ZJQ 75 Submersible Slurry Pump Motor Winding Moisture Diagnosis: Application of Polarization Index (PI) and Dielectric Absorption Ratio (DAR)

Subtitle: More accurate than single insulation resistance measurement – field test methods, interpretation criteria, and moisture treatment procedures

Introduction

The ZJQ 75 submersible slurry pump (75mm discharge) is widely used in mine dewatering, foundation pit drainage, and dredging. The motor operates in water or humid environments, making winding moisture ingress the most common cause of motor burnout. While insulation resistance testing with a megger is standard, it has limitations: readings are heavily influenced by temperature, humidity, and surface contamination, often failing to accurately reflect the true internal condition of the winding.

Polarization Index (PI) and Dielectric Absorption Ratio (DAR) are more reliable diagnostic methods. By measuring insulation resistance changes over time, they can distinguish between surface moisture, internal moisture, and insulation aging, avoiding misjudgment. Hebei Xingou Machinery Equipment Co., Ltd. has extensive field experience applying these techniques. This article covers the principles, test methods, interpretation criteria, and moisture treatment procedures for ZJQ 75 motor windings.

1. Why Single Insulation Resistance Measurement Is Insufficient

Insulation resistance testing (500V or 1000V megger) is common but has limitations:

PI and DAR eliminate surface leakage and temperature effects by measuring the rate of change of insulation resistance over time, providing much better diagnostic accuracy.

2. Principles of Polarization Index (PI) and Dielectric Absorption Ratio (DAR)

When DC voltage is applied to insulation, the current consists of three components:

• Capacitive charging current: decays rapidly (within seconds)

• Absorption current: caused by dielectric polarization, decays slowly (tens of seconds to minutes)

• Leakage current: steady, reflects true insulation resistance

Insulation resistance readings combine absorption and leakage currents. The ratio of readings at different times reflects the polarization characteristics of the insulation and thus indicates moisture level.

Polarization Index (PI) = Insulation resistance at 10 minutes / Insulation resistance at 1 minute

Dielectric Absorption Ratio (DAR) = Insulation resistance at 60 seconds / Insulation resistance at 15 seconds

For dry, good insulation, absorption current decays slowly, so PI and DAR are high. When moisture is present, leakage current increases, the absorption current proportion decreases, and PI/DAR become low.

3. Field Test Procedure

3.1 Tools Required

3.2 Test Steps

3.3 Precautions

• Measure and record winding temperature before the test

• Insulation resistance values need temperature correction to a common reference (e.g., 20°C), but PI and DAR are temperature‑insensitive

• For large motors, charging time is longer; ensure sufficient test duration

4. Interpretation Criteria

4.1 Polarization Index (PI) Criteria (for medium/large motors)

4.2 Dielectric Absorption Ratio (DAR) Criteria (all motors)

Note: PI/DAR values that are too low generally indicate moisture or contamination. Abnormally high PI (e.g., >4) may indicate brittle aging of insulation, also requiring attention. If insulation resistance itself is very high (>1000 MΩ) but PI is low, suspect surface contamination or improper test method – clean and retest.

4.3 Combined Assessment

5. Moisture Treatment Procedure

When PI or DAR is below standard, follow these steps:

5.1 Field Drying Methods

6. Case Study: ZJQ 75 Moisture Diagnosis at an Iron Mine

Background: A ZJQ 75 pump was idle for two weeks during the rainy season. Upon restart, it tripped. Field megger reading showed 8 MΩ (rated 380V, requirement ≥1 MΩ). By conventional standards, this seemed usable, but the user was concerned.

Further diagnosis: Hebei Xingou Machinery technicians performed a PI test.

Results:

• R15 = 4 MΩ

• R60 = 6.5 MΩ

• R600 = 8.2 MΩ

• DAR = 6.5 / 4 = 1.63 (pass)

• PI = 8.2 / 6.5 = 1.26 (below 2.0, fail)

Conclusion: Although insulation resistance met the minimum, the PI value indicated internal moisture. Drying was recommended.

Action: After 8 hours of hot air drying, PI rose to 2.5 and insulation resistance to 150 MΩ. The pump operated without further tripping.

Lesson: Relying only on insulation resistance would have misjudged the motor as acceptable, risking burnout during operation. PI testing identified the hidden issue in advance.

7. Routine Monitoring Recommendations

Conclusion

For diagnosing moisture in ZJQ 75 submersible slurry pump motor windings, single insulation resistance measurement has limitations. Polarization Index (PI) and Dielectric Absorption Ratio (DAR) provide more accurate assessment, distinguishing between surface moisture, internal moisture, and insulation aging, offering reliable guidance for field maintenance. Hebei Xingou Machinery Equipment Co., Ltd. recommends incorporating PI/DAR testing into routine maintenance and establishing insulation condition records to detect hidden issues before motor burnout.

For on‑site moisture diagnosis or drying services, please contact Hebei Xingou Machinery’s technical team.