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.
PЗ Stabilizing Load windings Damper feedback Ref Bias _P4 ¿ RE + Input signal Output signa
• +
d. c. bias Ref Bias Load windings Stabilizing Damper feedback winding From generator damper windings FIGURE 9-44. First stage of a magnetic amplifier voltage regulator.
These units are shown in figure 9-43. Potentiometer Pl is adjusted so that, at rated bus voltage, there is a zero potential difference between points A and B on the bridge circuit. For any other input voltage, the voltage drop across the glow tubes causes a potential to exist between points A and B.
For example, if the generator voltage is low, the current flow through the arms of the bridge will be reduced. The voltage across R4 will be less than the fixed voltage across V1; consequently, point B will be at a higher potential than point A.
This gives an error signal used as an input to the first mag-amp (magnetic amplifier) stage. For high input voltages the error signal polarity is reversed.
The second unit in the system is the magnetic amplifier. The circuitry for the first stage of a typical mag-amp voltage regulator is shown in figure 9-44. This unit consists of two reactors, supply voltage transformers and rectifiers, and the following windings: reference, d.c. bias, damper circuit, load circuit, and feedback circuit. The d.c.
bias winding fixes the operating level of the reactors and is adjusted by potentiometers P5 and P6.
Potentiometer P6 regulates the magnitude of the bias voltage, and P5 regulates the magnitude of biasing current on each reactor to overcome the slight differences in the two cores and the associated rectifiers. If the bias voltage is properly adjusted and if a zero error signal input exists, the voltages developed across R5 and R6 will be equal and the output will be zero.
The damper circuit is connected into the circuit and is used as a stabilizing winding. Its source of power is the damper winding of the generator.
The generator damper winding is energized through transformer action by a changing generator excitation current and is, therefore, proportional to the rate of change of excitation. This current is used as a feedback signal in the first magnetic amplifier stage because its polarity always opposes the error signal input.
R5 R6 DC Bias adj P7 To gen. exciter regulator winding E, = Input signal FIGURE 9-45. Second mag-amp stage of voltage regulator.
The magnitude of the damper feedback current is adjusted with potentiometer P4. Its function is to establish the recovery time of the regulator and to provide stable operation. The potentiometer should be adjusted to provide fast voltage recovery during stable operation under normal load conditions.
Next, the feedback winding receives a voltage that is proportional to the output voltage; this provides stability during steady load conditions.
A look at the circuit will disclose that the load winding receives its power from transformerrectifier terminals T1 and T2. The current fow through these windings and load resistors R5 and R6 is regulated by the degree of magnetization of the reactor cores, established by the current flow in the various control windings.
Figure 9-44 also illustrates that, when the input signal is not zero, the currents through R5 and Ro will not be equal. The unequal currents in these resistors provide a potential difference which is the output signal for this stage, the polarity of which depends on the polarity of the error signal input.
All of the units in the regulator have been discussed except the output stage, which is referred to as the second stage of the regulator.
This is a three-phase, full-wave, magnetic amplifier, as shown in figure 9-45. The output of the first stage, which we have just discussed, is fed into the control winding of the second stage. The output of this stage is the generator exciter-regulator field voltage. The magnitude of this voltage is established by the magnitude and polarity of the input signal, the bias current which is adjustable by P7, and also by the feedback current which is proportional to the output.
This type of regulator has a distinct advantage 422
FIGURE 9-46. Constant-speed drive.
over other types, since it will function on a very small change in voltage. Because of the operating characteristica of this type of regulator, variations in the output voltage will be within 1 percent.
The various adjustments on the unit, with the exception of those on P1, have been discussed.
Adjustments on PI are to be accomplished only on the bench, when the regulator is being calibrated.
Potentiometer P1 is located in the center of the front face of the regulator adjacent to the voltmeter jacks, The potentiometer may be adjusted while the regulator is installed on the aircraft to set the bus voltage to the desired value.
The voltage regulator is divided into three main parts: the voltage error detector, the preamplifier, and the power amplifier. These three units work together in a closed-loop circuit with the generator exciter regulator winding to maintain nearly constant voltage at the generator output terminals.
The function of the error detector is to sample the generated voltage, compare it with a fixed standard, and send the error to the preamplifier.
The detector includes a three-phase rectifier; a variable resistor for voltage adjustment, and a bridge consisting of two voltage reference tubes and two resistors. In operation, if the generator voltage ranges above or below its rated value, a current will flow either in one direction or the other, depending on the polarity developed in the bridge circuit.
The preamplifier receives an error signal from the voltage error detector. With the use of magnetic amplifiers, it raises the signal to a sufficient level to drive the power amplifier to full output, required for proper excitation.
The power amplifier delivers a signal to the exciter regulator winding; its magnitude depends on the signal from the preamplifer. This will raise or lower the voltage of the exciter regulator winding, which, in turn, will raise or lower the output voltage of the generator.
