Exam Question

### (a) Circuit Diagram for an Automatic Voltage Regulator (AVR) with SCR Control
A detailed circuit diagram is required to illustrate the functional blocks of an SCR-based Automatic Voltage Regulator for a brushless alternator. The diagram must clearly show the following key sections, interconnected to form a closed-loop control system:
1. **Sensing Circuit:** A three-phase step-down transformer (star-delta connection) connected to the alternator output terminals. The secondary of this transformer feeds a three-phase bridge rectifier, followed by an L-C (inductor-capacitor) smoothing filter. The output is a smooth DC voltage proportional to the alternator's terminal AC voltage.
2. **Reference and Comparator Circuit:** A Zener reference bridge circuit. This circuit takes the sensed DC voltage from the sensing circuit and compares it against a stable reference voltage provided by a Zener diode. The output of this stage is an 'error voltage', representing the deviation of the actual terminal voltage from the desired setpoint.