Engineering Article

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 electrical energy in the form of alternating current.
Most alternators use a rotating magnetic field with a stationary armature but occasionally, a rotating armature is used with a stationary magnetic field; or a linear alternator is used.
In principle, any AC electrical generator can be called an alternator, but usually the term refers to small rotating machines driven by automotive and other internal combustion engines. 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.

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 - each active conductor passed successively through regions where the magnetic field was in opposite directions.[1] The first public demonstration of a more robust "alternator system" took place in 1886.[2] 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 Hz. In 1891, Nikola Tesla patented a practical "high-frequency" alternator (which operated around 15 kHz).[3] After 1891, polyphase alternators were introduced to supply currents of multiple differing phases.[4] 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.[5]

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.
Alternators generate electricity using 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 induced EMF (electromotive force), as the mechanical input causes the rotor to turn.
The rotating magnetic field induces an AC voltage in the stator windings. 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.
The rotor's magnetic field may be produced by induction (as in a "brushless" alternator), by permanent magnets (as in very small machines), or by a rotor winding energized with direct current through slip rings and brushes. The rotor's magnetic field may even be provided by stationary field winding, with moving poles in the rotor. Automotive alternators invariably use a rotor winding,[citation needed] 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 machines than those used in automotive applications.
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.
Alternators used in central power stations may also control the field current to regulate reactive power and to help stabilize the power system against the effects of momentary faults.

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[6] gives some examples:
Poles RPM for 50 Hz RPM for 60 Hz RPM for 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
Key
More generally, 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 (RPM). 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.

Automotive alternators


Alternator mounted on an automobile engine with a serpentine belt pulley

Cut-away of an alternator, showing the claw-pole construction; two of the wedge-shaped field poles, alternating N and S, are visible in the centre and the stationary armature winding is visible at the top and bottom of the opening. The belt and pulley at the right hand end drives the alternator.
Alternators are used in modern automobiles to charge the battery and to power the electrical system when its engine is running. Until the 1960s, automobiles used DC dynamo generators with commutators. With the availability of affordable silicon diode rectifiers, alternators were used instead. They were first introduced by the Chrysler Corporation on the Valiant in 1960, several years ahead of Ford and GM. Alternators have several advantages over direct-current generators. They are lighter, cheaper and more rugged. They use slip rings providing greatly extended brush life over a commutator. The brushes in an alternator carry only excitation current, a small fraction of the current carried by the brushes of a DC generator, which carry the generator's entire output. A set of rectifiers (diode bridge) is required to convert AC to DC. To provide direct current with low ripple, a three-phase winding is used and the pole-pieces of the rotor are shaped (claw-pole) to produce a waveform similar to a square wave instead of a sinusoid. Automotive alternators are usually belt driven at 2-3 times crankshaft speed. The alternator runs at various RPM (which varies the frequency) since it is driven by the engine. This is not a problem because the alternating current is rectified to direct current.
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 like fingers of two hands interlocked with each other. Larger vehicles may have salient-pole alternators similar to larger machines. There are two separate types of alternators: the Delta set-up and the Wye[7] set-up.
Automotive alternators require a voltage regulator which operates by modulating the small field current to produce a constant voltage at the battery terminals. Early designs (c.1960s-1970s) used a discrete device mounted elsewhere in the vehicle. Intermediate designs (c.1970s-1990s) incorporated the voltage regulator into the alternator housing. Modern designs do away with the voltage regulator altogether; voltage regulation is now a function of the electronic control unit (ECU). The field current is much smaller than the output current of the alternator; for example, a 70 A alternator may need only 7 A of field current. The field current is supplied to the rotor windings by slip rings. The low current and relatively smooth slip rings ensure greater reliability and longer life than that obtained by a DC generator with its commutator and higher current being passed through its brushes.
The field windings are supplied power from the battery via the ignition switch and regulator. A parallel circuit supplies the "charge" warning indicator and is earthed via the regulator.(which is why the indicator is on when the ignition is on but the engine is not running). Once the engine is running and the alternator is generating power, a diode feeds the field current from the alternator main output equalizing the voltage across the warning indicator which goes off. The wire supplying the field current is often referred to as the "exciter" wire. The drawback of this arrangement is that if the warning lamp burns out or the "exciter" wire is disconnected, no current reaches the field windings and the alternator will not generate power. Some warning indicator circuits are equipped with a resistor in parallel with the lamp that permit excitation current to flow if the warning lamp burns out. The driver should check that the warning indicator is on when the engine is stopped; otherwise, there might not be any indication of a failure of the belt which may also drive the cooling water pump. Some alternators will self-excite when the engine reaches a certain speed.
Older automobiles with minimal lighting may have had an alternator capable of producing only 30 A. Typical passenger car and light truck alternators are rated around 50-70 A, though higher ratings are becoming more common, especially as there is more load on the vehicle's electrical system with air conditioning, electric power steering and other electrical systems. Very large alternators used on buses, heavy equipment or emergency vehicles may produce 300 A. Semi-trucks usually have alternators which output 140 A. Very large alternators may be water-cooled or oil-cooled.
In recent years, alternator regulators are linked to the vehicle's computer system and various factors including air temperature obtained from the intake air temperature sensor, battery temperature sensor and engine load are evaluated in adjusting the voltage supplied by the alternator.
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 partial load efficiency is between 50-62% depending on the size of alternator and varies with alternator speed.[8] This is similar to very small high-performance permanent magnet alternators, such as those used for bicycle lighting systems, which achieve an efficiency around 60%. Larger permanent magnet alternators can achieve higher efficiencies.[citation needed] Large AC generators used in power stations run at carefully controlled speeds and have no constraints on size or weight. They have much higher efficiencies, as high as 98%.
Hybrid automobiles replace the separate alternator and starter motor with one or more combined motor/generator(s) (M/Gs) that start the internal combustion engine, provide some or all of the mechanical power to the wheels, and charge a large storage battery. When more than one M/G is present, as in the Hybrid Synergy Drive used in the Toyota Prius and others, one may operate as a generator and feed the other as a motor, providing an electromechanical path for some of the engine power to flow to the wheels. These motor/generators have considerably more powerful electronic devices for their control than the automotive alternator described above.

