Aircraft technical Basics: TM 1-406, Aircraft Electrical Systems, 1940: 3. Generator and Regulating Systems

TM 1-406, TECHNICAL MANUAL,  AIRCRAFT ELECTRICAL SYSTEMS, Prepared under direction of the Chief of the Air Corps, WAR DEPARTMENT, WASHINGTON, October 18, 1940.

SECTION lII - GENERATOR AND REGULATING SYSTEMS

17. General.-a. An electric generator is a machine which trans-forms mechanical power into electrical energy by means of the movement of a conductor cutting across a magnetic field.

b. The purpose of the generator, as applied to aircraft, is to supply the energy required to maintain the storage battery of the installation in a charged condition, and at the same time to furnish the necessary energy to operate numerous other units in the electrical system.

c. There are two types of generators in general use, the alternating-current type and the direct-current type. The direct-current generator is used in aircraft at the present time, however, the alternating type is being developed for use in the larger aircraft.

(1) An alternating current changes its direction of flow through the circuit at regular intervals and is classified as single phase, two phase, or three phase.

(a) In the single phase, two reversals of the current occur (fig. 40), the phase being simply one single current flowing in a circuit consisting of two main conductors (fig. 41).

FIGURE 40.-Single-phase alternating current.

FIGURE 41.-Simple form of alternating current generator.

( b ) A two-phase alternating current is one in which two single-phase alternating currents flow in the network comprising the, circuit, which usually consists of four main wires. The two currents pass through the zero point, 90° apart, or one current leads the other by 90° (fig. 42).

FIGURE 42.-Two-phase alternating current.

(c) A three-phase alternating current is one in which three single-phase currents flow in the network forming the circuit, which has three main wires. The three currents pass through the zero point 120° apart. (fig. 43). In any alternating current, two reversals of current comprise a cycle, and are referred to as "frequency" of the current.

FIGURE 43.-Three-phase alternating current.

(2) In the direct current generator, the induced voltage that causes the current to flow in the armature coils is alternating. but the armature coils are connected by means of brushes and the commutator in such manner as to furnish a direct current to the external circuit.

18. Construction principles. - The principal units in the construction of a direct current generator are as follows:

a. Field frame.-This unit of the generator forms the greater portion of the magnetic circuit and is immediately associated with the field winding and pole pieces. The pole pieces of the field frame project inwardly between which the armature rotates. The field coils or windings are wound on the pole pieces and when current. is conducted through them produce an electromagnetic field. A generator may have any even number of field poles, and when only two poles are used the generator is called a bipolar type. When more than two poles are employed, it is called a multipolar generator. The field poles are arranged with the poles of opposite polarity alternated around the field ring assembly. All aircraft generators have multiple poles and have the decided advantage over the bipolar type in producing a more efficient flux distribution with much smaller field magnets. Figure 44 (1) and (2) illustrates the pole arrangement of a bipolar and multipolar generator, and figure 45 shows the various methods used in connecting the field coils together. Electromagnets are used in preference to permanent magnets because of their capability in producing a much stronger field and the ease in which the field current can be controlled to vary the strength of the magnetic field.

FIGURE 44.-Pole arrangements of generators.

FIGURE 45.-Arrangement of field coils in aircraft generators.

b. Armature assembly.-(1) The armature of a generator is the rotating part in which the e. m. f. is induced to furnish current to the closed external circuit. It incorporates a centrally located shaft, laminated soft iron core, the armature coils or windings, and a commutator. The commutator (fig. 46) is so constructed as to convey the current produced in the rotating armature to the commutator brushes which rest on its outside circumference. The brushes make contact with the commutator in such manner as to cause the alternating current produced by the armature to flow out through them as direct current.

FIGURE 46.-Sectional view of commutator

(2) There are two common types of armatures, the ring wound type, figure 47(1) and the drum wound type, figure 47(2). Aircraft generators incorporate the latter type in which the coils are all inter-connected forming a continuous closed circuit. The network of windings in the core slots is called wave or series wound, and a general lay-out of windings in a four-pole generator armature is shown in figure 48.

