Which of the following is/are not the test condition of the HV test on three-phase induction motor?

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Examoogle · Accepted Answer

when machine rotates at no-load Explanation: The High Voltage (HV) test, also known as the High Potential or Dielectric test, is a routine insulation withstand test performed on stationary electrical equipment to verify the integrity of the dielectric strength. Since the test is intended to stress the insulation between the windings and the earthed frame, it must be performed while the machine is at a standstill; rotation would introduce induced voltages and mechanical hazards, making it an invalid test condition. Working Principle: The principle relies on applying a test voltage higher than the rated operating voltage (typically $2V_{rated} + 1000V$) between the windings and the grounded frame. By ensuring all non-tested parts are earthed, the test safely isolates potential leakage paths and verifies that no breakdown occurs within the insulation system at the specified voltage level. Key Formulas: $V_{test} = 2V_{rated} + 1000V$ — Standard HV test voltage formula for machines Key Points: HV tests verify insulation integrity against dielectric breakdown.; The machine must be in a stationary state (at rest) to prevent insulation damage during the test.; All phases are typically coupled together during the test to check phase-to-earth insulation.; An insulation resistance (IR) test (Megger test) is a prerequisite to ensure the equipment is not already faulted before high voltage application. Advantages: Detects manufacturing defects in insulation.; Prevents potential in-service failures.; Confirms the safety of equipment grounding. Disadvantages: Non-destructive testing (NDT) can become destructive if the insulation is weak.; Requires high-voltage testing equipment and trained safety personnel. Applications: Post-installation of new 3-phase induction motors.; Post-maintenance verification for rewound motors.; Quality control in manufacturing plants. Additional Info: The HV test is defined under IS 325 (Indian Standards for Three-Phase Induction Motors).. Option D is the correct answer because rotating the motor during an HV test is technically incorrect and dangerous, as it may result in back-EMF interference and mechanical instability. Answer: D is correct — The High Voltage test is a dielectric withstand test performed on stationary equipment; rotating the motor at no-load is not a valid condition as it may lead to erroneous measurements and electrical hazards. Exam Tip: Always remember that the IR test is a 'diagnostic' test, whereas the HV test is a 'stress' test; you must verify that the machine is 'healthy' via IR test before applying the 'stress' of the HV test.

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