Insulation Resistance (IR) & Polarization Index (PI) Testing

Insulation resistance testing evaluates the integrity of ground-wall insulation separating phase windings from the motor frame.

Key Testing Metrics:

• Insulation Resistance (IR): Applies DC voltage (500V to 5,000V) to check for moisture ingress, contamination, or dielectric breakdown.
• Polarization Index (PI): Calculated as the ratio of 10-minute resistance to 1-minute resistance ($PI = R_{10\text{min}} / R_{1\text{min}}$). A target $PI \ge 2.0$ confirms clean, dry insulation.
• Dielectric Absorption Ratio (DAR): Compares 60-second to 30-second insulation resistance to detect early insulation aging.

Winding Resistance & Phase Balance Measurement

Milliohm winding resistance measurements detect loose internal connections, open turns, and phase imbalances caused by overheating.

Winding Balance Parameters:

• Phase-to-Phase Resistance Balance: Identifies copper line resistance deviations. A phase imbalance exceeding 2% indicates bad connection crimps, corroded terminal studs, or partial turn shorts.
• Temperature Compensation: All resistance values are normalized to 25°C or 75°C standard reference temperatures to ensure historical trend accuracy.

High-Voltage Surge Testing & Partial Discharge (PD) Analysis

Standard megger tests cannot detect turn-to-turn insulation weakness because turn-to-turn voltages remain low. Surge testing uses fast-rise voltage pulses to identify weak turn insulation before it develops into a phase-to-ground fault.

Advanced Diagnostic Features:

• Surge Comparison Waveform Analysis: Compares voltage pulse decay curves across phases. Mismatched pulse frequencies indicate turn-to-turn or coil-to-coil insulation shorts.
• Offline Partial Discharge Testing: Detects microscopic internal void discharges in High Voltage (HV) form-wound stator insulation prior to complete ground fault failure.

Rotor Influence Check (RIC) & Inductance Testing

Testing stator inductance across various rotor positions isolates squirrel-cage rotor defects, broken rotor bars, and air-gap eccentricities without dismantling the motor.

By recording phase inductance variation ($L$) as the rotor turns manually, engineers map out rotor bar symmetry and spot dynamic air-gap distortions between the rotor and stator core laminations.