The stator is the stationary part of a rotor system, found in an electric generator, electric motor and biological rotors. Depending on the configuration of a spinning electromotive device the stator may act as the field magnet, interacting with the armature to create motion, or it may act as the armature, receiving its influence from moving field coils on the rotor. The first DC generators (known as dynamos) and DC motors put the field coils on the stator, and the power generation or motive reaction coils on the rotor.This was necessary because a continuously moving power switch known as the commutator is needed to keep the field correctly aligned across the spinning rotor. The commutator must become larger and more robust as the current increases. The stator of these devices may be either a permanent magnet or an electromagnet.
Where the stator is an electromagnet, the coil which energizes it is known as the field coil or field winding. DC Armature (electrical engineering)In electrical engineering, an armature generally refers to one of the two principal electrical components of an electromechanical machine–generally in a motor or generator, but it may also mean the pole piece of a permanent magnet or electromagnet, or the moving iron part of a solenoid or relay. The other component is the field winding or field magnet. The role of the “field” component is simply to create a magnetic field (magnetic flux) for the armature to interact with, so this component can comprise either permanent magnets, or electromagnets formed by a conducting coil.The armature, in contrast, must carry current so it is always a conductor or a conductive coil, oriented normal to both the field and to the direction of motion, torque (rotating machine), or force (linear machine). The armature’s role is two-fold. The first is to carry current crossing the field, thus creating shaft torque in a rotating machine or force in a linear machine. The second role is to generate an electromotive force (EMF).
In the armature, an electromotive force is created by the relative motion of the armature and the field.When the machine is acting as a motor, this EMF opposes the armature current, and the armature converts electrical power to mechanical torque, and power, unless the machine is stalled, and transfers it to the load via the shaft. When the machine is acting as a generator, the armature EMF drives the armature current, and shaft mechanical power is converted to electrical power and transferred to the load. In an induction generator, these distinctions are blurred, since the generated power is drawn from the stator, which would normally be considered the field.Terminology The parts of an alternator or related equipment can be expressed in either mechanical terms or electrical terms. Although distinctly separate, these two sets of terminology are frequently used interchangeably or in combinations that include one mechanical term and one electrical term. This may cause confusion when working with compound machines such as brushless alternators, or in conversation among people who are accustomed to work with differently configured machinery.
In alternating current machines, the armature is usually stationary, and is known as the stator winding. In DC rotating machines other than brushless DC machines, it is usually rotating, and is known as the rotor. The pole piece of a permanent magnet or electromagnet and the moving, iron part of a solenoid, especially if the latter acts as a switch or relay, may also be referred to as armatures. Mechanical Rotor: The rotating part of an alternator, generator, dynamo or motor. Stator: The stationary part of an alternator, generator, dynamo or motor Electrical Armature: The power-producing component of an alternator, generator, dynamo or motor. The armature can be on either the rotor or the stator. Field: The magnetic field component of an alternator, generator, dynamo or motor. The field can be on either the rotor or the stator and can be either an electromagnet or a permanent magnet.
Relation of “armature” to usage in sculpture A sculpture with a medium that is not self-supporting, or requires reinforcement, is supported by armature[s], which are rigid frameworks.Armatures in electrical machines support windings of insulated wire. Considering that in wound-rotor machines the armature moves, by extension, moving parts attracted by stationary electromagnets are sometimes called armatures. Armature reaction in a DC machine In a DC machine, the main field is produced by field coils. In both the generating and motoring modes, the armature carries current and a magnetic field is established, which is called the armature flux. The effect of armature flux on the main field is called the armature reaction.The armature reaction: 1.
demagnetizes the main field, and 2. cross magnetizes the main field. The demagnetizing effect can be overcome by adding extra ampere-turns on the main field.
The cross magnetizing effect can be reduced by having common poles. Armature reaction is essential in Amplidyne™ rotating amplifiers Alternator 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 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. Alternators in power stations driven by steam turbines are called turbo-alternators.
