Alternator

Early 20th century Alternator made in Budapest, Hungary, in the power generating hall of a hydroelectric station (photograph by Prokudin-Gorsky, 1905-1915).

An alternator is an electromechanical device that converts mechanical energy to alternating current electrical energy. Most alternators use a rotating magnetic field but linear alternators are occasionally used. In principle, any AC electrical generator can be called an alternator, but usually the word refers to small rotating machines driven by automotive and other internal combustion engines. In UK, large alternators in power stations which are driven by steam turbines are called turbo-alternators.

History

Alternating current generating systems were known in simple forms from the discovery of the magnetic induction of electric current. The early machines were developed by pioneers such as Michael Faraday and Hippolyte Pixii. Faraday developed the "rotating rectangle", whose operation was heteropolar. The first public demonstration of a more robust "alternator system" took place in 1886. Large two-phase alternating current generators were built by a British electrician, J.E.H. Gordon, in 1882. Lord Kelvin and Sebastian Ferranti also developed early alternators, producing frequencies between 100 and 300 hertz. In 1891, Nikola Tesla patented a practical "high-frequency" alternator (which operated around 15,000 hertz). After 1891, polyphase alternators were introduced to supply currents of multiple differing phases. Later alternators were designed for varying alternating-current frequencies between sixteen and about one hundred hertz, for use with arc lighting, incandescent lighting and electric motors.

Theory of operation

Alternators generate electricity by the same principle as DC generators, namely, when the magnetic field around a conductor changes, a current is induced in the conductor. Typically, a rotating magnet called the rotor turns within a stationary set of conductors wound in coils on an iron core, called the stator. The field cuts across the conductors, generating an electrical current, as the mechanical input causes the rotor to turn.

The rotor magnetic field may be produced by induction (in a "brushless" alternator), by permanent magnets (in very small machines), or by a rotor winding energized with direct current through slip rings and brushes. The rotor magnetic field may even be provided by stationary field winding, with moving poles in the rotor. Automotive alternators invariably use a rotor winding, which allows control of the alternator 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, owing 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 machines than those used in automotive applications.

A rotating magnetic field is a magnetic field which periodically changes direction. This is a key principle to the operation of alternating-current motor. In 1882, Nikola Tesla identified the concept of the rotating magnetic field. In 1885, Galileo Ferraris independently researched the concept. In 1888, Tesla gained U.S. patent 0,381,968 for his work. Also in 1888, Ferraris published his research in a paper to the Royal Academy of Sciences in Turin.

Sine wave current in each of the coils produces sine varying magnetic field on the rotation axis. Magnetic fields add as vectors.

Vector sum of the magnetic field vectors of the stator coils produces a single rotating vector of resulting rotating magnetic field.

A diagram from a Tesla patent (U.S. Patent 381968) showing a revolving magnetic field created by two phase currents and two sets of field windings.

A symmetric rotating magnetic field can be produced with as few as three coils. Three coils will have to be driven by a symmetric 3-phase AC sine current system, thus each phase will be shifted 120 degrees in phase from the others. For the purpose of this example, magnetic field is taken to be the linear function of coil's current.

The result of adding three 120-degrees phased sine waves on the axis of the motor is a single rotating vector. The rotor (having a constant magnetic field driven by DC current or a permanent magnet) will attempt to take such position that N pole of the rotor is adjusted to S pole of the stator's magnetic field, and vice versa. This magneto-mechanical force will drive rotor to follow rotating magnetic field in a synchronous manner.

A permanent magnet in such a field will rotate so as to maintain its alignment with the external field. This effect was utilised in early alternating current electric motors. A rotating magnetic field can be constructed using two orthogonal coils with 90 degrees phase difference in their AC currents. However, in practice such a system would be supplied through a three-wire arrangement with unequal currents. This inequality would cause serious problems in standardization of the conductor size and in order to overcome it, three-phase systems are used where the three currents are equal in magnitude and have 120 degrees phase difference. Three similar coils having mutual geometrical angles of 120 degrees will create the rotating magnetic field in this case. The ability of the three phase system to create a rotating field utilized in electric motors is one of the main reasons why three phase systems dominated in the world electric power supply systems. Because magnets degrade with time, synchronous motors and induction motors use short-circuited rotors (instead of a magnet) following a rotating magnetic field of multicoiled stator. (Short circuited turns of rotor develop eddy currents in the rotating field of stator which (currents) in turn move the rotor by Lorentz force).

Note that the rotating magnetic field can actually be produced by two coils, with phases shifted 90 degrees. In case two phases of sine current are only available, four poles are commonly used.

Synchronous speeds

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 gives some examples:

Poles	RPM at 50Hz	RPM at 60Hz
2	3000	3600
4	1500	1800
6	1000	1200
8	750	900
10	600	720
12	500	600
14	428.6	514.3
16	375	450
18	333.3	400
20	300	360

Key

• Rotational speeds are given in revolutions per minute (RPM)
• Frequencies are given in Hertz (Hz)

Automotive alternators

Alternators are used in automobiles to charge the battery and to power a car's electric system when its engine is running. Alternators have the great advantage over direct-current generators of not using a commutator, which makes them simpler, lighter, less costly, and more rugged than a DC generator. The stronger construction of automotive alternators allows them to use a smaller pulley so as to turn twice as fast as the engine, improving output when the engine is idling. The availability of low-cost solid-state diodes from about 1960 allowed car manufacturers to substitute alternators for DC generators. Automotive alternators use a set of rectifiers (diode bridge) to convert AC to DC. To provide direct current with low ripple, automotive alternators have a three-phase winding.

