Manual Voltage Control
The generator requires excitation of the rotating field with d.c. power. This can be obtained from a small d.c. exciter generator as shown in (Refer to Systems Operation, "Alternating Current Generator" of this Manual - Illustrations 11 and 12), or from a static source of d.c. power, on type of which will be mentioned later. Generators with rotating d.c. exciters will be considered in the following description.
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| Illustration 1 | g01059185 |
Schematic of an generator with rotating D.C. exciter illustrating how the voltage control rheostat controls generator output. | |
Illustration 1 represents an generator with rotating d.c. exciter. The output of the generator is controlled by the power available from the engine driving it and the magnetic strength of the generator rotating field. The engine governor controls the engine power. The exciter output controls the magnetic strength of the rotating field. The exciter output is controlled by the magnetic strength of the exciter shunt field. The magnetic strength of the exciter shunt field is controlled by adjustment of a voltage control rheostat, or variable resistance, in the circuit between the exciter armature and the exciter field.
A single generator operating at constant speed will deliver a terminal voltage which is almost directly proportional to its excitation or the magnetic strength of its rotating field. When an generator is developing rated voltage on open circuit, its excitation level is a certain value. If it is desired to raise or lower the voltage, the excitation must be raised or lowered. When the excitation is raised there is more magnetizing power (or filed current) available than is required for generation of rated voltage. This power must be expended in some manner. In the case of the single unloaded generator this magnetizing power is expended in raising the terminal voltage. A very small part of it changes to heat. If the excitation is lowered from the value required to generate rated open circuit voltage, there is a shortage of magnetizing power and the voltage drops. It will require the addition of some magnetizing power to raise the voltage back to the rated value.
When as isolated generator developing rated open circuit voltage – with the excitation fixed at the value required to develop this voltage – has a load circuit connected to its terminals, the voltage will cause a current to flow through the load. This same current circulates back through the generator. Since all electrical circuits exhibit resistance to the flow of electric current, there will be a loss in voltage in the windings of the generator. This voltage drop will be proportional to the amount of current flowing and the impedance of the generator windings. This loss of voltage due to impedance and current flow resembles the loss in voltage due to a reduction in the magnetic strength of the generator rotating field.
Impedance exists in A.C. circuits in the same manner as resistance exists in D.C. circuits. The characteristics of an impedance is a combination of resistance and reactance. In D.C. circuits, the voltage drop across a circuit is equal to the current in amperes flowing through the circuit multiplied by the resistance in ohms of the circuit, E = IR. In A.C. circuits the voltage drop across a circuit is equal to the current in amperes flowing through the circuit multiplied by the impedance in ohms of the circuit, E = IZ.
If the connected load is not greater than the rated load of the unit, the voltage can be returned to rated value by increasing the excitation of the generator. This is done by reducing the resistance on the exciter field. This results in more excitation in the exciter. The exciter voltage increases and more current flows through the generator field. This increase in current causes an increase in the magnetizing power (field current) of the generator rotor and, correspondingly an increase in the generator generated voltage. The voltage will rise toward the rated value. Of course, more current will flow through the load and the generator as the voltage increases, but a setting of the exciter field control can be made which will result in the generator operating at rated voltage when connected to a load if the load in not greater than rated load. If the exciter control should be advanced beyond the point at which the increased excitation will compensate for the voltage drop at the generator terminals, the voltage will rise past the rated value. This indicates that too much magnetizing power is being supplied to the generator rotor. The excess is expended in increasing the terminal voltage.
Automatic Voltage Control
The changes in the control of the exciter field circuit can be made to occur automatically by the use of a simple automatic voltage regulator which has a circuit connected to the generator terminals as shown in Illustration 2. Usually the generator voltage is used to energize an electromagnet. The pull of the electromagnet is opposed by a spring. Both the magnet and the spring can act to move the control arm of a variable resistance element in the exciter field circuit. If the terminal voltage decreases, the pull of the electromagnet weakens and the spring pulls on the control arm, moving it in the direction to decrease the resistance in the exciter field circuit. This permits the excitation of the generator to increase and the voltage increases. Conversely, if the voltage is too high, the pull of the electromagnet increases and the control arm moves in the direction to decrease the excitation and thus decrease the terminal voltage. The opposing forces of the spring and the electromagnet are in balance when the generator voltage is the value set by the voltage level control.
Detailed descriptions of various automatic voltage control arrangements including static regulators (which consist of electronic circuitry with no moving parts) are available from Caterpillar Tractor Co.
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| Illustration 2 | g01059188 |
Schematic of an automatic voltage control regulator. | |