Alternators

AC Alternators

In addition to saving weight, the voltage in alternating current is easily stepped up or down, AC can carry current a long distance by raising the voltage with a step-up transformer. This saves additional weight because high-voltage AC is conducted through relatively small wires. At its destination, electricity passes through a step-down transformer where voltage is lowered and current is stepped up to value needed.

Alternators controls

Low-output alternators with vibrator controls increase and decrease the field current by opening a set of contacts The transistorized voltage regulator uses both vibrating points and transistors for voltage controls. In a completely solid-state voltage regulator, semi-conductor devices replace all of the moving parts. Because an alternator uses solid-state diodes for rectification, Current cannot flow from the battery into the alternator. There fore, there is no need for a reverse-current cutout relay.

Brush Assembly

The brush assembly in an alternator consists of two brushes and their respective brush springs and brush holders. The brushes in an alternator supply current to the field coils.

Rectifiers

The three-phase, full-wave rectifier in an alternator is made up of six heavy-duty silicon diodes. Three of the diodes are pressed into the slip-ring end frame, and the other three are pressed into the heat sink that is electrically insulated from the end frame.

Types of AC alternators

Alternating current alternators can be single-phase, two-phase, three-phase, And sometimes even six-phase or more. However, almost all aircraft electrical systems use three-phase alternators. In a single-phase alternator, the stator is made up of several windings connected in series to form a single circuit. The windings are also connected so the AC voltages induced into each winding are in phase. This means that to determine the total output of a single-phase alternator, you must add the voltage induced into each winding. Two-phase alternators have two or more single-phase windings spaces symmetrically around the stator, so that the AC voltage induced in one is 90 degrees out of phase with the voltage induced in the other. Most aircraft alternators use a three-phase, or polyphase, circuit. The three-phase alternator has three single-phase windings spaces so that the voltage induced in each winding is 120 degrees out of phase with the voltage in the other two windings. Another alternator classification distinguishes between the type of stator and rotor used. In this classification there are two types of alternators: the revolving-armature type, and revolving-field type. The revolving-armature type alternator is similar in construction to DC generator, in that the armature rotates within a stationary magnetic field. This setup is typically found only in alternators with a low power rating and generally is not used. The revolving-field alternator has stationary armature winding (stator) and a rotating-field winding (rotor).

Rotor

An alternator rotor consists of a wire coil wound on an iron spool between two heavy iron segments with interlacing fingers. Each finger forms one pole of the rotating magnetic field.

Stator

As the rotor turns, the load current is induced into stationary stator coils. The coils making up the stator are wound in slots around the inside periphery of the stator frame, which is made of thin laminations of soft iron. Most alternators are three-phase alternators. This means that the stator has three separate coils that are 120 degrees apart.

DC Alternators

Typically, one of two problems prevents an alternator from producing electrical power. The most likely problem is a shorted or open diode in rectifying circuit. The other is the possibility of an open circuit in the field windings. If the diodes are good, there will be relatively low resistance when the diodes are forward-biased and an infinite or very high reading when the diodes are reversed-biased. If the reading is not inibindo-te or very high, one or more of the diodes are shorted. Because the diodes in an alternator are connected in parallel, an ohmmeter cannot detect an open diode. However, an ohmmeter will identify an open diode if you check the diodes individually. Solid-state diodes are quite rugged and have a long life when property used. However, they can be damaged by excessive voltage or reverse current flow. (do not operate without load). Because alternators receive their field current from the aircraft bus and do not rely on residual magnetism to be started, never flash the field or polarize an alternator.