ALTERNATOR CONSTANT-SPEED DRIVE Alternators are not always connected directly to the airplane engine like d.c. generators. Since the various electrical devices operating on a.c. supplied by alternators are designed to operate at a certain voltage and at a specified frequency, the speed of the alternators must be constant; however, the speed of an airplane engine varies. Therefore, some alternators are driven by the engine through a constant-speed drive installed between the engine and the alternator.
A typical hydraulic-type drive is shown in figure 9-46. The following discussion of a constantspeed drive system will be based on such a drive, found on large multiengine aircraft.
The constant-speed drive is a hydraulic transmission which may be controlled either electrically or mechanically.
The constant-speed drive assembly is designed to deliver an output of 6,000 r.p.m., provided the input remains between 2,800 and 9,000 r.p.m. If the input, which is determined by engine speed, is below 6,000 r.p.m., the drive increases the speed in order to furnish the desired output. This stepping up of speed is known as overdrive.
In overdrive, an automobile engine will operate at about the same r.p.m. at 60 m.p.h. as it does in conventional drive at 49 m.p.h. In aircraft, this principle is applied in the same manner. The constant-speed drive enables the alternator to produce the same frequency at slightly above engine-idle r.p.m. as it would at takeoff or cruising r.p.m.
With the input speed to the drive set at 6,000
r.p.m., the output speed will be the same. This is known as straight drive and might be compared to an automobile in high gear. However, when the input speed is greater than 6,000 r.p.m., it must be reduced to provide an output of 6,000 r.p.m. This is called underdrive, which is comparable to an automobile in low gear. Thus, the large input, caused by high engine r.p.m., is reduced to give the desired alternator speed.
As a result of this control by the constant-speed drive, the frequency output of the generator varies from 420 c.p.s. at no load to 400 c.p.s. under full This, in brief, is the function of the constantspeed drive assembly. Before discussing the various units and circuits, the overall operation of the transmission should be discussed.
cylinder block gear, which it drives, as well as to the makeup and scavenger gear pumps E.
The makeup (charge) pump delivers oil (300
p.s.i.) to the pump and motor cylinder block, to the governor system, and to the pressurized case, whereas the scavenger pump returns the oil to the external reservoir.
The rotating cylinder assembly B consists of the pump and motor cylinder blocks, which are bolted to opposite sides of a port plate. The two other major parts are the motor wobbler A and the pump wobbler C. The governor system is the unit at the top of the left side in the illustration.
The cylinder assembly has two primary units.
The block assembly of one of the units, the pump, contains 14 cylinders, each of which has a piston and pushrod. Charge pressure from the makeup pump is applied to each piston in order to force it outward against the pushrod. It, in turn, is pushed against the pump wobble plate.
If the plate remained as shown in part A of figure 9-48, each of the 14 cylinders would have equal pressure, and all pistons would be in the same relative position in their respective cylinders.
But with the plate tilted, the top portion moves outward and the lower portion inward, as shown in part B of the illustration. As a result, more oil enters the interior of the upper cylinder, but oil will be forced from the cylinder of the bottom If the pump block were rotated while the plate remained stationary, the top piston would be forced inward because of the angle of the plate.
This action would cause the oil confined within the cylinder to be subjected to increased pressure great enough to force it into the motor cylinder block assembly.
Before explaining what the high-pressure oil in the motor unit will do, it is necessary to know something about this part of the rotating cylinder block assembly. The motor block assembly has 16 Hydraulic Transmission The transmission is mounted between the generator and the aircraft engine. Its name denotes that hydraulic oil is used, although some transmissions may use engine oil. Refer to the cutaway view of such a transmission in figure 9-47. The input shaft D is driven from the drive shaft on the accessory section of the engine. The output drive F, on the opposite end of the transmission, engages the drive shaft of the generator.
The input shaft is geared to the rotating cylinders, each with its piston and pushrod. These are constantly receiving charge pressure of 300
p.s.i. The position of the piston depends upon the point at which each pushrod touches the motor wobble plate. These rods cause the wobble plate to rotate by the pressure they exert against its sloping surface.
This is how the action works: the piston and pushrod of the motor are pushed outward as oil is forced through the motor valve plate from the pump cylinder. The pushrods are forced against the motor wobble plate, which is free to rotate but 424
FICURE 9-47. Cutaway view of a hydraulic transmission.
cannot change the augle at which it is set. Since the pushrods cannot move sideways, the pressure exerted against the motor wobble plate's sloping face causes it to rotate.
In the actual transmission, there is an adjustable wobble plate. The tilt of the pump wobble plate is determined by the control cylinder assembly. For example, it is set at an angle which causes the motor cyliaders to turn the motor wobble plate faster than the motor assembly, if the transmission is in overdrive. The result described is produced by the greater pressure in the pump and motor cylinders.
With the transmission in underdrive, the angle is arranged so there is a reduction in pumping action. The subsequent slippage between the pushrods and motor wobble plate reduces the output speed of the transmission. When the pump wobble plate is not at an angle, the pumping action will be at a minimum and the transmission will have what ia known as hydraulic lock. For this condition, the input and output speed will be about the same, and the transmission is considered to be in straight drive.
To prevent the oil temperature from becoming excessively high within the cylinder block, the 425