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 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 induced EMF, 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 three phase currents, displaced by one-third of a period with respect to each other. Automotive alternators Alternators are used in modern 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, and the slip rings allow for greatly extended brush life.The stronger construction of automotive alternators allows them to use a smaller pulley so as to turn faster than the engine, improving output when the engine is idling. The availability of low-cost solid-state diodes from 1960 onward allowed car manufacturers to substitute alternators for DC generators (major American manufacturers had made the transition to alternators by 1962, for example).
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.In addition, the pole-pieces of the rotor are shaped (claw-pole) so as to produce a voltage waveform closer to a square wave that, when rectified by the diodes, produces even less ripple than the rectification of three-phase sinusoidal voltages. 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. The automotive alternator is usually belt driven at 2-3 times the engine crankshaft speed.Automotive alternators are not restricted to a certain RPM because the alternating current is rectified to direct current and need not be any constant frequency. 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. The field current is supplied to the rotor windings by slip rings and brushes.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 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 excitation current reaches the alternator field windings and so the alternator, due to low residual magnetism in the rotor, will not generate any power. However, some alternators will self-excite when the engine is revved to a certain speed. Also, some warning light circuits are equipped with a resistor in parallel with the warning light that will permit excitation current to flow even if the warning light fails.The driver should check that the warning light is glowing when the engine is stopped; otherwise, there might not be any indication of a failure of the alternator drive belt which normally also drives the cooling water pump.
Typical passenger car and light truck alternators are rated around 50-70 amperes, though higher ratings are becoming more common, especially as there is more load on the vehicle’s electrical system with, for example, the introduction of electric power steering systems.Many alternator voltage regulators are today linked to the vehicle’s onboard computer system and, in recent years, other factors including air temperature (obtained from the intake air temperature sensor or battery temperature sensor in many cases) and engine load are considered in adjusting the battery charging voltage supplied by the alternator. 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) 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.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 in central power station use may also control the field current to regulate reactive power and to help stabilize the power system against the effects of momentary faults. Electrical generator In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. The reverse conversion of electrical energy into mechanical energy is done by a motor; motors and generators have many similarities.
A generator forces electrons in the windings to flow through the external electrical circuit. It is somewhat analogous to a water pump, which creates a flow of water but does not create the water inside. The source of mechanical energy may be a reciprocating or turbine steam engine, water falling through a turbine or waterwheel, an internal combustion engine, a wind turbine, a hand crank, compressed air or any other source of mechanical energy. Terminology The two main parts of a generator or motor can be described in either mechanical or electrical terms: Mechanical: Rotor: The rotating part of an electrical machine * Stator: The stationary part of an electrical machine Electrical: * Armature: The power-producing component of an electrical machine.
In a generator, alternator, or dynamo the armature windings generate the electric current. The armature can be on either the rotor or the stator. * Field: The magnetic field component of an electrical machine.
The magnetic field of the dynamo or alternator can be provided by either electromagnets or permanent magnets mounted on either the rotor or the stator.Because power transferred into the field circuit is much less than in the armature circuit, AC generators nearly always have the field winding on the rotor and the stator as the armature winding. Only a small amount of field current must be transferred to the moving rotor, using slip rings. Direct current machines (dynamos) require a commutator on the rotating shaft to convert the alternating current produced by the armature to direct current, so the armature winding is on the rotor of the machine.Vehicle-mounted generators Early motor vehicles until about the 1960s tended to use DC generators with electromechanical regulators. These have now been replaced by alternators with built-in rectifier circuits, which are less costly and lighter for equivalent output.
Automotive alternators power the electrical systems on the vehicle and recharge the battery after starting. Some cars now have electrically-powered steering assistance and air conditioning, which places a high load on the electrical system.Vehicle alternators do not use permanent magnets and are typically only 50-60% efficient over a wide speed range. Engine-generator An engine-generator is the combination of an electrical generator and an engine (prime mover) mounted together to form a single piece of self-contained equipment. The engines used are usually piston engines, but gas turbines can also be used. Many different versions are available – ranging from very small portable petrol powered sets to large turbine installations.Bibliographyhttp://en.wikipedia.org/wiki/Armature_(electrical_engineering) http://en.wikipedia.org/wiki/Statorhttp://en.wikipedia.org/wiki/Alternatorhttp://en.wikipedia.org/wiki/Electric_generatorScott James