field magnet

A field magnet is a permanent magnet or electromagnet installed in rotating electrical machinery to create the stationary magnetic field through which the armature rotates. In DC motors and generators, the field magnet is usually mounted on the stator (stationary part) and produces a constant or controlled flux density. This field provides the foundation for electromagnetic induction: when the armature coil rotates through it, voltage is generated in generators or torque is produced in motors.

Field magnets come in two main forms. Permanent field magnets, typically made from alnico or ferrite ceramics, are fixed and require no power input. Electromagnets, made from wound coils around an iron core, allow the field strength to be adjusted by varying the current through the coil, giving better control of motor speed or generator output. Alnico magnets retain strength up to about 450°C, while ferrite ceramics are cheaper but weaker and more brittle. The choice depends on the machine's power rating, duty cycle, and whether field adjustment is needed.

The strength of a field magnet is rated in gauss or tesla. A typical small DC motor might use a field magnet producing 0.3 to 0.8 tesla, while larger industrial machines can exceed 1.2 tesla. The flux density must be strong enough to produce useful torque or voltage without saturating the iron core, which wastes magnetomotive force and generates heat. Over time, permanent field magnets can lose strength through vibration, temperature cycling, or mechanical shock, reducing the machine's efficiency.

Field magnets in practice

In series-wound DC motors, the field coil carries the same current as the armature, so field strength varies with load, naturally limiting speed under light loads. In shunt-wound motors, the field is connected in parallel to the armature supply, keeping the field approximately constant. Compound-wound motors use both series and shunt windings to combine benefits of each. Permanent magnet brushless DC motors have replaced many electromagnet designs in modern applications because they eliminate the need for commutator brushes and reduce size.

Field magnet damage usually shows as loss of output power, overheating, or inability to reach rated speed. In permanent magnet machines, this points to magnet degradation or loss of mechanical contact with the pole pieces. In electromagnets, open circuits in the field winding or loss of field current are common faults. Field magnets are not field-replaceable in most motors; replacement typically requires removal and disassembly of the stator frame, making it economically preferable to retire or rewind the entire machine.