Brake resistor fault

What does A791 mean?

A general fault has been detected within the brake resistor circuit. This may indicate an open circuit, incorrect resistance, or an inability to dissipate regenerative energy, which could lead to DC link overvoltage and a drive trip.

• Open circuit in the brake resistor wiring (e.g., loose connection, broken wire) or the resistor element itself is open (e.g., burned out due to sustained overload).
• Incorrect brake resistor resistance value (e.g., too high) specified in P43.01, causing the drive to misinterpret the circuit impedance during diagnostic checks.
• Brake chopper (internal or external) is faulty, preventing it from switching correctly to sense or utilize the resistor.
• Auxiliary contact (if present) on an external brake resistor thermal switch is open, indicating an overtemperature condition on the resistor.
• Control wiring for the external brake resistor thermal protection switch (e.g., connected to DI1) is damaged or disconnected.

Click steps to track your progress.

1. 1

   1. Power down and lock out the drive. Disconnect the brake resistor from the drive's terminals (e.g., R+, R-).

2. 2

   2. Measure the actual resistance of the brake resistor directly at its terminals using an ohmmeter. Compare this value to the specified resistance in P43.01.

3. 3

   3. Inspect the brake resistor wiring for loose connections, corrosion, or signs of overheating at the drive terminals and the resistor itself.

4. 4

   4. If an external thermal switch is used, verify its wiring (e.g., to a digital input) and check its continuity. Ensure the thermal switch is not tripped due to resistor overtemperature.

5. 5

   5. Check the drive's internal brake chopper diagnostics if available (e.g., status bits in a diagnostic parameter group) for specific chopper faults.