Electirc Generator

Faraday discovered that emf is induced in a conductor if a magnet passes by the conductor since relative motion between the magnet and conductor cut through magnetic lines of flux

The direction of the induced emf depends on the direction of relative motion between the magnet and the conductor

The magnitude of the induced current depends on the magnitude of magnetic flux, velocity at which the magnet passes by the conductor, and the number of magnetic lines of flux that are cut

Revolving armature

rotor is an armature which is rotating inside a stationary electromagnetic field

seldom used since output power must be transmitted through slip-rings and brushes

Revolving field

dc current is supplied to the rotor which makes a rotating electromagnetic field inside the stator

more practical since the current required to supply a field is much smaller than the output current of the armature

Three-Phase Electrical PowerUses three sets of armature windings to produce three separate outputs

Armature windings are physically separated 120o from each other, and therefore, each phase is 120o apart from another

More power may be generated by a generator of a given size and weight

Provides continuous power to electrical equipment even if one phase is damaged

Classifying Generators

l Number of phases: most shipboard electrical power is 3 phase

l Frequency: most shipboard electrical power is 60 Hz, some electronic equipment operate at 400 Hz or higher

l Voltage: usually 450 V, smaller appliances use 120 V

l Power rating: measured in kW, most shipboard generators are 2,000 - 3,000 kW

AC MOTORS

l Widely used for constant speed applications (speed depends only upon the frequency for a given number of poles)

l Most AC motors are synchronous, 3-phase, induction motors

l Rotor is a "cage" with conductors arranged in a cylinder with short circuited ends

l Rotor currents are supplied by electromagnetic induction, and a rotating magnetic field is established by 3-phase stator windings