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The secret weapon of electromagnetic machines: Why are field coils so critical in motors and generators?
Field coils play a vital role in the operation of electromagnetic machines. They are essential components for generating electricity via wires, ensuring that the required magnetic fields are generated to drive electric motors and generators.
A field coil is an electromagnet used to generate a magnetic field in an electromagnetic machine, usually a rotating electric motor, and consists of a coil of wire through which current is passed.
Field coil designs typically consist of a coil of wire wound around an iron core that effectively directs the magnetic field lines. The magnetic core is divided into two parts: the stationary stator and the rotating rotor. The magnetic field lines form a continuous loop from the stator, through the rotor, and back to the stator.
The field coils can be mounted on the stator or rotor, depending on the economic benefits of the design. Most arrangements use one field coil per pole. Although they are most common in rotating machines, field coils are also found in many other electromagnetic machines, such as simple electromagnets and complex laboratory instruments such as mass spectrometers and nuclear magnetic resonance machines.
Field coils were once widely used in loudspeakers until the widespread availability of lightweight permanent magnets caused their use to decline.
Fixed and Rotating Fields
Most DC field coils produce a steady static magnetic field, while three-phase AC field coils are used as rotating magnetic fields for electric motors. Small motors are usually universal motors, which can use brushed DC motors, while large AC motors are mostly induction motors, either three-phase or single-phase.
In many rotating electrical machines, transferring electrical current to or from a moving rotor typically requires sliding contacts, such as brushes or slip rings. These contacts are often the most complex and least reliable part of the machine and can limit the maximum current the machine can handle.
In a brushed DC motor, the field is stationary, but the arm-ring currents on the rotor must be commutated in order for it to rotate continuously.
Stator and rotor
Depending on value, the field coils can be mounted on the rotor or the stator. The field coils in a brushed DC motor are stationary, but the arm coils on the rotor require brushes for commutation. Induction motors use field coils on the stator, and the current in the rotor is supplied by induction.
For a generator, the field current is usually smaller than the output current, so the field coils are mounted on the rotor and powered through slip rings. Instead, the output current is drawn from the stator, avoiding the need for high current slip rings.
Bipolar and Multipolar Fields
In the early days of generator development, the stator field gradually evolved from a single two-pole field to a multi-pole design. Bipolar generators were widely used before 1890, but were then replaced by multipole field magnets. The change was because, to increase output voltage, DC generators had to spin faster, but above a certain speed this became impractical for large power transmission generators.
The use of multi-pole generators can increase the output voltage without increasing the speed at the same time, which is particularly important for long-distance power transmission.
Winding Materials
The field coil is generally made of enameled copper wire, which has low resistance and can reduce the energy consumption of the field coil. As copper prices rise, the use of aluminum coils has become increasingly popular; although silver has lower resistance, it is rarely used due to cost issues.
During World War II, the Manhattan Project borrowed a large amount of silver from the U.S. Treasury to build low-resistance, high-efficiency field coils. These field coils were crucial in the electromagnetic devices of the time.
ConclusionThe development and application of field coils have affected the efficiency and performance of many electromagnetic devices. Understanding the importance of field coils is vital for engineers and scientists, whether in various types of electric motors or other electromagnetic devices. As technology advances, how will the application of field coils evolve in the future?
