Armature Reaction Current flowing through the armature sets up electromagnetic fields in the windings. These new fields tend to distort or bend the magnetic flux between the poles of the generator from a straight line path. Since armature current increases with load, the distortion becomes greater with an increase in load. This distortion of the magnetic 00000A Rotation офо a Neutral plane 000000 000000 A Field excited, armature unexcited Rotation B Armature excited, field unexcited 0000 Rotation 000000 C Both field and armature excited FIGURE 9-20. Armature reaction.
feld is called armature reaction and is illustrated in figure 9-20.
Armature windings of a generator are spaced in such a way that, during rotation of the armature, there are certain positions when the brushes contact two adjacent segments, thereby shorting the armature windings to these segments. Usually, when the magnetic field is not distorted, there is no voltage being induced in the shorted windings, and, therefore, no harmful results occur from the shorting of the windings. However, when the field is distorted, a voltage is induced in these shorted windings and sparking takes place between the brushes and the commutator segments. Consequently, the commutator becomes pitted, the wear on the brushes becomes excessive, and the output of the generator is reduced. To correct this condition, the brushes are set so that the plane of the coils which are shorted by the brushes is perpendicular to the distorted magnetic field, which is accomplished by moving the brushes forward in the direction of rotation. This operation is called shifting the brushes to the neutral plane, or plane of commutation. The neutral plane is the position where the plane of the two opposite coils is perpendicular to the magnetic field in the generator. On a few generators, the brushes can be shifted manually ahead of the normal neutral plane to the neutral plane caused by field distortion. On nonadjustable brush generators, the manufacturer sets the brushes for minimum sparking.
Interpoles may be used to counteract some of the effects of field distortion, since shifting the brushes is inconvenient and unsatisfactory, especially when the speed and load of the generator are changing constantly. An interpole is a pole placed between the main poles of a generator.
For example, a four-pole generator has four interpoles, which are north and south poles, alternately, as are the main poles. A four-pole generator with interpoles is shown in figure 9-21.
An interpole has the same polarity as the next main pole in the direction of rotation. The magnetic flux produced by an interpole causes the current in the armature to change direction as an armature winding passes under it. This cancels the electromagnetic fields about the armature windings. The magnetic strength of the interpoles varies with the load on the generator; and since field distortion varies with the load, the magnetic field of the interpoles counteracts the effects of the field set up around the armature windings 397
Terminal connections E FIGURE 9-21. Generator with interpoles.
and minimizes field distortion. Thus, the interpole tends to keep the neutral plane in the same position for all loads on the generator; therefore, field distortion is reduced by the interpoles, and the efficiency, output, and service life of the brushes are improved.
Generator Ratings A generator is rated in power output. Since a generator is designed to operate at a specified voltage, the rating usually is given as the number of amperes the generator can safely supply at its rated voltage.
Generator rating and performance data are stamped on the name plate attached to the generator. When replacing a generator, it is important to choose one of the proper rating.
The rotation of generators is termed either clockwise or counterclockwise, as viewed from the driven end. Usually, the direction of rotation is stamped on the data plate. If no direction is stamped on the plate, the rotation may be marked by an arrow on the cover plate of the brush housing. It is important that a generator with the correct direction of rotation be used; otherwise the voltage will be reversed.
The speed of an aircraft engine varies from idle
r.p.m. to takeoff r.p.m.; however, during the major portion of a flight, it is at a constant cruising speed. The generator drive is usually geared to revolve the generator between 18 and 1½ times the engine crankshaft speed. Most aircraft generators have a speed at which they begin to produce their normal voltage. Termed the "coming-in" speed, it is usually about 1,500 r.p.m.
Generator Terminals On most large 24-volt generators, electrical connections are made to terminals marked B, A, and E. (See figure 9-22.) The positive armature lead in the generator connects to the B terminal. The negative armature lead connects to the E terminal.
