The high voltage test of three-phase induction motor shall be applied between the

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Examoogle · Accepted Answer

both windings and the frame & each phase and the frame Explanation: The high voltage (HV) test for a three-phase induction motor, often called a High-Potential or Hi-Pot test, is a mandatory dielectric strength verification. It requires testing the insulation resistance between the combined phase windings and the earth (frame), and also individually between each phase and the frame to ensure no dielectric breakdown exists under simulated overvoltage conditions. Working Principle: The test applies a voltage significantly higher than the rated voltage (typically $2V + 1000$ V, where $V$ is the rated voltage) for a duration of one minute. This confirms the structural integrity of the stator winding insulation. If leakage current exceeds the safe threshold defined by Indian Standards (IS 325), the insulation is considered defective, preventing potential phase-to-ground faults during operation. Key Formulas: $V_{test} = 2V_{rated} + 1000 ext{ V}$ — Standard test voltage formula for machines with $V_{rated} > 100 ext{ V}$ Key Points: The test is essential to ensure personnel safety and prevent equipment damage.; The test voltage is applied for a standard duration of 60 seconds (1 minute).; It checks the dielectric strength of the insulation system (slot liners, varnish, and phase insulation).; IS 325 is the governing standard for three-phase induction motor testing in India. Advantages: Detects manufacturing defects like poor varnish impregnation.; Identifies insulation weaknesses before commissioning.; Reduces the probability of premature motor failure. Disadvantages: Destructive if insulation is already near failure point.; Requires specialized high-voltage testing equipment. Applications: Routine testing of new electrical machines.; Post-repair maintenance verification.; Periodic diagnostic testing for large industrial motors. Additional Info: If the motor has a neutral point brought out, it should also be tested against the frame.. For motors with $V_{rated} \leq 100 ext{ V}$, the test voltage is typically $500 ext{ V}$ RMS. Answer: C is correct — The high voltage test must verify insulation integrity between both windings and the frame, as well as between individual phases and the frame to ensure comprehensive protection against ground faults. Exam Tip: Always remember that for rotating machines, the Hi-Pot test is a 'Type Test' or 'Routine Test' depending on the standard; never perform this test on a machine with damaged windings as it will cause immediate breakdown.

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