When a megohmmeter is being used on a...

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Question 1 of 205eccefa4bacbc34ba4d48406

When a megohmmeter is being used on a alternating current machine, under certain conditions the meter pointer will dip toward 'zero' and then gradually rise to the true resistance value of the motor insulation. What does this indicate, in terms of the machine windings?

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🔍 Key Concepts

• Effect of capacitance in motor windings when testing with a megohmmeter on AC machines • Difference between an instant, steady low reading vs a temporary dip and gradual rise in insulation resistance tests • What a dirty or moisture-contaminated insulation surface does to resistance readings over time

💭 Think About

• Ask yourself: If the needle dips low at first, what physical process inside the insulation or on its surface could cause resistance to increase gradually instead of staying low? • Which condition (grounded, shorted, good, dirty) would most likely produce a constant low resistance rather than a temporary dip? • How would surface leakage paths versus a solid internal fault behave when voltage from the megohmmeter is first applied and then held?

✅ Before You Answer

• Compare which faults (grounded/shorted) would give a steady, consistently low megohmmeter reading with almost no recovery • Think about how moisture, dirt, or oil films on windings affect insulation and how they might dry or polarize under test voltage, changing the reading over time • Confirm that a truly good winding generally shows a high resistance that is stable, not one that must climb slowly from an initially low value

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