Excitation system of an electric motor - Working Principle

An excitation system provides controlled DC current to the rotor windings of a synchronous motor to create a rotating magnetic field, which is essential for motor operation. This DC field interacts with the stator's armature field to produce torque. It acts as an automatic voltage regulator (AVR) to control power factor and stability.

Working Principle

• DC Generation: The system converts AC voltage, typically from the motor terminals or a separate exciter machine, into DC current using rectifiers (rotating or static).
• Field Regulation: An Automatic Voltage Regulator (AVR) monitors the motor's performance (terminal voltage, current) and adjusts the DC current strength sent to the rotor.
• Magnetization: This DC current passes through slip rings and brushes (or via brushless exciters) into the rotor field coils, creating a magnetic field.
• Rotation: The interaction between this rotating magnetic field and the stator's armature magnetic field creates torque, allowing the motor to run.

• Exciter: A source of DC power (often a small DC generator or AC generator with rectifier).
• AVR (Automatic Voltage Regulator): Controls the excitation level.
• Rotor Winding: Receives DC current to produce magnetic flux.

• Brushless Excitation: Uses a rotating AC exciter on the same shaft, with rectifiers mounted on the rotor, eliminating brushes.
• Static Excitation: Uses a stationary rectifier powered directly from the motor terminals or a transformer

• Under-Excitation: The motor draws lagging reactive power from the grid.
• Normal Excitation: The motor operates at a unity power factor (most efficient).
• Over-Excitation: The motor supplies leading reactive power to the grid, often used in industrial settings to improve the overall plant power factor.