conveyor belt

[buzzp]

I have a conveyor belt application using 2 mechanically coupled 500HP motors with 2 VFD's (VFD1 and 2) and one 500HP motor/VFD (lets call it VFD3). They are setup as master/followers. VFD1 is the master. It takes a speed set point and in turn sends the required torque to VFD2 and 3. The conveyor will often be called upon to carry more material than the initial design. On some occasions, the VFD's will source more than nameplate amps (motors are 563 FLA). Sometimes as high as 650A for very short durations due to material load only. What do I need to consider if the VFD's are current limited to 600A? Would stall really the only thing to consider?

This is a hypothetical situation and not sure why you would do this given I can program the VFD for motor OL protection but is more a question for my own knowledge.
Thanks

 

[jraef]

What you have set up is called a “torque follower” situation and is the correct way of doing this when you have multiple motors that are mechanically coupled. If your overloads are occasional and of short duration, I would not get overly concerned because that is the nature of working with variable loads like material handling conveyors. But if you are getting repeated tripping of the drives from motor OL (or drive OL), then yes, you must set a current limit into your main drive only. Current and torque are virtually synonymous once running, so setting it on the master will follow through to the slaves.

What will happen though is that to keep the current below the threshold, the drive will override the commanded speed and lower it, which may affect other aspects of your system. So you need to thoroughly investigate those consequences.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[Drivesrock]

The two motors mechanically coupled will be in master/slave or torque follower. The third motor is perhaps coupled only via the conveyor at a small distance from the others? If this is the case, this is not a rigid coupling and I believe should not be a torque follower from the master drive. I suggest this VFD would receive the same speed reference as the master, and VFD#1 and VFD#3 are programmed with a droop factor so as to better share the load between all of them. This arrangement should also help avoid overloads of the VFDs fighting against each other.

 

[controlsdude]

You probably need some method to make sure that the feed rate onto the conveyor does not exceed a certain feed rate. Otherwise you get into a condition where your overfeeding material onto the conveyor and now you have a conveyor that is stopped and the only way to get it started is to get up there and offload the product with a shovel.

 

[crshears]

Not positively related...or maybe it is; nice link to a rather sophisticated drive control system by ABB, and no, I have no association with ABB; it's just that the site of the described application always makes for a nice day trip for technically inclined summertime/navigation season visitors...

Link:

https://library.e.abb.com/public/e92c567aa2eec49dc12571d90042ffe0/60-63 3M653_ENG72dpi.pdf

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[waross]

The third motor is perhaps coupled only via the conveyor at a small distance from the others? If this is the case, this is not a rigid coupling

It is either the equivalent of a rigid coupling or it is slipping.
If it a 500 HP drive is slipping the heat generated may soon do damage.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[buzzp]

Thanks for the replies. I have the OL situation handled but you guys have brought up another possible issue.
VFD3 operates separately from the two that are mechanically coupled, does seem to run either slower or faster than the other two by 20FPM. This is calculated from the motor RPM and is not physically measured. They are all close to one another and it seems having VFD3 run from a speed setpoint vs. the torque sent by the master makes more sense to me.
The coupling to the conveyors is via the roller so slip is possible between the drive roller and the belt. We are in the process of replacing the speed sensors so that actual speed is known, and therefore, slip sensing.