Hi everyone,

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)

Problem:

• 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).

What I have already done:

• 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)

Questions:

1. Since the imbalance moves with the output wires, is this almost certainly a cable / termination issue?
2. Could a brand new VFD still have a weak IGBT/module causing this?
3. Has anyone experienced intermittent current imbalance that comes and goes with heat/vibration on Unidrive M200?
4. Any other parameters I should check or adjust for better stability?

Any advice or similar experiences would be greatly appreciated. Thank you!

 

Hi everyone,

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)

Problem:

• 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).

What I have already done:
Since the imbalance follows the phase swap, it does suggest looking beyond the VFD itself and focusing on the motor, cabling, or measurement setup. Comparing winding resistance and insulation readings may help narrow it down. Troubleshooting critical systems requires reliable components too, especially when sourcing Authentic NCJ29D5DHN stock for industrial applications.

• 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)

Questions:

1. Since the imbalance moves with the output wires, is this almost certainly a cable / termination issue?
2. Could a brand new VFD still have a weak IGBT/module causing this?
3. Has anyone experienced intermittent current imbalance that comes and goes with heat/vibration on Unidrive M200?
4. Any other parameters I should check or adjust for better stability?

Any advice or similar experiences would be greatly appreciated. Thank you!

Since the imbalance follows the phase swap, I’d focus on the motor cable, terminations, and any junction boxes before suspecting the new VFD. A faulty IGBT usually causes the problem to stay with the same output phase. If possible, perform insulation resistance and phase-to-phase impedance tests while flexing cables to check for intermittent faults caused by vibration.