The positive end of the shunt field winding connects to terminal A, and the opposite end connects to the negative terminal brush. Terminal A receives current from the negative generator brush through the shunt field winding. This current passes through the voltage regulator and back to the armature through the positive brush. Load current, which leaves the armature through the negative brushes, comes out of the E lead and passes through the load before returning to the armature through the positive brushes.
REGULATION OF GENERATOR VOLTAGE Efficient operation of electrical equipment in an airplane depends on a constant voltage supply from the generator. Among the factors which determine the voltage output of a generator, only one, the strength of the field current, can be conveniently controlled. To illustrate this control, refer to the diagram in figure 9-22, showing a simple generator with a rheostat in the field circuit. If the rheostat is set to increase the resistance in the field circuit, less current flows through the field winding and the strength of the magnetic field in which the armature rotates decreases. Consequently, the voltage output of the generator decreases. If the resistance in the field Shunt field Rheostat Load Terminal connections Current flow FIGURE 9-22. Regulation of generator voltage by field rheostat.
ator systems require a different type of regulator, such as the carbon-pile voltage regulator.
Loac FIGURE 9-23. Vibrating-type voltage regulator.
circuit is decreased with the rheostat, more current flows through the field windings, the magnetic field becomes stronger, and the generator produces a greater voltage.
With the generator running at normal speed and switch K open (figure 9-23), the field rheostat is adjusted so that the terminal voltage is about 60 percert of normal. Solenoid S is weak and contact B is held closed by the spring. When K is closed, a short circuit is placed across the field rheostat. This action causes the field current to increase and the terminal voltage to rise.
When the terminal voltage rises above a certain critical value, the solenoid downward pull exceeds the spring tension and contact B opens, thus reinserting the field rheostat in the field circuit and reducing the field current and terminal voltage.
When the terminal voltage falls below a certain critical voltage, the solenoid armature contact B is closed again by the spring, the field rheostat is now shorted, and the terminal voltage starts to rise. The cycle repeats with a rapid, continuous action. Thus, an average voltage is maintained with or without load change.
The dashpot P provides smoother operation by acting as a damper to prevent hunting. The capacitor C across contact B eliminates sparking.
Added load causes the field rheostat to be shorted for a longer period of time and, thus, the solenoid armature vibrates more slowly. If the load is reduced and the terminal voltage rises, the armature vibrates more rapidly and the regulator holds the terminal voltage to a steady value for any change in load, from no load to full load, on the generator.
Vibrating-type regulators cannot be used with generators which require a high field current, since the contacts will pit or burn. Heavy-duty gener- Carbon-Pile Voltage Regulator The carbon-pile voltage regulator depends on the resistance of a number of carbon disks arranged in a pile or stack. The resistance of the carbon stack varies inversely with the pressure applied. When the stack is compressed under appreciable pressure, the resistance in the stack is less. When the pressure is reduced, the resistance of the carbon stack increases, because there is more air space between the disks, and air has high resistance. Pressure on the carbon pile depends upon two opposing forces: a spring and an electromagnet. The spring compresses the carbon pile, and the electromagnet exerts a pull which de- Adjusting knob Insulating tubes Carbon disks Metal A Field plug Armature D FIGURE 9-24. Illustrating the controlling effect of a carbon-pile voltage regulator.
• Adjusting screw . Mica Insulation Insulating tube Carbon stack Spring Compensating ring Spacer Solenoid housing Equalizing coil Voltage col
- Adjustable core FIGURE 9-25. A 24-volt voltage regulator showing internal circuits.
creases the pressure. The coil of the electromagnet, as represented in the diagram in figure 9-24, is connected across the generator terminal B and through a rheostat (adjustable knob) and resistor (carbon disks) to ground.
When the generator voltage varies, the pull of the electromagnet varies. If the generator voltage rises above a specific amount, the pull of the electromagnet increases, decreasing the pressure exerted on the carbon pile and increasing its resistance. Since this resistance is in series with the field, less current flows through the field winding, there is a corresponding decrease in field strength, and the generator voltage drops. On the other hand, if the generator output drops below the specified value, the pull of the electromagnet is decreased and the carbon pile places less resistance in the field winding circuit. In regulator includes a current limiter and a reversecurrent cutout in addition to a voltage regulator.