FIGURE 47.-Types of armatures.

FIGURE 48.-Lay-out of four-pole wave wound armature.

c. Brushes and brush holders.-(1) The commutator brushes, as previously mentioned, are held in contact with the rotating commutator by various types of holders and springs. The two common type holders are the pocket type (which is stationary) (fig. 49(1)) and the pivot or swinging type (fig. 49 (2) ). The springs used in aircraft generators to exert pressure on the brushes are adjustable. The brush material is either carbon or combined carbon and metal, and the brushes are usually provided with a flexible connection fastened to the external part of the circuit to insure positive current conduction.

FIGURE 49.-Types of brush holders.

(2) Several brush arrangements are shown in figure 50. The  arrangement at (1) is used in a two-pole generator; at (2) and (4) in four-pole generators; and at (3) in a six-pole generator. The reason only two brushes are shown at (4), in a four-pole generator, is because one-half of the conventional number may be dispensed with in certain types of armature windings. These two brushes, however, must be spaced on the commutator 90° apart.

FIGURE 50.-Various brush arrangements in two-, four-, and six-pole generators.

19. Operating principles. - a. Due to the simplicity of the ring wound generator figure 51(1), it will be used as an example in explaining the operating principles of the direct current generator; however, it must be remembered that all modern aircraft generators are of the drum wound type. The curves shown in figure 51(2) represent the e. in. f. induced in the coils at various positions of the armature. In all direct-current generators, the field magnets are self-excited, that is, they are excited or magnetized from current produced by the machine itself. If the field magnets were of perfectly pure soft iron, it is probable that no voltage would be induced when the armature is set in motion; however, there is always some residual magnetism left in the poles when the machine is not in motion. When the machine is started, this residual field is sufficient to induce a voltage in the armature coils, causing a current to flow through them. This action strengthens the magnetism in the poles, which in turn increases the armature voltage and the current flow through the field. It is in this manner that the generator field is built up to a high value.

FIGURE 51.-Operating principles of bipolar, ring wound generator

b. In figure 51(1), there are two divided circuits through the armature between the two main brushes. Assuming that the armature is rotating in a clockwise direction, armature coils 1 and 4 are in the center between the two magnetic poles N and S, moving parallel to the field flux, and not cutting across the magnetic lines of force; therefore, they are in a neutral plane, and no e. m. f. is being induced in these coils. Whenever the armature coils of a generator occupy the neutral plane, they must be short-circuited by the brush or brushes. As each brush is shown connecting two commutator segments, the coils 1 and 4 are short-circuited to eliminate sparking at the brushes. Coil 2 is shown within 30° of the point of maximum induction, while coil 3 is shown 30 degrees past the point of maximum induction; therefore, they are cutting across equal numbers of magnetic lines of force. Coils 2 and 3 are connected in series as are coils 5 and 6, and the induced e. m. f. is the sum of coils 2, 3, 5, and 6. However, since the two armature circuits (the coils of the right and left half of the armature) are in parallel, the combined e. m. f. is no greater than the induced e. m. f. in one circuit. Consequently, the total induced e. m. f. of the armature must be equal to the sum of the active coils on one-half of the armature. Inasmuch as the induced e. m. f. on both sides of the armature is equal in value, we need only consider what takes place in one side.

c. (1) The curves in figure 51(2) represent the induced e. m. f. in the coils of the right half of the armature shown in figure 51(1). The vertical lines at the point where they intersect a curve represent the induced e. m. f. in the armature coils for various positions of the armature.

