What is a wound-field (DC-excited) synchronous motor?
Working principle
A wound-field synchronous motor has an ordinary three-phase stator, but its rotor carries salient poles wound with field coils that are fed with direct current through two slip rings and carbon brushes. The stator produces a rotating field at 120 × f / p rpm, and the energised rotor poles lock onto it like two magnets pulling on each other. Once locked, the rotor turns at exactly synchronous speed, whatever the load, until the load is too heavy and it falls out of step.
Excitation controls the power factor
The field current sets the internal voltage E_f the rotor induces in the stator. When E_f is lower than the supply voltage the motor is under-excited: it takes magnetising current from the grid and runs at a lagging power factor, like an induction motor. Raising the field current brings the stator current into phase with the voltage (unity power factor), and raising it further makes the motor over-excited, so it delivers reactive power to the grid. Run unloaded and over-excited, it is a synchronous condenser that corrects the power factor of a plant.
Load angle, V curves and pull-out
Load does not change the speed; it changes the load angle δ between the rotor poles and the rotating field. Power follows P = 3·V·E_f·sin δ / X_s, so torque peaks at δ = 90°: the pull-out torque. It is proportional to both the supply voltage and the field current, which is why weak excitation or a voltage dip makes a loaded motor slip poles. Plotting armature current against field current at fixed load gives the V curve: the minimum is at unity power factor, and it rises on both sides.
Starting and where it is used
A synchronous motor has no net torque at standstill, so large machines start as induction motors on a damper (amortisseur) winding in the pole faces, and the field is applied close to full speed to pull the rotor into step. Wound-field machines drive big compressors, mills, crushers, pumps and motor–generator sets, where constant speed, high efficiency and power-factor correction justify the slip rings and the exciter.
Frequently asked questions
- How does the field current change the power factor of a synchronous motor?
- At constant load and supply voltage, the active component of the armature current stays the same while the field current changes its reactive component. Below the unity-power-factor excitation the motor is under-excited and lagging; above it, over-excited and leading, delivering reactive power. Move the field current slider and watch the power factor, the reactive power and the V curve.
- What is the pull-out torque and what happens when it is exceeded?
- Pull-out torque is the largest load the rotor can carry while staying locked to the rotating field, reached at a load angle of 90°. Beyond it the rotor slips a pole pitch, the average torque collapses and the current rises sharply, so the motor must be tripped. Raising the field current or the voltage raises the limit.
- Why does a synchronous motor run at constant speed regardless of load?
- The rotor is locked to the rotating field, whose speed is fixed by the supply frequency and pole count (n = 120 × f / p). More load only increases the load angle, so the speed stays exactly synchronous until the pull-out limit is passed.