The action of the voltage regulator unit is similar to the vibrating-type regulator described earlier. The second of the three units is a current regulator to limit the output current of the generator. The third unit is a reverse-current cutout that disconnects the battery from the generator. If the battery is not disconnected, it will discharge through the generator armature when the generator voltage falls below that of the Voltage Current Reverse current regulator limiter cut-out Loads S3 addition, the field strength increases and the generator output increases. A small rheostat pro- Armature vides a means of adjusting the current flow through the electromagnet coil. Figure 9-25 shows L2 a typical 24-volt voltage regulator with its internal L5 circuits. Ć1 { SI FRI {S2}R 2 Three-Unit Regulators Many light aircraft employ a three-unit regu- FIGURE 9-26. Three-unit regulator for lator for their generator systems. This type of variable-speed generators.
battery, thus driving the generator as a motor.
This action is called "motoring" the generator and, unless it is prevented, it will discharge the battery in a short time.
The operation of a three-unit regulator is described in the following paragraphs. (Refer to figure 9-26.)
The action of vibrating contact Cl in the voltage regulator unit causes an intermittent short circuit between points R1 and L2. When the generator is not operating, spring S1 holds Cl closed; C2 is also closed by S2. The shunt field is connected directly across the armature.
When the generator is started, its terminal voltage will rise as the generator comes up to speed, and the armature will supply the field with current through closed contacts C2 and C1.
As the terminal voltage rises, the current flow through Il increases and the iron core becomes more strongly magnetized. At a certain speed and voltage, when the magnetic attraction on the movable arm becomes strong enough to overcome the tension of spring SI, contact points Cl are separated. The field current now flows through RI and L2. Because resistance is added to the field circuit, the field is momentarily weakened and the rise in terminal voltage is checked. Also, since the L2 winding is opposed to the LI winding, the magnetic pull of Ll against SI is partially neutralized, and spring SI closes contact C1.
Therefore, Rl and L2 are again shorted out of the circuit, and the field current again increases;
the output voltage increases, and C1 is opened because of the action of LI. The cycle is rapid and occurs many times per second. The terminal voltage of the generator varies slightly, but rapidly, above and below an average value determined by the tension of spring S1, which may be adjusted.
The purpose of the vibrator-type current limiter is to limit the output current of the generator automatically to its maximum rated value in order to protect the generator. As shown in figure 9-26, L3 is in series with the main line and load.
Thus, the amount of current flowing in the line determines when C2 will be opened and R2 placed in series with the generator field. By contrast, the voltage regulator is actuated by line voltage, whereas the current limiter is actuated by line current. Spring S2 holds contact C2 closed until the current through the main line and L3 exceeds a certain value, as determined by the tension of spring 52, and causes C2 to be opened. The increase in current is due to an increase in load.
This action inserts R2 into the field circuit of the generator and decreases the field current and the generated voltage. When the generated voltage is decreased, the generator current is reduced. The core of L3 is partly demagnetized and the spring closes the contact points. This causes the generator voltage and current to rise until the current reaches a value sufficient to start the cycle again.
A certain minimum value of load current is necessary to cause the current limiter to vibrate.
The purpose of the reverse-current cutout relay is to automatically disconnect the battery from the generator when the generator voltage is less than the battery voltage. If this device were not used in the generator circuit, the battery would discharge through the generator. This would tend to make the generator operate as a motor, but 'because the generator is coupled to the engine, it could not rotate such a heavy load. Under this condition, the generator windings may be severely damaged by excessive current.
There are two windings, L4 and L5, on the soft-iron core. The current winding, LA, consisting of a few turns of heavy wire, is in series with the line and carries the entire line current. 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, 15 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 will 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 Cl 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 will not close again until the generator terminal voltage 401
Main contactor Gen • Batt.
