PLC to VFD communication

Hello All,

I am new to working with PLC's and am currently trying to use a PLC to control the speed of a VFD.

I am using a 1769-L32E processor with rslogix 5000. Also, I am currently trying to use the output of the analog (1769-IF4XOF2) to control the speed and use the digital output(1769-IQ6XOW4) to start and stop the VFD.

I found the raw decimal data that corresponds to 4-20 mA ( 6218 to 31086) and trying to move it to the VFD but I am having no luck. I'm having trouble understanding the concepts being applied in this communication.

Any help is greatly appreciated.

Thank you.

What drive are you using? What issue are you having? Have you check your 4-20mA signal with a meter to be sure your drive is getting the correct speed reference? Are you sure your drive is wired correctly? Do you have the drive parameters set correctly for your speed reference (4-20mA) and your start/stop inputs?

When you try starting it, does it go into "Running" mode and stay at 0 speed or just not do anything at all?

Or is the issue you are not sure how to program the later logic?

I am using a Culter Hammer AMF 10. The issue I'm having is that I am having confusion on how to program the ladder logic. I have wired the drive correctly and have set it to 4-20mA(TB13A) which i got from the manual. I am currently using a MOV instruction to try and send my speed referencing signal to the VFD. Not sure if i am on the right track or not.

Basically you need to take whatever value you are using in your program to represent speed which could be 0-100%, 0-1800 RPM, 0-60Hz, etc and scale it to 6218 to 31086 if you are using Raw/Proportional setup in your analog card. You will then need to take your scaled value and place it into the local tags for your card and channel. For example, if your analog card is in Slot 4 and you want to use Output channel 0, you will write your scaled value to the tag "Local:4:O.Ch0Data".

The easiest way for me to scale values in my opinion is to use a SCP (scale with parameters) add-on instruction that i have attached. The attached picture shows how to set it up to scale 0-100% to 4-20mA on the output card.

However, you can also use a CPT function or any other method of scaling you desire. There is nothing wrong with doing that or using a MOV instruction.

Okay, thank you for your help!

So when you use a meter and force a 4, 8, 12 and 20 ma the drive responds correctly?

yes, is responds correctly

Also i just want to check if my addresses are right.

I have the 1769-IF4XOF2 in slot 2 and my VFD is connected to Iout1+ and COM.
I have the digital 1769-IQ6XOW4 in slot 1 and VFD 1 and 2 connected to Out0 and VDC

Would my address be Local:2:O.Ch1data.0 and Local:1:O.Data.0?

i am getting confused with the channel and the bit.

Thanks

Your digital output address is correct, but your analog output address doesn't need the ".0" on the end.

Think of it this way. Your digital output card has four outputs, hence four bits. The output data image is Local:1:O.Data, which is a four-bit integer*. You are interested only in output 0, which is bit 0, so by writing "Local:1:O.Data.0", you are specifying "Local rack, Slot 1, Output Data Image, Bit 0".

By comparison, each of your two analog outputs are a whole 16-bit integer themselves - they're not just on and off; they're any value from -32767 to 32767 (or whatever scaling you have selected). By writing "Local:2:O.Ch1data" you are specifying "Local rack, Slot 2, Output Data, Channel 1 Output". If you specify ".0" on the end, you are addressing only the least significant bit of your integer, not the analog value as a whole.

Does that make sense?


*it's probably actually a 16-bit integer, as Logix doesn't have a "nibble" data type that I'm aware of, and the other 12 bits just wouldn't be used. But for the purposes of keeping it simple, thinking of it as a four-bit integer is probably fine

Okay, thank you for your help!