Electrical Static Automatic Voltage Regulator

Static Automatic Voltage Regulator
The carbon pile regulator uses a magnetic coil powered from the alternator output Strength of the field vanes with alternator voltage and this strength is tested against springs which are the voltage reference. The moving contact regulator employs a similar matching of alternator output effect through a magnetic coil against springs.
The availability of a small transformed, rectified and smoothed power supply from the alternator output makes possible the matching or it directly against an electronic reference in the static automatic voltage regulator. The direct current derived from the alternator output is applied to a bridge which has fixed resistances on two arms and variable resistances (zener diode voltage references) on the other two. The zeners operate in the reverse breakdown mode having been manufactured with a zener breakdown voltage of very low value. As can be seen from the earlier description of zener diodes, voltage remains constant once breakdown has occurred despite change in current. This implies, however, that changes in applied voltage while not affecting voltage across the diode will cause a change in resistance which permits change in current. .As with a Wheatstone bridge, imbalance of the resistances changes the flow pattern and produces in the voltage measuring bridge an error signal
The error signal can be amplified and used to control alternator excitation in a number of different ways. Thus it can control the firing angle of Thyristors (Fig. 3.14) through a triggering circuit to give the desired voltage in the brushless alternator described. It can be used in the statically excited alternator to correct small errors through a magnetic amplifier arrangement-The error signal has also been amplified through transistors in series, for Excitation control.