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.
2. An ordinary lamp bulb (24-volt type), with one lead from the center lamp contact and one ground lead attached to the lamp housing, can be used to test circuits with the main circuit power on.
3. A precision voltmeter is used to test circuits with the main circuit power on by placing the positive lead on the circuit point and the negative lead on any convenient ground.
Tests should be made at each terminal of the circuit. Between the last point at which voltage is indicated and the first point at which zero voltage is indicated, there is an open circuit or a voltage drop caused by unit operation or short to ground. If the same voltage reading is obtained on the negative terminal of a unit as was obtained on the positive terminal, an open ground is indicated. If a small voltage reading is obtained on the negative terminal of the unit, a high resistance is indicated between the unit and ground.
The following troubleshooting chart outlines the most commonly encountered malfunctions, a list of probable causes to isolate the malfunction, and the proper corrective action to be taken. This chart is a general guide for troubleshooting a twin-engine d.c. generator system, which utilizes carbon-pile voltage regulators.
• TROUBLE ISOLATION PROCEDURE CORRECTION No voltage indication on any one Check for defective generator Replace generator switch or field generator. switch or field switch. switch.
Determine if generator polarity Flash generator field.
is reversed.
Check for open, shorted, or Replace defective wiring.
grounded wiring.
Check for defective generator. Replace generator.
Low voltage on any one genera- Check voltage regulator adjust- Adjust voltage regulator.
Check for defective voltage reg. Replace voltage regulator.
ulator.
Check for defective wiring. Replace defective wiring.
Check for defective generator. Replace generator.
Generator cuts out. Check for defective reverse-cur- Replace reverse-current cutout rent cutout relay. relay.
Check for defective overvoltage Replace overvoltage relay.
relay.
Check for defective field control Replace field control relay.
relay.
Check for defective voltage reg- Replace voltage regulator.
ulator.
Check for defective wiring. Replace defective wiring.
Voltage unsteady for any one Check for defective wiring. Replace defective wiring.
generator.
Check for defective generator. Replace generator.
Check wear of generator bear- Replace generator.
ings.
No load indication on any one Check for defective reverse-cur- Replace reverse-current cutout generator. Voltage is normal. rent cutout relay. relay.
Check for defective generator Replace generator switch.
switch.
Check for defective wiring. Replace defective wiring.
TROUBLE ISOLATION PROCEDURE CORRECTION Low d.c. bus voltage. Check improper voltage regula- Adjust voltage regulator.
tor adjustment.
Check for defective reverse-cur- Replace reverse-current cutout rent cutout relays.
Voltage high on any one genera- Check for improper voltage reg- Adjust voltage regulator.
tor. ulator adjustment.
Check for defective voltage reg- Replace voltage regulator.
ulator.
Determine if generator field lead Replace shorted wiring or repair A is shorted to positive. connections.
Generator fails to build up more Check voltage regulator or base. Check regulator contacts where than approximately 2 volts. Take precision voltmeter read- they rest on the silver contact ing between A terminal and bar. Any signs of burning at this ground. No voltage reading in- point is cause for replacement of dicates trouble in either regulator or base. Reading of about 2 volts indicates regulator and base are OK.
Check for defective generator. Disconnect generator plug. Place Low ohmmeter reading indicates one lead of ohmmeter on A tercurrent is good and trouble must minal and the other lead on E be within the generator. terminal. High reading indicates that the generator field is open.
Replace generator.
Instrument panel voltmeter read- Check for short across A and B If shorted, change voltage reguing of excessive voltage. terminal of voltage regulator.
Check voltage regulator control. Replace voltage regulator.
Instrument panel voltmeter read- Check for defective voltmeter Place positive lead of voltmeter ing of zero volts. on positive terminal of instrument panel voltmeter and negative lead to ground. Reading should be 27.5 volts. If not, lead from regulator to instrument is defective. Replace or correct lead.
Place positive lead of voltmeter on negative terminal of instrument panel voltmeter and negative lead to ground. If voltmeter reading is zero, instrument panel voltmeter is defective. Replace Check for broken B or E lead.. High resistance most likely is Remove voltage regulator and caused by oil, dirt, or burning at take ohmmeter reading between connector plug or commutator.
B contact finger of regulator Replace generator.
TROUBLE ISOLATION PROCEDURE CORRECTION base and ground. Low reading indicates circuit is OK. High reading indicates that a high resistance is the trouble.
Check for loss of residual mag- Place flasher switch in ON position momentarily. Do not hold NOTE: If flasher switch is held ON rather than switched momentarily, damage may be done to generator field coils.
Voltage does not build up prop- Check for open field. Disconnect Check and repair lead or connecerly when field is flashed. generator connector and take tors.
ohmmeter reading between A and E terminals of generator connectors. High reading indicates field circuit is open.
Check for grounded field. Take Insulation on field winding is ohmmeter reading between A broken. Replace generator.
terminal of generator and generator housing. Low reading indicates field is grounded.
Check for open armature. Re- Replace generator.
move generator cover and inspect commutator. If solder is melted and has been thrown off, then armature is open (caused by generator overheating).
