VFD interface with PLC


Thread Starter


Hi all!

I am working with a Schlumberger Speedstar VFD and Control Logix PLC (I/O card rated for 24VDC). I currently have a D/O from the PLC to the VFD as a D/I for an ESD/Permissive. Will I need an interposing relay or is a dry contact sufficient?

Thanks in advance.
The inputs in that VFD are 5VDC logic level (REALLY bad idea in my opinion). So you have to either give it an external +5VDC signal, which will not come from the PLC so you would have to add MORE hardware, or you can configure each input to respond to a dry contact closure. Without knowing what I/O card you have in the PLC, it's hard to know. But assuming it's not a relay card (because you said 24VDC), I would use an interposing relay. Also, with the VFD inputs being TTL level, they are going to be VERY sensitive to noise. Isolation is your friend here.