Frequencies of the industrial currents

The frequencies of the industrial currents are divided by broad portions of electrical communications inter-connected with Alternative course, each frequency is that of the electric current which is transported unit of production to the end user. In the greatest part of the world, the frequency is of 50 Hz (Europe, Asia, Africa), against 60 Hz in North America (see illustration).

In the countries using the 50 Hz, the final tension (i.e at the point of consumption) is generally of 230 V , while it is often of 120 V for the countries using the 60 Hz. Except mention clarifies of the manufacturer mentioning the use potential indifferently of the 50 Hz or of the 60 Hz, the electricals appliance can not function correctly and especially to present security issues with a frequency different from that initially envisaged.

History

Operating time ratios

Several parameters influence the choice of the frequency of a system functioning with an alternative course. The lighting, the Driving S, the Transformer S, the generators and the lines of transport of the current have all of the characteristics which depend on the frequency of the current which feed them.

The first commercial applications of electrical energy were lighting with incandescence and the collector electrical motors. These two applications function well with continuous current , but the latter cannot be easily transferred onto long distances with the tension of use; however this tension cannot be easily modified.

Into alternate, the transformers can lower the tension of the alternative courses of the tension of transport (high voltage) until the tension of use. For a level of power determined, dimensions of the transformers are more or less inversely proportional to the frequency. An electrical installation including/understanding of the transformers would be much more economic with a high frequency.

If an incandescent lamp is supplied with a current having a low frequency, the filament cools with each half cycles, modifying in a perceptible way the luminosity of the lamp (flickering). This effect is pronounced more with the arc lamps, the last mercury discharge lamps and the fluorescent lamps. The commutator motors, known as driving universal do not function correctly with high frequency currents because them Inductance is opposed to the current food. Although at present the universal engines are common in the domestic applications, they are used only in ranges of low power (typically less than one kilowatt).

When the asynchronous motor (or with induction) were invented, it was noted that they functioned better at frequencies of 50 Hz with 60 Hz that at higher frequencies such as 133 Hz (with materials available to the XIXe century).

The transport of the electric current is made low frequency more effectively because the effects due to the capacitance and inductance of the line are less.

The generators turn at a speed which is under multiple of the frequency:

f/p

speed out of turns per minute

frequency of the network

many pairs of poles

Consequently, a number of standard revolutions limits the choice of the frequency (and reciprocally).

The generators motorized by low-speed engines produce lower frequencies, for a number of fixed poles, that those motorized, for example, by a Turbine with fast vapor. For low number of revolutions, it would be more expensive to build a generator with sufficient poles to produce a high frequency alternative course. Moreover, to synchronize two generators at the same speed is easier when their number of revolutions is low.

The generators can only be connected in parallel if they produce a of the same current frequency and of the same form. The standardization of the frequency in an geographical area, makes it possible to inter-connect the generators, improving reliability and allowing economies.

The continuous currents completely had not been eliminated by the alternative courses and were used in the electrochemical trains and processes. Before the invention of the rectifiers with mercury, the rotary rectifiers were used to produce D.C. current starting from alternative course. Like the other collector machines, they function better with low frequencies.

All these factors interact and make that the choice of the frequency of the current is crucial. The choice of the best frequency is a compromise between contradictory conditions.

Technological development

The networks of production of alternative course very isolated used arbitrary frequencies based on the design of the Steam engine, the water turbine or the electric Générateur. At the 19th century, the originators chose relatively high frequencies for the transformers and the arc lamps, in order to save on the matter; but chose lower frequencies for transport on long distances, engines and the rotary converters for the production of D.C. current. Frequencies between 16 2/3 Hz and 133 1/3 Hz were used by several systems of production. For example, the town of Coventry (England) had a delivery system single-phase current of 87 Hz which was under operation until 1906. Once the electrical motors became very current, it was important to standardize the frequency of the network in order to ensure compatibility with the equipment of the customers. The standardization on a single frequency, later, also allowed the interconnection of the production units in networks generating of the economies and bringing a reliability. Although several theories and legends exist, there exist some certainty on the history of the 60 Hz vs 50 Hz. First of all, frequencies very in lower part of 50 Hz generate flickerings of the incandescence or arc lamps. Little before 1892, Westinghouse in the United States chose the 60 Hz, whereas AEG in Germany chose the 50 Hz in 1899, leading to a world mainly cut into two.

Nikola Tesla would have had a very strong influence on the choice of Westinghouse. The use of the 60 Hz made it possible to use the engines with induction at the same speed as that of the steam engines standard, current machines at the XIXe century. However, the first generators, built by Westinghouse and installed on the falls of the Niagara, produced 25 Hz because the speed of the turbine had already been selected before the standardization was definitively fixed.

