Controlling a VFD using PID

Good day,

Can anyone give some feedbacks on what to use when controlling a VFD using PID. My application is on Flow and i will use a PID block in the PLC to adjust the VFD's frequency so what's the best to change the VFD frequency thru MODBUS TCP/IP or use Analog Output Card BTW i am using Phoenix Contact.

Thanks,
Joel

Communicating or using an analog signal has nothing to do with the PID at all. You should decide which technology suits you best; i.e.: if you want only to start the drive at a defined speed probably the analog signal will be better. If you have lots of drives then the communication method will save you money.

Hope this helps...

Hi Joel201!

I'd have a look at the VFD itself, the ones I've worked with all have internal PID (the ones designed for flow sometimes multiple, like the Danfoss FC202).

Analog or comm? Depending on the number of drives there could be a substancial efford in wiring up the analog signals, but communication modules are not cheap either.
When going the ModbusTCP/IP way, when having multiple drives you may also need a switch (some comms options offer a "pass-through" solution though).

For some drives you will get a "free" ModbusRTU communication, and here you can daisy chain your drives. But it is not neccesarily the way you will want to go.

Keep in mind that drives are noisy by nature and both analog signals as well as communication signals are succeptible to noise, so keep your signal (and control) wires a minimum of 200 mm (8 Inch) away from the motor cables. And always cross them at a 90 degree angle.

Michal

Hello,

you said that if i need the drive to run at a defined speed a analog module will be better. Ok now if i use the MODBUS TCP / IP in SIEMENS can it do Read and Write values to multiple VFD's at the same time? or is it one instruction at a time? cause if it can't write frequency values to multiple VFD's at the same time then i think that an analog module will be best to change frequency which lessens the delay or the delay of MODBUS TCP / IP is not that long?

Thanks,
Joel

IMO, if you are not familiar with digital communcation schemes, stick with using analog communication. Analog communication has a *HUGE* advantage in that almost any maintenance personnel with a multimeter can check the analog reference voltage and see if the run contact is made. When it comes to digital communication methods, they are just staring at a magical box whenever it fails to function properly.

Hello

Your absolutely right about that thanks for your feedback. I'll just stick with 4-20mA output for control and maybe just use the MODBUS for data acquisition.

Thanks,