Motor phase U missing

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Motor phase U is not detected between the frequency converter and the motor. This indicates a break in the motor wiring for phase U or a problem with the motor's winding. Operating with a missing phase can cause motor damage, instability, and inefficient operation, potentially damaging the frequency converter output stage due to imbalanced loading.

• Open circuit in the motor cable conductor for phase U due to physical damage or a severed wire.
• Loose or disconnected wire at the drive's output terminal T1 (U) or at the motor's terminal box for phase U.
• Internal open circuit in the motor winding for phase U, preventing current flow.
• Failure of the drive's internal output power stage (e.g., IGBT module) responsible for phase U output.
• Corrosion or poor contact on the terminal block for phase U, leading to high resistance.

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1. 1

   1. De-energize the drive, disconnect motor cables, and measure resistance between drive output terminal T1 (U) and motor terminal U to confirm cable continuity (e.g., less than 1 Ohm).

2. 2

   2. Tighten all connections for motor phase U at the drive output terminal T1 and inside the motor's terminal box, ensuring proper torque.

3. 3

   3. Measure the resistance across the motor windings (U-V, V-W, W-U) at the motor terminals; confirm that all three measurements are balanced and within the motor's datasheet specifications.

4. 4

   4. Perform an insulation resistance test (Megger test) on the motor winding U to ground and between phases (e.g., 500VDC, >100M Ohm) to detect winding faults.

5. 5

   5. If external wiring and motor are confirmed good, consult the drive's service manual for specific diagnostics related to output stage faults for phase U.