However, these machines usually exhibit high amount of sub-harmonic spatial fields, which will lead to high rotor eddy loss and low efficiency. In this paper, a tangential structure fractional-slot IPMSM with segmented magnets and multi-layer winding is proposed to reduce the sub-harmonics in magnetic field.
A field coil is an electromagnet used to generate a magnetic field in an electro-magnetic machine, typically a rotating electrical machine such as a motor or generator. It consists of a coil of wire through which a current flows.
In a rotating machine, the field coils are wound on an iron magnetic core which guides the magnetic field lines. The magnetic core is in two parts; a stator which is stationary, and a rotor, which rotates within it. The magnetic field lines pass in a continuous loop or magnetic circuit from the stator through the rotor and back through the stator again. The field coils may be on the stator or on the rotor.
The magnetic path is characterized by poles, locations at equal angles around the rotor at which the magnetic field lines pass from stator to rotor or vice versa. The stator (and rotor) are classified by the number of poles they have. Most arrangements use one field coil per pole. Some older or simpler arrangements use a single field coil with a pole at each end.
Although field coils are most commonly found in rotating machines, they are also used, although not always with the same terminology, in many other electromagnetic machines. These include simple electromagnets through to complex lab instruments such as mass spectrometers and NMR machines. Field coils were once widely used in loudspeakers before the general availability of lightweight permanent magnets (see Field coil loudspeaker for more).
Fixed and rotating fields[edit]
Most[note 1]DC field coils generate a constant, static field. Most three-phase AC field coils are used to generate a rotating field as part of an electric motor. Single-phase AC motors may follow either of these patterns: small motors are usually universal motors, like the brushed DC motor with a commutator, but run from AC. Larger AC motors are generally induction motors, whether these are three- or single-phase.
Stators and rotors[edit]
Many[note 1] rotary electrical machines require current to be conveyed to (or extracted from) a moving rotor, usually by means of sliding contacts: a commutator or slip rings. These contacts are often the most complex and least reliable part of such a machine, and may also limit the maximum current the machine can handle. For this reason, when machines must use two sets of windings, the windings carrying the least current are usually placed on the rotor and those with the highest current on the stator.
The field coils can be mounted on either the rotor or the stator, depending on whichever method is the most cost-effective for the device design.
In a brushed DC motor the field is static but the armature current must be commutated, so as to continually rotate. This is done by supplying the armature windings on the rotor through a commutator, a combination of rotating slip ring and switches. AC induction motors also use field coils on the stator, the current on the rotor being supplied by induction in a squirrel cage.
For generators, the field current is smaller than the output current.[note 2] Accordingly, the field is mounted on the rotor and supplied through slip rings. The output current is taken from the stator, avoiding the need for high-current sliprings. In DC generators, which are now generally obsolete in favour of AC generators with rectifiers, the need for commutation meant that brushgear and commutators could still be required. For the high-current, low-voltage generators used in electroplating, this could require particularly large and complex brushgear.
Bipolar and multipolar fields[edit]
Salient field bipolar generator | Consequent field bipolar generator |
Consequent field, four-pole, shunt-wound DC generator | Field lines of a four-pole stator passing through a Gramme ring or drum rotor. |
In the early years of generator development, the stator field went through an evolutionary improvement from a single bipolar field to a later multipole design.
Bipolar generators were universal prior to 1890 but in the years following it was replaced by the multipolar field magnets. Bipolar generators were then only made in very small sizes.[1]
The stepping stone between these two major types was the consequent-pole bipolar generator, with two field coils arranged in a ring around the stator.
This change was needed because higher voltages transmit power more efficiently over small wires. To increase the output voltage, a DC generator must be spun faster, but beyond a certain speed this is impractical for very large power transmission generators.
