Alternator

An alternator (or synchronous generator) is an electrical generator that converts mechanical energy to electrical energy in the form of alternating current (AC).[1] For reasons of cost and simplicity, most alternators use a rotating magnetic field with a stationary armature.[2] The rotor coil is externally excited by a small direct current (DC), while the AC is generated in the stationary coils. Occasionally, a linear alternator or a rotating armature with a stationary magnetic field is used. Although any AC electrical generator might be called an alternator, usually the term refers to small rotating machines driven by automotive and other internal combustion engines. The term usually applies those types with externally excited rotor coils. An alternator that uses a permanent magnet for its magnetic field is called a magneto.

Alternators in power stations driven by steam turbines are called turbo-alternators. Large 50 or 60 Hz three-phase alternators in power plants generate most of the world's electric power, which is distributed by electric power grids.[3]

## Contents

* 1 History
* 2 Principle of operation
* 3 Synchronous speeds
* 4 Classifications
* 4.1 By excitation
* 4.1.1 Direct-connected direct-current (DC) generator
* 4.1.2 Direct-connected alternating-current (AC) generator
* 4.1.3 Transformation and rectification
* 4.1.4 Brushless alternators
* 4.2 By number of phases
* 4.3 By rotating part
* 4.4 Cooling methods
* 5 Specific applications
* 5.1 Synchronous generators
* 5.2 Automotive alternators
* 5.3 Diesel–electric locomotive alternators
* 5.4 Marine alternators
* 5.5 Aviation
* 5.6 Radio alternators
* 6 See also
* 7 References
* 8 External links

## History

In what is considered the first industrial use of alternating current in 1891, workers pose with a Westinghouse alternator at the Ames Hydroelectric Generating Plant. This machine was used as a generator producing 3,000-volt, 133-hertz, single-phase AC, and an identical machine 3 miles (4.8 km) away was used as an AC motor.[4][5][6]

Alternating current generating systems were known in simple forms from the discovery of the magnetic induction of electric current in the 1830s. Rotating generators naturally produced alternating current, but since there was little use for it, it was normally converted into direct current via the addition of a commutator in the generator.[7] The early machines were developed by pioneers such as Michael Faraday and Hippolyte Pixii. Faraday developed the "rotating rectangle", whose operation was heteropolar – each active conductor passed successively through regions where the magnetic field was in opposite directions.[8] Lord Kelvin and Sebastian Ferranti also developed early alternators, producing frequencies between 100 and 300 Hz.

The late 1870s saw the introduction of the first large-scale electrical systems with central generation stations to power Arc lamps, used to light whole streets, factory yards, or the interior of large warehouses. Some, such as Yablochkov arc lamps introduced in 1878, ran better on alternating current, and the development of these early AC generating systems was accompanied by the first use of the word "alternator".[9][7] Supplying the proper amount of voltage from generating stations in these early systems was left up to the engineer's skill in "riding the load".[10] In 1883 the Ganz Works invented the constant voltage generator[11] that could produce a stated output voltage, regardless of the value of the actual load.[12] The introduction of transformers in the mid-1880s led to the widespread use of alternating current and the use of alternators needed to produce it.[13] After 1891, polyphase alternators were introduced to supply currents of multiple differing phases.[14] Later alternators were designed for various alternating current frequencies between sixteen and about one hundred hertz for use with arc lighting, incandescent lighting, and electric motors.[15] Specialized radio frequency alternators like the Alexanderson alternator were developed as longwave radio transmitters around World War 1 and used in a few high power wireless telegraphy stations before vacuum tube transmitters replaced them.

## Principle of operation

Diagram of a simple alternator with a rotating magnetic core (rotor) and stationary wire (stator) also showing the current induced in the stator by the rotating magnetic field of the rotor.

