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STANDARD TEST - Phase-to-phase Inductance

What does it tell you?

In AC motors, phase-to-phase inductance readings can:

  • Indicate the condition of the stator windings
  • Detect phase-to-phase and coil-to-coil current leakage paths
  • Reveal poor or incorrect rework

These readings can also be used to detect faults in power cables and main contacts in the power circuit. A rotor influence Check (RIC) can be performed to further troubleshoot the motor to reveal faults such as:

  • Broken/cracked rotor bars or end rings
  • Porosity and lamination damage
  • Eccentricity problems

In DC motors, inductance changes within the field or armature can indicate current leakage paths in the windings.

Inductance changes when leakage paths develop. These paths can be either within the winding coils, or directly to ground. Leakage paths result from mechanical, thermal, environmental, or electrical damage to the insulation system of the windings. Additionally phase-to-phase and turn-to-turn shorts can occur. In either case, current flow bypasses some coils, thereby reducing inductive reactance and increasing current in other phases of the stator. Temperature rises in the remaining conductors and in the surrounding insulation. This accelerates the deterioration, which can cause an avalanche effect, as heat produces more insulation failures, resulting in more leakage paths and more coils removed from the circuit, further increasing temperature.

As there are fewer winding turns in a given phase actively creating the magnetic field upon which the motor is functioning, the windings in the other phases compensate to meet the requirements of the load on the motor. These windings in turn draw more current than is normally supplied by a balanced motor.

Why is this important?

A large inductive imbalance causes torque-induced vibration can be linked to mechanical degradation. Also, inductive imbalance can contribute to other problems, among which are:

  • Bearing damage
  • Coupling damage
  • Loosened rotor bars
  • Insulation failure at winding end turns or at exit of stator slots

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