(2) Starting with coil 1, which occupies the neutral plane, the e. m. f. indicated by curve D is zero. At the same time, coils 2 and 3 occupy equal relative positions to and from the point of maximum induction ; therefore, the induced e. m. f. in these coils must be equal and is represented by curves E, and F. The induced e. m. f. in coil 2 is increasing, while in coil 3 it is decreasing, hence lines E. and F cross at point A. At this point in the operation of the armature, the total e. m. f. of the right half of the armature must be the sum of the individual voltage of coils 2 and 3, because they are connected in series, and is represented by line B which is drawn twice the height from point A to the zero line. When the armature is rotated 15° farther from this position, coil 1, which occupied the neutral plane, is starting to cut across the flux, and e. m. f. is being induced in the coil. In this position of the armature, the total induced e. m. f. of the right half of the armature will be the sum of the induced e. m. f. of coils 1, 2, and 3 and is represented by the vertical line C which is equal to the vertical distance of points D, E, and F from the zero line. A similar action is taking place in coils 4, 5, and 6 on the left of the armature. Each coil successively cuts out of and into the circuits in the two halves of the armature when two commutator segments, to which the coil is connected, come into contact with the brushes. This cutting out and coming in of the circuits in the armature coils takes place in the operation of all direct-current generators. The voltage varies to some extent, but due to the rapidity with which the variations occur it requires a very sensitive instrument to detect any pulsation.

FIGURE 52.-Distortion of field due to magnetic induction.

d. When a low value current is flowing through the armature coils of a generator, the magnetic lines of force produced by the field windings flow uniformly across the armature from the north pole piece to the south pole piece. An increase of current from the armature to the external circuit results in producing magnetic poles in the armature core which react with the magnetic field of the pole pieces. The result of this reaction causes the main field flux to be distorted (fig. 52), indicating that the neutral plane has shifted forward in the direction of rotation of the armature to position (A). Obviously the brush assemblies must be shifted to coincide with this plane, or slightly ahead, to prevent sparking at the brushes. The location of the brushes when shifted, with respect to the neutral plane, is called the commutating plane, and its angle in relation to the symmetrical plane is called the angle of lead. The brushes on aircraft generators are adjusted at the factory, and no shifting of the brushes is required.

e. Figure 53 represents four brush positions of the generator armature shown in figure 51 (1). The vertical line X indicates the neutral plane. When the armature is rotated in a clockwise direction, the current flow through the armature coils is in the direction as indicated by the arrows. In position (1) the generator brush has total contact with commutator segment (c) and the total current flow through the brush to the external circuit is half from coil 3 plus half from coil 2.

FIGURE 53. - Cur rent flow in ring wound armature.

(1) As the armature is rotated to position (2), the brush is still in contact with segment (c) and also starting to contact. segment (b). The current flow from the armature is now from coil 3 to segment (c) in the right half of the armature. In the left half, the current is divided as follows: a small but increasing value flows through coil 1 to segment (b), to the brush, and a greater but decreasing value from coil 2 to segment (c), to the brush. The reason that only a small portion of the current is flowing from coil 1 is because only a small area of the brush is in contact with segment (b) which offers considerable resistance. As the armature moves forward this resistance decreases, thereby increasing the current in coil 1 and proportionately decreasing the current in coil 2.

(2) When the armature is in position (3), the brush has equal contact with segments (b) and (c) ; therefore, the current flows through coil 3 on the right half of the armature, and coil 1 on the left half of the armature. The current in coil 2 is now zero because it occupies the neutral plane and is being short-circuited by the brush.

(3) When the armature is rotated to position (4), the current in coil 2 has reversed after passing the neutral plane. The current through the brush contact at segment (b) is increasing, and at the same time decreasing at segment (c). In this position, the total armature current is from coil 1 in the left half of the armature to segment (b). In the right half the current is divided as follows: a small but decreasing value flows through coil 3 to segment (c) (due to the increased contact resistance), and an increasing value flows from coil 2 to segment (b). When the armature is again in position (1) and the brush has full contact with segment (b), the same conditions occur as previously described, except that the total armature current now flows from coil 1 to segment (b) in the left half of the armature and from coil 2 to segment (b) in the right half of the armature. As each of the coils passes through the neutral plane, the current in the coils reverses at the same instant that the brush slides across the segments. However, the current still flows through the brush to the external circuit in the same direction.

f. There are three distinct methods used in winding the wire on the coils of the field magnets to obtain the desired number of ampere turns. The method of winding employed classifies the generator as series wound, shunt wound, or compound wound. A schematic drawing of these windings is illustrated in figure 54.

FIGURE 54. -Simple diagram of three types of generators.

