Signal Output Circuit
The signal output circuit, which controls field excitation of the AC generator, increases or
decreases field excitation to either raise or lower the AC output voltage.
The feedback circuit takes some of the output of the signal output circuit and feeds it back to the
amplification circuit. It does this to prevent overshooting or undershooting of the desired voltage
by slowing down the circuit response.
Changing Output Voltage
Let us consider an increase in generator load and, thereby, a drop in output voltage. First, the
sensing circuit senses the decrease in output voltage as compared to the reference and lowers its
input to the comparison circuit. Since the reference circuit is always a constant, the comparison
circuit will develop an error signal due to the difference between the sensed voltage and the
reference voltage. The error signal developed will be of a positive value with the magnitude of
the signal dependent on the difference between the sensed voltage and the reference voltage.
This output from the comparison circuit will then be amplified by the amplifier circuit and sent
to the signal output circuit. The signal output circuit then increases field excitation to the AC
generator. This increase in field excitation causes generated voltage to increase to the desired
If the load on the generator were decreased, the voltage output of the machine would rise. The
actions of the voltage regulator would then be the opposite of that for a lowering output voltage.
In this case, the comparison circuit will develop a negative error signal whose magnitude is again
dependent on the difference between the sensed voltage and the reference voltage. As a result,
the signal output circuit will decrease field excitation to the AC generator, causing the generated
voltage to decrease to the desired output.