The rotating magnetic field induces an AC voltage in the stator windings. Since the currents in the stator windings vary in step with the position of the rotor, an alternator is a synchronous generator.[2]

Alternators used in central power stations also control the field current to regulate reactive power and to help stabilize the power system against the effects of momentary faults. Often, there are three sets of stator windings, physically offset so that the rotating magnetic field produces a three-phase current, displaced by one-third of a period with respect to each other.[16]

## Synchronous speeds

One cycle of alternating current is produced each time a pair of field poles passes over a point on the stationary winding. The relation between speed and frequency is N=120f/P, where f is the frequency in Hz (cycles per second). P is the number of poles (2, 4, 6, ...), and N is the rotational speed in revolutions per minute (r/min).[17] Old descriptions of alternating current systems sometimes give the frequency in terms of alternations per minute, counting each half-cycle as one alternation; so 12,000 alternations per minute corresponds to 100 Hz. An alternator's output frequency depends on the number of poles and the rotational speed. The speed corresponding to a particular frequency is called the synchronous speed. This table[18] gives some examples:

Poles  | Rotation speed (r/min), giving...

|

50 Hz  | 60 Hz  | 400 Hz
2  | 3,000  | 3,600  | 24,000
4  | 1,500  | 1,800  | 12,000
6  | 1,000  | 1,200  | 8,000
8  | 750  | 900  | 6,000
10  | 600  | 720  | 4,800
12  | 500  | 600  | 4,000
14  | 428.6  | 514.3  | 3,429
16  | 375  | 450  | 3,000
18  | 333.3  | 400  | 2,667
20  | 300  | 360  | 2,400
40  | 150  | 180  | 1,200

## Classifications

Alternators may be classified by the method of excitation, number of phases, the type of rotation, cooling method, and their application.[19]

### By excitation

#### Direct-connected direct-current (DC) generator

This method of excitation consists of a smaller direct-current (DC) generator fixed on the same shaft as the alternator. The DC generator generates a small amount of electricity, just enough to excite the field coils of the connected alternator to generate electricity. A variation of this system is a type of alternator that uses direct current from a battery for initial excitation upon start-up, after which the alternator becomes self-excited.[19]

#### Direct-connected alternating-current (AC) generator

This method of excitation consists of a smaller alternating-current (AC) generator fixed on the same shaft as the alternator. The AC stator generates a small amount of field coil excitation current, which is induced in the rotor and rectified to DC by a bridge rectifier built in to the windings where it excites the field coils of the larger connected alternator to generate electricity. This system has the advantage of not requiring brushes, which increases service life, although with a slightly lower overall efficiency. A variation of this system is a type of alternator that uses direct current from a battery for initial excitation upon start-up, after which the alternator becomes self-excited.[19]

#### Transformation and rectification

This method depends on residual magnetism retained in the iron core to generate a weak magnetic field, which would allow a weak voltage to be generated. This voltage is used to excite the field coils so the alternator can generate stronger voltage as part of its build up process. After the initial AC voltage buildup, the field is supplied with rectified voltage from the alternator.[19]

#### Brushless alternators

A brushless alternator is composed of two alternators built end-to-end on one shaft. Until 1966, alternators used brushes with rotating field.[20] With the advancement in semiconductor technology, brushless alternators are possible. Smaller brushless alternators may look like one unit, but the two parts are readily identifiable in the large versions. The main alternator is the larger of the two sections, and the smaller one is the exciter. The exciter has stationary field coils and a rotating armature (power coils). The main alternator uses the opposite configuration with a rotating field and stationary armature. A bridge rectifier, called the rotating rectifier assembly, is mounted on the rotor. Neither brushes nor slip rings are used, which reduces the number of wearing parts. The main alternator has a rotating field and a stationary armature (power generation windings).

Varying the amount of current through the stationary exciter field coils varies the 3-phase output from the exciter. This output is rectified by a rotating rectifier assembly mounted on the rotor, and the resultant DC supplies the rotating field of the main alternator and hence alternator output. The result is that a small DC exciter current indirectly controls the output of the main alternator.[21]

### By number of phases

Main articles: Engineering:Single-phase generator and Polyphase coil

Another way to classify alternators is by the number of phases of their output voltage. The output can be single-phase or polyphase. Three-phase alternators are the most common, but polyphase alternators can be two-phase, six-phase, or more.[19]

### By rotating part

### Cooling methods

Many alternators are cooled by ambient air, forced through the enclosure by an attached fan on the shaft that drives the alternator. In vehicles such as transit buses, a heavy demand on the electrical system may require a large alternator to be oil-cooled.[22] In marine applications water-cooling is also used. Expensive automobiles may use water-cooled alternators to meet high electrical system demands.

