Electric Generator

An electric generator is a machine that converts mechanical energy into electrical energy. Most generators operate through electromagnetic induction: relative motion between conductors and a magnetic field produces an electrical voltage. Generators supply power in generating stations, vehicles, portable equipment, and standby installations. Their operation is closely related to that of an electric motor, which performs the reverse conversion, from electrical to mechanical energy. Neither machine creates energy; each transfers it between forms. (eia.gov)

Operating principle

Generator action is an application of electromagnetism. When the magnetic flux linked with a circuit changes, an electromotive force is induced. For a coil with $N$ turns sharing the same flux, Faraday’s law of induction gives

where $\mathcal{E}$ is the induced electromotive force and $\Phi_B$ is the magnetic flux through one turn. Rotation can change this flux even when the surrounding magnetic field is steady. A voltage can exist across disconnected terminals, but sustained current through an external load requires a conducting circuit. (openstax.org)

In an idealized generator, a flat coil of area $A$ rotates at constant angular speed $\omega$ in a uniform field of magnitude $B$. With an appropriate choice of initial orientation, its voltage is

This model explains why rotation produces alternating voltage and why its amplitude increases with field strength, coil area, number of turns, and rotational speed. Practical machines use distributed windings and shaped magnetic structures rather than a single rectangular coil. (openstax.org)

The conversion requires mechanical work. When a generator supplies a load, electromagnetic forces oppose the driving motion. The mechanical input supplies both the useful electrical output and the machine’s losses; increased electrical loading therefore requires increased input to maintain steady operation. (openstax.org)

Construction

A rotating generator has two principal mechanical assemblies: the rotor, which turns, and the stator, which remains stationary. These names describe motion, not electrical function. The armature contains the windings in which the output voltage is induced, while the field system establishes the magnetic flux. Either assembly can carry the armature or field, depending on the design. (energy.gov)

Large alternating-current generators commonly have a rotating electromagnetic field and a stationary armature. This arrangement permits the main output connections to remain stationary and simplifies insulation for high voltages. The field winding receives an excitation current; in some designs, this current passes through brushes contacting continuous slip rings on the shaft. Slip rings maintain electrical connections during rotation without reversing their polarity. (energy.gov)

Alternating- and direct-current generators

An alternating-current generator, or alternator, produces voltage whose polarity reverses periodically. Utility generators commonly provide three-phase power, using three sets of armature windings whose induced voltages are separated by 120 electrical degrees. The winding connections determine the relationship between phase voltages and terminal voltages. (energy.gov)

A conventional direct-current generator uses a commutator instead of continuous slip rings to collect its output. This segmented contact assembly switches connections as the armature rotates, making the external voltage unidirectional even though the voltage induced in individual rotating coils reverses. A single-coil example produces pulsating output; multiple coils and commutator segments make it smoother. Such machines are often called dynamos. (openstax.org)

For a synchronous alternator, electrical frequency is tied to rotor speed and magnetic pole count:

where $f$ is frequency in hertz, $P$ is the total number of poles, and $n$ is speed in revolutions per minute. Thus, a four-pole machine produces 60-hertz voltage at 1,800 revolutions per minute. Machines with more poles can produce the same frequency at lower speeds. (energy.gov)

Mechanical drives and applications

The machine supplying mechanical input is called the prime mover. A steam turbine drives generators in many thermal generating stations, including plants using fuel combustion or nuclear power to produce steam. Hydroelectric turbines extract energy from moving water, while wind turbines extract energy from air movement. These systems differ in their energy sources but use the same underlying mechanical-to-electrical conversion. (eia.gov)

An internal-combustion engine can drive a generator directly. Engine-generator combinations, commonly called generator sets or gensets, serve remote locations, construction sites, and emergency or backup installations. The engine and generator are distinct components: the engine converts fuel energy into shaft power, and the generator converts shaft power into electricity. By contrast, a solar cell converts light directly into electricity without a rotating generator. (eia.gov)

Grid operation and efficiency

Before a synchronous generator is connected to an energized electrical grid, its terminal voltage, frequency, and phase must match the system. Incorrect synchronization can produce large currents and damage equipment. Once connected, generators participate in maintaining the balance between electrical supply and demand. Frequency response, balancing, and voltage control are distinct but related functions of grid operation. (energy.gov)

Generator efficiency is the ratio of useful electrical output to total input power. Losses include winding resistance, magnetic hysteresis, and mechanical friction, including bearing friction and aerodynamic drag. These losses appear largely as heat, making cooling part of machine design. Ratings specify operating limits such as voltage, current, speed, power, and allowable temperature rise. Generator efficiency is separate from the overall efficiency of a generating plant, which also includes the conversion processes supplying its mechanical input. (energy.gov)

Historical development

Michael Faraday demonstrated electromagnetic induction in 1831 and constructed the Faraday disk, an early generator that produced electricity by rotating a conducting disk in a magnetic field. His experiments established the physical basis for later electromagnetic generating machines. The disk differed from the wound-coil alternators and commutated dynamos subsequently used in power systems, but demonstrated that mechanical motion could supply electrical output. (eia.gov)