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Hi everyone,
I need your help with a persistent output current imbalance on a newly installed VFD.
Equipment Details:
Any advice or similar experiences would be greatly appreciated. Thank you!
I need your help with a persistent output current imbalance on a newly installed VFD.
Equipment Details:
- VFD: New 45 kW Control Techniques / Nidec Unidrive M200 (replaced old unit that had Er.STO fault)
- Motor: Siemens 1LE0101-2BB23-4AB4, 45 kW (50 Hz) / 51 kW (60 Hz), 400V Delta, 84.2A @ 60Hz
- Application: Belt-driven scrubber system
- Supply: 380-400V, 60 Hz (input voltage balanced: 415/419/416V)
- Output current imbalance that fluctuates and moves between phases.
- Typical readings: 27A / 31A / 38A (around 18-25% imbalance)
- Imbalance comes and goes (sometimes normal for hours, then appears again).
- When imbalance occurs, the belt jumps / becomes unsteady.
- Motor temperature is stable at 47°C (good).
- VFD frequency display is stable and matches setpoint (running at 30 Hz).
- Replaced original VFD due to Er.STO (hardware fault).
- Bypassed output contactor (was causing part of the problem).
- Motor winding resistance balanced at 0.8 Ω on all three phases.
- Phase swap test at VFD output terminals (U & V swapped) — imbalance moves to different phases.
- Re-tightened all terminals multiple times.
- Motor parameters set correctly (Pr 06=84A, Pr 07=1770 rpm, Pr 08=400V, Pr 09=0.87, etc.).
- Auto-tune done (stationary)
- Since the imbalance moves with the output wires, is this almost certainly a cable / termination issue?
- Could a brand new VFD still have a weak IGBT/module causing this?
- Has anyone experienced intermittent current imbalance that comes and goes with heat/vibration on Unidrive M200?
- Any other parameters I should check or adjust for better stability?
Any advice or similar experiences would be greatly appreciated. Thank you!
