When checking windings, why should resistance readings from each terminal to ground be infinite?

When checking windings, why should resistance readings from each terminal to ground be infinite?

• A. It indicates there is no short to ground and insulation is intact.
• B. It indicates a fault in insulation.
• C. It indicates a short between windings.
• D. It indicates the motor is energized.

Answer: (A)

This checks insulation resistance from winding terminals to ground. When you measure from each terminal to the metal frame (ground) with a megohmmeter, the path to ground should be blocked by intact insulation, so the resistance should be effectively infinite (very high, often many megaohms). Seeing an infinite resistance means there’s no current leaking to ground, which indicates that the insulation is intact and there’s no short to ground. If the insulation were compromised, moisture, contamination, or damage could create a leakage path to ground, yielding a finite (lower) resistance. Shorts between windings, on the other hand, would show up in a different measurement—between winding terminals—rather than from a winding terminal to ground. This test is performed with the circuit de-energized, so an energized motor wouldn’t apply here. So the infinite resistance from each terminal to ground best indicates there is no short to ground and the insulation is intact.

This checks insulation resistance from winding terminals to ground. When you measure from each terminal to the metal frame (ground) with a megohmmeter, the path to ground should be blocked by intact insulation, so the resistance should be effectively infinite (very high, often many megaohms).

Seeing an infinite resistance means there’s no current leaking to ground, which indicates that the insulation is intact and there’s no short to ground. If the insulation were compromised, moisture, contamination, or damage could create a leakage path to ground, yielding a finite (lower) resistance. Shorts between windings, on the other hand, would show up in a different measurement—between winding terminals—rather than from a winding terminal to ground. This test is performed with the circuit de-energized, so an energized motor wouldn’t apply here.

So the infinite resistance from each terminal to ground best indicates there is no short to ground and the insulation is intact.