Powerflex 70 Fault #24 Decel inhibit, DC bus 650 ~ 750V

We use Powerflex 70 for a 3phase 480V motor that pulls a coil off a coil reel. A vertical laser (Allen Bradley 45cpd analog laser sensor) measures the coil slack to adjust the pull speed.

We have been getting fault #24 (decel inhibit) every night. I changed the deceleartion from 2 seconds to 3 and that did not solve the problem. DC bus voltage was going between 650 & 740Volt just before the faults.

However, when we unplugged and replugged the cable plug to the AB laser, that solved the problem (for tonight anyway). Now, the DC bus is steady at 640V.

Any ideas? Can erratic speed input change (due to the AB laser not working right) could raise the DC bus voltage? What parameters should I look? Thanks.

I had the same problem with some of the PF70.
After searching in forums and even talk to Rockwell, we decided
to remove the fault message.
Bit 6 from parameter 238 (Fault config 1).
But in our case its not important that the motor stops in a certain time, we have accel and decel parameters
set to 30sec.
In your case i think it seens to be important.

"Actual deceleration times canbe longer than commanded
deceleration times; however, a “Decel Inhibit” fault is generated if the drive stops decelerating altogether. If this condition is
unacceptable, the “adjust freq” portion of the bus regulator must be disabled (see parameters 161 and 162). In addition, installing a
properly sized dynamic brake resistor will provide equal or better
performance in most cases.
Note: These faults are not instantaneous and have shown test
results that take between 2 and 12 seconds to occur."
Page 2-49 of the user manual


Best regards,
crawler009

Removal of the cable to the laser transducer "fixing" it is an oddity that tends to point to noise or EMI issues. It might be that under some condition, something bleeding into that laser cable is being seen by the system as a false feedback, maybe an extreme and or rapid reference change. The drive attempts to respond, but if the values or rate of change are unreal, it cannot and it causes the Decel error when the drive makes the attempt.

Double check all of your grounding of shields etc.; signal cable shields should be grounded at only one end, power cable shields should be grounded at both ends (a common mistake is that people only ground one end on the shielded power cables).

If you are not using shielded power cables going from the drive to the motor, start with that.

crawler009 said:

I had the same problem with some of the PF70.
After searching in forums and even talk to Rockwell, we decided
to remove the fault message.
Bit 6 from parameter 238 (Fault config 1).
But in our case its not important that the motor stops in a certain time, we have accel and decel parameters
set to 30sec.
In your case i think it seens to be important.

"Actual deceleration times canbe longer than commanded
deceleration times; however, a “Decel Inhibit” fault is generated if the drive stops decelerating altogether. If this condition is
unacceptable, the “adjust freq” portion of the bus regulator must be disabled (see parameters 161 and 162).

Click to expand...

Thanks for pointing me to parameter 162. It was set at "Adjust Freq." The laser was indeed malfunctiong, ending up sending max voltage command to the VFD when there was no slack in the line at all. So, for situations like that, I think we would want the Frequency adjusting and tripping to let us know the problem. The PFLX we have don't even have dynamic braking so I guess I didn't even have to bother with the decel speed.

However, why does PFLX have two parameters, 161 & 162 for the same function?

jraef said:

It might be that under some condition, something bleeding into that laser cable is being seen by the system as a false feedback, maybe an extreme and or rapid reference change.

Double check all of your grounding of shields etc.; signal cable shields should be grounded at only one end, power cable shields should be grounded at both ends (a common mistake is that people only ground one end on the shielded power cables).

Click to expand...

We have been having these problems throughout the plant that were resolved when the low voltage sensors were power cycled by unplugging & replugging. I have to ask a dumb question. For sinal cable wiring, is the shield considered to be grounded at the device-end where I just plug in the cable to the device?

unsaint32 said:

The PFLX we have don't even have dynamic braking so I guess I didn't even have to bother with the decel speed.

Click to expand...

Most drives are capable of limited decel without dynamic braking. All decel is is a controlled ramp down. So long as it isn't aggressive enough for regen to raise the DC bus voltage dynamic braking isn't required. You only need braking when you want to ramp down something faster than it will coast down by itself.

