Applicable Systems
EFC
FEX
FEI
HNC
Symptoms
External brake resistor overcurrent. The drive has detected excessive current flowing through the external brake resistor circuit during regeneration.
Possible Causes
- Brake resistor value too low (below minimum rated resistance)
- Brake resistor short circuited internally
- Brake chopper (transistor) short circuit
- Excessive regenerative energy from high-inertia deceleration
Troubleshooting Steps
- Step 1: Measure brake resistor resistance and compare to nameplate value
Resistance within +/-10% of rated value; no short circuit - Step 2: Check brake resistor insulation to ground
No continuity between resistor element and frame/ground - Step 3: Verify brake resistor power rating is adequate for the application
Resistor power rating exceeds calculated regenerative power - Step 4: If resistor is shorted, replace with correct specification
New resistor operates without overcurrent - Step 5: If resistor is good, the brake chopper transistor in the drive may be shorted
Drive replacement or power module replacement required
How to Reset
1) Disconnect brake resistor. 2. Measure resistance. 3) Check for short to ground. 4) Replace resistor if defective. 5) Replace drive if chopper is shorted. Caution: Never operate the drive without a brake resistor if regeneration is expected; DC bus overvoltage will result.
Frequently Asked Questions
- Q: How do I calculate the correct brake resistor?
Use IndraWorks AS drive sizing tool or the Rexroth brake resistor selection guide. Key factors: motor power, inertia, deceleration time, and duty cycle.
- Q: What happens if the brake chopper transistor is shorted?
The resistor will continuously draw current, overheating and potentially causing fire. Disconnect the brake resistor immediately and replace the drive.
Source: Rexroth IndraDrive HCS/HMS Fault Code List, Bosch Rexroth AG
Category: PLC & VFD