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.
Rac s Rc
• R Wye connection Delta connection B FIGURE 9-37. Wye- and delta-connected alternators.
phase currents; and the line current is equal to
1.73 times the phase current, when the loads are balanced.
For equal loads (equal kw. output), the delta connection supplies increased line current at a value of line voltage equal to phase voltage, and the wye connection supplies increased line voltage at a value of line current equal to phase current.
Alfernator-Rectifier Unit A type of alternator used in the electrical system of many aircraft weighing less than 12,500 pounds is shown in figure 9-38. This type of power source is sometimes called a d.c. generator, since it is used in d.c. systems. Although its output is a d.c.
voltage, it is an alternator-rectifier unit.
This type of alternator-rectifier is a self-excited unit but does not contain a permanent magnet.
The excitation for starting is obtained from the battery, and immediately after starting, the unit is self-exciting. Cooling air for the alternator is conducted into the unit by a blast air tube on the air inlet cover (figure 9-38).
The alternator is directly coupled to the aircraft engine by means of a flexible drive coupling. The
d.c. output voltage may be regulated by a carbon pile voltage regulator. The output of the alternator portion of the unit is three-phase alternating current, derived from a three-phase, delta-connected system incorporating a three-phase, full-wave bridge rectifier (figure 9-39).
This unit operates in a speed range from 2,100 to 9,000 r.p.m., with a d.c. output voltage of 26-29 volts and 125 amperes.
BRUSHLESS ALTERNATOR One generator now in use is the brushless type.
It is more efficient because there are no brushes to wear down or to arc at high altitudes.
This generator consists of a pilot exciter, an exciter, and the main generator system. The necessity for brushes has been eliminated by utilizing an integral exciter with a rotating armature that has its a.c. output rectified for the main a.c.
field, which is also of the rotating type. A brushless alternator is illustrated in figure 9-40.
FIGURE 9-38. Exploded view of alternator-rectifier.
CR6 +
- B _+ FIGURE 9-39. Wiring diagram of alternatorrectifier unit.
The pilot exciter is an 8-pole, 8,000 r.p.m., 533
c.p.s., a.c. generator. The pilot exciter field is mounted on the main generator rotor shaft and is connected in series with the main generator field (figure 9-40). The pilot exciter armature is mounted on the main generator stator, The a.c.
output of the pilot exciter is supplied to the voltage regulator, where it is rectified and controlled, and is then impressed on the exciter field winding to furnish excitation for the generator.
The exciter is a small a.c. generator with its field mounted on the main generator stator and its 3-phase armature mounted on the generator rotor shaft. Included in the exciter field are permanent magnets mounted on the main generator stator between the exciter poles.
The exciter field resistance is temperature compensated by a thermistor. This aids regulation by keeping a nearly constant resistance at the regulator output terminals. The exciter output is rectified and impressed on the main generator field and the pilot exciter field. The exciter stator has a stabilizing field, which is used to improve stability and to prevent voltage regulator overcorrections for changes in generator output voltage.
The a.c. generator shown in figure 9-40 is a 6-pole, 8,000 r.p.m. unit having a rating of
31.5 kilovolt amperes (KVA), 115/200 volts, 400
c.p.s. This generator is 3-phase, 4-wire, wyeconnected with grounded neutrals. By using an integral a.c. exciter the necessity for brushes within the generator has been eliminated. The a.c.
output of the rotating exciter armature is fed directly into the 3-phase, full-wave, rectifier bridge located inside the rotor shaft, which uses hightemperature silicon rectifiers. The d.c. output from the rectifier bridge is fed to the main a.c. generator rotating field.
Voltage regulation is accomplished by varying the strength of the a.c. exciter stationary fields.
Polarity reversals of the a.c. generator are eliminated and radio noise is minimized by the absence of the brushes. Any existing radio noise is further reduced by a noise filter mounted on the alternator.
The rotating pole structure of the generator is laminated from steel punchings, containing all six poles and a connecting hub section. This provides optimum magnetic and mechanical properties.
Some alternators are cooled by circulating oil through steel tubes. The oil used for cooling is supplied from the constant-speed drive assembly.
Oil flow between the constant-speed drive and the generator is made possible by ports located in the flange connecting the generator and drive assemblies.
Voltage is built up by using permanent magnet interpoles in the exciter stator. The permanent magnets assure a voltage buildup, precluding the necessity of field flashing. The rotor of the alternator may be removed without causing loss of the alternator's residual magnetism.
COMBINED A.C. AND D.C. ELECTRICAL SYSTEMS Many aircraft, especially aircraft of more than 12,500 pounds, employ both a d.c. and an a.c.
electrical system. Often the d.c. system is the basic electrical system and consists of paralleled d.c.
generators with output of, for example, 300 amperes each.
The a.c. system on such aircraft may include both a fixed frequency and a variable frequency system. The fixed frequency system may consist of three or four inverters and associated controls, protective, and indicating components to provide single-phase, a.c. power for frequency sensitive
a.c. equipment. The variable frequency system may consist of two or more engine-driven alternators, with associated control, protective, and indicating components, to provide three-phase,
a.c. power for such purposes as resistive heating on propellers, engine ducts, and windshields.
