The dielectric absorption ratio (DAR) is one of the two motor-health statistics calculated during an insulation resistance test — commonly called a “Megger” test.
The DAR measures the amount of energy required to polarize (i.e., align) the dipoles (i.e., ions) in the winding insulation. Polarization of the dipoles occurs in the first minute or so of the insulation resistance test. For motors with good winding insulation, current draw drops off as a greater percentage of the dipoles become polarized; in motors with degraded insulation, leakage current outweighs absorption current, obscuring this drop-off.
Hence, DAR is calculated as the ratio of the insulation resistance at 2t seconds divided by the insulation resistance t seconds into the test.
DAR = IR(2t) ÷ IR(t)
Some resources define t as either 15 or 30 seconds, depending on the type of insulation, with most resources recommending t = 30 seconds for AC motors with epoxied or varnished windings.
Generally, a DAR below 1.25 indicates that the health of this motor’s windings are questionable; a DAR above 1.6, healthy. Note that these values are guidelines: a downward trend in values over time or a sudden drop in values can indicate of a problem — even though DAR may exceed 1.25.
Insulation condition |
Dielectric absorption ratio range |
|---|---|
| Excellent | 1.6 and above |
| Good | 1.40 to 1.59 |
| Adequate | 1.25 to 1.39 |
| Questionable | 1.00 to 1.24 |
| Poor | Below 1.00 |
Suggested actions
- If motor is wet, dry out and retest to confirm insulation health before returning to service. Repeat test while motor dries.
- For DAR values in decline, work with a repair shop to locate and repair the cause of downward trend, which may include rewinding the motor.
- For a sudden drop in DAR values, retest frequently until DAR values either drop to the point where it’s unsafe to continue operating the equipment, stabilize at a new (lower) level, or the cause of the decline is located and corrected.