GB/T 20833.4-2021 in English

This standard is equivalent to IEC 60034-27-4:2018, which specifies the test method and evaluation criteria for the insulation resistance and polarization index of stator and rotor windings of rotating electrical machines of 750W and above. It is applicable to the insulation status assessment of AC/DC motors and provides a standardized basis for manufacturing inspection, maintenance and overhaul.

Insulation resistance measurement is based on DC megger or indirect voltage-current method. The equivalent circuit contains four current components:

1. Capacitance current IC: Transient current that decays in milliseconds
2. Polarization current IP: Absorption current that decays in minutes
3. Conductance current IG: Reflects the conductive properties of the insulation body
4. Surface leakage current IL: Significantly affected by humidity/pollution

Temperature correction formula: Ric = RiT × 0.5^(40-T)/X, where X value needs to be determined through experiments (see Appendix D)

Typical problem handling solutions

• Low insulation resistance: Check the surface contamination of the winding, clean it with anhydrous ethanol and then retest
• Abnormal polarization index: PI<1.5 may indicate that the insulation is damp and needs to be dried with hot air at 80℃ for 72 hours
• Impact of end anti-corona layer: Continuous anti-corona layer design may result in PI≈1, which needs to be judged in combination with historical data

Essential elements of test report

Should include ambient temperature and humidity, class=instrument>Test instrument model, connection method, corrected Ri1/Ri10 value, and a list of auxiliary equipment connected to the equipment under test.

Main changes of this standard compared with the 2008 version:

• Added temperature correction exemption clause for synthetic resin insulation (when Ri1>5000MΩ)
• Specified special test conditions for water-cooled windings
• Supplemented theoretical description of end anti-corona layer current (Appendix C)

Future technical development directions include online polarization index monitoring, AI-based insulation aging prediction, etc.