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.
12-162 Exciter (if used) Voltage control Start-stop pushbutton Reset button VM 5W V A W AM 5W Alternator Motor starting panel Alternator control panel Input terminal Output terminal Motor Terminal connections Current flow Load Shunt field Rheostat B + – A E The voltage winding, L5, consisting of a large number of turns of fine wire, is shunted across the generator terminals. When the generator is not operating, the contacts, C3 are held open by the spring S3. As the generator voltage builds up, L5 magnetizes the iron core. When the current (as a result of the generated voltage) produces sufficient magnetism in the iron core, contact C3 is closed, as shown. The battery then receives a charging current. The coil spring, S3, is so adjusted that the voltage winding does not close the contact points until the voltage of the generator is in excess of the normal voltage of the battery. The charging current passing through L4 aids the current in L5 to hold the contacts tightly closed. Unlike C1 and C2, contact C3 does not vibrate. When the generator slows down or, for any other cause, the generator voltage decreases to a certain value below that of the battery, the current reverses through L4 and the ampere turns of L4 oppose those of L5.
Thus, a momentary discharge current from the battery reduces the magnetism of the core and C3 is opened, preventing the battery from discharging into the generator and motoring it. C3 does not close again until the generator terminal voltage exceeds that of the battery by a predetermined value. Differential Relay Switch Aircraft electrical systems normally use some type of reverse current relay switch, which acts not only as a reverse current relay cut-out but also serves as a remote control switch by which the generator can be disconnected from the electrical system at any time. One type of reverse current relay switch operates on the voltage level of the generator, but the type most commonly used on large aircraft is the differential relay switch, which is controlled by the difference in voltage 12-163 Load P K C B S Voltage regulator Armature Shunt field Storage battery
Current
limiter Reverse current cut-out Loads + – + – S3 L4L3L1 L2 C1 C2 C3 S1 S2R1 R2 L5 A between the battery bus and the generator. The differential type relay switch connects the generator to the main bus bar in the electrical system when the generator voltage output exceeds the bus voltage by 0.35 to 0.65 volt. It disconnects the generator when a nominal reverse current flows from the bus to the generator. The differential relays on all the generators of a multiengine aircraft do not close when the electrical load is light. For example, in an aircraft having a load of 50 amperes, only two or three relays may close.
If a heavy load is applied, the equalizing circuit lowers the voltage of the generators already on the bus and, at the same time, raise the voltage of the remaining generators, allowing their relays to close. If the generators have been paralleled properly, all the relays stay closed until the generator control switch is turned off or until the engine speed falls below the minimum needed to maintain generator output voltage. The differential generator control relay shown in Figure 12-331 is made up of two relays and a coil-operated contactor. One relay is the voltage relay and the other is the differential relay.
Both relays include permanent magnets that pivot between the pole pieces of temporary magnets wound with relay coils. V oltages of one polarity set up fields about the temporary magnets with polarities that cause the permanent magnet to move in the direction necessary to close the relay contacts; voltages of the opposite polarity establish fields that cause the relay contacts to open. The differential relay has two coils wound on the same core. The coil-operated contactor, called the main contactor, consists of movable contacts that are operated by a coil with a movable iron core. Closing the generator switch on the control panel connects the generator output to the voltage relay coil. When generator voltage reaches 22 volts, current flows through the coil and closes the contacts of the voltage relay. This action completes a circuit from the generator to the battery through the differential coil.
When the generator voltage exceeds the bus voltage by 0.35 volt, current flows through the differential coil, the differential relay contact closes and, thus, completes the main contractor coil circuit. The contacts of the main contactor close and connect the generator to the bus. When the generator voltage drops below the bus (or battery) voltage, a reverse current weakens the magnetic field about the temporary magnet of the differential relay. The weakened field permits a spring to open the differential relay contacts, breaking the circuit to the coil of the main contactor relay, opening its contacts, and disconnecting the generator from the bus. The generator battery circuit may also be broken by opening the flight deck control switch, which opens the contacts of the voltage relay, causing the differential relay coil to be de-energized.
Overvoltage & Field Control Relays Two other items used with generator control circuits are the overvoltage control and the field control relay. As its name implies, the overvoltage control protects the system when