Reverse current coil Pivot Pivot Differential - Voltage relay coil relay contacts -Voltage relay contacts Permanent magnet Permanent magnet Differential coil Temporary magnet Temporary magnet I SW FIGURE 9-27. Differential generator control relay.
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 cutout 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 reversecurrent 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 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 will lower 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 the illustration in figure 9-27 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, which pivot between the pole pieces of temporary magnets wound with relay coils. Voltages of one polarity set up fields about the temporary magnets with polarities which 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 coiloperated 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 will flow through the differential coil, the differential relay contact will close and, thus, complete 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 402
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 cockpit control switch, which opens the contacts of the voltage relay, causing the differential relay coil to be deenergized.
Overvoltage and 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 excessive voltage exists. The overvoltage relay is closed when the generator output reaches 32 volts and completes a circuit to the trip coil of the field control relay. The closing of the field control relay trip circuit opens the shunt field circuit and completes it through a resistor, causing generator voltage to drop; also, the generator switch circuit and the equalizer circuit (multiengine aircraft) are opened.
An indicator light circuit is completed, warning that an overvoltage condition exists. A "reset" position of the cockpit switch is used to complete a reset coil circuit in the field control relay, returning the relay to its normal position.
PARALLELING GENERATORS the same time to supply power for a load, they When two or more generators are operated at a part they are operated in parallel; that is, each supplies a proportional part of the ampere-load. Successful multigenerator operation requires that each generator share the load equally, since a very small increase in the voltage output of one generator will result in that generator's supplying the greater part of the power needed by the load.
The power supplied by a generator for a load is often referred to as ampere-load. Although power is actually measured in watts-the product of voltage and current-the term "ampere-load" is applicable because the voltage output of a generator is considered constant; therefore, the power is directly proportional to the ampere output of the generator.
Negative Lead Paralleling To distribute the load equally among generators operated in parallel, a special coil is wound on the same core as the voltage coil of the voltage regulator. This is part of the equalizing system shown in figure 9-28. A calibrated resistor is located in the lead from the generator negative terminal E to ground. The value of the resistance in this lead is such that when the generator is operating at full current output, there is a 0.5-volt drop across the resistor. This resistor may be a special resistor; it may be a ground lead long enough to have the required resistance, or a series winding of the generator.
The equalizing system depends upon the voltage drop in the individual calibrated resistors. If all generators are supplying the same current, the voltage drop in all ground leads is the same. If the current supplied by the generators is unequal, there is a greater voltage drop in the ground lead of the generator supplying more current. 'Thus, when the No. 1 generator is supplying 150 amperes and the No. 2 generator is supplying 300 amperes, the voltage drop in the negative lead of the No. 1 generator is 0.25 volt and that in the negative lead of the No. 2 generator is 0.5 volt. This means that point E of the No. 1 generator is at a lower voltage than point E of the No. 2 generator, and current will flow in the equalizing circuit from E of the No. 2 generator to E of the No. 1 generator.
The equalizing coil will aid the voltage coil in voltage regulator No. 2 and oppose the voltage coil in regulator No. 1. In this way, the voltage of generator No. 2 will be lowered and that of the other will be increased.
Positive Lead Paralleling The diagram in figure 9-29 shows two generators carrying a total load of 300 amperes. If the generators were sharing this load equally, the ammeters would each indicate 150 amperes. The generators would be "paralleled" and no current would flow in the equalizing coils between the K and D terminals on the regulators. Note, however, that the ammeter for the No. 1 generator indicates only 100 amperes, but the ammeter for the No. 2 generator indicates 200 amperes. This is an unbalanced load and causes current to flow through the equalizing circuit (dotted lines) in the direction indicated by the arrows. The reason is as follows:
With 200 amperes of current flowing through the No. 2 equalizing resistor (from Ohm's law, E = 403
Amp oar Bus Batt gen oSW Gen Voltmeter Gen No. 1 No. 2 150A 300A Equalizer bus D 25 V FIGURE 9-28. Generator equalizer circuits.
