Иностранный язык (английский язык)

SOME FACTS ABOUT FREQUENCY

      Very early isolated AC generating schemes used arbitrary frequencies based on convenience for steam engine, water turbine and electrical generator design. In the late 19th century, designers would pick a relatively high frequency for systems featuring transformers and arc lights, so as to economize on transformer materials, but would pick a lower frequency for systems with long transmission lines or feeding primarily motor loads or rotary converters for producing direct current. Frequencies between 16 2/3 Hz and 133 1/3 Hz were used on different systems. For example, the city of Coventry, England, in 1895 had a unique 87 Hz single-phase distribution system that was in use until 1906.

      Once induction motors became common, it was important to standardize frequency for compatibility with the customer's equipment. Standardizing on one frequency also, later, allowed interconnection of generating plants on a grid for economy and security of operation.

      Though many theories exist, and quite a few entertaining urban legends, there is little certitude in the details of the history of 60 Hz vs. 50 Hz. What is known is that Westinghouse in the US decided on 60 Hz and AEG in Germany decided on 50 Hz, eventually leading to the world being mostly divided into two frequency camps. Frequencies much below 50 Hz gave noticeable flicker of arc or incandescent lighting. Westinghouse decided on 60 Hz before 1892 and AEG decided on 50 Hz by 1899. Tesla is believed to have had a key influence in the choice of 60 Hz by Westinghouse. Use of 60 Hz allowed induction motors to operate at the same speeds as standardized steam engines common in the late 19th century.

      However, the first generators at the Niagara Falls project, built by Westinghouse, were 25 Hz because the turbine speed had already been selected before alternating current power transmission had been definitively selected.

      Westinghouse would have selected a low frequency of 30 Hz to drive motor loads, but the turbines for the project had already been specified at a speed which was incompatible with a generator designed for 30 Hz. Because the Niagara project was so influential on electric power systems design, 25 Hz prevailed as the North American standard for low-frequency AC. A Westinghouse study concluded that 40 Hz would have been a good compromise between lighting, motor, and transmission needs. Although frequencies near 40 Hz found some commercial use, this frequency never overcame the "installed base" of 25 Hz, 50 Hz and 60 Hz equipment.

      Frequency changers used to convert between 25 Hz and 60 Hz systems were awkward to design; a 60 Hz machine with 24 poles would turn at the same speed as a 25 Hz machine with 10 poles, making the machines large, slow-speed and expensive. A ratio of 60/30 would have simplified these designs, but the installed base at 25 Hz was too large to be economically opposed.

      AEG's choice of 50 Hz is thought by some to relate to a more "metric-friendly" number than 60. It may also have been an intentional decision to be incompatible, although since so many frequencies were used it may not have been clear that any one value was desirable. A plethora of frequencies continued in broad use (London in 1918 had 10 different frequencies), and it wasn't until after World War II with the advent of affordable electrical consumer goods that broader standards were enacted.

      Other frequencies were somewhat common in the first half of the 20th century, and remain in use in isolated cases today, often tied to the 60 Hz system via a rotary converter or static inverter frequency changer. Because of the cost of conversion, some parts of the distribution system may continue to operate on original frequencies even after a new frequency is chosen. 25 Hz power was used in Ontario, Quebec, the northern USA, and for railway electrification. In the 1950s, many 25 Hz systems, from the generators right through to household appliances, were converted and standardized. Some 25 Hz generators still exist at the Beck 1 and Rankine generating stations near Niagara Falls to provide power for large industrial customers who did not want to replace existing equipment; and some 25 Hz motors exist in New Orleans' floodwater pumps.

      In the United States, the Southern California Edison company had standardized on 50 Hz and did not completely change frequency of their generators and customer equipment to 60 Hz until around 1948.