Marine Replacement – Service Inboard & Outboard Alternators For Your OMC, Mercruiser, Chrysler, Yamaha, Suzuki, And Tohatsu
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Alternating current generating systems were known in simple forms from the discovery of the magnetic induction of electric current. The early machines were developed by pioneers such as Michael Faraday and Hippolyte Pixii. Faraday developed the “rotating rectangle”, whose operation was heteropolar.  The first public demonstration of a more robust “alternator system” took place in 1886. Large two-phase alternating current generators were built by a British electrician, J.E.H. Gordon, in 1882. Lord Kelvin and Sebastian Ferranti also developed early alternators.  These electromechanical alternators produced frequencies between 100 and 300 hertz. In 1891, Nikola Tesla patented a practical “high-frequency” alternator (which operated around 15,000 hertz). 
After 1891, polyphase alternators were introduced to supply currents of multiple differing phases.  Early alternators were designed for varying alternating-current frequencies between sixteen and several hundred hertz, for use with arc lighting, incandescent lighting and electric motors.
Alternators generate electricity by the same principle as DC generators, namely, when the magnetic field around a conductor changes, a current is induced in the conductor. In a typical modern alternator, a rotating electromagnet called the rotor turns within a stationary set of conductors wound in coils on an iron core, called the stator. The field cuts across the conductors, generating an electrical current, as the mechanical input causes the rotor to turn.
The rotor magnetic field may be produced by induction (in a “brushless” generator), by permanent magnets (usually in very small machines), or by a rotor winding energized with direct current through slip rings and brushes. Automotive alternators invariably use brushes and slip rings, which allows control of the alternator generated voltage by varying the current in the rotor field winding. Permanent magnet machines avoid the loss due to magnetizing current in the rotor but are restricted in size owing to the cost of the magnet material. Since the permanent magnet field is constant, the terminal voltage varies directly with the speed of the generator. Brushless AC generators are usually larger machines than those used in automotive applications.
Source: Wikipedia – The Free Encyclopedia