Utility frequency: Difference between revisions

Content deleted Content added

Inline

Revision as of 03:07, 17 January 2007

The utility frequency (American English) or mains frequency (British English) is the frequency at which alternating current (AC) is transmitted from a power plant to the end user.

In most parts of the Americas, it is typically 60 Hz, and in most parts of the rest of the world it is typically 50 Hz. Precise details are shown in the list of countries with mains power plugs, voltages and frequencies.

Places that use the 50 Hz frequency tend to use 230 V RMS, and those that use 60 Hz tend to use 117 V RMS.

Unless specified by the manufacturer to operate on either 50 or 60 Hz, appliances may not operate efficiently or even safely if used on other than the intended supply frequency.

History

Operating factors

Several factors influence the choice of frequency in an AC system. Lighting, motors, transformers, generators and transmission lines all have characteristics which depend on the power frequency.

The first applications of commercial electric power were incandescent lighting and commutator-type electric motors. Both devices operate well on DC, but DC cannot be easilly transmitted long distances at utilization voltage and also cannot be easily changed in voltage.

Transformers can be used to step down high transmission voltages to lower utiliztion voltage. Since, for a given power level, the dimensions of a transformer are roughly inversely proportional to frequency, a system with many transformers would be more economical at a higher frequency.

If an incandescent lamp is operated off a low-frequency current, the filament cools on each half-cycle of the alternating current, leading to perceptible change in brightness and flicker of the lamps; the effect is more pronounced with arc lamps, and the later mercury-vapor and fluorescent lamps.

Commutator-type motors do not operate well on high-frequency AC since the rapid changes of current are opposed by the inductance of the motor field; even today, althogh commutator-type universal motors are common in household appliances, they are universally of low ratings less than 1 kW. Once the induction motor had been developed, it was found to work well on frequencies around 50 to 60 Hz but with the materials available in the late 1800's would not work well at a frequency of, say, 133 Hz. There is a fixed relationship between the number of magnetic poles in the induction motor field, the frequency of the alternating current, and the rotation speed; so, a given standard speed limits the choice of frequency (annd the reverse).

Electric power transmission over long lines favors lower frequencies. The effects of the distributed capacitance and inductance of the line are less at low frequency.

Generators operated by slow-speed engines or turbines will produce lower frequencies, for a given number of poles, than those operated by, for example, a high-speed turbine. For very slow prime mover speeds, it would be costly to build a generator with enough poles to provide a high AC frequency. As well, synchronizing two generators to the same speed was found to be easier at lower speeds. Generators can only be interconnected to operate in parallel if they are of the same frequency and wave-shape. By standardizing the frequency used, generators in a geographic area can be interconnected, providing reliability and cost savings.

Direct-current power was not entirely displaced by alternating current and was useful in railway and electrochemical processes. Prior to the development of mercury arc valve rectifiers, rotary converters were used to produced DC power from AC. Like other commutator-type machines these worked better with lower frequencies.

All of these factors interact and make selection of a power frequency a matter of considerable importance. The best frequency is a comprimise between contradictory requirements.

Development

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 comprimise between lighting, motor, and transmission needs, but this frequency never overcame the "installed base" of 25 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. 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, much of this electrical system, from the generators right through to household appliances, was converted and standardised to 60 Hz. 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 [1].

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.

Utility Frequencies in Use in 1897 in North America

Cycles	Description
140	Wood arc-lighting dynamo
133	Stanley-Kelly Company
125	General Electric single-phase
66.7	Stanley-Kelly company
62.5	General Electric "monocyclic"
60	Many manufacturers, becoming "increasing common" in 1897
58.3	General Electric Lachine Rapids
40	General Electric
33	General Electric at Portland Oregon for rotary converters
27	Crocker-Wheeler for calcium carbide furnaces
25	Westinghouse Niagara Falls 2-phase - for operating motors

Railways

Other utility frequencies are used. Germany, Austria, and Switzerland use traction power networks for railways, distributing single-phase AC at 16.7 Hz. A frequency of 25 Hz was used for the German railway Mariazeller Bahn and some railway systems in New York and Pennsylvania (Amtrak) in the USA. Other railway systems are energized at the local commercial power frequency, 50 Hz or 60 Hz. Traction power may be derived from commercial power supplies by frequency converters, or in some cases may be produced by dedicated generating stations.

400Hz

Frequencies as high as 400 Hz are used in aerospace and some special-purpose computer power supplies and hand-held machine tools. Such high frequencies cannot be economically transmitted long distances, so 400 Hz power systems are usually confined to the building or vehicle. Transformers and motors for 400Hz are much smaller and lighter than at 50 or 60 Hz, which is an advantage in aircraft and ships.

