Fig. 70.—One of the 1,065-kilowatt Motor-generators in the Shawinigan Sub-station at Montreal.

[Larger illustration] (71 kB)

For street-car motors using direct current at about 500 volts, the sub-station equipment includes either step-down transformers and converters or motor-generators with or without transformers. It is the practice in some cases where both lighting and street-railway service are drawn from the same transmission system to keep these two kinds of service entirely separate, devoting independent generators and transmission lines, as well as independent transformers and converters or motor-generators, to the street-car work. This is done in the transmission system centring at Manchester, N. H., in which each one of the four water-power plants, as well as the sub-station, has a double set of bus-bars on the switchboard; and from each water-power plant to the sub-station there are two transmission circuits. In operation, one set of generators, bus-bars, transmission circuits, and transformers supply converters or motor-generators for the street-car motors; and another set of generators, bus-bars, transmission circuits, and transformers are devoted to lighting and stationary motors in this system. Where street-car motors draw their energy from the same generators and transmission lines that supply commercial incandescent lamps, some means must be adopted to protect the lighting circuits from the fluctuations of voltage set up by the varying street-car loads. One way to accomplish this purpose is to operate the lighting circuits with generators driven by synchronous motors in the sub-stations. These generators may, of course, be of either direct or alternating type and of any desired voltage. The synchronous motors driving these generators take their current from the transmission line either with or without the intervention of step-down transformers. By this use of synchronous motors the lighting circuits escape fluctuations of voltage corresponding to those on the transmission line, because synchronous motors maintain constant speeds independently of the voltage of the circuits to which they are connected. This plan was followed at Buffalo, where the street-car system and the lighting service are operated with energy from the Niagara Falls stations over the same transmission line. In one of the sub-stations at Buffalo, both 2,200-volt, two-phase alternators, and 150-volt continuous-current generators for lighting service, are driven by synchronous motors connected to the Niagara transmission line through transformers. At other sub-stations in Buffalo, the 500-volt continuous current for street-car motors is obtained from the same transmission system through transformers and converters. Another solution of the problem of voltage regulation where street-railway and commercial lighting service are to be drawn from the same transmission line is found in the operation of 500-volt continuous-current generators in the sub-stations by synchronous motors fed from the line either directly or through transformers. This plan has been adopted in the transmission system of the Boston Edison Company, which extends to a number of cities and towns within a radius of twenty-five miles. The sub-stations at Natick and Woburn in this system, where there are street-railway as well as lighting loads, contain 500-volt continuous-current generators driven by synchronous motors connected directly to the three-phase transmission lines. In a case like this the synchronous motors maintain their speed irrespective of the voltage on the line and thus tend to hold that voltage steady in spite of the variable losses due to fluctuating loads.

Stationary motors should not as a rule be operated from the same distribution lines that supply incandescent lamps, especially in sizes above one horse-power, and this is the better practice. Motor circuits of about 2,400 volts and two- or three-phase, alternating, or 500 volts, alternating or direct current, may be supplied at a sub-station either by transformers alone in the first case or by transformers and converters in the second. In either case no especial provision is usually necessary for the regulation of constant pressure on the motor circuits.

Fig. 71.—1,100-kilowatt, 2,300-volt, Three-phase, 30-cycle, Synchronous Motor at Sub-station of Shawinigan Line in Montreal.

In some transmission systems the distribution circuits for stationary motors are not fed by the same transmission lines that carry the lighting load, but draw their energy from lines that do no other work. This practice is certainly desirable, as it frees the lighting circuits from all fluctuations of voltage due to line losses with changing motor loads. Examples of this sort may be seen at Springfield, Mass., and Portland and Lewiston, Me., in each of which the load of stationary motors is operated over independent transmission as well as distribution lines.

In transmission systems series arc and incandescent lamps for street lighting are commonly operated either by direct-current arc dynamos or by constant-current transformers or constant-pressure transformers with automatic regulators at the sub-stations. The arc dynamos are driven by either induction or synchronous motors supplied directly from the transmission line or through transformers. As the arc dynamos regulate automatically for constant current no further regulation is required. If the series arc and incandescent lamps are to be supplied with alternating current, the constant-current transformer or the constant-current regulator come into use. This type of transformer and regulator alike depend for their regulating effect on the movement of a secondary coil on a transformer core in such a way that the current in this coil, which is in series with the lamps, is held nearly constant. Such constant-current transformers and regulators are usually supplied from the transmission line through regular constant-pressure transformers, and they hold their currents sufficiently constant for the purposes of their use.

The main problem of regulating thus comes back to the 250- or 2,200-volt, constant-pressure circuits for incandescent lighting, supplied from transmission lines through transformers or motor generators or both at the sub-station. For this regulation one of the most reliable instruments is the hand of a skilful attendant, guided by voltmeters connected with pressure wires from minor centres of distribution, and adjusting the regulating transformers above mentioned, or other regulating devices.