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Electronics Engineering QUESTION #11737
Question 1
The polarisation index (PI) of a motor winding insulation is defined as:
  • $\text{PI} = R_{10\,\text{min}} / R_{1\,\text{min}}$✔️
  • $\text{PI} = R_{1\,\text{min}} / R_{10\,\text{min}}$
  • $\text{PI} = R_{10\,\text{min}} \times R_{1\,\text{min}}$
  • $\text{PI} = (R_{10} - R_1) / R_1$
Correct Answer Explanation
$\text{PI} = \dfrac{R_{10\,\text{min}}}{R_{1\,\text{min}}}$ — the ratio of insulation resistance at 10 minutes to 1 minute of applied test voltage. Good, dry insulation absorbs charge over time (polarisation current decreases), giving PI > 2. Contaminated or wet insulation has low PI (~1). Per IEEE 43, PI < 1.0 is dangerous; 1.0–2.0 is questionable; > 2.0 is good.