The more I learn about the decel inhibit fault, I realize that the laser has nothing to with it. The laser may or may not send decel command to the PFLX, but either way, the problem is that the DC bus voltage was already high at that point. High enough for the voltage regulation. So the root problem is something that is causing the high DC bus voltage even before the decel command, such as poor grounding. The input voltage was stable at 470V every time.

What puzzles me is that the problem had gone away after we plug/unplug the laser (which senses uncoiling coil slack and sends proper speed reference to the PFLX). The same fault did come back few times when we start the machine again, but the laser plug/unplug again solves the problem. How can that little voltage of the laser affect the DC bus voltage of the drive power?

Originally posted by unsaint32:

Any ideas? Can erratic speed input change (due to the AB laser not working right) could raise the DC bus voltage?

Click to expand...

That is in fact the most likely culprit. A change in the speed command to the drive is requiring a deceleration that the drive can't follow. This will cause the drive to indicate a decel inhibit state.

Originally posted by crawler009:

After searching in forums and even talk to Rockwell, we decided
to remove the fault message.
Bit 6 from parameter 238 (Fault config 1).

Click to expand...

In your case this is the solution. The response to a decel inhibit action is configurable. The drive come as default set to fault on a decel inhibit. You can change this action up to and including "ignore". I have always set the PF70 drive to ignore a decel inhibit fault. But it especially important on any application where I don't have a dynamic braking resistor.

Have you ever heard the expression "The road to hell is paved with good intentions"? The fault on decel inhibit action is the AC drive embodiment of that expression.

Keith

unsaint32 said:

...

However, why does PFLX have two parameters, 161 & 162 for the same function?

Click to expand...

The "A" abd "B" allow you to have two different ways of having the drive deal with bus regulation. "A" is how it normally handles it, then you can select "B" based on a digital input assigned to this if there are different operating conditions that might require a different response.

unsaint32 said:

The more I learn about the decel inhibit fault, I realize that the laser has nothing to with it. The laser may or may not send decel command to the PFLX, but either way, the problem is that the DC bus voltage was already high at that point. High enough for the voltage regulation. So the root problem is something that is causing the high DC bus voltage even before the decel command, such as poor grounding. The input voltage was stable at 470V every time.

What puzzles me is that the problem had gone away after we plug/unplug the laser (which senses uncoiling coil slack and sends proper speed reference to the PFLX). The same fault did come back few times when we start the machine again, but the laser plug/unplug again solves the problem. How can that little voltage of the laser affect the DC bus voltage of the drive power?

Click to expand...

You are still thinking that the laser is "telling" the drive to decel as if it is a Stop function, that misses the point. All the laser input must do is to CHANGE SPEED, just as if you were using a pot on the door and someone came by and immediately turned it all the way down. If the Decel rate is telling the drive to change that output speed to follow that command slower than the motor and load can actually change speed, the motor becomes a generator and charges up the DC bus.

unsaint32 said:

We have been having these problems throughout the plant that were resolved when the low voltage sensors were power cycled by unplugging & replugging. I have to ask a dumb question. For sinal cable wiring, is the shield considered to be grounded at the device-end where I just plug in the cable to the device?

Click to expand...

No way to answer that, it totally depends on how they are wired. The point of bringing this up was to let you know that you have to CHECK on how it is done. Conventional wisdom is to NOT do the grounding at the sensor end, because you are not 100% sure that what the sensor might be able to ground to is actually grounded itself. By grounding it at the other end, where the sensor leads connect to whatever the sensor is communicating with, your ground is consistent. That's why you see sensor terminal blocks for control panels where there are 3 wires plus a common ground terminal where all of the sensor shields are grounded to the same point, keeps it simple.

unsaint32 said:

So the root problem is something that is causing the high DC bus voltage even before the decel command, such as poor grounding. The input voltage was stable at 470V every time.

Click to expand...

Your DC bus voltage isn't high. 650 V DC bus is normal for a 470 V AC input.

What I think is happening is that under normal operation the sensor is commanding speed changes slower than the drives decel setting so no problem. Now the sensor is malfunctioning it's commanding speed changes which are causing the drives decel curve to kick in faulting the drive.