I X R), there will be a 0.5 volt drop in voltage across the No. 2 resistor. Since there are only 100 amperes flowing through the No. 1 equalizing resistor, there will be a one-fourth volt drop across that resistor, and a difference in voltage of onefourth volt will exist between the two resistors.
Since current flows from a high pressure (potential) to a lower pressure and from negative to positive, it will be in the direction indicated by the arrows.
When the load is equal, there will not be a difference in voltage between the two resistors.
The current can be traced through the equalizing circuit and through the voltage regulator coils to show the effect on the electromagnets.
With the current in the direction shown, the equalizing coil and voltage coil of the No. 1 voltage regulator set up magnetic fields opposing each other, thus weakening the electromagnet of the No. 1 voltage regulator. This allows the spring to compress the carbon disks, decreasing their resistance and allowing more current to flow in the field circuit of the No. 1 generator. As a result, the voltage output of that generator increases, but at the same time, the current through the equalizing coil and voltage coil of No. 2 voltage regulator sets up magnetic fields that aid each other, thus increasing the strength of that electromagnet. This decreases the spring pressure on the carbon disks, increasing their resistance and allowing less current to flow in the field circuit of the No. 2 generator. As a result, the voltage output of this generator will decrease. With the voltage output of the No. I generator increased, the voltage drop across No. 1 equalizing resistor increases; and with a decrease in voltage output of the No. 2 generator, the voltage drop across the No. 2 equalizing resistor decreases. When the voltage output of the two generators is equal, the voltage drop across the equalizing resistors will also be equal. No current will flow in the equalizing circuit, the load will be balanced, and the ammeters will read approximately the same.
The generators are then paralleled.
The purpose of the equalizing circuit is to help the voltage regulators automatically by lowering the voltage of the high generator and raising the voltage of the low generator, so that the total will be shared equally by the generators.
D. C. GENERATOR MAINTENANCE Inspection The following information about the inspection and maintenance of d.c. generator systems is general in nature because of the large number of differing aircraft generator systems.
These procedures are for familiarization only.
Always follow the applicable manufacturer's instructions for a given generator system.
In general, the inspection of the generator in- 404
100 amps 200 amps No. 1 equalizing No. 2 equalizing resistor .0025 ohms resistor 0025 ohms.
½ V 28 V 300 amps load No. 1 voltage reg. No. 2 voltage reg.
equalizing coil equalizing coil Equalizing bus bar Gen. No. 1 Dotted lines represents the equalizer circuit Gen. No. 2 FIGURE 9-29. Generator and equalizer circuits.
stalled in the aircraft should include the following items:
1. Security of generator mounting.
2. Condition of electrical connections.
3. Dirt and oil in the generator. If oil is present, check engine oil seal. Blow out dirt with compressed air.
4. Condition of generator brushes.
5. Generator operation.
6. Voltage regulator operation.
A detailed discussion of items 4, 5, and 6 is presented in the following paragraphs.
Condition of Generator Brushes Sparking of brushes quickly reduces the effective brush area in contact with the commutator bars.
The degree of such sparking should be determined.
Excessive wear warrants a detailed inspection.
The following information pertains to brush seating, brush pressure, high-mica condition, and brush wear.
Manufacturers usually recommend the following procedures to seat brushes which do not make good contact with slip rings or commutators.
The brush should be lifted sufficiently to permit the insertion of a strip of No. 000, or finer, sandpaper under the brush, rough side out (figure 9-30). Pull sandpaper in the direction of armature rotation, being caretul to keep the ends of the sandpaper as close to the slip ring or commutator surface as possible in order to avoid rounding the edges of the brush. When pulling the sandpaper back to the starting point, the brush should be raised so it does not ride on the sandpaper. The brush should be sanded only in the direction of rotation.
