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Modern (c. 2009) low-cost universal motor, from a vacuum cleaner. Field windings are dark copper colored, toward the back, on both sides. The rotor's laminated core is gray metallic, with dark slots for winding the coils. The commutator (partly hidden) has become dark from use; it's toward the front. The large brown molded-plastic piece in the foreground supports the brush guides and brushes (both sides), as well as the front motor bearing.

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 the field 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.

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-phase 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

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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]
Main article: Electromagnetic coil

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 resistive heating. Excess heat in the windings is a common cause of failure. Owing to the increasing cost of copper, aluminium windings are increasingly used.[citation needed]

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]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Field coil

View on Grokipedia
from Grokipedia
A field coil is an composed of multiple turns of insulated wire wound around a ferromagnetic core, through which a (DC) is passed to generate a controlled . These coils are essential components in (DC) electric motors and generators, where they produce the stationary or rotating that interacts with the armature windings to generate in motors or electrical output in generators. Typically mounted on the poles, field coils enable precise control of machine performance by adjusting the field strength via current variation. Field coils function as electromagnets, where the DC excitation current creates a density ranging from 0.6 to 0.9 Wb/m² in typical designs, directing the flux through air gaps and armature cores. They dissipate less than 1% of the machine's rated power as heat, making them efficient for sustained operation. Common configurations include shunt field coils (connected in parallel with the armature for constant ), series field coils (in series with the armature for varying field with load), separately excited coils (powered by an independent DC source for maximum flexibility), and compound windings (combining shunt and series for balanced characteristics). In addition to main field coils, auxiliary elements like interpoles—smaller coils placed between main poles—improve commutation by neutralizing armature reaction and reducing sparking at the brushes. Field coils are constructed from wire insulated with materials like enamel or , wound on laminated cores to minimize losses, and are critical in applications such as industrial drives, traction systems, and legacy power generation equipment. While modern permanent magnet machines have partially supplanted them, field coils remain vital for adjustable-speed and high-power DC systems due to their ability to dynamically alter .

Fundamentals

Definition and Principle

A field coil is a coil of insulated wire wound around a core, typically composed of iron or other ferromagnetic , that generates a when an electric current flows through it; it is used primarily in electric machines to produce the necessary for operation. These coils function as electromagnets, concentrating within the core to enhance compared to air-core coils. The basic principle of a field coil relies on Ampère's law, which relates the to the producing it, and , which governs the interaction of changing s with conductors in machines. For a solenoid-like field coil, the magnetic field strength HH inside the coil is given by H=nIH = n I, where nn is the number of turns per unit length and II is the current; the magnetic flux density BB is then B=μHB = \mu H, with μ\mu as the permeability of the core material. This relationship allows the field to be precisely controlled by adjusting the current, enabling variable flux in response to operational needs. Unlike permanent magnets, which provide a fixed inherent to their material properties, field coils offer controllable through current variation, facilitating adjustable performance in devices. Field coils are commonly integrated into stators or rotors to establish the primary for electromechanical conversion. In a simple cross-section of a solenoid-like field coil, the lines form closed loops that pass longitudinally through the core, emerging from one end and returning externally to the other, with the ferromagnetic core guiding and intensifying the flux lines for efficient field production.

