PowerFlex 40 Won't Start Automatically After Stopping


Join Date
Jul 2021
I have a PowerFlex 40 drive that communicates with a 1756-L61 PLC via an 22-COMM-E card. There's an auxiliary contactor attached to the motor disconnect that interrupts the input to terminal 1 on the drive for the Stop when the disconnect is off. P036 and P038 are set to Comm Port. The VFD is being controlled by an AOI that I'm honestly not quite following along with. Or at least don't quite understand how the drive inputs and outputs interface with the logic. But there's a .Iterlocked_By bit that eventually turns on the Drive_Start bit in the AOI that won't allow the motor to run unless it's high.

On this conveyor that the drive runs, there's a labeler with a verification camera that checks a barcode. If the wrong barcode is read 5 times, there's a fault bit in the logic leading to the interlock output that drops out. I did some testing yesterday and if the interlock output goes high again within 30 seconds, the motor will start up automatically. If the interlock output stays low for more than 30 seconds, I have to turn the disconnect off at the motor, interrupting the stop input to the drive, turn it back on and then the motor runs again. I tested this a dozen times and anything under 30 seconds starts right back up.

The VFD is tagged PF40_5M0. I've noticed that when this happens, PF40_50M0:I.Ready drops out and when I turn the motor disconnect off and back on, it goes high again. I'm just not understanding what the relation is to the Stop input or why there's a 30 second threshold. I'm sure it's something simple I'm missing. I'm much better with logic than I am electrical. This is happening on multiple drives in the same area but from what I can tell, we have a handful of overhead conveyors with the same drive that no one has to go up with a lift to cycle the motor disconnects off and back on to get the motors to run if the line backs up and the conveyors stop. However, the drive parameters look the same.

Hopefully that all makes sense. Any help would be appreciated.

I think this is a case where a verbal description, or even a screenshot, won't do the trick. You'll have to post the AOI and the related motor control code.

It makes sense to have an interlock contact that interrupts Digital Input 1, when that's set for the default function of "STOP". On an A-B drive, a STOP command from any source always overrides the START commands.

I can't say where this 30 second threshold is coming from.

But I will say that a classic programming error with PowerFlex on EtherNet/IP or ControlNet is that if the Start bit goes from false to true when there is still an active STOP command, the drive will "ignore" the Start command.

The most common error is to write your STOP bit (which =1 to Stop over the network) so that it only goes =0 when the START bit goes =1. If the cyclic I/O update timeout happens to occur in between those logic rungs or instructions, then both START and STOP are =1 at the same time, and STOP wins.

If your logic is set up that it might send a START command over the network while the STOP command is still asserted by a physical Digital Input, that would explain why the Start appears to be "ignored" or will not start the motor like you expect it to.
Also: welcome to the PLCTalk forum community !

We like long-form posts, links to resources and documents, and screenshots.

Not like those buncha jabronis over at the reddit/r/PLC site.

I appreciate you getting back to me and the warm welcome. I've been looking through these forums since I got into this field 3 years ago and your answers to others questions have helped me more times than I can count. So, thank you.

I've left work for the weekend but I've asked for a coworker to send me a picture of the AOI logic and the rung with the Interlock logic on it. If I hear back, I'll post what he sends me. Otherwise I'll get that on here as soon as I get back to work on Monday.

Here's a copy of the AOI and a picture of the Interlock logic.

_Prg_BOOL[1].2 is just a start command for a group of conveyors from an HMI.

Motor_50M0.Ready is the bit that drops out after 30 seconds and won't go high again until the motor disconnect is cycled.

Labelers_FAULT_A is a fault for a barcode inspection camera. If the camera records 5 bad reads in a row, this fault comes on and is reset with a local push button.

SS_50M4 is an auto / manual selector switch for a downstream conveyor.

Please let me know if you need any other information.

I got caught up in other projects at work and forgot about this but I was just reminded it's still an issue by a coworker. Any chance the AOI helps indicate what might be going on?

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