PLC5 - Scanport - 1336II - what happens to the VFD if the PLC goes?

Hi,

I have a setup where a fan is being controlled by a VFD (1336 Plus II) which interfaces via a scanport module (1203-FM-1) to the PLC (PLC-5/40).

I would like to know what happens to the VFD if the VFD is put into LOCAL mode AND it loses communication with the PLC (which is supplying the speed reference) for whatever reason.

Will the VFD continue to function? Will it change speed to a default or retain the last held values it received from the PLC?

Any ideas based on experiences with the VFD would be appreciated.

Thanks,
Nadeem

You have a complicated interface because you're using the 1203-SM1 and some variety of network adapter (RIO,DNet,CNet).

If the power supply to the 1203-FM1 gets interrupted, or it gets unplugged from the drive, the drive will definitely fault. You'll have to reset the fault (generally toggling the STOP input is the preferred method) and command the drive to take its Reference from another source (analog, HIM module, etc) to start it again.

If the PLC-5 faults, or the FLEX I/O adapter fails, the 1203-FM1 can bet set to command the drive to a safe or alternate operation mode by itself.

The difference between FLEX adapter failure, PLC-5 failure, and disconnection is why I prefer to use onboard 1336-GMx adapters, which get their control power from the drive and have little chance of disconnection, as they are plugged directly into the main control board.


Your specific question was what happens "if the VFD is put into LOCAL mode AND it loses communication with the PLC"

Let's assume that your "Local Mode" means a Remote/Local selector switch that is wired to a Discrete Input on the drive that selects a local Analog Input instead of the usual SCANport adapter for frequency reference.

If the 1203-FM1 physically gets disconnected from the drive, the drive is going to fault. The only way around that is to Mask out the -FM1, and you can't do that while you are also relying on it for command and reference.

If the 1203-FM1 stays connected and powered up, but the connection to the PLC-5 fails, the 1203-FM1 can be configured to assert the Local Reference bit itself, and leave the Command bits at their last state. The drive will continue operating the way it was before the communication failure, but will switch to a local speed reference.

Clear as mud ? The important thing to learn is that you can configure the 1203-FM1 to not fault the drive if it loses communication to the PLC, but rather to set the drive to a local speed reference.

Wow. Thanks for the indepth answer Ken.

I was wondering if you knew what would happen if say the 1203-FM isn't configured for asserting the local reference, would the last speed reference from the PLC before losing communications be retained?

The 1203-SM1/FM1 can be configured to Hold Last State or to assert a Fault State Command and Fault State Reference when it loses communication with the PLC.

I once actually configured a 1203-SM1/FM1 to run the drive backwards at 1/4 speed until it hit a reverse overtravel in the event of a controller network failure. That allowed a system to gently open up the gap between some massive refiner discs if the controller or network suffered damage but the drive survived. The drive was at a higher elevation than the controller, which has a high-water flood mark inside the cabinet !

But if the 1203-SM1/FM1 gets physically disconnected from the 1336+II drive, the drive will fault. It does not keep the last speed command when there is a fault; you have to give it some kind of selection instruction to have it use the HIM or an analog input.