Prime Mover drive system

At the level of machinery manufacturing at that time, the cost of speed change system was high. So the speed of the prime mover directly becomes the speed of the generator. The number of poles of the generator is limited by materials, and there are many restrictions on the improvement of frequency.

Westinghouse Company first built Nicaragua Waterfall Hydropower Station at 1895, and the selected frequency was 25 Hz( 12 pole 250RPM), mainly due to the limited speed of the turbine. Because of the importance of power station and the dominant position of Westinghouse, 25 Hz once became the frequency standard of low-frequency alternating current in North America (some distribution systems in the northern United States, Ottawa, Canada and Quebec were used until the 1950s).