field coil
An electromagnet, consisting of a coil of wire through which a current flows, used to generate a magnetic field in an electromagnetic machine, typically a rotating machine such as a motor or generator.
field coil: the magnet that powers electric motors and generators
A field coil is a wound wire assembly that generates the stationary magnetic field in rotary electrical machines. Current flowing through the coil creates a magnetic field strong enough to interact with a rotor and produce mechanical torque (in motors) or induce electrical current (in generators). The coil sits in slots carved into the stator, the non-rotating iron core that forms the machine's frame.
Field coils come in two main configurations. In series-wound machines, the field coil is in series with the armature, so load current passes through both. In shunt-wound machines, the field coil connects in parallel across the supply, receiving constant voltage independent of load. Separately-excited machines use an independent power source for the field. The choice determines how the machine responds to changing load and whether its speed is self-regulating.
The wire itself is typically copper or aluminium, insulated with enamel or cloth. Coil diameter, wire gauge, and turn count are calculated for the desired field strength and permissible heating. Large industrial motors may have field coils wound with square or rectangular conductor to improve space efficiency. The iron core beneath the coil is laminated from thin steel sheets to reduce eddy current losses at alternating frequencies.
Common failure modes and design trade-offs
Field coils fail when insulation breaks down under heat, moisture, or voltage stress. A short between turns raises current and winding temperature catastrophically. Open circuits (broken wire or cold solder joint) kill field generation entirely, causing the machine to stall or lose synchronism. Thermal cycling, moisture ingress, and contamination with conductive dust all degrade insulation life. Industrial machines are often rated for 20 to 40 kV per millimetre of insulation thickness, a safety margin that defines coil voltage class.
Design involves balancing copper loss (I²R heating in the wire), iron loss (hysteresis and eddy currents in the core), and the cost of materials. Larger wire reduces resistive heating but increases coil mass and cost. More turns raise field strength but also copper volume. Lamination thickness affects core loss but complicates manufacture. The field coil must be robust enough to survive the mechanical forces generated when the rotor torque peaks, especially during motor starting or generator short-circuit transients.