Excitation current is a specialized electrical input required by large rotating electrical machines. This current is a small, continuous stream of direct current (DC) that serves a single, foundational purpose: generating a controlled magnetic field. Without this electrical energy, devices like generators and synchronous motors cannot create the necessary magnetic flux to convert mechanical power into electrical power, or vice versa. The precise regulation of this current allows operators to manage the quality of the electricity produced or the machine’s efficiency.
The Role of the Field Winding
Excitation current is frequently called field current because it is routed exclusively through the machine’s field winding. In most large modern alternators and synchronous motors, the field winding is located on the rotor, the rotating component of the machine. This setup is preferred because the relatively low-power DC excitation can be easily transferred to the rotor, while the high-power output is generated in the stationary stator, simplifying insulation and power collection.
The primary function of this current is to produce a powerful, adjustable electromagnet, forming the machine’s magnetic field. The strength of this magnetic field is directly proportional to the magnitude of the DC field current supplied to the coils. This field circuit is separate from the armature circuit, which is the high-power winding where the main energy conversion takes place. The armature circuit carries the generated alternating current (AC) power output in a generator, while the field circuit only consumes the small amount of DC power required to sustain the magnetic field.
Regulating Voltage and Power Factor
The ability to precisely control the excitation current is the single most important mechanism for managing the operational output of synchronous machines. In a generator, the strength of the magnetic field dictates the magnitude of the voltage induced in the armature windings. Therefore, increasing the field current strengthens the magnetic field, which directly raises the generated output voltage.
Maintaining a stable output voltage is accomplished by an Automatic Voltage Regulator (AVR), which continuously monitors the generator’s terminal voltage. If the load increases, the output voltage momentarily dips, and the AVR instantly increases the field current to restore the voltage to the set point. This automatic adjustment ensures the generator maintains a consistent voltage despite varying electrical demand. The excitation system also manages the Volts per Hertz limit, preventing excessive magnetic flux density that could overheat the generator’s core or connected transformers.
In synchronous motors, the control over the excitation current is used not to regulate voltage but to manage the machine’s power factor. Power factor is a measure of how efficiently the machine uses the electrical power it draws from the system. By adjusting the field current, an operator can make the motor either consume or supply reactive power to the electrical grid.
When the synchronous motor’s field current is increased above a certain baseline, known as over-excitation, the motor acts like a capacitor, drawing a leading power factor and supplying reactive power to the system. Conversely, when the field current is decreased, or under-excited, the motor acts like an inductor, drawing a lagging power factor and consuming reactive power. This capability allows synchronous motors to be used as synchronous condensers, specialized devices used to inject or absorb reactive power to stabilize voltage and improve power factor.
Practical Excitation Systems
The supply of excitation current is achieved through two main categories of hardware systems: brushless and static direct systems. Brushless excitation systems are the preferred choice for many modern, high-speed machines because they eliminate the need for physical contact components. In this setup, a small AC exciter machine is mounted on the main generator shaft, with its stationary field coils inducing AC power into its own rotating armature windings. This induced AC power is immediately converted into the required DC field current by a set of rotating semiconductor diodes, known as a rotating rectifier, which is also fixed to the main shaft. This DC is then fed directly to the main generator’s rotor field winding, creating a closed, maintenance-free system.
Static or direct excitation systems, however, rely on external power electronics and require brushes and slip rings. This system draws AC power from the generator’s output terminals via a step-down transformer or a separate dedicated source. This AC power is then converted to a highly controllable DC current by a stationary power converter, typically a bridge of thyristors, or Silicon-Controlled Rectifiers (SCRs). The controlled DC current is then transferred to the rotating field winding through stationary carbon brushes riding on smooth, conductive slip rings mounted on the shaft. This design offers a faster response time for voltage regulation due to the quick switching capabilities of the thyristors, but it necessitates periodic maintenance of the brushes and slip rings due to wear.