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, a key principle to the operation of an alternating-current induction motor. In 1885, Galileo Ferraris built a working model of a rotating magnetic field induction motor and published his research in a paper to the Royal Academy of Sciences in Turin in 1888. Nikola Tesla used the same concept in his induction motor patented in 1888[9], an idea he claimed to have conceived in 1882.

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 two coils, but three or more are usually used. 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 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.

Diesel electric locomotive alternators

In diesel electric locomotives, and in diesel electric multiple units, a diesel engine turns an alternator which in turn provides electricity for the traction motors (ac or dc) and, optionally, the head end power (HEP), however, the HEP option requires a constant engine speed, 900 rpm for a 480 volt 60 Hz HEP application, even when the locomotive is not moving.
The traction alternator usually incorporates integral silicon diode rectifiers to provide the traction motors with 1200 volts dc (dc traction, which is used directly) or the common inverter bus (ac traction, which is first inverted from dc to three-phase ac).
Although ac traction motors are emerging for very heavy drag service, particularly in western North America, and its supply of Powder River Basin coal to other areas of the U.S., the simpler dc traction motor system remains the most popular, with Union Pacific alone having over 1,500 current model SD70Ms with dc traction motors (plus an additional 1,000 SD70MACs with ac traction motors). Both of these models share an ac traction alternator with integral rectification, as the common bus is 1200 volts dc, from which dc traction motors may be directly powered, or ac traction motors may be powered through inverters.

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 explosion proof so that brush sparking will not ignite explosive gas mixtures in an engine room 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 or house battery (or batteries) by use of a split-charge diode (battery isolator) 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.

Brushless alternators

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. Neither brushes nor slip rings are used, which reduces the number of wearing parts. The main alternator has a rotating field as described above 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 of all this is that a small DC exciter current indirectly controls the output of the main alternator.

Radio alternators

Main article: Alexanderson alternator
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.

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