Historical Development

The concept of field coils emerged from foundational 19th-century advancements in , building on William Sturgeon's invention of the in 1825, which utilized a coil of wire wrapped around an iron core to amplify magnetic fields when energized. This device provided the basic structure for generating controllable magnetic fields in electrical apparatus. Michael Faraday's experiments in 1831 further advanced the field by demonstrating through rotating coils in magnetic fields, laying the groundwork for dynamic electrical machines and highlighting the potential of coiled conductors to produce and interact with . Early electrical generators, such as Hippolyte Pixii's in 1832, relied on permanent magnets to create the necessary , but limitations in magnet strength prompted the transition to electromagnets. In 1864, Henry Wilde pioneered the use of field coils by replacing permanent magnets with energized electromagnets in a , allowing for stronger and adjustable fields powered by a separate exciter. This was refined in 1866 by , who developed the first self-excited generator, where the machine's own output current energized the field coils via residual magnetism, enabling efficient, self-sustaining operation without external power sources. These innovations marked the practical adoption of field coils in DC generators, evolving from bipolar to multipolar arrangements for increased power output. By the late , field coils facilitated the shift from DC to AC machines, as seen in synchronous generators where rotor field coils produced rotating magnetic fields interacting with stator windings. The early saw widespread integration of field coil-based motors and generators in industrial applications, powering factories and transportation systems amid the boom. Refinements continued during , with field coils employed in robust DC generators and servomechanisms for military equipment, including radar systems requiring stable magnetic fields for precise control. In the up to 2025, while rare-earth permanent magnets have largely supplanted field coils in compact, high-efficiency applications like electric vehicles due to their superior , field coils remain essential in large-scale, high-power systems such as utility-scale synchronous generators and traction motors, where adjustable fields and cost-effectiveness in bulk production are prioritized.

Configurations

Fixed versus Rotating Fields

In electric machines, fixed field coils are electromagnets mounted on stationary components, such as the , to generate a with a constant spatial orientation relative to the machine's frame. This configuration simplifies design by eliminating the need for electrical connections to for the field excitation, reducing wear from mechanical contacts and avoiding stresses due to . For instance, in DC machines, field coils are typically placed on the stator poles, providing a steady (DC) field that interacts with the rotating armature to produce . Rotating field coils, in contrast, are positioned on the , where they physically rotate with the machine's moving element while carrying DC current to maintain the field. Supplying current to these coils necessitates slip rings and brushes, introducing challenges such as electrical sparking, maintenance requirements, and exposure to centrifugal forces that demand robust mechanical retention, like wedges or rings, to prevent deformation at high speeds. A common example is the rotor field coils in synchronous machines, where DC excitation creates a that locks with the stator's for synchronous operation. Comparing the two, fixed field coils suit applications emphasizing reliability and ease of excitation, particularly in DC systems or small AC machines where the field remains stationary while the armature rotates. Rotating field coils, however, are preferred in large synchronous generators for handling high voltages on the stationary armature, though they require careful design to manage rotational stresses. A key design consideration in both configurations is production, which arises from the acting on current-carrying conductors in the . The force on a conductor of length LL carrying current II in field BB () is given by: F=IL×B\mathbf{F} = I \mathbf{L} \times \mathbf{B} with magnitude F=ILBF = I L B when the angle is 90 degrees, driving the relative motion between field and armature. This interaction ensures stable field orientation in fixed setups for consistent , while in rotating setups, it synchronizes the field's motion with the to maintain phase alignment.

Integration with Stators and Rotors

In electric machines such as DC motors and generators, field coils are integrated into the by winding them directly around the salient poles of the core, creating a stationary that interacts with the rotating armature on the . This fixed positioning of the field coils allows for straightforward electrical connections via terminals to a DC excitation source, enabling stable field strength without the need for rotary electrical interfaces. In contrast, synchronous machines typically place field coils on the to produce a rotating relative to the stator's multi-phase armature windings, which generate the overall rotating field through AC currents displaced by 120 electrical degrees. For rotor integration in synchronous machines, field coils are mounted on the rotor core—either concentrated on salient poles for low-speed applications like hydroelectric generators or distributed in slots for high-speed turbo-generators— and supplied with DC excitation current through slip rings and brushes mounted on the rotor shaft. This setup requires managing rotational electromotive force (EMF) in the field windings, which is minimized by the DC supply but can induce minor asymmetries; additionally, brush contact resistance at the slip rings must be controlled to limit voltage drops and heating, typically kept below 2 volts through proper maintenance. The interplay between stator and rotor components introduces armature reaction, where currents in the armature windings (on the stator in synchronous machines or rotor in DC machines) produce a cross-magnetizing field that distorts the main field flux, shifting the magnetic neutral plane and potentially causing commutation issues or reduced efficiency. In DC machines, compensation techniques, such as interpoles—small auxiliary poles placed between main poles and wound in series with the armature to generate an opposing field proportional to armature current—neutralize these effects, ensuring linear commutation and minimizing sparking at the brushes. Assembly of field coils involves precise techniques to ensure mechanical integrity and electrical isolation, including slot insulation where coils are placed in or slots, using materials like fish paper or to prevent short circuits between windings and the laminated core. Coil pitching, the span of the coil sides within slots or around poles, is selected as full-pitch (180 electrical degrees) for maximum in concentrated windings on salient poles or fractional-pitch to reduce harmonics and end-winding length in distributed configurations. End-winding supports, such as blocking and bracing with insulated wedges or impregnation, are essential to secure the overhanging portions of the coils against vibrational forces during operation, thereby preventing fatigue and insulation degradation.

