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So I have 3 VFDs being run off of a PLC. I am Mechanical and new to this stuff but I think I need 1digital 1analog to each of the motors.. digital for ON/OFF and then the analog is 4-20 so i can control the speed though that, correct? I have external sensors that will be recording the rate and flow of my pumps. I just need to know is 1DO and 1AO each is enough to control the pumps... I am running a Wago PLC and the VFDs are Baldor VS1SP series.
Thanks for your time I hope u can help


Most (if not all) VFD's can be run in 2-wire mode that only require a single closed contact (or DC output) to start the drive. Most (if not all) VFD's can have their speed controlled by a single analog out. This is the most basic control possible. Without the use of more I/O you can't monitor drive status or running frequency, for example. I always like to have some sort of "status" feedback to the PLC. If your application is simple enough, most VFD's can have can have their speed controlled via the drive keypad or the drive digital inputs so an analog signal is not necessary.

FYI, in today's industrial environments, many VFD's are being controlled via a network cable such as Ethernet, Devicenet, Profibus, etc. There are no digital or analog "wires" going to the drive.


Most drives have several speed settings.  Each contact closure selects another speed.  So technically you could control the speed digitally by contact closures.  You would first set what contact closure means what speed in the VFD.

A lot of jobs could be adequately run by selecting slow, med, fast, faster, if you know what I mean.

Anyway this would result in speed control with only digital outs which are often more available or cheaper.

Keith Cress
kcress -


Methinks if you are a mechano, you may be biting off a little more than you can chew. Running pumps off of VFDs usually means you want to vary the speed to match some sort of process condition. For example, you want to maintain a particular flow rate (you mentioned flow). To do that effectively, you need what is called a PID control loop. That could be in the PLC, but now days, chances are really good that it is in the VFD as well. In fact, if this is all you are doing, I'd venture to say you can do everything you need to inside the VFD, forget the PLC.

But all that aside, assuming you have some other valid reason for having the PLC, you can usually do on-off control through the analog input as well, eliminating the DI altogether. What you do is program the VFD t turn itself off if the AI drops below some threshold value, and wake up to run if it increases above it. So for example, let's say you are using 0-10VDC as your speed command. First you program the AI speed response to vary from 0-100% speed on the AI going from 1-10V, then tell it that if it is even below 1V, shut down. It's done all the time, I'm sure the Baldor drive has that capability.

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I agree with jraef - the interface between PLC and drive depends a lot on the application, industry, reliability needs, etc.  I'd really recommend getting an I&C engineer with some experience in this area to help you out.  It would be money well spent.  



Holding an exact Flow rate is not that important. I will be using my three VFD's to fill pressurize and drain different size pipe for hydropressure testing. SO really I just need to be able to slow down the pressure pump at the end of a cycle, to stop at desired mark. We need to keep the fill pump from filling at a rate faster than the pipe can handle if we are testing smaller pipe.
Its just that I have not played with these before and we dont have a PLC guy so I am left with the task.... even if it is more than i can chew. Right now we are assigning addresses to the PLC. We have two PLCs, a master and a slave and i am having one hell of a time getting them to talk to each other.... any ideas. Its a Wago PLC



I'm sorry, but I am not familiar with Wago PLC's. I have been in your shoes before though. I am a PLC tech for a steel mill. We have mostly allen bradley's and some large siemens plc's. For someone familiar with these PLC's it is simple to get tyhe communcations working. But for the novice, you will probably have a hell of a time. What I did before I had any experience or training, is had my local distributors help me out. Find out who is a distributor in your area for Wago, call them up and they should be able to get an Engineer down there to help you out.  


That was my thinking as well and I contacted the distributor in November and the soonest they could get a Engineer out to me is Jan 15 ... This is a time sensitive project that must be completed by the 1st of Jan.

And what is everyones take on Wago. Its seems people dont like it to much


TYPICALLY, you need a MINIMUM of the following PLC I/O per pump;
1- DO to command the VFD to Run / Stop,
1- AO to command the VFD speed,
1- AI for the VFD speed feedback signal to the PLC,
1- AI as your process input,
1- DI as a Run/Stop command to the PLC, and
1- DI as a feedback from the VFD for Run / Fault status. But I would do it with a little more than that, or do serial communications to the VFD.

Generally, if you are wanting to do serial communications between a PLC and a VFD, the best approach is to use a VFD/combo from a PLC mfr., such as Siemens, A-B, Schneider etc. because they make it easy for the novice. The difficulty you would have with something like a Wago PLC (I didn't know they even made them!) is that any direct serial communications will need to be done via the serial port and you will need to write commands in the PLC code that the VFD comm port can understand. For experienced programmers this is a no-brainer, but for a novice it can be daunting.


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Great Post jraef thank you that helps a ton.


Quote (Greenes):

I contacted the distributor in November and the soonest they could get a Engineer out to me is Jan 15 ... This is a time sensitive project that must be completed by the 1st of Jan.
If I may be so bold, several of the regulars at this forum have a world of experience in such things and do contract work, even from afar. Some of them* would I'm sure be willing to do this for you under contract.

*[link]itsmoked[/link] comes to mind...

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Forum Policies  


Well anyone in the Laffayette louisiana area?.


I take it that you have the software, do you know how to upload the files or have a copy? Like Jraef said, you could possibly email an Engineer the file, have him write the code and send it back electronically for a fee. If you have to install new hardware, than some harware configuration will be needed. But maybe someone could walk you through that over the phone. The best solution is to get someone there at the site.

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