Alternator Fundamentals and Classification

12-154 Three-phase winding produces a rotating magnetic field Slip rings + N S S – with one lead disconnected refuses to turn and “growls.” A knocking sound generally indicates a loose armature coil, a shaft out of alignment, or armature dragging because of worn bearings. In all cases, the inspection and maintenance of all AC motors should be performed in accordance with the applicable manufacturer’s instructions.

Alternators

Basic Alternators & Classifications An electrical generator is a machine that converts mechanical energy into electrical energy by electromagnetic induction. A generator that produces alternating current is referred to as an AC generator and, through combination of the words “alternating” and “generator,” the word “alternator” has come into widespread use. In some areas, the word “alternator” is applied only to small AC generators. This handbook treats the two terms synonymously and uses the term “alternator” to distinguish between AC and DC generators. The major difference between an alternator [Figure 12-317] and a DC generator is the method of connection to the external circuit. The alternator is connected to the external circuit by slip rings, but the DC generator is connected by a commutator.

Method of Excitation One means of classification is by the type of excitation system used. In alternators used on aircraft, excitation can be affected by one of the following methods: 1. A direct connected, DC generator. This system consists of a DC generator fixed on the same shaft with the AC generator. A variation of this system is a type of alternator that uses DC from the battery for excitation, after which the alternator is self-excited. 2. By transformation and rectification from the AC system. This method depends on residual magnetism for initial AC voltage buildup, after which the field is supplied with rectified voltage from the AC generator.

3. Integrated brushless type. This arrangement has a DC generator on the same shaft with an AC generator. The excitation circuit is completed through silicon rectifiers rather than a commutator and brushes. The rectifiers are mounted on the generator shaft and their output is fed directly to the AC generator’s main rotating field. Number of Phases Another method of classification is by the number of phases of output voltage. AC generators may be single-phase, two-phase, three-phase, or even six-phase and more. In the 12-155 Compensating winding Main field Compensating winding Main field motor.

electrical systems of aircraft, the three-phase alternator is by far the most common. Armature or Field Rotation Still another means of classification is by the type of stator and rotor used. From this standpoint, there are two types of alternators: the revolving armature-type and the revolving field-type. The revolving armature alternator is similar in construction to the DC generator in that the armature rotates through a stationary magnetic field. The revolving armature alternator is found only in alternators of low-power rating and generally is not used. In the DC generator, the EMF generated in the armature windings is converted into a unidirectional voltage (DC) by means of the commutator.

In the revolving armature-type of alternator, the generated AC voltage is applied unchanged to the load by means of slip rings and brushes. The revolving field type of alternator has a stationary armature winding (stator) and a rotating field winding (rotor). [Figure 12-318] The advantage of having a stationary armature winding is that the armature can be connected directly to the load without having sliding contacts in the load circuit. A rotating armature would require slip rings and brushes to conduct the load current from the armature to the external circuit. Slip rings have a relatively short service life and arc over is a continual hazard; therefore, high voltage alternators are usually of the stationary armature, rotating field-type. The voltage and current supplied to the rotating field are relatively small, and slip rings and brushes for this circuit are adequate. The direct connection to the armature circuit makes possible the use of large cross-section conductors, adequately insulated for high voltage. Since the rotating field alternator is used almost universally in aircraft systems, this type is explained in detail, as a single-phase, two-phase, and three-phase alternator.

Single-Phase Alternator Since the EMF induced in the armature of a generator is alternating, the same sort of winding can be used on an alternator as on a DC generator. This type of alternator is known as a single-phase alternator, but since the power delivered by a single-phase circuit is pulsating, this type of circuit is objectionable in many applications. A single-phase alternator has a stator made up of a number of windings in series, forming a single circuit in which an output voltage is generated. [Figure 12-319] The stator has four polar groups evenly spaced around the stator frame. The rotor has four poles with adjacent poles of opposite polarity. As the rotor revolves, AC voltages are induced in the stator windings. Since one rotor pole is in the same position relative to a stator winding as any other rotor pole, all stator polar groups are cut by equal numbers of magnetic lines of force at any time.

As a result, the voltages induced in all the windings have the same amplitude, or value, at any given instant. The four stator windings are connected to each other so that the AC voltages are in phase or “series adding.” Assume that rotor pole 1, a South pole, induces a voltage in the direction indicated by the arrow in stator winding 1. Since rotor pole 2 is a North pole, it induces a voltage in the opposite direction in stator coil 2 with respect to that in coil 1. For the two induced voltages to be in series addition, the two coils are connected as shown in Figure 12-319 . Applying the same reasoning, the voltage induced in stator coil 3 (clockwise rotation of the field) is the same direction (counterclockwise) as the voltage induced in coil 1.