Motor stall.

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Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
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EC - retired
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?
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
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.
 

LMAO

Senior Member
Location
Texas
How in the world a VFD trip on over-braking fault when motor shaft is locked? Where is the reverse power coming from...??? VFD might have tripped on over-current or over-torque (whichever limit is reached faster) but there is no reason for a VFD to trip on over-brake (DC overvoltage) when shaft is not spinning.
 

kwired

Electron manager
Location
NE Nebraska
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.)

Funny how elecrticians are always at fault or at least have the problem until they can prove there is a mechanical problem.

I have many times had to convince a customer/operator of equipment that if the belt is slipping (but is not unusually loose) that the motor likely still is putting out usual power but it is something about the driven load that has changed. I'd say that 75% never believe me until I find the load problem myself.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
How in the world a VFD trip on over-braking fault when motor shaft is locked? Where is the reverse power coming from...??? VFD might have tripped on over-current or over-torque (whichever limit is reached faster) but there is no reason for a VFD to trip on over-brake (DC overvoltage) when shaft is not spinning.
It's a belt drive, likely to a large drive pulley on the gearbox to get high torque at low speed, which acts like a flywheel. With the rotor locked, the belts are strained against that flywheel. Then when the motor torque can't hold it, the flywheel responds to the belt tension by spinning backward. Mining draglines have the same issue.
 
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
I set it up years ago and no doubt had factory help and most likely used defaults for the type of load we have. The regenerative condition that developed is the only way it could fault while braking. The load would normally stop almost immediately with no deceleration ramp time. I had it set for 6 seconds just to make it a nice stop. Hotline explained what was going on, with one of the temp solutions to extend the ramp time. That kept it from faulting but.... Most likely we will add a break resistor and/or change the way it stops on a fault. Good thing nothing turns my crank like the depths of a VFD programming manual.

Thank you for the CEUs. I wish the State would give me credit for these. Seems like I get enough of them.
 
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