By increasing the number of pole faces surrounding the Gramme ring, the ring can be made to cut across more magnetic lines of force in one revolution than a basic two-pole generator. Consequently, a four-pole generator could output twice the voltage of a two-pole generator, a six-pole generator could output three times the voltage of a two-pole, and so forth. This allows output voltage to increase without also increasing the rotational rate.
In a multipolar generator, the armature and field magnets are surrounded by a circular frame or 'ring yoke' to which the field magnets are attached. This has the advantages of strength, simplicity, symmetrical appearance, and minimum magnetic leakage, since the pole pieces have the least possible surface and the path of the magnetic flux is shorter than in a two-pole design.[1]
Winding materials[edit]
Coils are typically wound with enamelled copper wire, sometimes termed magnet wire. The winding material must have a low resistance, to reduce the power consumed by the field coil, but more importantly to reduce the waste heat produced by ohmic heating. Excess heat in the windings is a common cause of failure. Owing to the increasing cost of copper, aluminium windings are increasingly used.
An even better material than copper, except for its high cost, would be silver as this has even lower resistivity. Silver has been used in rare cases. During World War II the Manhattan project to build the first atomic bomb used electromagnetic devices known as calutrons to enrich uranium. Thousands of tons of silver were borrowed from the U.S. Treasury reserves to build highly efficient low-resistance field coils for their magnets.[2][3]
See also[edit]
References[edit]
- ^ abField coils are found in a vast array of electrical machines and so any attempt to categorise them in a readable manner is likely to exclude some obscure examples.
- ^Strictly it is the output power that is greater than the field power, although in practice this usually implies that the current is greater too.
- ^ abHawkins Electrical Guide, Volume 1, Copyright 1917, Theo. Audel & Co., Chapter 14, Classes of Dynamo, page 182
- ^'The Silver Lining of the Calutrons'. ORNL Review. Oak Ridge National Lab. 2002. Archived from the original on 2008-12-06.
- ^Smith, D. Ray (2006). 'Miller, key to obtaining 14,700 tons of silver Manhattan Project'. Oak Ridger. Archived from the original on 2007-12-17.
In electrical engineering, an armature is the component of an electric machine which carries alternating current.[1] The armature windings conduct AC even on DC machines, due to the commutator action (which periodically reverses current direction) or due to electronic commutation, as in brushless DC motors. The armature can be on either the rotor (rotating part) or the stator (stationary part), depending on the type of electric machine.
The armature windings interact with the magnetic field (magnetic flux) in the air-gap; the magnetic field is generated either by permanent magnets, or electromagnets formed by a conducting coil.
The armature 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 twofold. The first is to carry current across 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 or motor is used as a motor, this EMF opposes the armature current, and the armature converts electrical power to mechanical power in the form of torque, and transfers it via the shaft. When the machine is used as a generator, the armature EMF drives the armature current, and the shaft's movement is converted to electrical power. In an induction generator, generated power is drawn from the stator.
A growler is used to check the armature for short and open circuits and leakages to ground.
Terminology[edit]
The word armature was first used in its electrical sense, i.e. keeper of a magnet, in mid 19th century.[2]
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 like brushless alternators, or in conversation among people who are accustomed to work with differently configured machinery.
In most generators, the field magnet is rotating, and is part of the rotor, while the armature is stationary, and is part of the stator.[3] Both motors and generators can be built either with a stationary armature and a rotating field or a rotating armature and a stationary field. 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.
Armature reaction in a DC machine[edit]
In a DC machine, two sources of magnetic fluxes are present; 'armature flux' and 'main field flux'. The effect of armature flux on the main field flux is called 'armature reaction'. The armature reaction changes the distribution of the magnetic field, which affects the operation of the machine. The effects of the armature flux can be offset by adding a compensating winding to the main poles, or in some machines adding intermediate magnetic poles, connected in the armature circuit.
Armature reaction is essential in amplidyne rotating amplifiers.
