Modbus to control VFD speed.

Elcan

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Hi all,
We use the PLC analog outputs cards mostly to control the speed of VFDs. We are considering the alternative of using Modbus to communicate the PLC with the VFD and set the speed.
The pros I can think of are:

  • Getting rid of PLC analog outputs (speed control)
  • Getting rid of PLC analog inputs (current reading from the VFDs, for example)
  • Getting rid of digital inputs and outputs (enable signal, running aux signal).
  • Being able to read a lot of VFD's parameters (faults, for example)
The cons in my opinion would be:

  • Dealing with Modbus (I don't have experience with it)
  • We will have a single failure point (if the Modbus communication fails all the VFDs will be out of control)
  • One VFD could affect the communication with the rest of the chain
  • Troubleshooting it remotely
What is your opinion about this?

Thank you!

PS: If this helps, we use Allen Bradley PLCs (mainly Micrologix) and ABB VFDs (but AB could be an option).
 
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Well, it does not have to be Modbus, I just want to use any communication protocol to decrease our dependence on PLC I/Os.
 
I´ve setup devicenet with micrologix 1500 & compactlogix, setup it´s easy. You have to use rsnetworx for devicenet, you have to get EDS file for your devices. If you decide to work with devicenet I got a lot of info i can share with you.
 
Thank you for your help, widelto!
Why did you choose Devicente instead of Modbus?
I heard Devicenet is more expensive than Modbus, but faster.
 
We typically use MODBUS to VFD's (Powerflex 4). We still have a hard enable contact output from the PLC to the VFD. All else (frequency command and status readback) is via MODBUS. Once you've done one it is fairly easy to duplicate. The newest AB messaging makes a MODBUS connection fairly easy.

And yes, MODBUS should not be used for higher speed commands (frequency change or start/stop)
 
We've done Modbus to A-B VFDs among others. We use a hardwired E-Stop and run permissive, all other commands are over the comm link.

Regarding throughput, for most applications Modbus is plenty fast enough. We used 19.2 kBaud and had no problem. Remember the VFD has an accell/decell time setting that will dictate its response time to speed command changes. Starting and stopping response was also fast enough. Unless you are doing motion control or some other machine that requires very very fast response I don't think you should have a problem.
 
And yes, MODBUS should not be used for higher speed commands (frequency change or start/stop)
Click to expand...

To clarify my own statement - I didn't mean you should do these commands. Just that you may want to consider speed of response in a network with multiple drives. We typically have many and are updating status from them all so getting a command in a timely fashion, for us, can sometimes be problomatic so we ten to keep step frequency changes and faster start/stop to hard control lines.
 
I have lots of VFD's (Altivar 61/71) being controlled by ML's via modbus.

Our usual setup is ML-comport -> 1761-CBL-AM00 -> 1761-NET-AIC -> Modbusnetwork

Easy to set up
 
Going back to my original dilemma, does everybody agree that controlling the VFD speed using Modbus (or any other protocol) is better than using a 4-20mA signal from the PLC analog output card?
Nobody has said a word against it so far, so I could assume this is the way to go for everybody.
 
I have lots of VFD's (Altivar 61/71) being controlled by ML's via modbus.

Our usual setup is ML-comport -> 1761-CBL-AM00 -> 1761-NET-AIC -> Modbusnetwork
Click to expand...
Jesper,

Which MicroLogix are you using?
 
Elcan said:
Going back to my original dilemma, does everybody agree that controlling the VFD speed using Modbus (or any other protocol) is better than using a 4-20mA signal from the PLC analog output card?
Nobody has said a word against it so far, so I could assume this is the way to go for everybody.
Click to expand...

"Better" depends on your objectives. Using a comm link allows you access to more information, such as kW and fault codes, and allows you to set drive parameters such as accell time and min Hz from the HMI and PLC. If you have a comm link in place adding speed control and start/stop control is not an incremental cost, and reduces wiring.

On the other hand, hardwired is in some respects more reliable. You can set the drive to go to zero speed if you loose your 4-20 mA signal or run contact, for example. With a comm link, loss of "signal" means the drive will keep going at its present settings forever. And, as Bernie suggests, hardwired control may be faster, and it is certainly more deterministic.

Bottom line, either works and you have to weigh the pros and cons against your specific application.
 
as a designer.. i second tom's design... i've tried using all modbus but during failures. its very hard to keep track of them esp in the comm wires... the implementation should depend on your project scale size and design..
 
To AJZ - we have used the Micrologix 1500LRP (the 2 port model) and the Micrologix 1100. The 1100 already has RS485 and just needs a breakout. "nettogrisen" detailed the hookup we use from the 1500 for the RS232 - RS485 conversion.

The CPU's port is just configured as MODBUS. Then the MSG commands are set up with the information needed for the information transfer. If the AB PLC will be a master its fairly painless. If it will be a slave then additional setup is necessary to map internal (N, B) registers as MODBUS registers.

Adressing Tom's objection:

With a comm link, loss of "signal" means the drive will keep going at its present settings forever.
Click to expand...

The Powerflex 4's which we use have an option on what to do in a comm loss. They can be set to continue with the last command, stop, coast to a stop or fault.
 
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