5.2. AC generator: construction, principal scheme of operation
AC generator: construction, principal scheme of operation
A generator is a machine that transforms mechanical energy into electrical energy. A generator can be AC or DC. The AC one is more common on board.
Its operation is based on Faraday’s law of electromagnetic induction.
Faraday’s law of electromagnetic induction states, that if a coil is moving in a magnetic field, electromotive force (emf) will be induced in that coil. The magnetic field can be stationary, and the coil can move or the magnetic field can move and the coil stay stationary – the result is the same. The induced electromotive force (emf) generates AC (alternating) current in the coil.
A basic AC generator consists of an armature, made of a set of wires, permanent magnet, slip rings and brushes. A Generating unit (GU) usually consists of a prime mover, which rotates the shaft of the generator. The armature is connected to the shaft. As the armature rotates, the AC current is induced by emf in it. The current flows to the brushes, slip rings and galvanometer, that shows the electrical power produced.
The permanent magnet has constant magnetic field;
therefore, it cannot be adjusted. In addition, its power is limited. To solve
it, an electromagnet is used instead of a permanent magnet in another structure
of an AC generator The generator
consists of a stator and a rotor. A stator is an armature coil wound round a
rectangular shape of stator frame. It has a rheostat, that can increase or
decrease the excitation circuit resistance so altering the excitation current.
The change of the excitation current and excitation field created by it affects
the output voltage of the generator.
The rotor consists of a shaft and rotor windings. The
prime mover rotates the shaft and the stationary excitation field will induce
emf in the coils of the rotor. The induced AC current flows through the brushes
and slip rings to the galvanometer which shows the presence of AC current.
The typical construction of AC generator on board is a brushless AC generator.