Stator Terminal block Terminal block Oil inlet cover Rotor Ball bearing Filter assembly Exciter field Pilot exciter Stabilizing armature field Radio noise filter Thermistor Rotor T Pilot Main a.c exciter Main a.c. I generator field Exciter generator field armature stator windings 3-phase bridge rectifier FIGURE 9-40. A typical brushless alternator.
Such combined d.c. and a.c. electrical systems normally include an auxiliary source of d.c. power to back up the main system. This generator is often driven by a separate gasoline or turbinepowered unit.
Alternator Rating The maximum current that can be supplied by an alternator depends upon the maximum heating loss (IR power loss) that can be sustained in the 416
armature and the maximum heating loss that can be sustained in the field. The armature current of an alternator varies with the load. This action is similar to that of d.c. generators. In a.c. generators, however, lagging power factor loads tend to demagnetize the field of an alternator and terminal voltage is maintained only by increasing
d.c. field current. For this reason, alternating current generators are usually rated according to KVA, power factor, phases, voltage, and frequency. One generator, for example, may be rated at 40 KVA, 208 volts, 400 cycles, three-phase, at 75 percent power factor. The KVA indicate the apparent power. This is the KVA output, or the relationship between the current and voltage at which the generator is intended to operate.
The power factor is the expression of the ratio between the apparent power (volt-amperes) and the true or effective power (watts). The number of phases is the number of independent voltages generated. Three-phase generators generate three voltages 120 electrical degrees apart.
Alternator Frequency The frequency of the alternator voltage depends upon the speed of rotation of the rotor and the number of poles. The faster the speed, the higher the frequency will be; the lower the speed, the lower the frequency becomes. The more poles on the rotor, the higher the frequency will be for a given speed. When a rotor has rotated through an angle so that two adjacent rotor poles (a north and a south pole) have passed one winding, the voltage induced in that winding will have varied through one complete cycle. For a given frequency, the greater the number of pairs of poles, the lower the speed of rotation will be.
A two-pole alternator rotates at twice the speed of a four-pole alternator for the same frequency of generated voltage. The frequency of the alternator in c.p.s. is related to the number of poles and the speed, as expressed by the equation F = 2 P × 60 N = 120 PN where P is the number of poles and N the speed in r.p.m. For example, a two-pole, 3,600-r.p.m.
2 X 3,600 alternator has a frequency of = 60 120
c.p.s.; a four-pole, 1,800-r.p.m. alternator has the same frequency; a six-pole, 500-r.p.m. alternator has a frequency of X 500 = 25 c.p.s.; and a 12-pole, 4,000-r.p.m. alternator has a frequency of 12 × 4,000 = 400 c.p.s. 120
VOLTAGE REGULATION OF ALTERNATORS The problem of voltage regulation in an a.c.
system does not differ basically from that in a d.c.
system. In each case the function of the regulator system is to control voltage, maintain a balance of circulating current throughout the system, and eliminate sudden changes in voltage (anti-hunting) when a load is applied to the system. However, there is one important difference between the regulator system of d.c. generators and alternators operated in a parallel configuration. The load carried by any particular d.c. generator in either a two- or four-generator system depends on its voltage as compared with the bus voltage, while the division of load between alternators depends upon the adjustments of their speed governors, which are controlled by the frequency and droop When a.c. generators are operated in parallel, frequency and voltage must both be equal. Where a synchronizing force is required to equalize only the voltage beween d.c. generators, synchronizing forces are required to equalize both voltage and speed (frequency) between a.c. generators. On a comparative basis, the synchronizing forces for a.c.
generators are much greater than for d.c. generators. When a.c. generators are of sufficient size and are operating at unequal frequencies and terminal voltages, serious damage may result if they are suddenly connected to each other through a common bus. To avoid this, the generators must be synchronized as closely as possible before connecting them together.
The output voltage of an alternator is best controlled by regulating the voltage output of the
d.c. exciter, which supplies current to the alternator rotor field. This is accomplished as shown in figure 9-41, by a carbon-pile regulator of a 28-volt system connected in the field circuit of the exciter. The carbon-pile regulator controls the exciter field current and thus regulates the exciter output voltage applied to the alternator field. The only difference between the d.c. system
Exciter field Carbon CT pile M Exciter DC I regulator voltage (generator) Alt field Alternator Damping Pilot's voltage transformer adjustment rheostat Transformer rectifier Alt. reactor L- FIGURE 9-41. Carbon-pile voltage regulator for an alternator.
and the a.c. system is that the voltage coil receives its voltage from the alternator line instead of the
d.c. generator. In this arrangement, a three-phase, step-down transformer connected to the alternator voltage supplies power to a three-phase, full-wave rectifier. The 28-volt, d.c. output of the rectifier is then applied to the voltage coil of the carbonpile regulator. Changes in alternator voltage are transferred through the transformer rectifier unit to the voltage coil of the regulator and vary the pressure on the carbon disks. This controls the exciter field current and the exciter output voltage.
The exciter voltage anti-hunting or damping transformer is similar to those in d.c. systems and performs the same function.