## Specific applications

### Synchronous generators

Most power generation stations use synchronous machines as their generators. The connection of these generators to the utility grid requires synchronization conditions to be met.[23] Synchronous generators provide several important system services. Their inherent rotational inertia slows the initial frequency decline after a sudden loss of generation, allowing turbine governors and other frequency-control reserves time to respond and restore balance. This role is increasingly significant as the share of non-synchronous generation, such as solar photovoltaics and many modern wind plants, grows in many power systems.[24]

### Automotive alternators

Alternator mounted on an automobile engine with a serpentine belt pulley (belt not present.)

Alternators are used in modern internal combustion engine automobiles to charge the battery and to power the electrical system when its engine is running.[25]

Until the 1960s, automobiles used DC dynamo generators with commutators. With the availability of affordable silicon-diode rectifiers, alternators were used instead.[26]

### Diesel–electric locomotive alternators

In later diesel–electric locomotives and diesel–electric multiple units, the prime mover turns an alternator which provides electricity for the traction motors (AC or DC).[27][28]

The traction alternator usually incorporates integral silicon diode rectifiers to provide the traction motors with up to 1,200 volts DC.[29]

The first diesel electric locomotives, and many of those still in service, use DC generators as, before silicon power electronics, it was easier to control the speed of DC traction motors. Most of these had two generators: one to generate the excitation current for a larger main generator.[30]

### Marine alternators

### Aviation

### Radio alternators

High-frequency alternators of the variable-reluctance type were applied commercially to radio transmission in low-frequency radio bands. These were used for transmitting Morse code and, experimentally, for transmitting voice and music. In the Alexanderson alternator, both the field winding and armature winding are stationary, and current is induced in the armature by the changing magnetic reluctance of the rotor (which has no windings or current-carrying parts). Such machines were made to produce radio frequency current for radio transmissions, although the efficiency was low.[31]

## See also

* Bottle dynamo
* Dynamo
* Electric generator
* Engine-generator
* Flux switching alternator
* Folsom Powerhouse State Historic Park
* Hub dynamo
* Induction generator
* Jedlik's dynamo
* Linear alternator
* Magneto
* Polyphase coil
* Revolving armature alternator
* Single-phase generator
* Synchronverter