Armature reaction drop is the effect of a magnetic field on the distribution of the flux under main poles of a generator.[4]
Since an armature is wound with coils of wire, a magnetic field is set up in the armature whenever a current flows in the coils. This field is at right angles to the generator field and is called cross magnetization of the armature. The effect of the armature field is to distort the generator field and shift the neutral plane. The neutral plane is the position where the armature windings are moving parallel to the magnetic flux lines, that is why an axis lying in this plane is called as magnetic neutral axis (MNA).[5] This effect is known as armature reaction and is proportional to the current flowing in the armature coils.
The geometrical neutral axis (GNA) is the axis that bisects the angle between the centre line of adjacent poles. The magnetic neutral axis (MNA) is the axis drawn perpendicular to the mean direction of the flux passing through the centre of the armature. No e.m.f. is produced in the armature conductors along this axis because then they cut no flux.[6] When no current is there in the armature conductors, the MNA coincides with GNA.
The brushes of a generator must be set in the neutral plane; that is, they must contact segments of the commutator that are connected to armature coils having no induced emf. If the brushes were contacting commutator segments outside the neutral plane, they would short-circuit 'live' coils and cause arcing and loss of power.
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Without armature reaction, the magnetic neutral axis (MNA) would coincide with geometrical neutral axis (GNA). Armature reaction causes the neutral plane to shift in the direction of rotation, and if the brushes are in the neutral plane at no load, that is, when no armature current is flowing, they will not be in the neutral plane when armature current is flowing. For this reason it is desirable to incorporate a corrective system into the generator design.
These are two principal methods by which the effect of armature reaction is overcome. The first method is to shift the position of the brushes so that they are in the neutral plane when the generator is producing its normal load current. in the other method, special field poles, called interpoles, are installed in the generator to counteract the effect of armature reaction.
The brush-setting method is satisfactory in installations in which the generator operates under a fairly constant load. If the load varies to a marked degree, the neutral plane will shift proportionately, and the brushes will not be the correct position at all times. The brush-setting method is the most common means of correcting for armature reaction in small generators (those producing approximately 1000 W or less). Larger generators require the use of interpoles.
Winding circuits[edit]
Coils of the winding are distributed over the entire surface of the air gap, which may be the rotor or the stator of the machine. In a 'lap' winding, there are as many current paths between the brush (or line) connections as there are poles in the field winding. In a 'wave' winding, there are only two paths, and there are as many coils in series as half the number of poles. So, for a given rating of machine, a wave winding is more suitable for large currents and low voltages.[7]
Windings are held in slots in the rotor or armature covered by stator magnets. The exact distribution of the windings and selection of the number of slots per pole of the field greatly influences the design of the machine and its performance, affecting such factors as commutation in a DC machine or the waveform of an AC machine.
Winding materials[edit]
Armature wiring is made from copper or aluminum. Copper armature wiring enhances electrical efficiencies due to its higher electrical conductivity. Aluminum armature wiring is lighter and less expensive than copper.
See also[edit]
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References[edit]
- ^Stephen D. Umans, Fitzgerald's and Kingsley's Electric Machinery - 7th ed, McGraw Hill, 2014, ISBN978-0-07-338046-9, pp. 190
- ^'armature'. definition of armature in English from the Oxford dictionary. Retrieved July 17, 2015.
- ^'Basic AC electrical generators'(PDF). American Society of Power Engineers. Archived from the original(PDF) on 2016-03-03. Retrieved 2016-01-02.Cite journal requires
|journal=(help) - ^A.Van Valkenburgh (1993). Basic Electricity. Thomson Delmar Learning. ISBN978-0-7906-1041-2.
- ^Armature reaction in DC machines, | electricaleasy.com
- ^'Armature Reaction in DC Generator'. www.studyelectrical.com. September 2014. Retrieved 2018-11-09.
- ^Gordon R. Slemon, Magnetoelectric Devices: Transducers, Transformers and Machines, John Wiley and Sons, 1966, no ISBN, pp. 248-249