Definition
A field magnet is a magnet—either permanent or electromagnet—whose primary function is to generate a magnetic field that interacts with other components of a device, such as the rotor of an electric motor, the armature of a generator, or the magnetic circuit of a magnetic bearing. The term is commonly used in the engineering of electromechanical machines to distinguish the stationary source of magnetic flux (the field) from the moving or induced parts.
Overview
In electric machines, the magnetic field produced by the field magnet is essential for converting electrical energy to mechanical energy (as in motors) or mechanical energy to electrical energy (as in generators). The field may be created by a permanent magnet assembly (e.g., rare‑earth neodymium magnets) or by a field winding—an electromagnet energized with direct current. The choice between permanent‑magnet and electromagnet field designs influences the machine’s size, efficiency, cost, controllability, and thermal performance.
Field magnets are also employed in magnetic resonance imaging (MRI) systems, magnetic separators, and certain types of magnetic levitation (maglev) devices, where a stable, well‑characterized magnetic field is required.
Etymology/Origin
The word magnet derives from the Greek μαγνήτης λίθος (magnes lithos), “Magnesian stone,” referring to naturally occurring magnetite from the region of Magnesia in ancient Thessaly. The qualifier field in technical contexts arises from the physics concept of a magnetic field—the spatial region in which magnetic forces act. The compound term “field magnet” therefore literally denotes a magnet that produces a magnetic field for a specific functional “field” in a system.
Characteristics
| Characteristic | Description |
|---|---|
| Type | Permanent‑magnet or electromagnet (field winding). |
| Material | Permanent magnets: NdFeB, SmCo, ferrite; electromagnets: laminated steel cores with copper windings. |
| Flux Density | Typically ranging from 0.1 T to >1.5 T for high‑performance machines. |
| Location | Fixed (stator) in rotating machines; can be positioned externally (e.g., in MRI) or internally. |
| Control | Permanent‑magnet fields are constant; electromagnet fields can be varied by adjusting current. |
| Thermal Considerations | Electromagnet windings generate heat; permanent magnets may lose coercivity at elevated temperatures. |
| Size & Weight | Determined by required flux, material energy product, and machine power rating. |
Related Topics
- Permanent magnet – a magnet that retains its magnetization without external power.
- Electromagnet – a coil of wire that becomes magnetized when electric current passes through it.
- Magnetic field – the vector field surrounding magnetic materials and currents.
- Stator and rotor – the stationary and rotating parts of an electric machine, respectively.
- Field winding – the coil(s) that generate the magnetic field in an electromagnetic field magnet.
- Magnetic circuit – an arrangement of magnetic materials that guides magnetic flux.
- Magnetic bearing – a bearing that uses magnetic forces, often produced by field magnets, to support rotating shafts without mechanical contact.