Customers drag locked up and belts to gear reduction were slipping and squealing so they tigtened them up. Drag was full of product but this should not have been a problem but no go. Eventually we found the broken part locking it up. (Old electricians still have to shovel corn.)
When I got there the Altivar 71 vfd was in a Over Braking Fault. Hotline was good and helped fill in the blanks.
The motor would attempt a start until it stalled, the tension on the drag chain would pull it backwards then repeat about 2 more times before faulting again.
I am guessing that the motor essentially lost the ability to provide forward torque when it went LR, with the tension then spinning it backwards. Someone have a better explanation?
Could be. For the drive to trip on Over Braking Fault, I think that means the motor was regenerating back into the drive as you were thinking, which was then tracking the capability of the dynamic braking transistor and resistor circuit, calculating that they were about to burn up one or the other of them. But does this drive have an external braking resistor?
If not, that points to someone not fully understanding how braking works with VFDs and expecting the drive to do something it cannot really do. Most drives have a SMALL amount of dynamic braking capacity by using the bleed-off resistors for the bus caps, but it's typically a tiny fraction of the available power and not that useful. But because it's there, the drive allows users to set up dynamic braking functions even though they haven't added that feature, it's up to the user to know if it's appropriate or not. I would check out the programming of the drive with regard to how the braking functions are set up.
There may also be an issue with how the drive is programmed to stop and/or respond to a fault. For the motor to regenerate back into the drive, the motor windings must remain excited, which comes from the drive in the form of a Decel profile. Most drives will allow you to program the response to a fault condition to IGNORE the Decel profile and immediately turn off all of the transistors so as to avoid this exact situation. In other words to protect itself from the Locked Rotor condition the drive would have likely gone into Current Limit, until it timed out (any drive can only do that for so long). But then when it decided to shut down, instead of turning completely off, it went into Decel mode, which lowered the torque output yet kept the windings excited. The lower torque output was then overcome by the belt tension, which allowed the backspin, which allowed the regen into the DC bus of the drive, which then tried to do something with it and could not. All of that was set in motion by the inappropriate programmed response to the trip; the drive should be set up to ignore the Decel profile if it trips.
If the drive DOES have a dynamic braking resistor setup then it was doing its job, protecting the components from an unusual situation that may have otherwise caused collateral damage. It still might be the wrong way for the drive to respond to a trip, but the DB system was the correct way to deal with that, it was just too much because it likely kept repeating and repeating trying to make the motor accelerate, not knowing that it never would.
