Question 33 QMED02 - Electrician-Refrigerating Engineer

The Correct Answer is D **Explanation for Option D (Correct):** A diode is a semiconductor device that allows current to flow primarily in one direction. In a rectifier assembly (like those used in a generator's excitation field), the diodes are arranged specifically to convert alternating current (AC) into direct current (DC). This assembly requires diodes of two specific polarities: half the diodes are typically cathode-base (allowing current in one direction), and the other half are anode-base (allowing current in the opposite direction, necessary for the full-wave rectification bridge). If a replacement diode has the wrong polarity (e.g., installing a negative/cathode-based diode where a positive/anode-based one belongs, or vice versa), it will short circuit the AC phase or prevent current from flowing correctly, immediately damaging the new diode, potentially damaging other components, and causing the rectifier assembly to fail its function (i.e., the generator will not build excitation field power). Therefore, matching the polarity is the single most critical electrical factor when replacing a faulty diode. **Why the Other Options Are Incorrect:** * **A) Replacement of a diode also requires balancing of the rotor with a one-piece rotor lamination to be shrunk fit and keyed to the shaft.** This statement confuses the simple replacement of a stationary electrical component (the diode, which is part of the non-rotating rectifier assembly) with major mechanical repairs involving the rotor, lamination, and balancing. Diode replacement does not affect the physical balance of the rotating mass (rotor). * **B) Never alter the diode alignment to cause a change in the neutral plane.** The concept of the "neutral plane" is critical in DC brush machines (commutators and brushes) where it defines the optimal position for the brushes. Diodes in a rectifier assembly do not have an "alignment" that determines or influences the generator's neutral plane. Diode replacement is a straight electrical substitution; it doesn't involve mechanical alignment adjustments related to magnetic fields or brushes. * **C) The replacement diode must be dipped in varnish prior to installation to protect against humidity.** While electrical components sometimes receive protective coatings, dipping an installed diode in varnish is not standard or necessary practice for general replacement, especially in modern generators. Diodes are typically sealed units, and excessive varnish could interfere with heat dissipation, potentially leading to premature failure. If protective coating is required, it is usually applied uniformly over the entire circuit board or assembly after installation, not by individually dipping the diode.