VFD controls

Rson · Mar 26, 2019

So, I have a customer with a VFD system - pretty simple single VFD (Emerson) - and he wants us to just send direct signals from the PLC output to the VFD drive. (VFD is in another room)

I've always used interposing relays with the VFD's control power to drive VFD I/O - it is just how things were done everywhere I have been.

Are there any disadvantages you have seen to using a sourcing output from a PLC into a VFD's digital input? Shouldn't I isolate these signals somehow?

kamenges · Mar 26, 2019

You will need to check the drive data but these inputs are sometimes already isolated through opto-isolators. If all the inputs come from one source the input common simply becomes the source common and the inputs go straight in. It just gets tricky when you are tying together commons from multiple sources.

Keith

rupej · Mar 26, 2019

Nothing wrong with using the PLC's 24V instead of the drive's. Much easier to do multiple drives that way.

James Mcquade · Mar 26, 2019

we always use a contact closure to start / stop the drive.
you should also use an operator start/stop/estop at the drive.
you will need a disconnect at the drive since the drive is in another room.

what is the vfd voltage and the distance from the drive to the motor?
you may need to include line and load reactors.

james

shooter · Mar 26, 2019

in most cases it is done with optocoupler so no worries it is separate.

rQx · Mar 26, 2019

I wouldn't tie seperate power supplys together like that, it's just not pretty

drforsythe · Mar 26, 2019

I do this all the time. Connect the DC Common of each device together, then source the +24v signal from the PLC. DO NOT connect the +24V of each device together!

jaden · Mar 26, 2019

How about using relay PLC outputs?

VFD controls

Rson

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