Conveyor with driven head roll and tail roll.

Hi all,




I have a customer with a metal mesh conveyor. They have a need to reverse the conveyor. To accomplish this they had to remove the motor and gearbox from one end and install it on the other end. It took too much time, so they bought a second motor and gearbox to install on the second shaft. I was asked to make the system work without adding a clutch. The motors turn 40 to 1 gearboxes. Each motor is driven with an Allen Bradley 525 VFD, and there is no feedback (ie encoder or tachometer) on either motor. There is no room for a dancer bar or. I set the lead motor in torque mode and the tail motor in velocity mode. This causes the tail motor to run faster than the head motor and eventually removes all the slack from the lower side of the conveyor. After looking through many drive advanced parameters, I found "Torque current" to change the most when one side of the conveyor started to tighten. I wrote logic that sped up the head motor to relieve the tension on the lower side of the belt. It runs fine this way. The difference in current from 10 to 60 Hz is not linear but exponential, so I limited them to 5 speeds (10, 20, 30, 40, 50, and 60Hz). They are fine with only 5 speeds. My question is has anyone ever setup a conveyor in this way, and if so how did you accomplish the task? Does my setup make sense or does someone have a better approach?




Brian

If it's functional with no issues, I'd say it's good.

The torque setting and the ability to monitor and change is a good application. I've done a few wind/unwind setups with the same speed signal being used and tweaking it using an offset array.

Cant you just swap the leads or put in a reversing starter ?

Set one VFD up as a torque follower the other as a master
I think the discharge end VFD should be the follower

Bright,




Theoretically a reversing contactor should be all you need, but it won't work. The slight differences in the motors and gearboxes (even though they are the same make, model, and ratio's) will always cause one to outrun the other. This will eventually tear apart the conveyor.



Gary,


you are correct the head motor (outfeed) is set to torque mode and the follower (infeed) motor is set to Velocity mode, until the conveyor is reversed. When the conveyor is reversed the (infeed) motor is the lead and is set to torque mode, and the follower(outfeed)is set to velocity mode.



I was curious if anyone had experience with the setup and how they achieved the goal.

Squints
When you reverse the conveyor the infeed becomes the outfeed
the setup would be the same each VFD would have to switch Master / Follower
but operation would be the same the outfeed would torque follow (Load Share)

To answer you question I has set up torque follower system years ago.
I also had an old DC load share system on a printing press (Common Shaft)
2 DC motor in series on a single DC drive
that was before you could do it with AC drives

Gary I understand what you mean thanks for clearing it up. I will look into it and see how to set up the load sharing. I'm sure it's fairly easy.

I just remembered years ago I worked on a system ( re-setup after drive repair )
As system of 2 DC motors and drives . The maser was about 100 HP and follower as about 40 HP
The 100HP was the master that set the speed and 40 HP provided the additional torque necessary for the added printing unit. All units were connected with a common shaft to sync the units.
The follower followed the torque output signal of the master both drives would supply the same percentage torque , master 50% torque of the master, the follower supplies 50% of it’s torque
The combine torque drove the full load.
It worked very well for years.
That was before you could do torque follower on an AC system
And before you could do Digital Line shaft Sync.
Now most printing presses us a separate drive for each unit and use digital line shaft sync
So many advantages to the new system it hard to envision still doing it the old wa

GaryS said:

Set one VFD up as a torque follower the other as a master
I think the discharge end VFD should be the follower

Click to expand...

The 525 does not do torque control unfortunately. You need a 7 class drive for that. In a perfect world, you would do this with Line Regenerative drives, like the PF755TR, so that you can continuously apply tension with the follower drive.

If what you have is working, I would leave it. Just be aware that if the drive that is trying to hold back on tension goes into regen, you will need to have somewhere for that energy to go, i.e. braking resistors. The down side is that typically, braking resistors cannot be energized on a continuous basis, they will burn up.

While Regenerative drive are always good to gave but expensive
and will be of no value in this application
in this application there is NO regenerative component all the load is motoring
LOAD SHEARING both drives will be pulling the load equally
you should change the drives to ones that can do the job

In this case you don't need line regeneration. You have a closed system that is a net energy user. Any drive that gives you access to the DC bus to allow bus sharing would do anything you need in this case.

However, I guess the biggest question I have is where do you want the belt slack to end up? Since you can't push a noodle I wouldn't necessarily think you would want to do straight-up torque sharing. In fact, since the system used to work with only one drive point, I would think ideally you want the "off-side" motor to just idle.

And as jraef said, the PF525 isn't capable of any kind of torque control. It uses the typical sensorless vector control that is really just voltage vector control. That does a pretty good job of keeping you on speed but it will use whatever current is available up to the current limit to do it.

If you aren't looking for perfection you can get a general feel for load by looking at overall current from both drives and adjusting the speed command of one to even out the current. That should get you fairly close if you really are looking for load sharing.

Keith

If I was designing the system I would just power one motor at a time, and use a sprag clutch or similar to avoid back driving the idle unit.

https://en.wikipedia.org/wiki/Sprag_clutch

From the original post:

I was asked to make the system work without adding a clutch.

Click to expand...

I never said customers were reasonable. Just that we either have to do what they ask or convince them they are wrong. And for a group who by anecdotal definition believe they are always right, convincing is not always the easiest thing to do.

Keith

Kamenges
Common Buss is what you are referring to, that where the DC buss of both VFDs are connected together
But not as you think to shear the load, but to reduce to overall energy consumption of the system
For that to work at least one of the VFD’s must be in regenerative mode The energy absorbed in the regenerative unit is put in the buss for the motoring unit to use. I have put more than a few of them together over the years. I started with the old Powertec Drives
You also need to add some hardware to the system to make it work properly, buss choke, fusing on both drives and let’s not forget the buss loader to absorb any excess energy. The do work very well in certain applications. But this is not one of them.
In this case nether vfd will be in regenerative mode.

As the question about conveyor slack it will be the same place it is now in return side of the conveyor in the underside of the conveyor
Most it not all belt or chain conveyors have a mechanism to adjust the slack. It would still be necessary

I agree that with 2 drives it could get to be expensive, you might want to look at a one way slip clutch on each gearbox. Between the gearbox and drive roll. In this application the motors will be changing the direction when you change the conveyor direction. One motor for each direction. One motor will be running clockwise the other will be running counterclockwise when they are loaded. If the conveyor drives the clutch in reverse it just slips and you can get them to slip in either direction. The all you have to do is just switch the power from one motor to the other. This co