A brushless alternator is composed of two alternators constructed from end to end on one axis. Smaller brushless alternators may look like a unit, but the two parts are easily identifiable in large versions. The larger of the two sections is the main alternator and the smaller is the exciter. The exciter has stationary field coils and a rotating armature (power coils).
The main alternator uses the opposite configuration with a rotating field and a stationary armature. A bridge rectifier, called a rotary rectifier assembly, is mounted on a plate attached to the rotor. No brushes or slip rings are used, which reduces the number of wear parts. The main alternator has a rotating field as described above and a stationary armature (power generation windings).
Alternator Operating principle
A conductor moving with respect to a magnetic field develops an electromotive force (EMF) in it (Faraday's law). This emf inverts its polarity when it moves under magnetic poles of opposite polarity. Typically, a rotary magnet, called a rotor, rotates within a stationary array of conductors wound in coils on an iron core, called a stator. The field cuts through the conductors, generating an induced EMF (electromotive force), as the mechanical input causes the rotor to rotate.
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.
The magnetic field of the rotor can be produced by permanent magnets, or by a field coil electromagnet. The automotive alternators use a rotor winding that allows the control of the voltage generated from the alternator by varying the current in the winding of the rotor field. Permanent magnet machines prevent loss due to the magnetizing current in the rotor, but are of a limited size, due to the cost of the magnet material. Since the field of the permanent magnet is constant, the voltage of the terminal varies directly with the speed of the generator. Brushless AC generators are usually larger than those used in automotive applications.
An automatic voltage control device controls the field current to maintain the output voltage constant. If the output voltage of the stationary armature coils drops due to an increase in demand, more current is fed to the rotating field coils through the voltage regulator (VR). This increases the magnetic field around the field coils which induces a higher voltage in the coils of the armature. In this way, the output voltage is restored to its original value.
Alternators used in power plants also control the field current to regulate reactive power and help stabilize the power system against the effects of momentary failures. Often there are three sets of stator windings, physically displaced so that the rotating magnetic field produces a three-phase current, displaced one third of a period with respect to each.