proportion of three to one, and the resistance of the field in the proportion of nine to one. By these progressive changes the potential difference at the armature terminals is raised. the field losses with any given current are reduced, and while the speed of the machine is increased the saturation of the field is kept very high. In the last position on the switch, which is the normal one for the machine when operating under a steady and large load, the combined resistances of the field coils as arranged is practically equal to that of the armature, that is '63 ohm. The resistance of the three coils is 1.27. and '87 ohms respectively. and the resistance of the field varies in the following order: 7-4, 4.86, 3'14, 1'40, and 77 ohms; while the resistance of the field and armature together is : 8'03, 5'49, 2-03, and 1.40 ohms respectively. By this arrangement the field mag netization is the same at the first notch of the switch with 10 amperes of current as it is on the last with 30 amperes, and the torsional effort in the first position with a given current is about equal to that on the last with double amount. In later forms or machine the coils are of equal resistance and number of turns of wire. While this interferes somewhat with the perfect gradation of resistance, it is not a serious objection, and the additional sim plicity in manufacture is important.
In actual operation in street-ear work. tile plates shown in Fig. 8 are duplicated on the switch-cylinder, and the lower connections reversed, so that a single movement of the one switch, without the op eration of any other switches, a c c omplishes the full set of changes in and the re versal of the current in the armature required for going ahead or revers ing. In this way, no matter how sudden the reversal of the machine. as from full speed ahead to full :Teed back, the movement or the switch cylinder through an are of about 3tH1) effects a pro gressive change through the entire rang-colt-on) mu teflon in either direction, In equipping a car the usual practice is to con nect the terminals to all parts of the two motor circuits to a three-way, cut-out box placed in the body of the ear, to which are also brought the mill tiple-areed connections of two switches—one placed on each of the platforms. In one position of the switch the circuits of one motor are cut off from the switches ; in another those of the other motor, and in a third those of neither, The object of this arrangement is to facilitate testing of motors, and also to cut a machine out of circuit in case of accident. Either switch, therefore, has control over either or both motors, as desired.
With heavy grades an arrangement for using the machines as dynamic brakes has been adopted. This consists in connecting lever switches at each platform, so that either the conductor or driver can cut loose from the trolley connection, and close the circuit of the machines upon themselves. In descending a grade, in case the brake chains part, or the supplying system should fail, this gives instant and positive braking control of the car. This latter system is now in use in Florence, Italy.
The Thomson-Houston railway motors, Fig. 9, have the seine general appearance as the Sprague motors, but differ somewhat in construction. Ordinarily two 15-horse-power motors are supplied to each truck, and are run in parallel at a pressure of about 500 volts. While the Sprague system had depended entirely on the high resistance of the three-field windings in series to choke down the initial rush of current during the time the motor was starting from rest, the Thomson-Houston engineers preferred to employ an external rhe ostat for this purpose. The motor fields are wound with what are practically double coils, one or both being employed, as occasion demands. On starting, the rheostat, semi-circular in form, and controlled by a sprocket-wheel, generally operated by a handle on the car plat• form, offers sufficient resistance to check the initial current. Afterward, as the motor gets up speed, more or less of this rheostat is cut out, and finally the motor coils alone arc in series.