ocn Over-Current during Constant Speed fault in delta drive

ocn Over-Current during Constant Speed fault in delta drive troubleshooting

Description

The ocn fault occurs when the drive detects an over-current condition while the motor is running at a steady, constant speed. This is arguably the most problematic of the OC faults because the system is supposed to be in a stable state, implying a sudden change in conditions.

Causes

Sudden spikes in current during steady operations are usually external:
1. Sudden Load Change: A jam in a conveyor, a sudden blockage in a pump, or a mechanical binding in a gearbox can instantly spike torque demand and current.
2. Insulation Breakdown: As the motor heats up during operation, weak insulation might fail, causing an intermittent arc/short that triggers the protection.
3. Loose Wiring: Vibrations at constant speed might cause a loose connection to arc, creating current noise.
4. Drive Output Failure: The VFD's current sensors or drive circuit might be failing thermally.

Solution

Troubleshooting requires monitoring the system while running:
1. Load Analysis: Check the mechanical system for jams, tight bearings, or obstructions. Monitor the drive's current display (Amps) to see if it fluctuates before tripping.
2. Thermal Inspection: Check the motor temperature. If the motor is overheating, insulation resistance may drop. Let it cool and Megger test it.
3. Tighten Connections: Power down and retighten all power terminals (R, S, T, U, V, W).
4. Drive Swap: If the motor and load are verified good, the drive’s internal current sensor may be drifting. Try running the motor on a different drive.