Alternator

In principle, any AC electrical generator can be called an alternator, but usually, the term refers to small rotating machines driven by automotive and other internal combustion engines.

Faraday developed the "rotating rectangle", whose operation was heteropolar – each active conductor passed successively through regions where the magnetic field was in opposite directions.

[9] Lord Kelvin and Sebastian Ferranti also developed early alternators, producing frequencies between 100 and 300 Hz.

The late 1870s saw the introduction of the first large-scale electrical systems with central generation stations to power Arc lamps, used to light whole streets, factory yards, or the interior of large warehouses.

[10][8] Supplying the proper amount of voltage from generating stations in these early systems was left up to the engineer's skill in "riding the load".

[citation needed] A conductor moving relative to a magnetic field develops an electromotive force (EMF) in it (Faraday's Law).

The field cuts across the conductors, generating an induced EMF (electromotive force), as the mechanical input causes the rotor to turn.

[citation needed] The rotating magnetic field induces an AC voltage in the stator windings.

Since the currents in the stator windings vary in step with the position of the rotor, an alternator is a synchronous generator.

Since the permanent magnet field is constant, the terminal voltage varies directly with the speed of the generator.

Often, there are three sets of stator windings, physically offset so that the rotating magnetic field produces a three phase current, displaced by one-third of a period with respect to each other.

[17] One cycle of alternating current is produced each time a pair of field poles passes over a point on the stationary winding.

[citation needed] An alternator's output frequency depends on the number of poles and the rotational speed.

[citation needed] This method of excitation consists of a smaller direct-current (DC) generator fixed on the same shaft as the alternator.

[19] This method of excitation consists of a smaller alternating-current (AC) generator fixed on the same shaft as the alternator.

The AC stator generates a small amount of field coil excitation current, which is induced in the rotor and rectified to DC by a bridge rectifier built in to the windings where it excites the field coils of the larger connected alternator to generate electricity.

This system has the advantage of not requiring brushes, which increases service life, although with a slightly lower overall efficiency.

Smaller brushless alternators may look like one unit, but the two parts are readily identifiable in the large versions.

The main alternator has a rotating field and a stationary armature (power generation windings).

The result is that a small DC exciter current indirectly controls the output of the main alternator.

In vehicles such as transit buses, a heavy demand on the electrical system may require a large alternator to be oil-cooled.

Expensive automobiles may use water-cooled alternators to meet high electrical system demands.

Larger marine diesels may have two or more alternators to cope with the heavy electrical demand of a modern yacht.

Due to the high cost of large house battery banks, Marine alternators generally use external regulators.