Siemens SINAMICS G120 Fault F30055 Braking Chopper Overcurrent

Siemens G120 displaying Fault F30055? This indicates a braking chopper overcurrent. Learn how to inspect your braking resistor, check for shorts, and avoid trip-inducing deceleration.

 

When your Siemens SINAMICS G120 drive decelerates a heavy load, the motor acts like a generator, sending energy back into the drive (regenerative energy). This energy must be dissipated, usually via an external braking resistor controlled by the drive's braking chopper. If the drive trips with Fault F30055, it means the current flowing through that chopper circuit is far too high.

This is a protective trip. If the chopper were to remain active during an overcurrent event, it would rapidly destroy the drive’s internal power components. Here is how to find the source of the fault.

Common Causes of F30055

• Short-Circuited Braking Resistor: The most common cause is a dead short in the external braking resistor or its connecting cables.
• Incorrect Resistor Sizing: If the resistance value (Ohms) is too low for the drive's output capacity, the chopper will pull excessive current.
• Excessive Regenerative Energy: The load is decelerating too quickly or is too heavy, causing a current spike that the resistor cannot dissipate fast enough.
• Ground Fault: A short-to-ground in the wiring between the drive and the braking resistor.

Step-by-Step Troubleshooting Guide

Safety Warning: Always disconnect main power and wait at least 5 minutes for the DC link capacitors to discharge before touching the braking resistor terminals (usually labeled BR or R1/R2).

1. Inspect the Braking Resistor

The braking resistor is often located outside the cabinet and can be exposed to environmental hazards.

• Check for Shorts: Use a multimeter to measure the resistance of the braking resistor at the drive's terminals. Compare this value to the rating provided by the manufacturer. If the resistance is near 0 Ohms, the resistor is shorted.
• Check for Grounds: Measure the resistance between each resistor lead and the machine frame (earth). Any continuity indicates a short-to-ground.
• Physical Damage: Look for burnt spots, melted insulation, or signs of overheating on the resistor and its wiring.

2. Review Deceleration Parameters

If the resistor is physically healthy, the drive might be trying to handle more energy than the system can manage.

• Deceleration Time (P1121): If your decel time is set too short (e.g., trying to stop a massive fan in 2 seconds), the drive will dump a massive amount of energy into the resistor. Try doubling the deceleration time to see if the fault clears.
• Application Load: Has the process recently changed? Are you stopping heavier loads than before?

3. Verify Resistor Sizing

If you recently replaced the resistor, ensure it matches the drive requirements.

• If the resistor Ohm value is too low, the current (I = V/R) will be too high.
• Ensure the resistor power rating (Watts) is sufficient for the energy generated by your specific application duty cycle.

4. Inspect Drive Hardware

If the resistor and cables test perfectly but the F30055 fault triggers immediately upon a start command or during decel, the internal braking chopper transistor within the G120 Power Module may have failed (shorted internally).

How to Reset Fault F30055

1. Resolve the electrical short or adjust your deceleration parameters.
2. Acknowledge the fault via the Control Unit (IOP/BOP keypad) or the fieldbus reset command.
3. Perform a test run with a longer deceleration time to ensure the system is stable.

Pro-Tip: Monitoring

If this fault occurs intermittently, use the SINAMICS Startdrive or Starter software to create a trace of the "DC link voltage" and "Braking chopper current." This will show you exactly which part of your process—acceleration, steady state, or deceleration—is triggering the spike, allowing you to fine-tune your settings.