The alternator equalizing circuit is similar to that of the d.c. system in that the regulator is affected when the circulating current supplied by one alternator differs from that supplied by the others.
Alternator Transistorized Regulators Many aircraft alternator systems use a transistorized voltage regulator to control the alternator output. Before studying this section, a review of transistor principles may be helpful.
A transistorized voltage regulator consists mainly of transistors, diodes, resistors, capacitors, and, usually, a thermistor. In operation, current flows through a diode and transistor path to the generator field. When the proper voltage level is reached, the regulating components cause the transistor to cut off conduction to control the alternator field strength. The regulator operating range is usually adjustable through a narrow range. The thermistor provides temperature compensation for the circuitry. The transistorized voltage regulator shown in figure 9-42 will be referred to in explaining the operation of this type of regulator.
The a.c. output of the generator is fed to the voltage regulator, where it is compared to a reference voltage, and the difference is applied to the control amplifier section of the regulator. If the output is too low, field strength of the a.c.
exciter generator is increased by the circuitry in the regulator. If the output is too high, the field strength is reduced.
The power supply for the bridge circuit is CRI, which provides full-wave rectification of the threephase output from transformer T1. The d.c. output voltages of CRI are proportional to the average phase voltages. Power is supplied from the negative end of the power supply through point B, R2, point C, zener diode (CR5), point D, and to the parallel hookup of V1 and R1. Takeoff point C of the bridge is located between resistor R2 and the zener diode. In the other leg of the reference bridge, resistors R9, R7, and the temperature-compensating resistor RTI are connected in series with Vl and Rl through points B, A, and D. The output of this leg of the bridge is at the wiper arm of R7.
As generator voltage changes occur, for example, if the voltage lowers, the voltage across 418
CRI RI V1 CR5 + 28V Special bus R7 R29 R9 R28 3 CR14 R27 Switch underspeed _F+ At CR4 - F _ CR2 R6 S AC Gen.
exciter Gen. neg " bus Y-^ CRATI RIOZ R4 CR3 FIGURE 9-42. Transistorized voltage regulator.
Rl and V1 (once V2 starts conducting) will remain constant. The total voltage change will occur across the bridge circuit. Since the voltage across the zener diode remains constant (once it starts conducting), the total voltage change occurring in that leg of the bridge will be across resistor R2.
In the other leg of the bridge, the voltage change across the resistors will be proportional to their resistance values. Therefore, the voltage change across R2 will be greater than the voltage change across R9 to wiper arm of R7. If the generator output voltage drops, point C will be negative with respect to the wiper arm of R7. Conversely, if the generator voltage output increases, the polarity of the voltage between the two points will be reversed.
The bridge output, taken between points C and A, is connected between the emitter and the base of transistor QI. With the generator output voltage low, the voltage from the bridge will be negative to the emitter and positive to the base.
This is a forward bias signal to the transistor, and the emitter to collector current will therefore increase. With the increase of current, the voltage across emitter resistor RIl will increase. This, in turn, will apply a positive signal to the base of transistor Q4, increasing its emitter to collector current and increasing the voltage drop across the emitter resistor R10.
This will give a positive bias to the base of Q2, which will increase its emitter to collector current and increase the voltage drop across its emitter resistor R4. This positive signal will controi output transistor Q3. The positive signal on the base of Q3 will increase the emitter to collector current.
The control field of the exciter generator is in the collector circuit. Increasing the output of the exciter generator will increase the field strength of the a.c. generator, which will increase the generator output.
P6 → Bias adj T2.# T3 •
• Bias adj , R4 + Input to first
- mag-amp stage FIGURE 9-43. Voltage reference circuits of a typical magnetic amplifier voltage regulator.
To prevent exciting the generator when the frequency is at a low value, there is an underspeed switch located near the F+ terminal. When the generator reaches a suitable operating frequency, the switch will close and allow the generator to be excited.
Another item of interest is the line containing resistors R27, R28, and R29 in series with the normally closed contacts of the KI relay. The operating coil of this relay is found in the lower left-hand part of the schematic. Relay KI is connected across the power supply (CR4) for the transistor amplifier. When the generator is started, electrical energy is supplied from the 28-volt d.c.
bus to the exciter generator field, to "flash the field" for initial excitation. When the field of the exciter generator has been energized, the a.c.
generator starts to produce, and as it builds up, relay KI is energized, opening the "field flash" circuit.
Magnetic Amplifier Regulator Because of their lack of moving parts, this type of voltage regulator is referred to as a static voltage regulator. Some static regulators employ electron tubes or transistors as amplifiers to achieve the necessary high energy gain, but the most commonly used static regulator utilizes a magnetic amplifier.
The magnetic amplifier voltage regulator is somewhat heavier and larger than a carbon-pile regulator of the same rating. Because of the absence of moving parts, regulators of this type do not require shock or vibration mounts.
This regulator consists of a voltage reference circuit, a two-stage magnetic amplifier, and the associated power transformer and rectifier. The reference circuit consists of a three-phase rectifier, a potentiometer (P1), and a bridge circuit made up of two fixed resistors and two glow tubes.