20. Series wound generator.-a. In the series wound generator, the entire armature current passes through the field coils, figure 55 (1). A study of the curve in figure 55(2) reveals that as the generator armature is first revolved there is no current flowing in the field windings; however, there is a small voltage induced due to the residual magnetism in the field magnet. This residual voltage is the factor which causes current to start flowing through the field windings and external circuit, building up the field magnetism, as well as the induced e. m. f. This occurs very rapidly at first, but as the field frame nears its saturation point the voltage and cur-rent become constant. With the armature operating at a constant speed, the voltage and current from a series generator vary with every change in the resistance of the external circuit, since each current change in this external circuit alters the field magnetism current and, consequently, the voltage induced in the armature. In actual practice, the required current from a series generator is constant, irrespective of the amount of the external resistance, while the voltage is altered to suit conditions of the circuit by some means of regulation. The resistance of an external circuit of a series generator must be very low to permit sufficient current to flow through the field windings and produce the necessary field strength.

FIGURE 55.-Circuits and resistance curve of series wound generator

b. Should the resistance of the external circuit be increased to the critical point after the generator has reached its maximum out-put, the voltage and current in the external circuit will immediately drop to practically a zero value. The reason for this is that the current in the field windings controls the strength of the field magnetism, and, if the resistance of the external circuit is increased beyond the critical point, the current in the field will be proportionately reduced. This results in a drop in voltage in the armature and a further decrease in current to the external circuit. It is in this manner that the generator output is decreased to practically a zero value and will not build up again until the resistance of the external circuit is reduced. This type of generator is very seldom used except for industrial purposes.

FIGURE 56.-Circuit diagram of shunt wound generator

21. Shunt wound generator.-In the shunt generator, the field winding is connected directly across or parallel to the armature (fig. 56). In a generator of this type, the current value in the field winding is always directly proportional to the. voltage induced in the armature. The resistance of the shunt field winding is high compared to that of the armature. The induced voltage of the armature of a shunt wound generator builds up rapidly at first, due to an increase in both the field magnetism and speed, but after the field poles become saturated magnetically the increase in the induced voltage beyond this point is in direct proportion to the speed at which the armature is driven. The voltage is at a maximum. when there is no current in the external circuit. As the load is increased, due to a decrease in the external resistance, the voltage falls off regularly until a final point is reached where a further decrease in the external resistance causes both current and voltage to drop and, if all resistance is removed, the generator output will fall to practically zero, as shown by the curve in figure 57.

FIGURE  57.-Effect of decreasing resistance in external circuit of shunt wound generator.

When the resistance of the external circuit is below a certain value, the current flow is excessive and causes a drop in voltage in the armature. Further lowering of the resistance causes a further drop in the voltage across the generator terminals and results in the shunt field current being abnormally reduced. This eventually results in practically a zero output of the generator and is called the critical point. Both the series and shunt generators have critical points; however, the characteristics of the two generators are opposite. The series generator maintains its load only when the external resistance is below a certain value; whereas the shunt generator maintains its load only when the external resistance is above a certain value. Many aircraft generators are of the shunt wound type.

22. Compound wound generator.-a. Figure 58 illustrates the circuits in a compound wound generator. This type generator has a series field winding and a shunt field winding, both wound on the field poles in the same direction, and has the combined characteristics of both the series and shunt types. The shunt. field winding is connected in parallel with the armature, and is acted upon at all times by the total voltage induced in the armature. The current in the shunt field winding is governed by its resistance and voltage; there-fore, as the resistance of the shunt field remains practically constant, with the exception of a variation resulting from temperature changes, the current will always be proportional to the voltage. The series field winding is in series with the armature and the external circuit; consequently, the current in the series field depends upon the current which flows through the external circuit. If the external circuit is open, the generator builds up iii the same manner as the ordinary shunt generator, and the current flows through the series wound field. The drop in voltage in the armature of a shunt generator is compensated for in the compound generator by the current in the series field increasing the field magnetism.

FIGURE 58.-Circuit diagram of compound wound generator.

b. If the magnetizing power of the shunt and series field windings of the compound generator is directly proportional, a drop in voltage occurring in the shunt field is compensated for by an increase in field magnetism, thus maintaining constant voltage at the generator terminals. The later models of aircraft generators are of the compound wound type.

