Fig. 50.—1065-Kilowatt, 2300 volt Generator Connected to Motor in Shawinigan Sub-station at Montreal.
On the other hand, the cost of transformers is greater with current at 25 cycles per second than with a higher frequency, and this current is only just bearable for incandescent lighting and quite unsuited for arc lamps, because of the fluctuating character of the light produced. At 15 cycles per second a current can be employed for incandescent lighting with satisfactory results only by means of some special devices, as lamps with very thick filaments, to avoid the flicker. Very low fluctuations cut down undesirable effects in the way of inductance and resonance, but these effects can be avoided to a large degree in other ways.
Where power is the most important element in the service of an electric water-power and transmission system there is a decided tendency to adopt a rather small number of periods for the system, even at some disadvantage as to lighting facilities. This is illustrated by the transmission from St. Anthony’s Falls, Minn., at 35 cycles, from Cañon City to Cripple Creek, Col., at 30 cycles, by the Sault Ste. Marie plant of 32,000 kilowatts at 30 cycles, as well as by the two Niagara Falls plants of 78,750 kilowatts at 25 cycles.
Where the main purpose of a transmission system is the supply of light and power for general distribution, sixty periods per second are adopted as the standard in many cases. This number of periods in comparison with a smaller one tends to increase the cost of rotary converters but decreases the cost of transformers, and is suitable for both incandescent and arc lighting.
Fig. 51.—Efficiency Curves for Motor Generators at Montreal Sub-station of the Shawinigan Transmission Line.
Few, if any, transmission systems have recently been installed for frequencies above sixty cycles, and the older plants that worked at higher figures have in most cases been remodelled.
During the past decade the voltages of alternators have been greatly increased, but have not caught up with the demand for high pressures on long-transmission lines. Ten years ago when the first long transmissions were going into operation 2,000 volts was considered high for an alternator. As this voltage is too low for economy of conductors longer than three or four miles, the important early transmissions were all carried out with the aid of step-up transformers at generating stations. The practice then was, and to a large extent still is, to design the alternators for a transmission with a voltage well suited to their economical construction, and then give the step-up transformers any ratio necessary to attain the required line voltage.
Alternators in Transmission Systems.
| Location of System. | Num- ber at Plant. | Kilo- watts Each. | Alter- nator Volt- age. | Phase. | Cycles. | R. P. M. | Type of Magnet. | Method of Connec- tions. | ||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Niagara Falls[[A]] | 16 | 3,750 | 2,300 | 2 | 25 | 250 | External re- volving | Direct | ||||
| Niagara Falls[[A]] | 5 | 3,750 | 2,300 | 2 | 25 | 250 | Internal | „ | ||||
| Colgate to Oakland | 3 | 2,250 | 2,400 | 3 | 60 | 285 | Inductor | „ | ||||
| Colgate to Oakland | 4 | 1,125 | 2,400 | 3 | 60 | 400 | „ | „ | ||||
| Electra to S.Francisco | 5 | 2,000 | .... | 3 | 60 | 240 | „ | „ | ||||
| Portsmouth to Pelh’m | 1 | 2,000 | 13,200 | 3 | 25 | 83 | .3 | Internal | „ | |||
| Portsmouth to Pelh’m | 2 | 1,000 | 13,200 | 3 | 25 | 94 | „ | „ | ||||
| Virginia City | 2 | 750 | 500 | 3 | 60 | 400 | External | „ | ||||
| Ogden & Salt Lake | 5 | 750 | 2,300 | 3 | 60 | 300 | Internal | „ | ||||
| Chaudière Falls | 2 | 750 | 10,500 | 3 | 66 | .6 | 400 | „ | „ | |||
| Yadkin River Falls | 2 | 750 | 12,000 | 3 | 66 | 166 | „ | „ | ||||
| Lewiston, Me. | 3 | 750 | 10,000 | 3 | 60 | 180 | „ | „ | ||||
| Farmington River | - | 2 | 750 | 500 | 3 | 60 | ... | „ | „ | |||
| to | ||||||||||||
| Hartford, Conn. | 2 | 600 | 500 | 2 | 60 | ... | „ | „ | ||||
| Cañon Ferry to Butte | 10 | 750 | 500 | 3 | 60 | 157 | „ | „ | ||||
| Apple Riv. to St. Paul | 4 | 750 | 800 | 3 | 60 | 300 | „ | „ | ||||
| Edison Co., L. Angeles | 4 | 700 | 750 | 3 | 50 | ... | „ | „ | ||||
| Madrid to Bland | 2 | 600 | 605 | 3 | 60 | 90 | „ | „ | ||||
| Cañon City to Cripple Creek | 3 | 450 | 500 | 3 | 30 | ... | ..... | „ | ||||
| Sault Ste. Marie | 80 | 400 | 2,400 | 3 | 30 | 180 | „ | „ | ||||
| St. Hyacinthe, Que. | 3 | 180 | 2,500 | 3 | 60 | 600 | „ | „ | ||||
| Great Falls to Portland, Me. | 4 | 500 | 10,000 | 3 | 60 | 225 | „ | „ | ||||
| [A] Niagara Falls Power Company. | ||||||||||||