## References

1. ↑ Aylmer-Small, Sidney (1908). "Lesson 28: Alternators". Electrical railroading; or, Electricity as applied to railroad transportation. Chicago: Frederick J. Drake & Co.. pp. 456–463. https://books.google.com/books?id=jdk7AAAAMAAJ.
2. ↑ 2.0 2.1 Gordon R. Selmon, Magnetoelectric Devices, John Wiley and Sons, 1966 no ISBN pp. 391-393
3. ↑ "List of Plug/Sockets and Voltage of Different Countries". http://www.worldstandards.eu/electricity/plug-voltage-by-country/.
4. ↑ D. M. Mattox, The Foundations of Vacuum Coating Technology, page 39
5. ↑ "Charles C. Britton, An Early Electric Power Facility in Colorado". Colorado Magazine 49 (3): 185. Summer 1972. http://www.historycolorado.org/sites/default/files/files/Researchers/ColoradoMagazine_v49n3_Summer1972.pdf. Retrieved 15 August 2016.
6. ↑ "Milestones:Ames Hydroelectric Generating Plant, 1891". IEEE Global History Network. IEEE. http://www.ieeeghn.org/wiki/index.php/Milestones:Ames_Hydroelectric_Generating_Plant,_1891.
7. ↑ 7.0 7.1 Christopher Cooper, The Truth about Tesla: The Myth of the Lone Genius in the History of Innovation, Quarto Publishing Group USA – 2015, page 93
8. ↑ Thompson, Sylvanus P., Dynamo-Electric Machinery. p. 7.
9. ↑ Jill Jonnes, Empires of Light: Edison, Tesla, Westinghouse, And The Race To Electrify The World, Random House – 2004, page 47
10. ↑ Donald Scott McPartland, Almost Edison: How William Sawyer and Others Lost the Race to Electrification, ProQuest – 2006, page 135
11. ↑ American Society for Engineering Education (1995). Proceedings, Part 2. p. 1848. https://books.google.com/books?id=EZVRAAAAMAAJ&q=ganz+"constant+voltage+generator".
12. ↑ Robert L. Libbey (1991). A Handbook of Circuit Math for Technical Engineers. CRC Press. p. 22. ISBN 9780849374005. https://books.google.com/books?id=b6dD_bqZNyoC&q="constant+voltage+generator"&pg=PA22.
13. ↑ Thompson, Sylvanus P.. "Milestones:Alternating Current Electrification, 1886". IEEE Global History Network. http://www.ieeeghn.org/wiki/index.php/Milestones:Alternating_Current_Electrification,_1886.
14. ↑ Thompson, Sylvanus P., Dynamo-Electric Machinery. pp. 17
15. ↑ Thompson, Sylvanus P., Dynamo-Electric Machinery. pp. 16
16. ↑ B. M. Weedy. Electric Power Systems Second Edition, John Wiley and Sons, 1972, ISBN 0 471 92445 8, p. 141
17. ↑ Grainger, John J.; Stevenson, William D. (1994). Power system analysis. McGraw-Hill. p. 91. ISBN 978-0-07-061293-8. https://books.google.com/books?id=NBIoAQAAMAAJ.
18. ↑ The Electrical Year Book 1937, published by Emmott & Co. Ltd., Manchester, England, page 72
19. ↑ 19.0 19.1 19.2 19.3 19.4 Aviation Maintenance Technician Handbook—General (FAA-H-8083-30). Federal Aviation Administration. 2008. pp. 10_160–10_161. http://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/amt_handbook/media/FAA-8083-30_Ch10.pdf. Retrieved 6 September 2013.
20. ↑ "Cummins Generator Technologies". Cummins Generator Technologies. https://www.stamford-avk.com/heritage/cummins-generator-technologies.
21. ↑ G. K. Dubey, Fundamentals of Electrical Drives, CRC Press, 2002, ISBN 084932422X, page 350
22. ↑ Gus Wright, Fundamentals of Medium/Heavy Duty Diesel Engines, Jones & Bartlett Publishers, 2015, ISBN 128406705X page 1233
23. ↑ Soft synchronization of dispersed generators to micro grids for smart grid applications
24. ↑ https://eepublicdownloads.entsoe.eu/clean-documents/SOC documents/Inertia and RoCoF_v17_clean.pdf?
25. ↑ "How It Works: The Charging System". 2023-01-01. https://www.hotrod.com/features/how-it-works-december-1993-982-1510-38-1/.
26. ↑ "A History of Alternators". Elreg. https://www.elreg.com/blog/a-history-of-alternators/.
27. ↑ Kapetanović, Marko; Vajihi, Mohammad; Goverde, Rob M. P. (2021). "Analysis of Hybrid and Plug-In Hybrid Alternative Propulsion Systems for Regional Diesel-Electric Multiple Unit Trains". Energies 14 (18): 5920. doi:10.3390/en14185920.
28. ↑ Saadat, M.; Esfahanian, M.; Saket, M.H. (2017). "Reducing fuel consumption of diesel-electric locomotives using hybrid powertrain and fuzzy look-ahead control". Sage Publishing 231 (4): 406–418. doi:10.1177/0954409716631010. https://journals.sagepub.com/doi/10.1177/0954409716631010. Retrieved 2025-04-16.
29. ↑ "Global Traction Alternators Market". 2024-04-17. https://www.precisionreports.co/global-traction-alternators-market-27225755.
30. ↑ Guss, Chris (2022-08-03). "What Locomotive Generators Do". https://www.trains.com/trn/railroads/locomotives/what-locomotive-generators-do/.
31. ↑ Wilson, Keith (2020-03-17). "Have you ever heard of the Alexanderson alternator?". https://www.megger.com/en-us/et-online/march-2020/have-you-ever-heard-of-the-alexanderson-alternator.