23. Generator regulation.-a. Since all generators are rated for a given power output, it is necessary to regulate or control the generator to prevent it from delivering energy beyond its capacity. The electrical equipment dependent on the generator for power is of a given capacity and, if overloaded, may overheat causing probable failure.

(1) According to Ohm's law, the current that flows in a closed circuit is directly proportional to the voltage acting upon the circuit, and inversely proportional to the resistance of the circuit. Therefore, the power, conducted from the generator to the electrical equipment is in direct proportion to the voltage of the generator and inversely proportional to the resistance of the circuit.

(2) The voltage of a generator depends upon three factors: number of conductors or armature windings; speed or rate of cutting magnetic lines of force; and strength of magnetic field.

b. On all aircraft generators, the voltage is varied by increasing or decreasing the number of lines of force, and is accomplished by varying the current strength in the field windings which changes the number of ampere turns. The method of varying the current strength in the field windings may be accomplished as follows:

(1) Regulation by means of a variable resistance or rheostat in series with the shunt circuit of a shunt wound generator (fig. 59). In a series wound generator, the field may be varied by a rheostat parallel with the field (fig. 60).

FIGURE 59.-Rheostat regulation in shunt wound generator.

FIGURE 60.-Rheostat regulation in series wound generator.

(2) Regulation by means of field distortion, commonly called third brush regulation (fig. 61).

FIGURE 61.-Third brush regulation.

(3) Regulation by means of vibrating type regulators. These are classified as voltage, current, and combination voltage current type regulators. All generator systems used in connection with aircraft utilize the vibrating type regulators; therefore, the subsequent paragraphs will deal specifically with this type.

24. Vibrating voltage regulator.-a. The term "voltage regulation" applies to methods whereby the voltage of a generator, after reaching a predetermined value, is maintained at that value. Figure 62 shows a typical circuit diagram of a. voltage regulator. It. is constructed of a soft iron core and a winding of fine wire (called the voltage winding). At one end of the core is a movable armature carrying one of the contact points, the other contact being fixed or stationary. These contacts are held closed by means of an adjust-able spring. In the modern type voltage regulators, the rapidity of contact, functioning is improved by including a secondary or reverse winding wound on the iron core in the reverse direction of the winding on the voltage coil. Since the demagnetizing winding does not affect, the principle of operation, it is not shown.

FIGURE 62.-Vibrating type voltage regulator. ( In my copy the words "voltage regulator" have been striked out by pen and are replace by "current limiter")

b. When armature (A), figure 62, is rotated, cutting across the residual magnetic field, a low value e. m. f. is induced in the armature windings causing current to flow from the generator through lead (C) to the voltage regulator. Some of the current divides through the contact points (F), through the shunt field winding (B), thence to the negative brush of the generator, and through the voltage winding (D) to the ground, back to the negative brush of the generator. The increase in field strength and in armature speed further increases the induced armature e. m. f., which forces more current through the shunt field (B), and through the voltage winding (D), to the ground. A continuation of this increase in field strength, along with an increase of armature speed, causes the generator e. m. f. to build up rapidly until a predetermined value is reached. This predetermined voltage value or peak must be maintained after the field magnetism is at a maximum, regardless of variations in armature speed; however, without some method of regulation, after the field poles reach a saturation point and the speed of the armature is increased, a higher generator voltage would result. If continued, this high voltage would damage the generator or other equipment connected in or across the circuit. As previously stated, after a predetermined generator voltage is reached (above battery voltage), if line switch (H) is closed, the charging circuit is complete and allows current to flow out of the armature circuit to charge battery (I). The current flowing through the voltage winding (D) is in direct proportion to the voltage of the generator and, is not governed by the amount of current flowing in the charging circuit. Consequently, the regulator will operate when the generator voltage reaches a predetermined value, whether the charging circuit is open or closed. The purpose of the voltage winding is to magnetize the soft iron core sufficiently to open the points (F), normally held closed by means of spring (J). When the voltage of the generator has reached a predetermined value, the current flowing through the voltage winding magnetizes the core to sufficient strength to overcome the spring tension (J) and open the points. If, when contact points (F) open, there was no other path provided for the field current, it would result in a total field collapse and the generator voltage would drop to a residual value. To prevent this total field collapse, a resistance coil (G) is connected across the points, with the result that when the points are opened the field current flows to the ground through this resistor, and due to the increased resistance in the field circuit a consequent weakening of the field magnetism occurs. The weakening of the magnetic field strength results in a drop in the voltage produced in the armature of the generator. When the magnetic effect of the core holding the points open is less than the spring tension, the spring pulls them together and the generator voltage rises again, causing this series of operations to be repeated very rapidly. It is these operations of increasing and decreasing the magnetic field strength which maintain a constant voltage of the generator.