Typical passenger vehicle and light truck alternators use Lundell or claw-pole field construction, where the field north and south poles are all energized by a single winding, with the poles looking rather like fingers of two hands interlocked with each other. Larger vehicles may have salient-pole alternators similar to larger machines. The automotive alternator is usually belt driven at 2-3 times the engine crankshaft speed.

Modern automotive alternators have a voltage regulator built into them. The voltage regulator operates by modulating the small field current in order to produce a constant voltage at the stator output. The field current is much smaller than the output current of the alternator; for example, a 70-amp alternator may need only 2 amps of field current.

Efficiency of automotive alternators is limited by fan cooling loss, bearing loss, iron loss, copper loss, and the voltage drop in the diode bridges; at part load, efficiency is between 50-62% depending on the size of alternator, and varies with alternator speed. In comparison, very small high-performance permanent magnet alternators, such as those used for bicycle lighting systems, achieve an efficiency of around only 60%. Larger permanent magnet alternators can achieve much higher efficiency.

A typical automotive alternator mounted in a spacious pickup truck engine bay.

The field windings are initially supplied via the ignition switch and charge warning light, which is why the light glows when the ignition is on but the engine is not running. Once the engine is running and the alternator is generating, a diode feeds the field current from the alternator main output, thus equalizing the voltage across the warning light which goes out. The wire supplying the field current is often referred to as the "exciter" wire. The drawback of this arrangement is that if the warning light fails or the "exciter" wire is disconnected, no priming current reaches the alternator field windings and so the alternator will not generate any power. However, some alternators will self-excite when the engine is reved to a certain speed. The driver may check for a faulty exciter-circuit by ensuring that the warning light is glowing with the engine stopped.

Very large automotive alternators used on buses, heavy equipments or emergency vehicles may produce 300 amperes. Very old automobiles with minimal lighting and electronic devices may have only a 30 ampere alternator. Typical passenger car and light truck alternators are rated around 70 amperes, though higher ratings are becoming more common. Very large automotive alternators may be water-cooled or oil-cooled.

Many alternator voltage regulators are today linked to the vehicle's on board computer system, and in recent years other factors including air temperature (gained from the mass air flow sensor in many cases) and engine load are considered in adjusting the battery charging voltage supplied by the alternator.

Marine alternators

Marine alternators as used in yachts are normally versions of automotive alternators, with appropriate adaptations to the salt-water environment. They may be 12 or 24 volt depending on the type of system installed. Larger marine diesels may have two or more alternators to cope with the heavy electrical demand of a modern yacht. On single alternator circuits the power is split between the engine starting battery and the domestic battery (or batteries) by use of a split-charge diode or a mechanical switch. Because the alternator only produces power when running, engine control panels are typically fed directly from the alternator by means of an auxiliary terminal. Other typical connections are for charge control circuits[[1]].

Brushless Alternators

Terminology

The stationary part of a motor or alternator is called the stator and the rotating part is called the rotor. The coils of wire that are used to produce a magnetic field are called the field and the coils that produce the power are called the armature. These coils are commonly called the “windings”.

Construction

A brushless alternator is composed of two alternators built end-to-end on one shaft. Smaller brushless alternators may look like one unit but the two parts are readily identifiable on the large versions. The larger of the two sections is the main alternator 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 a plate attached to the rotor.
No brushes and slip rings are used which results in a more reliable alternator as the only parts subject to wear are the bearings.

Main Alternator

The main alternator has a rotating field as described above and a stationary armature (power generation windings).

Control System

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.

Automatic Voltage Regulator (AVR)

The AVR regulates the alternator's output voltage by varying the amount of current in the stationary exciter field coils.

Hybrid automobiles

Hybrid automobiles replace the separate alternator and starter motor with a combined motor/generator that performs both functions, cranking the internal combustion engine when starting, providing additional mechanical power for accelerating, and charging a large storage battery when the vehicle is running at constant speed. These rotating machines have considerably more powerful electronic devices for their control than the simple automotive alternator described above.

Radio alternators

Main article: Alexanderson alternator

Extending Tesla's work on high-frequency 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.

See also

• Electrical generator as in pre-1960 motor cars
• Linear alternator

References

• Thompson, Sylvanus P., Dynamo-Electric Machinery, A Manual for Students of Electrotechnics, Part 1, Collier and Sons, New York, 1902
• White, Thomas H.,"Alternator-Transmitter Development (1891-1920)". EarlyRadioHistory.us.

Alternators

• "Alternators". Integrated Publishing (TPub.com).
• "Wooden Low-RPM Alternator". ForceField, Fort Collins, Colorado, USA.
• "Understanding 3 phase alternators". WindStuffNow.
• Author unknown, "Alternator secrets". date unknown. Possibly from Lindsay Publications: "Lindsay Booklist, Updated 16 January 2008"
• "Alternator, Arc and Spark. The first Wireless Transmitters". The G0UTY Homepage.
• Tesla, Nikola, "The Ewing High-Frequency Alternator and Parson's Steam Engine". 12-17-1892. (Pepe's Tesla Pages, DOC)
• First practical alternating current with transformers system demonstrated (by William Stanley)