VFD Fail to Start

On a system consisting of several VFDs and servos, I have one VFD that approximately once every couple hours gives me a fail to start. This is a fail to start alarm that I create if the VFD is receiving the start command and not running, it is a fail to start. I believe that because it only will start on a rising edge and maybe the start and stop happen so quickly that the code will start and then stop so quickly that it doesn't catch the rising edge of the start and never starts, but the alarm I created sees the start command and no run status and alarms. I copy all of my inputs such as photoeyes and reed switches to a buffer to make sure the I/O is synchronous to the code and have a slight delay on/off for each of them. I attached an image of the logic. Any ideas? Thanks guys!

What brand and model of vfd?

james

(Looks like some version of Powerflex)

#1) Do not leave your "Start" and "Stop" commands active. Trigger them to do their jobs and shut them off.

#2) Put the "Running" status as XIC between your stop logic and the Stop output. Put the "Running" status as an XIO between your start logic and the Start output. (This takes care of #1 if you don't sequence the commands properly).

#3) If the worst case, if you still have issues, put a free running timer bit just before your "Start" and "Stop" outputs. This will insure that they can get multiple rising edge triggers if needed.

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Personally, I just use a one shot before the start or stop command to the drive and don't have issues.

It's a Powerflex 525, I notice that in Rockwell's AOI for the Plant PAX faceplate, they keep retriggering the start or stop if they don't work the first time. That seems very sloppy to me.

Are you using ethernet for connection and starting drive ??

rdrast's advice is good, what you're seeing as a common issue with powerflex drives if the start/stop is not done quite right.

My approach with all powerflex drives is something like this:

Obviously there's slightly more to it than that in that I make sure the drive is ready/online/etc etc first, but that's the gist of it.

Make sure your firmware in the PF525 is up to date. There was a bug in an early version that had an issue with how the Stop command was given or not given, they changed the firmware to make it so that you couldn't make the mistake that was causing it. As I recall, the problem existed at version 2.002 and older. The current version is 3.001

Well, i would personaly call the tech support of the VFD brand you have. Talk with him about it and see what is his feedback.

If needed ask for a tech to come see it for you, it will cost for the first time but you will learn it for the next time you have a similar problem with your drive.

But still, i find it strange that you have a fail to start alarm and not a Overload or something like this alarm.

Also look in your troubleshooting manual for action to do and idea to try.

rdrast, jraef, and JSF are correct; logic with an "if not Start, then Stop" rung is a classic way to create a "missed start" when the RPI timer for the VFD's Ethernet connection expires in between the execution of the two instructions.

The fact that the PowerFlex 525 had a start logic bug just makes it worse, but that code will cause "missed starts" on any A-B drive on ControlNet or EtherNet/IP with a ControlLogix.

Depending on which Firmware you are using!

Another simple solution to the "if not Start, then Stop" rung is to add the drive's "Active" bit in between using an XIC instruction. This way, the Stop can only be asserted while the drive is already Active. Once the Stop is asserted, the Active instruction is FALSE and so the Stop will no longer be asserted by the XIO instruction for the Start. This will help to ensure that the Start rung can be successfully executed for the next Start operation.

Regards,
George