The use of a larger number of units than would otherwise be necessary in order to avoid a very low speed is further illustrated by the 7,500-kilowatt plant of the Missouri River Power Company, at Cañon Ferry, Mont. This capacity is made up of ten generators, each rated at 750 kilowatts and direct-connected to a pair of pressure turbine wheels operating at 157 revolutions per minute, under a head of about 32 feet.
Under comparatively high heads of water pressure turbines operate at speeds that are ample for direct-connection to even the largest dynamos.
[Illustration: FIG. 41.--Interior of Power-house No. 2, Niagara Falls.]
Thus in the Niagara Falls plant, where the head of water is 136 feet, each pair of turbines drives a direct-connected dynamo of 3,750 kilowatts at 250 revolutions per minute. In the rare case where the power to be developed is so great that the number of generators necessary to give security and reliability to the service leaves each generator with a capacity larger than is desirable for structural reasons, the number must be increased simply to reduce the size of each generator. Such a state of facts existed at Niagara Falls, where the first station contains ten dynamos of 3,750 kilowatts each, and the second station contains eleven units of like capacity.
In the greater number of transmission systems the generators are direct-connected to either steam-engines or water-wheels, and their speeds of rotation are largely determined by the requirements of these prime movers. Steam-engines can be designed with some regard to the desirable speeds for direct-connection to dynamos, but water-wheels are less flexible in this particular. Each type of wheel has its peripheral speed mainly determined by the head of water under which it may be required to operate, and variation from this speed means serious loss of efficiency.
[Illustration: FIG. 42.--10,000 H. P. 12,000 Volt Generator in Canadian Power-house at Niagara Falls.]
Under heads of much more than 100 feet pressure turbines operate at rather high speeds in all except very large sizes. It is much the more common to see water-wheels at a lower speed belted to dynamos at a higher speed; but in some instances, as at the lighting plant of Spokane, Wash., wheels of a higher speed are belted to dynamos of a lower speed. Another plan by which moderate dynamo speeds are obtained with water-wheels under rather high heads mounts a dynamo at each end of the shaft of a large turbine or pair of turbines. This plan is followed at the plant of the Royal Aluminum Company, Shawinigan Falls, Quebec, where there are two pairs of horizontal turbine wheels, each pair developing 3,200 horse-power under a head of 125 feet, and driving a dynamo direct-coupled on each end of its shaft. Where vertical wheels are employed it is sometimes more desirable to drive some standard type of dynamo with horizontal shaft by means of bevel gears than to design a special dynamo to mount directly on the vertical shaft. This latter plan is warranted in very large work like that at two of the Niagara Falls generating stations, where the twenty-one 3,750-kilowatt dynamos are direct-connected, each on the vertical shaft of a turbine. This type of connection is not one that will be frequently followed, but at one other point--Portland, Ore.--each dynamo is mounted directly on the shaft of its vertical turbine wheel.