Allen Bradely Powerflex 700 drives with modbus

Start with the prints...

Could it be that the PLC run command goes through the HOA switch, so when the PLC is shut down, that output goes off, killing your control even in Hand?

I have never set a drive up to run that way, but from my experiences any stops that a drive is set up to take is always active. Like if a drive is set for network control and you press the stop button on the keypad, it will stop. If a digital input is set as a stop input, if it is not closed the drive will not run on network control.

Andrew_W said:

Good afternoon,

...Now when the pump is running in hand and the plc is rebooted (not exactly sure why the operator did this) the pump that is running in hand stops working, this shouldn't happen. As far as I can tell the wiring is correct and the drive was suspossedly setup correct by the Allen Bradley rep that commisioned the drives? so can anyone give me any tips on where to start troubleshooting this?...

Click to expand...

The first thing that I would check for is to see if the Modbus link between the two devices is working. If your RS422/485 converter has blinky lights, then for a properly formed Modbus command the slave will respond with something.

That means that you will see the TX LED blink as the command goes out to the slave, and then the RX LED blink when the slave responds. There's some caveats to this (for example, if your converter is wired up for loopback, you will always see the RX LEDs mimic the TX LEDs).

The upshot of all this is that if your comm setup has gotten hosed, then the slave won't understand the command coming from the master and thus will not respond with anything.

OkiePC said:

Start with the prints...

Could it be that the PLC run command goes through the HOA switch, so when the PLC is shut down, that output goes off, killing your control even in Hand?

Click to expand...

Okie is thinking same as I.

I think I would take a real hard look at my motor control wiring. I would rewire such that in
AUTO the PLC controls the motor and hand is completely turned off ie switched out.
HAND again the PLC control is competely switched out of the motor control circuit. My guess is you do NOT have this.

Dan Bentler

OkiePC said:

Start with the prints...

Could it be that the PLC run command goes through the HOA switch, so when the PLC is shut down, that output goes off, killing your control even in Hand?

Click to expand...

I have double checked that and the hand control is only wired to the VFD

I have never set a drive up to run that way, but from my experiences any stops that a drive is set up to take is always active. Like if a drive is set for network control and you press the stop button on the keypad, it will stop. If a digital input is set as a stop input, if it is not closed the drive will not run on network control.

Click to expand...

we set all the drives we work with (whether they use a communication protocol or just plain old analog and digital signals). we do this because the installation is always a water reservoir and pump house, or a sewage lift station and they need a backup to run the pumps if the PLC was to fail, and we have found that the operator's have a easier time with the HOA and a speed pot to control the speed rather then using the keypad on the drive. I'm confident that the cause of the issue is that when the PLC reboots and goes through the first scan sequence is that the drive see's a stop command from the PLC even though its in hand and it shuts off. I just don't know how to stop this. My main concern is having the PLC die and send out a random stop bit which wont allow the pumps to be run manually.

The first thing that I would check for is to see if the Modbus link between the two devices is working. If your RS422/485 converter has blinky lights, then for a properly formed Modbus command the slave will respond with something.

That means that you will see the TX LED blink as the command goes out to the slave, and then the RX LED blink when the slave responds. There's some caveats to this (for example, if your converter is wired up for loopback, you will always see the RX LEDs mimic the TX LEDs).

The upshot of all this is that if your comm setup has gotten hosed, then the slave won't understand the command coming from the master and thus will not respond with anything.

Click to expand...

I know the modbus link is working as the pumps have worked flawlessly for the last couple of months while being controlled via the link from the PLC.

I think I would take a real hard look at my motor control wiring. I would rewire such that in
AUTO the PLC controls the motor and hand is completely turned off ie switched out.
HAND again the PLC control is completely switched out of the motor control circuit. My guess is you do NOT have this.

Click to expand...

That is the way that it was designed to be wired, and to my knowledge is wired, but at the moment I cannot confirm as the contractor hasn't supplied the as-built drawings yet. If I recall correctly the Auto side is wired to activate DI 5 on the drive which should make the drive look at the modbus link for start/stop commands and speed command, when in the hand position the run command comes from the switch and the speed reference comes from the speed pot.

thanks for all the help so far and keep the great responses coming.

Andrew

I have double checked that and the hand control is only wired to the VFD


Andrew
I think you miss Okie and my point. You already know the VFD runs OK in hand. What you do not know is why the PLC can interrupt that.

Okie and I are saying to check and ensure that when in hand all PLC to VFD signals are interrupted / switched out ie with open contact.

Dan Bentler

This may be apples to oranges.

We used a modbus and Powerflex 4s on a system last year. Whenever the PLC was cycled off or program was changed the drives would not restart. They had to be manually reset. I am guessing that you could program the PLC to do this as a normal startup routine and thought about doing it but for this company, once the operators understood, they just reset the drives.

I'm by no means a drive expert, and have never set up a drive on Modbus. But I have to agree with bguinn. You cannot disable a stop command to a PowerFlex drive. So if the PLC issues a stop over the network to the drive, even if it's in hand, the drive will stop. To prevent this on first scan, the PLC stop command will have to be conditioned so that it is not sent if the drive is in hand.

Also, check parameter 23 of the 20-COMM-M module. That parameter tells the adapter what to do if communication with the PLC is interrupted. The default is to fault the drive.