Question 8 QMED02 - Electrician-Refrigerating Engineer

The Correct Answer is D. **Why option D ("using a storage battery or battery charger to 'flash' the field") is correct:** Alternators and generators rely on a small amount of **residual magnetism** in the iron core of their field poles to initiate the generation process. This residual magnetism induces a tiny voltage when the rotor starts turning, which feeds back into the field windings, strengthening the magnetic field until the machine reaches full output (a process called "self-excitation"). If this residual magnetism is lost (often due to disuse, physical shock, or nearby strong magnetic fields), the machine cannot self-excite. The solution is to artificially reintroduce magnetism by applying an external DC voltage source (like a storage battery or battery charger) directly across the field windings for a short period. This process is universally known as **"flashing the field"** or "field flashing" and is the standard corrective procedure for restoring residual magnetism. **Why the other options are incorrect:** * **A) running the rotor in the opposite direction for 5 minutes:** Reversing the direction of rotation does not restore residual magnetism. While it might be relevant for certain types of motors, it is typically detrimental or irrelevant for standard alternators/generators and could damage cooling fans or internal components designed for unidirectional rotation. * **B) running the generator at normal speed with the field rheostat fully counterclockwise:** The field rheostat controls the resistance in the field circuit, thereby controlling the current and output voltage. Turning it fully counterclockwise (usually maximizing the resistance, minimizing the current) would result in the lowest possible field current. If the machine already has no residual magnetism, reducing the field current further will certainly not help it self-excite. * **C) allowing the generator to run at 10% of normal speed for 5 minutes:** Running the generator at a low speed (10% of normal) generates an extremely low frequency and voltage. While running the machine at any speed may eventually allow very weak residual magnetism to build up (if any exists), running it very slowly significantly reduces the efficiency of the induction process, making it highly unlikely to restore the magnetism within a short timeframe, especially if the magnetism is completely lost. Field flashing is required to restore the necessary initial magnetic strength quickly.