What could be a cause of an overbraking fault (ObF) on the secondary (or follower) ATV71 when in a primary/secondary (leader/follower) configuration?


02 January 2024

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 secondary drive does not match the speed reference of the primary.   This often indicates the analog output from the primary is fed to the secondary but the scaling is not correct.
When a drive is set up for a 60Hz system (bFr: Standard mot. freq), the Max Frequency (tFr) is changed by default to 72 Hz. 
When programming an analog output for Motor Frequency, the max scaling is the Max frequency setting.   
So if the speed range of the drive may be 0 to 60 Hz, but the scaling of the analog output will be 0 to 72 Hz...which results in the follower drive being commanded slower than the load is actually running and causing it to be in Regen.
Change the maximum frequency (tFr) of the primary from 72 hertz (default) to match the High Speed (HSP) setting, usually 60 hertz in North America will resolve this. 

If the maximum frequency is at 72, the analog output will not reach full scale (10 V or 20mA) when the Primary drive is at 60 Hz, so the speeds of both drives will not be synchronized.  The secondary drive 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 secondary drive could rise above the allowable voltage threshold, causing an overbraking fault.