Motor Winding Inter-Turn Short Circuit Testing Methods and Precautions

What is an Inter-Turn Short Circuit?

An inter-turn short circuit occurs when adjacent turns in a transformer or motor winding develop an unintended electrical connection. Normally, each turn should be insulated from others to ensure proper current flow. However, due to insulation aging, physical damage, or manufacturing defects, this isolation can fail, leading to short circuits between turns.

Testing Solution: METRAHIT IM E-DRIVE & COIL ADAPTER XTRA

Key Features:

✔ Wide inductance range coverage:

• Low range: 10 μH to 50 mH (for 3-phase devices: 380 V, ~80 MVA to ~15 kVA)

• High range: 5 mH to ~5 H (for 3-phase devices: 380 V, ~160 kVA to ~160 VA)
  ✔ Simultaneous 3-phase coil testing (L1-L2-L3, U1-V1-W1)
  ✔ Rotary switch for coil selection and polarity reversal
  ✔ Integrated plug connector for METRAHIT IM E-DRIVE/XTRA
  ✔ Automatic signal transmission of switch position to the tester

Testing Method: 1 kV Surge Voltage Test

1. Fault simulation under operational conditions:

• A capacitor is charged to the test voltage.

• Discharge cycles automatically through the motor windings, forming a resonant circuit.

• Decay voltage is analyzed (see waveform diagram).

2. Measurement principle:

• Time between the first two zero-crossings is measured and displayed on METRAHIT IM E-DRIVE.

• Time constant ∝ inductance.

3. Fault detection criteria:

• Healthy motors (with squirrel-cage rotors) show minor asymmetry (~1%).

• Deviation >10% → Fault (e.g., inter-turn short).

• Time = 0 → Short circuit detected.

• No discharge → Open coil (display: "OL").

4. Procedure for permanent-magnet or high-inductance motors:

• Test each coil pair in both polarities (U-V, U-W, V-W → then V-U, W-U, W-V).

• The instrument averages results to cancel rotor-position dependency.

Critical Precautions

⚠ For 10 kV motors:

• Residual magnetism may be comparable to the 1 kV surge field, causing polarity reversals and skewed readings.

• 1 kV testing is insufficient for 10 kV-rated motors (10× lower than operating voltage), potentially missing faults that occur at full voltage.

• No reliable results for squirrel-cage rotors at 10 kV.

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• METRAHIT IM E-DRIVE Inter-Turn Short Tester

• Testing Modes:

• 3-phase single-pole

• 3-phase multi-pole

• Coil (N-winding)

• Additional Features:

• Tabular/graphical result display

• Repeat single measurements

• Built-in data storage

• METRAHIT IM E-DRIVE Inter-Turn Short Tester

• Testing Modes:

• 3-phase single-pole

• 3-phase multi-pole

• Coil (N-winding)

• Additional Features:

• Tabular/graphical result display

• Repeat single measurements

• Built-in data storage