VFD Loadsharing

I know this is a PLC forum but I also know there are lots of smart guys here so I thought I'd ask this question.
How important is encoder feedback compared to open loop?
I have 4 15HP motors controled by Allen Bradly Powerflex drives, One is the master and other 3 are slaves, they are driving a large chain in an auto plant that carries approx 100 full sized suv's through an ecoat sysem.
I'm loosing speed feedback on master drive causing over currents on the master. I switch speed feedback to open loop (removes encoder) and problem goes away, drive maintains speed through it's own current/voltage calculations. Slaves don't seem to care because there speed is driven by the torque output of master, so they maintain (follow) speed no problem.
So why the encoder feedback ?????? why not just open loop, speed is not that important in application....any feedback would be appreciated. FYI, chain used to be 2 seperated chains...but modified now into one chain.....maybe speed sync was more important then. Thank you.

There is an intermediate level of speed control called sensorless vector. It does not use an encoder but instead depends upon a speed feedback signal generated from a motor model in the drive's memory.

All of these increased speed regulation methods serve to do just that---keep the speed stable as the load conditions change. Whether you need this and to what extent is entirely an application issue.

You will not have an answer to your question until you develop a fairly complete understanding of your load and its need for stable speed.

Or, to say it another way, its a load issue, not a drive issue.

In other words, load up the chain and see how well speed is maintained....If open loop doesn't do it I may try sensorless vector.

Just one further caution: Use of an encoder (flux vector) or a motor model (sensorless vector) generally allows good torque control at very low speeds in addition to the other things mentioned previously.

So, if the chain conveyor has to move very slowly and smoothly occasionally, open loop V/Hz operation is probably not good enough. Otherwise, simple is good!

IMO i would look at the root cause if this has been a working application. If it is not a new install then I would be looking at why you are loosing speed feedback. Most encoders are pretty reliable.

Look at the wiring and the shape of the encoder itself. Like the bearings and such.

Don't try to reengineer the application when it may just be a maintenance issue.

Just a thought.

I have proven that encoder is bad, and part doesn't exist anymore. Not trying to re-design, just simplify if I can. Chain is slow moving and torque control is important. Chain is pulling SUV's in and out of large baths, so lots of torque needed. It's a complex drive operation but we have no engineering staff here. Thanx for all you input.

When you go open loop then you are doing Scalar or Volts/ Hz control.

This is well known not to work well for high and precise torque control at low speeds. Thats why they had a encoder on the system to begin with I would imagine.

Sensorless vector would be your next best option and that can have issues depending on the application and also distance between the motor and drive.

I would bet if you replace that encoder and put the original settings back in all would be well.

Drives are not designed to have precision high torque at low speeds in scalar/ volts/HZ mode.

You are trying to make the system and the components of it do something it was not designed to do.

I have proven that encoder is bad, and part doesn't exist anymore.

Click to expand...

If you can find the specifications for the original encoder (maybe in an old catalog) then you can order a new encoder of a different brand that still matches the performance of the old encoder.

Switched to open loop and speed is maintained very well with chain fully loaded, still going to see if I can find another encoder just in case. Thanx for all your help.