Final answer:
In a shunt generator, the critical resistance is the resistance at which the load current is maximum. The electronic speed control mechanism of a DC motor involving a Thyristor and DIAC is known as a Chopper Circuit.
Step-by-step explanation:
A critical resistance is a value of resistance in a shunt generator at which the load current is maximum. It is the resistance that allows maximum power to be delivered to a load. When the resistance is less than the critical resistance, the generator functions as a constant voltage source, while when the resistance is greater than the critical resistance, the generator functions as a constant current source.
The electronic speed control mechanism of a DC motor involving a Thyristor and DIAC is commonly known as a Chopper Circuit. In this mechanism, the Thyristor acts as a switch that controls the current flow to the motor. The DIAC is used to trigger the Thyristor at specific points in the AC voltage cycle, allowing the motor speed to be controlled by adjusting the conduction angle of the Thyristor.