Sorry in advance for the long post, but this requires a little back story.
Now for my question (s).....
- I work in a facility with a couple hundred VFDs
- Most are in a web handling application
- Some are driving an extruder (plastic)
- Drives are primarily AB (1336 and PF755) and Parker/SSD (690+ and 890)
- Sizes are from 2HP - 500 HP. Most are in the 5-20 range
- Most drives are running closed loop with either a load cell or pressure transducer for feedback
- Many (most?) of the drives have the feedback instrument wired directly into the drive. The PLC provides the speed reference, and the instrument is used for trim
- Almost every drive has been outfitted with an encoder
- We have found that this is a bad deal for several reasons
- Stuck in older/obsolete drive hardware because upgrading involves engineering work
- Obsolete drive software requiring older OS for programming & troubleshooting PCs (our IT department hates this)
- Having the control divided between two components with two different operating environments (the PLC and the drive) makes troubleshooting a PITA
- All of our upgrades are PF755 drives with the PLC controlling the drive 100% over Ethernet/IP.
- Makes the drive as "dumb" as possible so it can be replaced or upgraded easier in the future
- Keeps all of the PID in the PLC where it is easier to manage
- There's no perceivable effect on the process when doing this vs having the feedback hooked directly to the drive
Now for my question (s).....
- Do I even need the encoders?
- What/how much advantage do I get?
- If PLC scan time plus Ethernet/IP comms are fast enough for the process, is the encoder helping at all?
- There are some drives that have fixed setpoints (the line-speed "master"). How much is the speed control in these improved with a fixed/open-loop control?