Altivar ATV320 OBF fault (braking overvoltage): causes

• A deceleration ramp too short for the inertia the machine is moving.
• A high-inertia load (a large fan, a flywheel, a centrifuge) that keeps pushing the motor while it is already braking.
• A braking resistor that is missing, undersized, disconnected or open.
• Overhauling loads, such as a downhill conveyor, where the motor becomes a permanent generator.
• Sustained high mains voltage, which leaves the bus close to the limit even before braking.

1. 1Reproduce the fault and note the exact moment it appears. If it is always when stopping, it is textbook OBF and there is no need to look elsewhere.
2. 2Lengthen the deceleration ramp and test again. If the error disappears, the diagnosis is confirmed; what remains is deciding whether that stopping time works for the process.
3. 3If the stop cannot be slower, the answer is the braking resistor: check that it exists, that it is connected and that its value matches the power and duty cycle of the machine.
4. 4Measure the resistor with the unit de-energized. An open resistor leaves the drive exactly as if it had none.
5. 5Check the mains voltage. If it sits at the high end of the range, the braking margin is smaller and the fault shows up with ramps that used to work.

Work on an energized panel: apply lockout/tagout and follow your plant procedure. This sheet guides the diagnosis, it does not replace it, and the manufacturer manual overrides anything written here.