Hooking up 2 wires on a PLC output. (Siemens S7-200)

[AutomationTechBrian]

Very simple/basic question, but I want to get rid of my uncertainty. I don't think it would be an issue, but when I'm the one writing the program, I always separate things out. Here, I'd be piggy-backing an output. I'm wondering if you guys ever do that, and if there is a reason to avoid doing that.

The ladder Network (Siemens) says Q0.6 is the "Auto Light", which I'm assuming is a 24vdc light powered by the PLC's connection to the 24v power supply. I don't know how much current is being used. ...it's one of those light stacks. I'm creating a "Manual" mode in the VFD, so I can use a pot for speed control in "Manual." Easiest way to do that (without stopping production) is to designate a VFD input as Auto/Manual. I'll just change the speed reference source based on that input. The amp draw must be negligible for the VFD input. I'm trying to keep this modification simple to avoid stopping production.

Any thoughts?

 

[Steve Bailey]

Whenever I have connected PLC outputs to VFD inputs I have used contact outputs on the PLC. The contacts close a circuit between the VFD's "Common" terminal and the input point.
Are you sure the VFD's inputs are 24 VDC? The safest way (least likely to let the smoke out) would be to use an interposing relay in parallel with the "Auto Light" device and use a contact from the relay for the VFD input.

 

[AutomationTechBrian]

I know what you're saying, Steve. I usually don't use an external power supply on a VFD, but this one is wired this way. I service over 1,500 VFDs a year (counting PM checks) and very, very few are wired for external power supply. The "run", "direction", and "enable" are all wired to the external power supply. And the common is wired to the power supply common.

...I did think about you the other day. I was back at a customer with a 90-30. All went smoothly, though.