Text-only reference. Published from the recorded official FAA General Chapter 12 PDF. Diagrams, photographs, and figure artwork are not reproduced here; use the official FAA PDF for those materials.
12-156 A B P9 P8 P7 P6 P5 P4 P3 P2 P1 aircraft. Similarly, the direction of the voltage induced in winding 4 is opposite to the direction of the voltage induced in coil 1. All four stator coil groups are connected in series so that the voltages induced in each winding add to give a total voltage that is four times the voltage in any one winding. Two-Phase Alternator Two-phase alternators have two or more single-phase windings spaced symmetrically around the stator. In a two-phase alternator, there are two single-phase windings spaced physically so that the AC voltage induced in one is 90° out of phase with the voltage induced in the other. The windings are electrically separate from each other. When one winding is being cut by maximum flux, the other is being cut by no flux. This condition establishes a 90° relation between the two phases.
Three-Phase Alternator A three-phase, or polyphase circuit, is used in most aircraft alternators, instead of a single or two-phase alternator. The three-phase alternator has three single-phase windings spaced so that the voltage induced in each winding is 120° out of phase with the voltages in the other two windings. A schematic diagram of a three-phase stator showing all the coils becomes complex and difficult to see what is actually happening. A simplified schematic diagram showing each of three phases is illustrated in Figure 12-320. The rotor is omitted for simplicity. The waveforms of voltage are shown to the right of the schematic. The three voltages are 120° apart and are similar to the voltages that would be generated by three single-phase alternators whose voltages are out of phase by angles of 120°.
The three phases are independent of each other. Wye Connection (Three-Phase) Rather than have six leads from the three-phase alternator, one of the leads from each phase may be connected to form a common junction. The stator is then called wye or star connected. The common lead may or may not be brought out of the alternator. If it is brought out, it is called the neutral lead. The simplified schematic shows a wye connected stator with the common lead not brought out. [Figure 12-321A] Each load is connected across two phases in series. Thus, RAB is connected across phases A and B in series; RAC is connected across phases A and C in series; and RBC is connected across phases B and C in series. Therefore, the voltage across each load is larger than the voltage across a single phase. The total voltage, or line voltage, across any two phases is the vector sum of the individual phase voltages. For balanced conditions, the line voltage is 1.73 times the phase voltage. Since there is only one path for current in a line wire and the phase to which it is connected, the line current is equal to the phase current.
Delta Connection (Three-Phase) A three-phase stator can also be connected so that the phases are connected end to end. [Figure 12-321B] This arrangement is called a delta connection. In a delta connection, the voltages are equal to the phase voltages; the line currents are equal to the vector sum of the phase currents; and the line current is equal to 1.73 times the phase current when the loads are balanced. For equal loads (equal output), the delta connection supplies increased line current at a value of line voltage equal to phase voltage, and the wye connection supplies increased line voltage at a value of line current equal to phase current.
Alternator Rectifier Unit A type of alternator used in the electrical system of many aircraft weighing less than 12,500 pounds is shown in a DC generator, since it is used in DC systems. Although its output is a DC voltage, it is an alternator rectifier unit. This type of alternator rectifier is a self-excited unit but does not contain a permanent magnet. The excitation for starting is