The problem of voltage regulation in an ac system does not differ basically from that in a dc 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 (antihunting) when a load is applied to the system. However, there is one important difference between the regulator system of dc generators and alternators operated in a parallel configuration. The load carried by any particular dc 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 circuits.
When ac generators are operated in parallel, frequency and voltage must both be equal. Where a synchronizing force is required to equalize only the voltage between dc generators, synchronizing forces are required to equalize both voltage and speed (frequency) between ac generators. On a comparative basis, the synchronizing forces for ac generators are much greater than for dc generators. When ac 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 dc 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 dc system and the ac system is that the voltage coil receives its voltage from the alternator line instead of the dc generator. In this arrangement, a three phase, step down transformer connected to the alternator voltage supplies power to a three phase, fullwave rectifier. The 28 volt, dc output of the rectifier is then applied to the voltage coil of the carbon pile 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 antihunting or damping transformer is similar to those in dc systems and performs the same function.
The alternator equalizing circuit is similar to that of the dc 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 ac 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 ac 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 CR1, which provides fullwave rectification of the three phase output from transformer T1. The dc output voltages of CR1 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 RT1 are connected in series with V1 and R1 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 R1 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 Q1. 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 R11 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 control 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 ac generator, which will increase the generator output.
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 K1 relay. The operating coil of this relay is found in the lower left-hand part of the schematic. Relay K1 is connected across the power supply (CR4) for the transistor amplifier. When the generator is started, electrical energy is supplied from the 28 volt dc bus to the exciter generator field, to "flash the field" for initial excitation. When the field of the exciter generator has been energized, the ac generator starts to produce, and as it builds up, relay K1 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.
These units are shown in figure 9-43. Potentiometer P1 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, dc bias, damper circuit, load circuit, and feedback circuit. The dc 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.
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 transformer rectifier terminals T1 and T2. The current flow 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 R6 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. 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 over other types, since it will function on a very small change in voltage. Because of the operating characteristics 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 P1 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 preamplifier. 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.