After the generator has run for a short period, brushes should be inspected to make sure that pieces of sand have not become embedded in the brush and are collecting copper.
Under no circumstances should emery cloth or similar abrasives be used for seating brushes 405
Unseated brush No. 000 sandpaper (Sand side next to brush) „1/32" to 1/16" Proper seated brush FIGURE 9-30. Seating brushes with sandpaper.
(or smoothing commutators), since they contain conductive materials which will cause arcing between brushes and commutator bars. se To til presle, one, real "bouncing" of the brushes, resulting in burned and pitted surfaces.
A carbon, graphite, or light metalized brush should exert a pressure of 1½ to 2½ p.s.i. on the commutator. The pressure recommended by the manufacturer should be checked by the use of a spring scale graduated in ounces. Brush spring tension is usually adjusted between 32 to 36 ounces; however, the tension may differ slightly for each specific generator.
When a spring scale is used, the measurement of the pressure which a brush exerts on the commutator is read directly on the scale. The scale is applied at the point of contact between the spring arm and the top of the brush, with the brush installed in the guide. The scale is drawn up until the arm just lifts off the brush surface.
At this instant, the force on the scale should be read.
Flexible low-resistance pigtails are provided on most heavy-current-carrying brushes, and their connections should be securely made and checked at frequent intervals. The pigtails should never be permitted to alter or restrict the free motion of the brush.
The purpose of the pigtail is to conduct the current, rather than subjecting the brush spring to currents which would alter its spring action by overheating. The pigtails also eliminate any possible sparking to the brush guides caused by the movement of the brushes within the holder, thus minimizing side wear of the brush.
Carbon dust resulting from brush sanding should be thoroughly cleaned from all parts of the generators after a sanding operation. Such carbon dust has been the cause of several serious fires as well as costly damage to the generator.
Operation over extended periods of time often results in the mica insulation between commutator bars protruding above the surface of the bars.
This condition is called "high mica? and interferes with the contact of the brushes to the commutator.
Whenever this condition exists, or if the armature has been turned on a lathe, carefully undercut the mica insulation to a depth equal to the width of the mica, or approximately 0.020 inch.
Each brush should be a specified length to work properly. If a brush is too short, the contact it makes with the commutator will be faulty, which can also reduce the spring force holding the brush in place. Most manufacturers specify the amount of wear permissible from a new brush length.
When a brush has worn to the minimum length permissible, it must be replaced.
Some special generator brushes should not be replaced because of a slight grooving on the face of the brush. These grooves are normal and will appear in a.c. and d.c. generator brushes which are installed in some models of aircraft generators.
These brushes have two cores made of a harder material with a higher expansion rate than the material used in the main body of the brush.
Usually, the main body of the brush face rides on the commutator. However, at certain temperatures, the cores extend and wear through any film on the commutator.
Generator Operation If there is no generator output, follow a systematic troubleshooting procedure to locate the malfunction. The following method is an example.
Although this method may be acceptable for most 28-volt, twin-engine or four-engine d.c. generator systems using carbon-pile voltage regulators, the applicable manufacturer's procedures should be followed in all cases.
Bus tact surfaces. Examine the base or housing for cracks. Check all connections for security. Remember that the voltage regulator is a precision Batt Sw Batt instrument and cannot withstand rough treat- Relay ment. Handle it with care. To adjust a voltage Gen. Neg. Generator switch regulator, a precision portable voltmeter is re- (Open) quired. This, too, must be handled with care, since it will not maintain accuracy under conditions of mishandling, vibration, or shock.
Detailed procedures for adjusting voltage regulators are given in applicable manufacturer's instructions. The following procedures are guidelines for adjusting the carbon-pile voltage regulator in a multiengine 28-volt d.c. electrical system:
Generator I. Start and warm up all engines which have
• at operating R.P.M. installed generators.
Mounting base 2. Turn all generator switches to the "off" (Voltage regulator removed) position.
FIGURE 9-31. Checking generator by shorting 3. Connect a precision voltmeter from the B terminals A and B. terminal of one voltage regulator to a good ground.