25. Vibrating current regulator.–a. The term "current regulation" applies to methods whereby the current strength of the generator, after reaching a predetermined value, is maintained at that value. The maintenance of a constant current strength from a generator regulated in this manner is dependent upon the resistance of the external circuit, which causes the voltage of the generator to rise until the current strength becomes high enough to operate the regulator, after which the voltage and current values of the generator become practically constant. Unlike voltage regulation, a current regulator always tends to maintain a constant generator current value. If the resistance of the armature circuit is increased, the generator voltage is proportionately increased, and in this manner a current of constant value in the armature circuit is maintained. The only difference in construction between the voltage regulator and the current regulator is that the actuating coil of the voltage regulator consists of many turns of fine wire and is shunted or connected in parallel across the armature circuit whereas, the actuating coil of the current regulator consists of a few turns of comparatively heavy wire and is connected in series with the armature circuit.

FIGURE 63.- Vibrating type  current regulator. ( In my copy the words "current regulator" have been striked out by pen and are replace by "voltage regulator")

b. When armature (A), figure 63, is rotated, cutting across the residual magnetic field, a low value e. m. f. is induced in the armature winding, causing current to flow from the generator through lead (C) to the current regulator. Some of the current divides through the contact points (F) and the shunt field circuit (B), back to the negative brush of the generator, to complete the field circuit. The current is also conducted through the series coil (D) to the battery and ground. Some current flows through the shunt coil (K) to ground (E) and back to the negative brush of the generator to complete the armature circuit. An increase in field strength, plus an increase in armature speed, further increases the induced e. m. f., which forces more current through the shunt field, and more current through armature lead (C), through series coils (D), battery and (K), to the ground. A continuation of this increase in field strength and armature speed causes the generator e. m. f. to build up rapidly, until a value is reached whereby the current in series coil (D) magnetizes the soft iron core, overcoming spring tension (J) to open the contact points (F). With the contact points open, the field current to the ground flows through resistance (G), increasing the resistance in the field circuit and weakening the field magnetism. This weakening of the magnetic field results in a drop of the voltage produced in the armature of the generator, decreasing the current in the series coil. When the magnetic effect on the core holding the points open is less than spring tension (J), the spring closes the points and the generator voltage increases. This cycle of operation is repeated very rapidly. According to this explanation, the shunt field current is decreased in proportion to the rapidity at which. the regulator contacts vibrate.

26. Combination regulator.-In a combined voltage and current regulator, the current regulator is used only as a safety feature and is adjusted to limit, the current output of the generator within the maximum current capacity of the generator. A typical circuit diagram of a combination regulator is shown in figure 64.

FIGURE 64.-Combination voltage and current type regulator circuit.

