- Synchronous Motor Excitation Definition: Synchronous motor excitation is the DC supply given to the rotor to produce the necessary magnetic flux.
- Role of Field Current: The field current in a synchronous motor determines the motor’s power factor and overall performance.
- Normal Excitation: When the field current matches the required air gap flux, the motor operates at unity power factor.
- Under-Excitation: Insufficient field current causes the motor to draw additional magnetizing current, resulting in a lagging power factor.
- Over-Excitation: Excessive field current leads to excess flux, causing the motor to operate at a leading power factor.
In a wound-field machine, synchronous motor excitation controls rotor field current. An ideal magnetising component is nearly 90o from terminal voltage, but stator current also contains the active-power component and effects of winding impedance. Stator and rotor magnetomotive forces combine to establish air-gap flux in the magnetic circuit. A wound-field synchronous motor is a doubly excited electrical motor: the armature winding receives AC and the rotor field receives controlled DC. Permanent-magnet and reluctance motors use different rotor-excitation principles.
Synchronous motor excitation includes the equipment and control that establish DC field current and rotor magnetic flux in a wound-field motor.
At constant shaft load and terminal voltage, changing excitation changes internal generated voltage, stator-current magnitude, reactive power, power factor and load angle. Operation remains limited by field heating, armature current and stability rather than covering any arbitrary power factor.
CASE 1: Normal excitation is the field current that gives unity stator power factor at the specified load and terminal voltage. It corresponds to the minimum point of that load’s V-curve, not one fixed field-current value for every load.
CASE 2: Reducing field current below the normal-excitation value makes the motor under-excited. At the same load and voltage, stator current becomes lagging and the motor absorbs reactive power. The resulting power factor and stability margin depend on the operating point.
CASE 3: Increasing field current above the normal-excitation value makes the motor over-excited. Stator current becomes leading and the motor supplies reactive power, subject to its field-current, armature-current and thermal limits.
In over-excited motor operation, the reactive component of stator current leads terminal voltage by about 90o under the sinusoidal steady-state model. Total stator current also carries the component needed for shaft power and losses.
A V-curve plots armature-current magnitude against field current at constant terminal voltage, frequency and shaft load. A family of curves is needed because the unity-PF field current and minimum armature current change with load.
Conclusion: For a wound-field motor at a fixed load and voltage, normal excitation gives unity power factor, under-excitation gives lagging operation and over-excitation gives leading operation, provided the machine stays within its capability and stability limits.
