Question 61

The Correct Answer is C ### Why Option C (Megohmmeter) is Correct A **Megohmmeter** (often called a "Megger") is specifically designed to measure very high electrical resistance (insulation resistance), typically in units of megohms. To locate a **grounded stator coil** (meaning the winding insulation has failed and the conductor is making contact with the motor frame/ground), you must test the integrity of the insulation. By applying a high DC voltage (e.g., 500V or 1000V) between the winding terminal and the motor casing (ground), the Megohmmeter will reveal a very low resistance reading (close to zero ohms, or below the accepted minimum threshold), confirming that the coil is indeed grounded. This is the standard diagnostic tool for checking winding insulation integrity in large electrical machines. ### Why Other Options Are Incorrect * **A) Frequency meter:** A frequency meter measures the frequency of an AC power supply (Hertz). This measurement provides no information about the insulation integrity or resistance of a grounded coil. * **B) Voltmeter:** A standard voltmeter measures voltage. While a voltmeter is crucial for many electrical checks, it cannot safely or accurately measure the very high resistance values necessary to assess insulation quality or pinpoint a ground fault in a de-energized high-voltage motor winding. * **D) Infrared temperature meter:** An infrared temperature meter measures surface temperature. It could potentially locate a coil that is overheating due to a short (or ground fault) while the motor is running, but it is an indirect method. It cannot be used to *test* or *confirm* the insulation failure when the motor is stopped (the standard safety procedure for testing grounds), making the Megohmmeter the definitive and best tool for the specific task of *locating* the insulation breakdown.