series-wound
Of an electric motor, having the windings connected in series.
series-wound: motor winding that handles all current in one path
A series-wound motor connects its field coils and armature coil in a single electrical circuit, so the full load current flows through both in sequence. This contrasts with shunt-wound motors, where field and armature circuits run in parallel, or compound-wound types, which use both arrangements. The series connection creates a direct relationship between load and magnetic field strength: as mechanical load increases, current rises, and so does the magnetic field.
Series-wound motors deliver very high starting torque and can accelerate heavy loads from rest with modest initial current relative to the torque produced. This makes them the standard choice for traction applications: electric trains, trams, and forklift trucks rely on series-wound designs. The torque-current characteristic is steep, meaning the motor naturally adapts to load variation without external control. A light load causes the motor to speed up; a heavy load slows it down and increases current and torque automatically.
Trade-offs and practical limits
The penalty for high starting torque is poor no-load performance. A series-wound motor with no mechanical load will accelerate dangerously toward runaway speeds, drawing excessive current and risking damage. For this reason, series-wound motors always require a load connection or mechanical brake and should never be operated unloaded. They are also less stable at constant speed than shunt-wound motors and more sensitive to supply voltage fluctuations.
Modern series-wound motors are wound for direct current (DC) or operate as universal motors on AC mains, where they perform well because the series connection works equally on alternating current. AC series-wound motors generate audible noise and radio interference at higher speeds due to commutator sparking. Series-wound designs are less common in fixed-speed industrial applications today, having been displaced by three-phase induction motors and variable-frequency drives, but remain standard in portable power tools, vehicle starters, and rail traction where high torque density and inherent load adaptation are essential.