Field Arrangements

Bipolar Configurations

Bipolar configurations of field coils produce a with a single pair of north and south poles, typically achieved using one or two coils wound on salient pole structures. These setups generate field lines that form closed loops traversing the air gap between the and , establishing a uniform distribution essential for basic electromagnetic interactions in the . In design, the coils employ concentrated windings directly on the pole pieces to maximize flux linkage, with the magnetomotive force (MMF) given by F=NI\mathcal{F} = NI, where NN represents the total number of turns and II is the excitation current in amperes. This MMF drives the magnetic circuit, concentrating the field within the poles for efficient energy transfer across the air gap. The resulting flux per pole is Φ=B×A\Phi = B \times A, where BB is the magnetic flux density and AA is the effective pole face area, ensuring a straightforward path for the magnetic circuit in simple geometries. Such configurations yield a relatively uniform field in small air gaps, minimizing distortions and supporting stable operation under moderate loads. They are particularly suitable for low-speed, high- applications where the simpler pole arrangement allows for robust torque production without the need for complex management. However, limitations arise at higher speeds due to intensified armature reaction, which shifts the magnetic and induces commutation sparking from elevated reactance voltages in the armature coils. Representative examples include early brushed DC motors, which relied on two-pole field coils for foundational demonstrations of rotary motion, and simple synchronous alternators used in low-power generation systems. These applications highlight the configuration's role in foundational electric machinery, though modern designs often extend to multipolar setups for enhanced performance.

Multipolar Configurations

Multipolar configurations of field coils employ multiple pairs of north and south magnetic poles, such as four or six poles, arranged around the or with concentrated windings on each pole to generate the excitation field. This setup reduces the pole pitch compared to bipolar designs, leading to a more uniform distribution across the air gap and enhanced efficiency in the machine. In design, the field coils are interconnected in series or parallel across the poles, ensuring the total (MMF) is distributed proportionally to produce balanced flux per pole. The synchronous speed of the is determined by the n=120fpn = \frac{120 f}{p}, where nn is the speed in , ff is the supply in hertz, and pp is the number of poles; increasing the number of poles lowers the speed for a given frequency, facilitating operation at reduced rotational rates. These configurations enable higher effective frequency operation at lower mechanical speeds, yielding smoother output due to the finer of the and making them suitable for high-power, high-speed applications. Challenges arise in coil balancing, as uneven current distribution or manufacturing variations can cause imbalances, leading to vibrations or reduced efficiency, necessitating precise excitation control. Multipolar field coils are exemplified in large synchronous generators for hydroelectric plants, where 20 or more poles accommodate low-speed turbines, and in traction motors for rail systems, representing an from simpler bipolar setups to achieve greater power scaling and output capacity.