ALTERNATORS An electrical generator is a machine which converts mechanical energy into electrical energy by electromagnetic induction. A generator which produces alternating current is referred to as an a.c.
generator and, through combination of the words alternating" and "generator," the word "alternator" has come into widespread use. In some areas, the word "alternator" is applied only to small a.c. generators. This text treats the two terms synonymously and uses the term "alternator" to distinguish between a.c. and d.c.
generators.
The major difference between an alternator and a d.c. generator is the method of connection to the external circrit; that is, the alternator is connected to the external circuit by slip rings, but the d.c. generator is connected by a commutator.
Types of Alternators Alternators are classified in several ways in order to distinguish properly the various types.
One means of classification is by the type of excitation system used. In alternators used on aircraft, excitation can be affected by one of the following methods:
1. A direct-connected, direct-current generator.
This system consists of a d.c. generator fixed on the same shaft with the a.c. generator. A variation of this system is a type of alternator which uses d.c. from the battery for excitation, after which the alternator is selfexcited.
2. By transformation and rectification from the
a.c. system. This method depends on residual magnetism for initial a.c. voltage buildup, 411
after which the field is supplied with rectified voltage from the a.c. generator. Armature Circuit
3. Integrated brushless type. This arrangement has a direct-current generator on the same shaft with an alternating-current generator.
The excitation circuit is completed through Rotating silicon rectifiers rather than a commutator field and brushes. The rectifiers are mounted on the generator shaft and their output is fed directly to the alternating-current gener- Another method of classification is by the number of phases of output voltage. Alternatingcurrent generators may be single-phase, twophase, three-phase, or even six-phase and more.
In the electrical systems of aircraft, the threephase alternator is by far the most common.
Still another means of classification is by the type of stator and rotor used. From this standpoint, there are two types of alternators: the revolving-armature type and the revolving-field type. The revolving-armature alternator is similar in construction to the d.c. generator, in that the armature rotates through a stationary magnetic field. The revolving-armature alternator is found only in alternators of low power rating and generally is not used. In the d.c. generator, the e.m.f.
generated in the armature windings is converted into a unidirectional voltage (d.c.) by means of the commutator. In the revolving-armature type of alternator, the generated a.c. voltage is applied unchanged to the load by means of slip rings and brushes.
The revolving-field type of alternator (figure 9-34) has a stationary armature winding (stator) and a rotating-field winding (rotor). The advantage of having a stationary armature winding is that the armature can be connected directly to the load without having sliding contacts in the load circuit. A rotating armature would require slip rings and brushes to conduct the load current from the armature to the external circuit. Slip rings have a relatively short service life and arcover is a continual hazard; therefore, high-voltage alternators are usually of the stationary-armature, rotating-field type. The voltage and current supplied to the rotating field are relatively small, and slip rings and brushes for this circuit are adequate.
The direct connection to the armature circuit makes possible the use of large cross-section conductors, adequately insulated for high voltage.
Since the rotating-field alternator is used almost To exciter FIGURE 9-34. Alternator with stationary armature and rotating field.
universally in aircraft systems, this type will be explained in detail, as a single-phase, two-phase, and three-phase alternator.
Single-Phase Alternator Since the e.m.f. induced in the armature of a generator is alternating, the same sort of winding can be used on an alternator as on a d.c. generator.
This type of alternator is known as a single-phase alternator, but since the power delivered by a single-phase circuit is pulsating, this type of circuit is objectionable in many applications.
A single-phase alternator has a stator made up of a number of windings in series, forming a single circuit in which an output voltage is generated.
Figure 9-35 illustrates a schematic diagram of a single-phase alternator having four poles. The stator has four polar groups evenly spaced around the stator frame. The rotor has four poles, with adjacent poles of opposite polarity. As the rotor revolves, a.c. voltages are induced in the stator windings. Since one rotor pole is in the same position relative to a stator winding as any other rotor pole, all stator polar groups are cut by equal numbers of magnetic lines of force at any time. As a result, the voltages induced in all the windings have the same amplitude, or value, at 412
te ime axis FIGURE 9-35. Single-phase alternator.
any given instant. The four stator windings are connected to each other so that the a.c. voltages are in phase, or "series adding." Assume that rotor pole 1, a south pole, induces a voltage in the direction indicated by the arrow in stator winding 1. Since rotor pole 2 is a north pole, it will induce a voltage in the opposite direction in stator coil 2 with respect to that in coil 1.
For the two induced voltages to be in series addition, the two coils are connected as shown in the diagram. Applying the same reasoning, the voltage induced in stator coil 3 (clockwise rotation of the field) is the same direction (counterclockwise) as the voltage induced in coil 1. 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 singlephase windings spaced symmetrically around the stator. In a two-phase alternator there are two single-phase windings spaced physically so that the a.c. 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 9-36. 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 which 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.
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 (A of figure 9-37) shows a wye-connected stator with the common lead not brought out. 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.
A three-phase stator can also be connected so that the phases are connected end-to-end as shown in B of figure 9-37. 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 te ° B -e FIGURE 9-36. Simplified schematic of three-phase alternator with output waveforms.