27. Cut-out relay.-a. The purpose of a cut-out relay is to protect the battery from wasteful discharge of energy. It is connected to the armature or charging circuit between the generator and battery. The contact points of the relay are normally open, so that whenever the voltage of the generator is less than that of the battery, the circuit between the generator and the battery will open. This prevents the battery from wastefully discharging back through the low resistance of the generator armature circuit. This unit also functions in closing the circuit between the generator and battery when the generator volt-age rises above the battery voltage, thus permitting a charge to flow to the battery.

b. The cut-out relay includes a compound winding, a soft iron core, and a contact point assembly. The compound winding consists of a shunt winding connected in parallel with the generator armature circuit and a series winding wound over the shunt winding. A movable armature at one end of the core carries one of the contact points, and t he other contact point is mounted on a fixed plate attached to the core adjacent to the movable armature. The contact points are normally held in an open position by spring tension when the generator is inoperative.

e. The electrical circuits of the cut-out relay are shown in figure 65. The contact points (H) are shown being held open by spring (B) breaking the circuit between the battery and generator. When the generator is operated and its voltage begins to increase, the current is conducted through the circuits (C) and (D) and the shunt coil (E) to the ground. When this current reaches a certain value, the magnetized core closes the contact points, "cutting-in" the battery circuit with the generator through series coil (F). Whenever the generator voltage drops below the battery voltage, the current flow will be reversed, causing a collapse of magnetism in the series coil. This allows the contact points to be immediately pulled open by spring tension, and in this way the battery is prevented from wasteful discharge through the generator.

FIGURE 65.-Cut-out relay circuits.

28. Aircraft generator circuits.-With a thorough understanding of the various circuits described in the preceding paragraphs, the complete generator circuit diagrams incorporated in any aircraft electrical system should be easily interpreted. Figures 66 and 67 illustrate typical generator and control systems used in single- and twin-engine aircraft installations.

FIGURE 66.-Typical single-engine generator and control circuits

FIGURE 67. Typical twin-engine generator and control-circuit diagram.

29. Maintenance.-a. General.-This section includes the maintenance of the control system, in addition to the generator, as the adjustment of the control panel is made in conjunction with the generator and storage battery. The information applies to both the 12- and the 24-volt aircraft systems.

b. Generator.-(1) Check the external leads, connections, and terminals for security and see that they are connected to the properly marked terminal posts.

(2) Remove the generator brush strap and check for worn or loose brushes; binding brushes in brush holders; improper brush spring tension; and dirty or rough commutator.

NOTE.-Improper brush functioning usually results in a low generator voltage output, and a rough or dirty commutator causes excessive arcing at the brushes.

(a) When replacing a brush, the new brush must be seated to the commutator by inserting a strip of No. 000 sandpaper between the brush and the commutator, with the sanded side next to the brush. The sandpaper is then pulled in the direction of generator rotation until the brush is seated properly.

(b) Binding brushes are wiped clean with a lint-free cloth moistened with unleaded gasoline.

(c) Brush springs are adjustable on most types of generators; however, if proper spring tension cannot be obtained the generator should be replaced.

(d) In smoothing a rough or dirty commutator, use No. 000 sand-paper; however, if extremely rough, burned, or pitted replace the generator.

(3) No lubrication of the generator is required between overhaul periods, and if oil is noted in the generator at any time it must be removed and the engine oil seal checked for leakage.

c. Control system.-Check the control panel for security of mounting and condition of the vibration absorbing mounts; terminals, cables, and the contact points of the voltage and current regulators and cut-out relay.

(1) When checking the contact points, the main-line switch should be open. If they require cleaning, the foreign particles can be re-moved by inserting a clean piece of paper between the contacts, pressing them together, and pulling the paper out. After cleaning, note that. all paper particles are removed, otherwise the voltage output will be affected. Do not use a file to resurface the contact points as this operation requires highly skilled personnel.

(2) When an adjustment of the control system is required and the proper test instruments are available, proceed as follows :

(a) Start the engine and with the main-line switch open increase the throttle opening until the rated generator speed is reached. Connect the test voltmeter to the control-panel terminals marked "Pos. Gen." and "B minus," and adjust the voltage regulator by increasing or decreasing the spring tension until the normal voltage of the system is indicated on the test voltmeter. After adjustment, recheck the setting by opening the contacts two or three times, and if normal voltage is maintained, the adjustment is correct.

(b) To adjust the cut-out relay, close the main line switch and slowly increase the engine speed until the test voltmeter indicates the normal cut-in voltage. Adjust the cut-out relay spring to close the contact points at this voltage.

(c) When adjustment is completed, check the ammeter for charging indications.