Braking resistor phase unsymmetry

What does A06921 mean?

This alarm indicates that the three phases of the braking chopper resistors are not symmetrical, meaning there's an imbalance in their resistance values. This could be due to issues with the feeder cables or an incorrectly set monitoring threshold. Asymmetry can lead to uneven current distribution and reduced effectiveness of the braking chopper, potentially causing other faults.

• One or more elements of the external braking resistor bank have failed (e.g., open circuit) or their resistance has drifted significantly.
• Loose, corroded, or high-resistance connections in the wiring between the drive's braking chopper output (e.g., X1, X2, X3) and the braking resistors.
• Incorrect wiring of multiple parallel or series braking resistor modules, leading to an imbalance in current sharing.
• Damage to the insulation of braking resistor feeder cables, causing partial shorts or resistance changes.
• Internal fault or component degradation within the braking chopper monitoring circuit of the power unit.

Click steps to track your progress.

1. 1

   Power off the drive and lock out. Disconnect all braking resistor cables from the drive's output terminals (e.g., X1, X2, X3).

2. 2

   Measure the resistance of each individual braking resistor element or phase using an ohmmeter; compare to manufacturer specifications (P0250).

3. 3

   Inspect all connections for the braking resistors (at the drive and at the resistor bank) for tightness, corrosion, and proper seating.

4. 4

   Examine the braking resistor cables for any physical damage, chafing, or discoloration that indicates overheating or insulation breakdown.

5. 5

   If multiple resistors are used, verify their configuration matches the system design and manufacturer recommendations for balanced loads.

6. 6

   With braking resistors disconnected, power on the drive and check if alarm A06921 clears (other faults for missing resistor may appear).