Stability

The frequency of large interconnected power distribution systems is tightly regulated so that, over the course of a day, the average frequency is maintained at the nominal value within a few hundred parts per million. While this allows simple electric clocks, relying on synchronous electric motors, to keep accurate time, the primary reason for accurate frequency control is to allow the flow of alternating current power from multiple generators through the network to be controlled.

Frequency of the system will vary as load is added to the system or as generators are shut down; other generators are adjusted in speed so that the average system frequency stays nearly constant. During a severe overload caused by failure of generators or transmission lines, the power system frequency will decline. Loss of an interconnection carrying a large amount of power (relative to system total generation) will cause system frequency to rise. Special protection relays in the power system network sense the decline and may automatically initiate load shedding or tripping of interconnection lines, to preserve the operation of at least part of the network. Quite small frequency deviations, on the order of 0.5 Hz on a 50 Hz or 60 Hz network, will result in automatic load shedding or other control actions to restore system frequency. Smaller power systems, not extensively interconnected with many generators and loads, may not maintain frequency with the same degree of accuracy.

Audible noise and interference

AC-powered appliances can give off a characteristic hum (often referred to as the "60 cycle hum" or "mains hum"), at the multiples of the frequencies of AC power that they use. This often occurs in poorly made speakers. Most countries have chosen their television standard to approximate their mains supply frequency. This helps prevent powerline hum and magnetic interference from causing visible beat frequencies in the displayed picture.

References

Edwin J. Houston and Arthur Kennelly, Recent Types of Dynamo-Electric Machinery, copyright American Technical Book Company 1897, published by P.F. Collier and Sons New York, 1902
Central Station Engineers of the Westinghouse Electric Corporation, Electrical Transmission and Distribution Reference Book, 4th Ed., Westinghouse Electric Corporation, East Pittsburgh PA, 1950, no ISBN
Donald G. Fink and H. Wayne Beaty, Standard Handbook for Electrical Engineers, Eleventh Edition,McGraw-Hill, New York, 1978, ISBN 0-07-020974-X

@@ Line 8: / Line 8: @@
 ==History==
 ===Operating factors===
-Several factors influence the choice of frequency in an AC system. The first applications of commericial electric power were incandescent lighting and [[commutator]]-type electric motors.  Both devices operate well on DC, but DC cannot be easilly transmitted long distances at utilization voltage and also cannot be easily changed in voltage.
+Several factors influence the choice of frequency in an AC system. Lighting, motors, transformers, generators and transmission lines all have characteristics which depend on the power frequency.
+The first applications of commercial electric power were incandescent lighting and [[commutator]]-type electric motors.  Both devices operate well on DC, but DC cannot be easilly transmitted long distances at utilization voltage and also cannot be easily changed in voltage.
 Transformers can be used to step down high transmission voltages to lower utiliztion voltage. Since, for a given power level, the dimensions of a transformer are roughly inversely proportional to frequency, a system with many transformers would be more economical at a higher frequency.
@@ Line 19: / Line 20: @@
 Commutator-type motors do not operate well on high-frequency AC since the rapid changes of current are opposed by the inductance of the motor field; even today, althogh commutator-type ''universal'' motors are common in household appliances, they are universally of low ratings less than 1 kW. Once the induction  motor had been developed, it was found to work well on frequencies around 50 to 60 Hz but with the  materials available in the late 1800's would not work well at a frequency of, say, 133 Hz. There is a fixed relationship between the number of magnetic poles in the induction motor field, the frequency of the alternating current, and the rotation speed; so, a given standard speed limits the choice of frequency (annd the reverse).
-Electric power transmission over long lines favors lower frequencies; the effects of the distributed capacitance and inductance of the line are less at low frequency.
+[[Electric power transmission]] over long lines favors lower frequencies. The effects of the distributed capacitance and inductance of the line are less at low frequency.
 Generators operated by slow-speed engines or turbines will produce lower frequencies, for a given number of poles, than those operated by, for example, a high-speed [[turbine]].  For very slow prime mover speeds, it would be costly to build a generator with enough poles to provide a high AC frequency.  As well, synchronizing two generators to the same speed was found to be easier at lower speeds. Generators can only be interconnected to operate in parallel if they are of the same frequency and wave-shape. By standardizing the frequency used, generators in a geographic area can be interconnected, providing reliability and cost savings.
+Direct-current power was not entirely displaced by alternating current and was useful in railway and electrochemical processes.  Prior to the development of [[mercury arc valve]] [[rectifier]]s, rotary converters were used to produced DC power from AC.  Like other commutator-type machines these worked better with lower frequencies.
-All of these factors interact and make selection of a power frequency a matter of considerable importance.
+All of these factors interact and make selection of a power frequency a matter of considerable importance. The best frequency is a comprimise between contradictory requirements.
 ===Development===
-Very early 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 [[transformer]]s 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 converter]]s 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.
+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 [[transformer]]s 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 converter]]s 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 [[electric motors|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 [[power transmission|grid]] for economy and security of operation.