## External links

* White, Thomas H.,"Alternator-Transmitter Development (1891–1920)". EarlyRadioHistory.us.
* Alternators at Integrated Publishing (TPub.com)
* Wooden Low-RPM Alternator, ForceField, Fort Collins, Colorado, USA
* Understanding 3 phase alternators at WindStuffNow
* Alternator, Arc and Spark. The first Wireless Transmitters (G0UTY homepage)

* v
* t
* e

Electric motors

* AC - Alternating current
* DC - Direct current
* PM - Permanent magnet
* SC - Self-commutated

Fundamental types|

* AC motor
* DC motor

DC motors|

* Homopolar
* Electrically-excited field or PM field brushed DC
* Unipolar

AC SC mechanical
commutator

* Repulsion
* Universal

AC SC electronic
commutator

* Brushless DC (BLDC)
* Switched reluctance (SRM)

AC asynchronous
(induction (IM))

* Shaded-pole (single-phase)
* Dahlander pole changing motor
* Squirrel-cage rotor (SCIM)
* Wound-rotor (WRIM)
* Linear induction

AC synchronous (SM)|

* Hysteresis
* Interior / Surface PM (IPMSM / SPMSM)
* Reluctance (SyRM)
* Wound-rotor (WRSM)

Special magnetic
machines

* Doubly-fed
* Linear
* Servomotor
* Stepper
* Traction

Non-magnetic|

* Electrostatic
* Piezoelectric
* Ultrasonic

Enclosure Type

* Hermetic
* TEFC

Components and
accessories

* Armature
* Braking chopper
* Brush
* Commutator
* DC injection braking
* Field coil
* Rotor
* Slip ring
* Stator
* Winding

Motor controllers|

* AC/AC converter
* Amplidyne
* Adjustable-speed drive (ASD)
* Cycloconverter
* Direct torque control (DTC)
* Metadyne
* Motor controller
* Motor soft starter
* Motor starter (engine)
* Power inverter
* Thyristor drive
* Variable-frequency drive (VFD)
* Vector control
* Voltage controller
* Ward Leonard control

History, education,
recreational use

* Timeline of the electric motor
* Ball bearing motor
* Barlow's wheel
* Lynch motor
* Mendocino motor
* Mouse mill motor

Experimental, futuristic|

* Coilgun
* Railgun
* Superconducting machine

Related topics|

* Blocked-rotor test
* Circle diagram
* Closed-loop control
* Electromagnetism
* Open-circuit test
* Open-loop controller
* Power-to-weight ratio
* Torque and speed of a DC motor
* Two-phase system

People|

* Arago
* Baily
* Barlow
* Botto
* Cook
* Davenport
* Davidson
* Dolivo-Dobrovolsky
* Faraday
* Ferraris
* Froment
* Gramme
* Henry
* Jacobi
* Jedlik
* Lenz
* Maxwell
* Ørsted
* Park
* Pixii
* Saxton
* Siemens
* Sprague
* Steinmetz
* Stimpson
* Sturgeon
* Tesla

See also|

* Alternator
* Electric generator
* Inchworm motor

SI electromagnetism
units

* C \- Capacitance (F)
* Q \- Charge (C)
* G, B, Y \- Conductance, susceptance, admittance (S)
* κ, γ, σ \- Conductivity (S/m)
* I \- Current (A)
* D \- Electric displacement field (C/m2)
* E \- Electric field (V/m)
* ΦE \- Electric flux (V·m)
* χe \- Electric susceptibility
* U, ΔV, Δφ; E \- Emf (V)
* L, M \- Inductance (H)
* H \- Magnetic field (A/m) strength
* Φ \- Magnetic flux (Wb)
* B \- Magnetic flux density (T)
* χ \- Magnetic susceptibility
* μ \- Permeability (H/m)
* ε \- Permittivity (F/m)
* P \- Power (W)
* R, X, Z \- Resistance, reactance, impedance (Ω)
* ρ \- Resistivity (Ω·m)

0.00

(0 votes)

Original source: https://en.wikipedia.org/wiki/Alternator. Read more

|

Retrieved from "https://handwiki.org/wiki/index.php?title=Physics:Alternator&oldid=4369474"
*[v]: View this template
*[t]: Discuss this template
*[e]: Edit this template