The information provided in this article is not necessarily specific to PF525; it can also apply to all other drives controlled by Allen Bradley PLCs. This article gives a general overview on how to monitor the raw inputs and outputs communicated with the PLC and their meaning.

It can be a challenge sometimes to know exactly the status of the VFD from the ladder logic. VFD controls can be complex, and often times have dedicated AOIs. Unlike contactor-controlled motors (ON & OFF operation), VFDs are communicated with via various network protocols (Ethernet/IP, Profibus, etc) and exchange information packets back and forth with the controller. Information such as drive status, output frequency, rotation direction, faults, etc.

Below is an example of PF525 controlled by AB PLC via Ethernet network. 

The array tags below show the raw data sent and received from the drives. The "I" and "O" indicate Input and Output respectively. "COMMS" tags monitor the VFD status within the ethernet network. The datatypes used below are custom datatypes used to filter data received/sent to VFDs.

On the input side, the controller receives a doubleword size data (32 bits) each representing a specific condition. This will show the raw information received from the VFD. This can be helpful when you are unsure of the VFD status from the logic. The received feedback (Output Frequency) is converted from binary to integers. This represents the actual running speed the VFD is running at.

Similarly, we can view the raw output data sent out from the PLC to the VFD. You could simply see from here if the controller is sending out the "start" command, and the direction of rotation, along with other useful information such as commanded frequency value. The highlighted bits can be useful when troubleshooting VFDs.

Note: the commanded frequency value can be different from one VFD to another. This value is calculated within the logic. To know exactly the FPM or Hz value it represents, you will need to check the AOI for drive controls. You should not need to make changes on those parameters unless drive speed calibration is required. Consult with higher level engineers before making changes.

Lastly, The VFD comms. This is a custom array and you may not see it in every logic. This monitors Enet comms and keeps records of faults counts, time, etc. The highlighted tag below would be the go-to when troubleshooting network related issues, with "1 / TRUE" indicating good status.

For VFDs raw inputs and outputs, they are largely the same across all Intelligrated AB PLCs. You can go straight to controller tags and look the VFDs IOs instead of spending some time navigating through the logic.