Westinghouse would have selected a relatively low frequency of 30 Hz for the engines, but the turbines for the project had already been ordered and were incompatible for an operation with a generator with 30 Hz. As the project of the falls of the Niagara was very influential on the systems design of power, the 25 Hz was essential like American northern standard for the low frequency alternative courses. A Westinghouse study concluded that a good compromise between lighting, the engines and the problems of the transport of the current would have been the 40 Hz. Although frequencies around 40 Hz found some applications commercial, this frequency taken forever the top on the frequencies already used (25 Hz, 50 Hz and 60 Hz).

Switches of frequency used to convert 25 Hz in 60 Hz (and reciprocally) were difficult to conceive; a machine functioning with 60 Hz with 24 poles would have turned at the same speed as a machine functioning to 25 Hz with 10 poles, making the machines bulky and expensive. A ratio of 60/30 would have simplified the design, but the installations with 25 Hz were too largely established so that this concept is economically viable. Some think that the choice of AEG of the 50 hertz would have been made to have a value " ronde". That also could have been an intentional decision intended to be incompatible with the other frequencies, although it is necessary to relativize (multiple frequencies were employed, so that any really did not emerge). A plethora of frequencies continued to coexist (In 1918, London had 10 different frequencies). It is only after the Second world war, with the diffusion of the electric consumer goods, that standards were established. Other frequencies which were relatively common in first half of the 20th century, remain in service today in isolated cases, often attached to the system of 60 Hz via a switch produced by a revolving converter or a static reverser. Because of the costs, the standardization was not done immediately and some parts of the distribution network could continue to operate with original frequencies. The 25 Hz was employed in Ontario, with the Quebec, and in the north of the the United States. In the Fifties, much of equipment (of the generators to the apparatuses electric household appliances) functioning with 25 Hz, were put at the standard. However, of the generators of 25 Hz always exist in the stations of the falls of the Niagara and provide power to the large industrial customers who did not want to replace the existing equipment. In the same way, of the engines with 25 hertz equip some pumps water installed at the time of believed of the New-Orleans.

In the United States, the company of electricity Southern California Edison, which had as a standard the 50 Hz, did not rock with the 60 Hz that in 1948.

electric Frequencies of use in 1897 in North America

In the middle of the XXe century, the frequencies in service had not been completely standardized with the current values (50 Hz and 60 Hz). In 1946, a handbook of reference for the design of the radio operator equipment listed (still) the obsolete electric frequencies!

electric Frequencies of use in 1946 (others that 50 and 60 Hz)

In the zones marked with (*), the frequency indicated was the only one in service.

Railroads

Other frequencies are used for the railroads. The Germany, the Austria and the Suisse use alternative course single-phase current with 16 2/3 Hz for the power channel. The frequency of 25 Hz was used on certain lines of the German railroads ( Mariazeller Bahn ) and some lines in the states of New York and Pennsylvania (Amtrak) to the the United States.

The other railway lines are fed at industrial frequencies (50 Hz or 60 Hz), and in this case the current intended for the chain of traction is delivered directly via lines standards. In the countries using the 16 2/3 Hz it is delivered either via frequency converters, or by dedicated production units.

400 Hz

400 Hz is used in the aerospace field, in computer supplies in special applications and portable machine tools. Indeed, transformers and engines designed for 400 Hz are much more compact and light that those using of the 50 or 60 Hz.

But the current at such frequencies cannot be economically transferred onto long distances, this is why the use of the 400 Hz remains generally limited to the vehicles or the buildings.

Stability of the frequency

The frequency of the great inter-connected distribution networks is narrowly regulated so that the average frequency is always maintained with the face value except for some hundred Ppm. This allows the operation of simple electric clocks based on synchronous electrical motors. But the reason first of the accurate check of the frequency is to allow the piloting of the generators coupled with the networks.

The frequency varies when the load of the networks varies, or then when generators are coupled or uncoupled from the network. More precisely, an addition of consumption not compensated by the production makes fall the frequency. The number of generators coupled with the network is thus permanently adjusted so that the frequency of the network is constant.

In the event of important fall of the frequency, due to an important overload, two solutions can then be committed:

if the overload comes from the loss of production unit (following the loss of an interconnection for example), automatic systems then launch the Délestage of certain portions of the network.

if the overload does not come from the network or the production units, but only of one increased consumption, the production units is brought into service and is connected to the network.

A contrario , if consumption drops, the frequency will increase, it is then necessary to reduce the production.

The frequency is thus the factor of regulation of the networks. Frequencies industrial (50 and 60 Hz) are consequently controlled in an interval of tolerance of ± 0.5 Hz (for networks with the national scales).

Audible noise and interferences

The food can emit a buzz (which one names also whirr with 50 Hz) at a multiple frequency of that of the network to which they are connected. This noise can mean on some pregnant active of bad qualitées.

The beginnings of the Télévision, the electronic systems did not make it possible to filter perfectly this noise due to the frequency of the network, which involved distortions of the image (stroboscopic undulations, beats, effects of the illuminations). The standards of diffusion thus chose multiples of the frequency of the network like frequency of vertical synchronization of the image. That explains the 25 images/second for the European standards and 30 images/second for the American standard. Since, progress of electronics allows an effective filtering of the noise coming from the network.

Appendices

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