SEAL 0773676 - Caterpillar

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.
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