Construction and Materials

Winding Materials and Techniques

Field coils are typically wound using wire due to its superior electrical conductivity, characterized by a resistivity of ρ=1.68×10−8 Ω⋅m\rho = 1.68 \times 10^{-8} \, \Omega \cdot \mathrm{m} at 20°C, which minimizes resistive losses and enables efficient current flow to generate strong magnetic fields. Aluminum serves as a lighter and more cost-effective alternative, though it exhibits approximately 61% of copper's conductivity, necessitating larger cross-sections to achieve comparable performance. Core materials for field coils prioritize high magnetic permeability and low eddy current losses; laminated silicon steel is commonly used, with thin sheets (typically 0.23–0.35 mm thick) stacked to interrupt eddy current paths while providing a relative permeability μr\mu_r up to 5000, facilitating efficient flux concentration. Soft iron cores offer a high saturation flux density of approximately 2 T, allowing the coil to operate near maximum magnetic intensity before nonlinearity sets in, though lamination is essential to mitigate hysteresis and eddy losses. Winding techniques for field coils include helical winding, where wire is coiled in a continuous spiral to form uniform turns that maximize uniformity along the axis, and layered winding, which stacks multiple helical layers for higher turn counts in compact designs. Optimizing the pitch factor reduces end-turn overhang, minimizing material use and inter-turn while improving field . Automatic winding machines are preferred for precision in production, ensuring consistent tension, turn spacing, and layer alignment compared to hand-winding, which suits prototypes but risks variability in high-volume applications. Key performance metrics include coil resistance, calculated as R=ρL/AR = \rho L / A where ρ\rho is resistivity, LL is total wire length, and AA is cross-sectional area, which directly influences and efficiency. Power losses arise primarily from I2RI^2 R heating, where II is current, proportional to the square of the operating current and necessitating effective cooling to prevent degradation of materials.

Insulation, Cooling, and Assembly

Field coils in electric machines require robust insulation to withstand electrical stresses, thermal cycling, and mechanical vibrations. High-temperature insulation systems, typically Class F (155°C maximum ) or Class H (180°C), are employed to ensure longevity and reliability under demanding conditions. These systems often incorporate combined with synthetic for their superior properties and resistance to partial discharges. provides exceptional endurance against electrical erosion, while enhances and resistance. Groundwall insulation, which separates the coil from the core or frame, typically ranges from 2 to 4 mm in thickness depending on voltage ratings, ensuring adequate barrier against ground faults. testing, such as high-potential (hi-pot) or surge tests, verifies the insulation's ability to endure voltages up to 1 kV or more without breakdown, following procedures outlined in IEEE standards for form-wound coils. Effective cooling is essential to prevent overheating in field coils, particularly in high-power applications where current densities can exceed 5 A/mm². Natural air suffices for low-power designs, relying on ambient to dissipate heat generated by I²R losses. For higher ratings, cooling via fans or blowers increases rates, while liquid cooling—using or —circulates through channels or jackets surrounding the coils to manage elevated loads. International standards, such as IEC 60034-1, limit rises to 105 K for Class F insulation in indirectly air-cooled rotating machines, measured by resistance or embedded sensors to maintain hotspot temperatures below critical thresholds and extend insulation life. Assembly processes finalize the structural integrity of field coils post-winding. Varnish impregnation, often via vacuum pressure impregnation (VPI), encapsulates the coil to eliminate voids, enhance mechanical rigidity, and improve thermal conductivity by filling interstices with or resins. In slotted configurations, coils are secured using banding—high-strength tapes or wires wrapped around the coil ends—and wedging, where insulating wedges are driven into slots to prevent radial movement under centrifugal forces. includes surge comparison testing, which applies high-voltage pulses to detect shorted turns by comparing waveforms across coils; deviations indicate insulation weaknesses or faults. This non-destructive method aligns with IEEE recommendations for turn-to-turn insulation integrity. Maintenance of field coil assemblies focuses on preserving performance over time, especially in rotating setups. In machines with slip rings, carbon brushes deliver excitation current to the rotating field; regular inspection for wear—typically 6-12 mm remaining length—prevents arcing and ensures consistent contact, with replacement intervals based on operating hours and dust accumulation. Field weakening, achieved by inserting rheostats in series with the shunt field circuit, reduces flux to extend speed range in DC machines, though it demands monitoring to avoid excessive armature reaction. Periodic and checks, per IEEE guidelines, help identify degradation early.

