Generators are machines used for the large-scale production of electrical energy. In electricity generation, an electrical generator is a device that converts mechanical energy to electrical energy. The generator is based on the principle of electromagnetic induction discovered in 1831 by Michael Faraday. Faraday discovered that if an electric conductor, like a copper wire, is moved through a magnetic field, electric current will flow in the conductor. So the mechanical energy of the moving wire is converted into the electric energy of the current that flows in the wire. Generators were earlier called dynamos, a shortened form of the term dynamoelectric.

The size of large generators is usually measured in kilowatts. One kilowatt equals 1,000 watts. A giant generator can produce more than 1 million kilowatts of electric power. There are two main types of generators. Direct-current (DC) generators produce electric current that always flows in the same direction. Alternating-current (AC) generators, or alternators, produce electric current that reverses direction many times every second.

Their operation is based on principle of electrical induction, whereby a periodic flow of electricity is produced in a loop-type conductor as a result of the periodic variation of the flux of the magnetic lines of force passing through this loop. In order to implement this, we can either cause the loop to rotate in a constant magnetic field or, alternatively the loop can be kept stationary and the magnetic field rotated.

In above mentioned arrangement loop is formed by the armature windings on the rotor which revolves between the fixed magnetic poles of the stator. In the latter arrangement the armature is stationery, and the magnetic poles on magnet wheel revolve instead; the stator consists of an iron ring with induction coils mounted on the inside; the magnetic poles on the rotor move past the ends of these coils at a very short distance from them (Figs 1 and 3).

In this case the current produced by the generator is taken direct from the stator, without the aid of special current collectors (brushes). Due to this reason this form of construction is particularly suitable for the generation of high-voltage alternating current. The reverse conversion of electrical energy into mechanical energy is done by a motor, and motors and generators have many similarities. A generator forces electric charges to move through an external electrical circuit, but it does not create electricity or charge, which is already present in the wire of its windings.

The sparking that occurs at high voltages i.e. around 20,000 volts in large generators would destroy the brushes. The relatively low output of direct current needed for producing the rotating magnetic field is fed to the rotor by means of slip-rings and carbon or copper-mesh brushes (Fig.3). The successive coils in Fig.1 are wound in alternate directions, which ensure that the generated current always flows in the same direction.

The source of mechanical energy may be a reciprocating or turbine steam engine, water falling through a turbine or waterwheel, an internal combustion engine, a wind turbine, a hand crank, the sun or solar energy, compressed air or any other source of mechanical energy. High-duty generators are usually coupled directly on the same shaft to steam or water turbines.

A small direct current dynamo for producing the magnetic field is also mounted on the driving shaft (Fig.2). In the older type of power station with reciprocating steam engines, the rotor of the generator is generally constructed as a flywheel with the magnetic pole windings round its rim. Fig.3 shows a smaller generator which likewise operates on the principle described above (rotating magnetic field, stationery armature winding). In this case the magnet wheel is in the form of a two-part T-rotor.