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.
12-167 Since it is driven by a drive gear in the transmission, the tachometer (often called tach) generator, a three-phase unit, has a voltage proportional to the speed of the output drive. The rectifier changes its voltage from AC to DC. After rectification, the current flows through the resistor, rheostat, and valve and solenoid. Each of these units is connected in series. [Figure 12-338] Under normal operating conditions, the output of the tach generator causes just enough current to enter the valve and solenoid coil to set up a magnetic field of sufficient strength to balance the spring force in the valve. When the alternator speed increases as the result of a decrease in load, the tach generator output also increases. Because of the greater output, the coil in the solenoid is sufficiently strengthened to overcome the spring force. Thus, the valve moves and, as a result, oil pressure enters the reduced speed side of the control cylinder.
In turn, the pressure moves the piston, causing the angle of the pump wobble plate to be reduced. The oil on the other side of the piston is forced back through the valve into the system return. Since the angle of the pump wobble plate is smaller, there is less pumping action in the transmission. The result is decreased output speed. To complete the cycle, the procedure is reversed. With the output speed reduction, tach generator output decreases; consequently, the flow of current to the solenoid diminishes. Therefore, the magnetic field of the solenoid becomes so weak that the spring is able to overcome it and reposition the valve.
If a heavy load is put on the AC generator, its speed decreases. The generator is not driven directly by the engine; the hydraulic drive allows slippage. This decrease causes the output of the tach generator to taper off and, as a result, weakens the magnetic field of the solenoid coil. The spring in the solenoid moves the valve and allows oil pressure to enter the increase side of the control cylinder and the output speed of the transmission is raised. There are still two important circuits that must be discussed: the overspeed circuit and the load division circuit. The generator is prevented from overspeeding by a centrifugal switch (S) in Figure 12-339 and the overspeed solenoid coil (R), which is located in the solenoid and valve assembly.
The centrifugal switch is on the transmission and is driven through the same gear arrangement as the tach generator. 12-168 AB E C D F The aircraft DC system furnishes the power to operate the overspeed coil in the solenoid and coil assembly. If the output speed of the transmission reaches a speed of 7,000 to 7,500 rpm, the centrifugal switch closes the DC circuit and energizes the overspeed solenoid. This component then moves the valve and engages the latch that holds the valve in the underdrive position. To release the latch, energize the underdrive release solenoid. The load division circuit’s function is to equalize the loads placed on each of the alternators, which is necessary to assure that each alternator assumes its share; otherwise, one alternator might be overloaded while another would be carrying only a small load.
In Figure 12-340, one phase of the alternator provides power for the primary in transformer (G), whose secondary supplies power to the primaries of two other transformers (J 1 and J2). Rectifiers (K) then change the output of the transformer secondaries from AC to DC. The function of the two capacitors (L) is to smooth out the DC pulsations. The output of the current transformer (F) depends upon the amount of current flowing in the line of one phase. In this way, it measures the real load of the generator. The output voltage of the current transformer is applied across resistor (H). This voltage is added vectorially to the voltage applied to the upper winding of transformer (J) by the output of transformer (F). At the same time as it adds vectorially to the upper winding of transformer (J), it subtracts vectorially from the voltage applied to the lower winding of (J).
This voltage addition and subtraction depends on the real load of the generator. The amount of real load determines the phase 12-169 A B angle and the amount of voltage impressed across resistor (H). The greater the real load, the greater the voltage across (H), and hence, the greater the difference between the voltages applied to the two primaries of transformer (J). The unequal voltages applied to resistor (M) by the secondaries of transformer (J) cause a current flow through the control coil (P). The control coil is wound so that its voltage supplements the voltage for the control coil in the valve and solenoid assembly. The resulting increased voltage moves the valve and slows down the generator’s speed. Why should the speed be decreased if the load has been increased? Actually, systems using only one generator would not have decreased speed, but for those having two or more generators, a decrease is necessary to equalize the loads.
The load division circuit is employed only when two or more generators supply power. In such systems, the control coils are connected in parallel. If the source voltage for one of these becomes higher than the others, it determines the direction of current flow throughout the entire load division circuit. As explained before, the real load on the generator determines the amount of voltage on the control coil; therefore, the generator with the highest real load has the highest voltage. As shown in Figure 12-341, current through No. 1 control coil, where the largest load exists, aids the control coil of the valve and solenoid, thereby slowing down the generator. (The source voltage of the control coils is represented by battery symbols in Figure 12-341.) The current in the remaining control coils opposes the control coil of the valve and solenoid, in order to increase the speed of the other generators so the load is more evenly distributed.
On some drives, instead of an electrically-controlled governor, a flyweight-type governor is employed, which consists of a recess-type revolving valve driven by the output shaft of the drive, flyweights, two coil springs, and a nonrotating valve stem. Centrifugal force, acting on the governor flyweights, causes them to move outward, lifting the valve stem against the opposition of a coil spring. The valve stem position controls the directing of oil to the two oil outlines. If the output speed tends to exceed 6,000 rpm, the flyweights lift the valve stem to direct more oil to the side of the control piston, causing the piston to move in a direction to reduce the pump wobble plate angle. If the speed drops below 6,000 rpm, oil is directed to the control piston so that it moves to increase the wobble plate angle.
Overspeed protection is installed in the governor. The drive starts in the underdrive position. The governor coil springs are fully extended and the valve stem is held at the limit of its downward travel. In this condition, pressure is directed to the side of the control piston giving minimum wobble plate angle. The maximum angle side of the control piston is open to the hollow stem. As the input speed increases, the flyweights start to move outward to overcome the spring bias. This action lifts the valve stem and starts directing oil to the maximum side of the control piston, while the minimum side is opened to the hollow stem.
At about 6,000 rpm, the stem is positioned to stop drainage of either side, and the two pressures seek a balance point as