Applications and Comparisons

Use in Electric Machines

In direct current (DC) machines, field coils serve as the primary means of excitation, generating the stationary essential for the operation of the armature and proper commutation of the current. These coils are wound around the stator poles and energized with to produce the required flux density. Configurations vary to suit specific performance needs: series field windings, connected in series with the armature, deliver high starting but variable speed under load; shunt field windings, connected in parallel with the armature, maintain relatively constant speed; and compound windings combine both series and shunt arrangements for balanced and speed control across varying loads. In (AC) synchronous machines, field coils are typically mounted on the and supplied with excitation to create a that synchronizes with the stator's alternating field, enabling constant-speed operation. This DC supply, often provided via slip rings or brushless exciters, ensures the rotor locks into step with the stator's at synchronous speed. By adjusting the excitation current, operators can control the machine's , shifting it from lagging to leading to optimize reactive power in electrical grids. Bipolar and multipolar field arrangements are commonly employed in these machines to match the desired pole count for and speed characteristics. Field coils also find application in various other electromechanical devices beyond traditional rotating machines. In loudspeakers, a stationary field coil acts as an to produce a constant , within which a moving voice coil attached to the diaphragm interacts with audio-frequency currents to generate sound waves. Although windings function similarly by creating alternating s for energy transfer, they are not classified as true field coils, which are typically DC-energized for steady flux in motor or generator contexts. In particle accelerators, field coils form the basis of superconducting or conventional s used to bend and focus beams along curved paths, with currents up to 11,000 amperes generating fields of several tesla. The contribution of field coils to overall machine efficiency is significant in large-scale units, where copper losses in the windings represent a small fraction—typically less than 1% of rated power in series configurations—enabling total efficiencies exceeding 95% through minimized stray and excitation losses. Historically, field excitation was controlled using rheostats to adjust current manually, but since the , solid-state exciters have become standard, offering precise, automated regulation via thyristors and digital controls for improved stability and response.

Advantages over Permanent Magnets

Field coils offer significant advantages over permanent magnets in electric machines, primarily through their ability to provide adjustable strength. By varying the excitation current, field coils enable precise control of levels, facilitating variable speed operation and flux weakening, which extends the constant power range in applications like traction motors. This adjustability is particularly beneficial in wound-field synchronous machines (WFSMs), where field current can be modulated via control systems such as circuits to optimize performance across speed ranges. Unlike permanent magnets, field coils eliminate the risk of demagnetization under high temperatures, overloads, or fault conditions, as the is generated electrically and can be immediately reduced or reversed if needed. Permanent magnets, such as neodymium-iron-boron types, are susceptible to irreversible demagnetization when flux densities exceed thresholds like -436 kA/m, whereas field coils maintain stability without such limitations. Additionally, in large machines, field coils can achieve higher peak flux densities, up to 1.64 T limited by steel saturation, surpassing the typical 1.1 T of permanent magnets and enabling greater output in high-power designs. However, field coils require a continuous DC power supply for excitation, typically consuming about 1% or less of the machine's rated power in series configurations, which adds to operational losses compared to the zero-excitation needs of permanent magnets. This power draw necessitates additional components like DC-DC converters, increasing mechanical complexity, especially with traditional brush-and-slip-ring systems that can introduce wear and maintenance issues. Initial costs for field coil systems are higher due to these extra elements, though long-term expenses may be lower in high-power applications by avoiding dependencies and price volatility. Trade-offs between field coils and permanent magnets are evident in post-2010 trends within electric vehicles (EVs) and renewables, where WFSMs persist for their superior and flux weakening capabilities, allowing efficiency gains of up to 8% in medium- to high-speed, low-torque regions despite slightly lower overall than permanent magnet synchronous machines (PMSMs). In EVs, this enables broader speed ranges without demagnetization risks at high temperatures, while in renewables like wind turbines, field coils support variable adaptation. Permanent magnets excel in and compactness but lack the same flux adjustability, making field coils preferable where controllability outweighs peak . Looking ahead, hybrid designs integrating field coils with permanent magnets are emerging for 2025 and beyond, particularly in generators, to combine adjustable control with high baseline densities for improved (up to 97.9%) and adaptability to grid disturbances in renewable systems. These hybrids reduce reliance on full excitation power while mitigating permanent magnet limitations, positioning them as viable for large-scale applications.

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