Alternator
An alternator is an
An alternator that uses a
History
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.[8] 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.[9] Lord Kelvin and Sebastian Ferranti also developed early alternators, producing frequencies between 100 and 300 Hz.[citation needed]
The late 1870s saw the introduction of first large scale electrical systems with central generation stations to power
Principle of operation
A conductor moving relative to a magnetic field develops an electromotive force (EMF) in it (Faraday's Law). This EMF reverses its polarity when it moves under magnetic poles of opposite polarity. Typically, a rotating magnet, called the rotor, turns within a stationary set of conductors, called the stator, wound in coils on an iron core. The field cuts across the conductors, generating an induced EMF (electromotive force), as the mechanical input causes the rotor to turn.[citation needed]
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.[3]
The rotor's magnetic field may be produced by permanent magnets, or by a field coil electromagnet. Automotive alternators use a rotor winding which allows control of the alternator's generated voltage by varying the current in the rotor field winding. Permanent magnet machines avoid the loss due to magnetizing current in the rotor, but are restricted in size, due to the cost of the magnet material. Since the permanent magnet field is constant, the terminal voltage varies directly with the speed of the generator. Brushless AC generators are usually larger than those used in automotive applications.[citation needed]
An automatic voltage control device controls the field current to keep output voltage constant. If the output voltage from the stationary armature coils drops due to an increase in demand, more current is fed into the rotating field coils through the voltage regulator (VR). This increases the magnetic field around the field coils which induces a greater voltage in the armature coils. Thus, the output voltage is brought back up to its original value.[citation needed]
Alternators used in central
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 , where is the frequency in Hz (cycles per second). is the number of poles (2, 4, 6, …) and is the rotational speed in revolutions per minute (r/min). Very 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.[citation needed]
The output frequency of an alternator depends on the number of poles and the rotational speed. The speed corresponding to a particular frequency is called the synchronous speed for that frequency. 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 method of excitation, number of phases, the type of rotation, cooling method, and their application.[19]
By excitation
There are two main ways to produce the magnetic field used in the alternators, by using
In other alternators, wound field coils form an electromagnet to produce the rotating magnetic field.[citation needed]
A device that uses permanent magnets to produce alternating current is called a permanent magnet alternator (PMA). A permanent magnet generator (PMG) may produce either alternating current, or direct current if it has a commutator.[citation needed]
Direct-connected direct-current (DC) generator
This method of excitation consists of a smaller direct-current (DC) generator fixed on the same shaft with 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 which 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 weak magnetic field which would allow a weak voltage to be generated. This voltage is used to excite the field coils for the alternator to 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.
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 of all this is that a small DC exciter current indirectly controls the output of the main alternator.[21]
By number of phases
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
The revolving part of alternators can be the
Cooling methods
Many alternators are cooled by ambient air, forced through the enclosure by an attached fan on the same 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.[citation needed]
Specific applications
Electric generators
Most power generation stations use synchronous machines as their generators. Connection of these generators to the utility grid requires synchronization conditions to be met.[23]
Automotive alternators
Alternators are used in modern
Until the 1960s, automobiles used DC dynamo generators with commutators. With the availability of affordable silicon-diode rectifiers, alternators were used instead.[citation needed]
Diesel-electric locomotive alternators
In later
The traction alternator usually incorporates integral silicon diode rectifiers to provide the traction motors with up to 1,200 volts DC.[citation needed]
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.[citation needed]
Optionally, the generator also supplies
Marine alternators
Marine alternators used in yachts are similar to automotive alternators, with appropriate adaptations to the salt-water environment. Marine alternators are designed to be
Aviation
Radio alternators
High frequency alternators of the variable-reluctance type were applied commercially to radio transmission in the low-frequency radio bands. These were used for transmission of Morse code and, experimentally, for transmission of voice and music. In the Alexanderson alternator, both the field winding and armature winding are stationary, and current is induced in the armature by virtue of 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.[citation needed]
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
References
- ^ "Abraham Ganz at the Hindukush". Poemas del río Wang. Studiolum. Archived from the original on 11 February 2016. Retrieved 30 September 2015.
- ^ Aylmer-Small, Sidney (1908). "Lesson 28: Alternators". Electrical railroading; or, Electricity as applied to railroad transportation. Chicago: Frederick J. Drake & Co. pp. 456–463.
- ^ a b Gordon R. Selmon, Magnetoelectric Devices, John Wiley and Sons, 1966 no ISBN pp. 391-393
- ^ "List of Plug/Sockets and Voltage of Different Countries". World Standards. World Standards.
- ^ D. M. Mattox, The Foundations of Vacuum Coating Technology, page 39
- ^ "CHARLES C. BRITTON, An Early Electric Power Facility in Colorado" (PDF). Colorado Magazine. Vol. 49, no. 3. Summer 1972. p. 185. Archived from the original (PDF) on 28 July 2016. Retrieved 15 August 2016.
- ^ "Milestones:Ames Hydroelectric Generating Plant, 1891". IEEE Global History Network. IEEE. Retrieved 29 July 2011.
- ^ a b Christopher Cooper, The Truth about Tesla: The Myth of the Lone Genius in the History of Innovation, Quarto Publishing Group USA – 2015, page 93
- ^ Thompson, Sylvanus P., Dynamo-Electric Machinery. p. 7.
- ^ Jill Jonnes, Empires of Light: Edison, Tesla, Westinghouse, And The Race To Electrify The World, Random House – 2004, page 47
- ^ Donald Scott McPartland, Almost Edison: How William Sawyer and Others Lost the Race to Electrification, ProQuest – 2006, page 135
- ^ American Society for Engineering Education (1995). Proceedings, Part 2. p. 1848.
- ISBN 9780849374005.
- ^ Thompson, Sylvanus P. "Milestones:Alternating Current Electrification, 1886". IEEE Global History Network. Retrieved 22 September 2013.
- ^ Thompson, Sylvanus P., Dynamo-Electric Machinery. pp. 17
- ^ Thompson, Sylvanus P., Dynamo-Electric Machinery. pp. 16
- ISBN 0 471 92445 8, p. 141
- ^ The Electrical Year Book 1937, published by Emmott & Co. Ltd., Manchester, England, page 72
- ^ a b c d e Aviation Maintenance Technician Handbook—General (FAA-H-8083-30) (PDF). Federal Aviation Administration. 2008. pp. 10_160–10_161. Archived from the original (PDF) on 6 September 2013. Retrieved 6 September 2013.
- ^ "Cummins Generator Technologies". stamford-avk.com. Cummins Generator Technologies. Retrieved 18 August 2022.
- ISBN 084932422X, page 350
- ISBN 128406705Xpage 1233
- ^ Soft synchronization of dispersed generators to micro grids for smart grid applications
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)