ocA Over-Current during Acceleration fault in delta drive

ocA Over-Current during Acceleration fault in delta drive troubleshooting

Description

The ocA fault indicates that the output current of the Variable Frequency Drive (VFD) has exceeded the rated current limit (typically 200% to 300% depending on the model) specifically during the acceleration ramp-up phase. This implies that as the drive attempts to increase the frequency to speed up the motor, the load or a system defect is drawing excessive amperage instantaneously.

Causes

The primary causes for an ocA fault generally fall into three categories: parameter settings, mechanical load, or electrical shorts.
1. Acceleration Time too Short: If the acceleration time (Parameter 01-09 or similar) is set too low, the drive attempts to overcome the inertia of the motor and load too quickly, requiring massive current.
2. Short Circuit or Ground Fault: There may be a phase-to-phase short or a phase-to-ground short in the motor windings or the cable connecting the drive to the motor.
3. Mechanical Jam/High Inertia: The motor might be mechanically locked, the brake may not have released, or the starting torque required by the load is higher than the V/F curve provides.

Solution

To resolve this, follow a systematic diagnostic approach:
1. Disconnect Motor: Remove the motor leads (U, V, W) from the drive and run the drive empty. If the fault persists, the internal IGBT module of the drive is damaged. If the fault clears, the issue is external (cable or motor).
2. Adjust Parameters: Increase the Acceleration Time. If the load is heavy, adjust the V/F curve to provide more torque at low speeds or enable Torque Boost.
3. Check Motor/Cable: Use a Megger to check for ground faults and a multimeter to check phase resistance balance. Replace damaged cables or rewind the motor if insulation failure is detected.
4. Check Capacity: Ensure the drive power rating matches the motor and load requirements.