What could be a cause of an overbraking fault (ObF) on the slave ATV71 when in a master-slave configuration?


23 January 2021

Overbraking fault

Product Line:
Altivar 71

All models and serial numbers

Maximum frequency is set to 72 hertz

This can happen if the speed reference of the slave drive does not match the speed reference of the master.   This often indicates the analog output from the Master is fed to the slave but the scaling is not correct.
Change the maximum frequency (tFr) of the master from 72 hertz (default) to match the High Speed (HSP) setting.  This is usually 60 hertz. 
The analog output of the drive is scaled from 0 to the maximum frequency parameter when assigned as Motor Freq.  If the maximum frequency is at 72, the analog output will not reach full scale (10 V or 20mA) so the speeds of both drives will not be synchronized.  The slave will run 20% slower than the master.  If the two motors are coupled together (either directly or through the load) the stored energy in the DC bus of the Slave drive could rise above the allowable voltage threshold, causing an overbraking fault.