If the generator is not producing voltage, remove the voltage regulator and, with the engine running at approximately 1,800 r.p.m., short circuit terminals A and B at the subbase of the regulator as shown in the diagram of figure 9-31. Batt If this test shows excessive voltage, the generator is not at fault, but the trouble lies in the voltage Batt regulator. If the test fails to produce voltage, the Relay Generator generator field may have lost its residual Gen. Neg. switch momentarily as shown magnetism. (open) To restore residual magnetism, flash the gener- Connect jumper ator field by removing the regulator and connect terminal A of the voltage regulator base to the battery at a junction box or a bus bar as indicated by the dotted line in the diagram of figure 9-32, while running the engine at cruising r.p.m. If there is still no voltage, check the leads for continuity shorts and grounds. If the generator is located where the brushes and commutator can be inspected, check each for proper condition as prescribed in the applicable manufacturer's procedures. If necessary, replace the brushes and clean the commutator. If the generator is located so that it cannot be serviced in the airplane, Generator at remove it and make the inspection. operating R.P.M.
Mounting base VOLTAGE REGULATOR OPERATION (voltage regulator removed) To inspect the voltage regulator, remove it from FIGURE 9-32. A method of flashing the subbase and clean all the terminals and con- generator field.
• Adjust knob FIGURE 9-33. Adjustment knob on carbon-pile voltage regulator.
4. Increase the engine speed of the generator being checked to normal cruising r.p.m.
Operate remaining engines at idling speed.
5. Adjust the regulator until the voltmeter reads exactly 28 volts. (The location of the adjustment knob on a carbon-pile voltage regulator is shown in figure 9-33.)
6. Repeat this procedure to adjust all voltage regulators.
7. Increase the speed of all engines to normal cruising r.p.m.
8. Close all generator switches.
9. Apply a load equivalent to approximately one-half full load rating of one generator when checking a two-generator system or a load comparable to the rating of one generator when checking a system that has more than two generators.
10. Observe the ammeters or load meters. The difference between the highest and lowest generator current should not exceed the value listed in the manufacturer's maintenance instructions.
11. If the generators are not dividing the load equally (unparalled), first lower-the voltage of the highest generator and slightly raise the voltage of the lowest generator by adjusting the corresponding voltage regulators.When the generators have been adjusted to share the load equally, they are in "parallel."
12. After all adjustments have been made, make a final check of bus voltage from positive bus to ground, with a precision voltmeter.
The voltmeter should read 28 volts, (+0.25 volt on most 28-volt systems). If the bus voltage is not within the proper limits, readjust all voltage regulator rheostats and recheck.
When inspecting the generator relay switch, examine the relay for cleanness and security of mounting and see that all electrical connections are tightly fastened. Look for burned or pitted contacts. Never close the relay manually by pressing the contacts together; this might severely damage the relay or cause an injury. Never adjust the differential type relay, since it closes when the generator voltage exceeds the system voltage by a specified value and is not checked to close at any set voltage; however, check it for proper closing by noting the ammeter indication with the battery generator control switches turned on while running the engines. It is sometimes necessary to put a slight load on the system before the ammeter will show a positive indication when the engine is run up to cruising speed. If the ammeter does not indicate, the relay is probably defective;
therefore, remove it and replace it with a new relay. Check the reverse-current relay for proper opening value. If the relay fails to close when the engine speed is increased or fails to disconnect the generator from the bus bar, the relay is defective.
Troubleshooting If a generator system malfunctions, there are two general possibilities: (1) The generator itself may be at fault (burned out, damaged mechanically, etc.), or (2) that part of the circuit leading to or from the generator may be at fault.
Continuity testing refers to checking for the existence of a complete electrical system between two points. The three main types of continuity testers are:
1. The portable dry cell tester, having a buzzer or a 3-volt lamp to indicate the completed circuit, is used to test circuits with the main circuit power off.