20 November 2020
If the drive indicates the OBF fault it is caused by over braking.
This is caused when the drive's DC bus voltage exceeds the permitted voltage threshold within the drive. The drive indicates the OBF fault and puts the motor in to freewheel stop mode.
There are three possible solutions:
1. If the load can be stopped in a longer ramp down time then a simple adjustment of the DEC ramp time may be all that is required.
2. Brake ramp adaptation
This is a internal parameter inside the drive which monitors the DC bus level, if the load is being ramped down and the DC bus voltage increases too much , the BRA parameter takes effect. The drive no longer brakes the load, which then switches to "freewheel" mode, so there is no more excess energy to be absorbed by the drive and the capacitors are no longer charged. On the contrary, as they discharge and the DC bus voltage decreases until it reaches a minimum threshold below which braked stopping on ramp is re-enabled.
The load is stopped by a succession of braked stops and freewheel stops, the braking is optimized but has a variable duration if the inertia of the load is constantly changing. The effect you see on the load is that the motor takes longer to stop than the programmed Deceleration time, which in some applications is undesirable.
To set this function:
Settings (SET) > Drive menu(DRC) => Stt = Stn = > brA = yes
3. Braked stop by using a braking resistor
In this option the extra energy generated is transmitted to a braking resistor and is dissipated by the Joule effect. If the braking resistor is correctly sized, the load can be stopped according to the period configured by the deceleration ramp parameter, irrespective of the inertia of the load.
The wiring of the resister is between terminals PA and PB. and the drive settings are as follows:
Drive menu => Stt = Stn => brA = no.