How to control a 4 AC motor drived Chain conveyor?

M

Thread Starter

Mauricio

Hello All!

I am involved in a retrofit of a long overhead chain conveyor (aprox. 1,5 miles) to replace 4 old and problematic DC motors connected in a single DC drive for a 4 new AC motors equiped with encoders and controlled by 4 inverters.

I noticed in field measurements that the old DC motors current are quite similar and the system chain tightener has almost no oscillations which means that the load is pretty well distributed.
I've read the chain conveyor posts here but I really don't believe that a simple "Load Sharing" operation will work without excessive movements over the conveyor chain tightener.

I would like to know if anyone have some experience about this kind of application, because the posts about it here here don't have the tests and results of their authors. I want to keep this post updated with our results.

I will be grateful for any help.
Mauricio
 
S
Look at ABB ACS-800. Multiple drives can be linked by fiber optic and the things they can do to track each other in real time are nothing short of amazing. They can do two motors on the same shaft with one responding to a speed signal with comms and response quick enough for the second drive to match the torque the first motor is applying cycle for cycle. In your case, with a long chain, you might find it better to match speed than torque, I'm not sure the details of your mechanics, but my point is that those drive are very capable. Talk to the ABB guys and get them on your site.
 
there are only so many ways to make motors follow each other... i never worked on a conveyor like this ... this big, so you may have to chew on it a bit.

Since they are all currently being run by one drive it appears to me that simple speed followers with the option of torque limiting on the 3 followers would work. this requires you to pick one drive as a master (the first motor is a logical choice). the remainder would set up to follow the speed reference of the first... you have a lot options there on brands, one person has stated ABB with Fiber... but really you got a lot of choices. I would set up the 3 follower drives to have the option of a torque limit mode to be enabled, so when you get to the field... if you see the conveyor being pulled too tight you can flip a bit enable that mode and test it out....

you could also do position following (electronic line shafting)... but it sounds like they didn't do that before and you shouldn't have to go to great lengths to make this work well as crude as it was set up before.
 
S
I think you misunderstood the capability I was describing. I was saying that you can set one drive, call it the master for a given speed and the fiber is actually fast enough to give cycle by cycle torque information from the master to the slave drives in time for them to use it, while set in torque mode.

Not saying your approach won't work or that it's not a good approach. Just saying the coordination capabilities I was describing require a lot more speed and capability than a speed-setpoint sharing scheme, which is what I believe you thought I was suggesting.
 
steve,
i have to admit i did read through your post kind of fast. on a second go round you're right you did speak of passing torque on the same clock cycle... However again, there are a lot of drives that can do that these days.

I don't think that kind of overkill is necessary for coordinating a few motors on a conveyor though.

I guess my point was: before these motors all had effectively the same speed reference (one drive) and shared the load (one drive)... there's not much simpler way to do that today with multiple drives by doing a simple speed follower with a torque limit... there are even more drives out there that can do that than there are with high data rates on proprietary comms.
 
S
Nor did I intend to demean line shafting with a shared speed reference and torque limiting. I've done it many times myself, and it could well be a better solution for this app.

I also don't mean to impugn by omission other drive brands; just that I don't have the same degree of familiarity with them. If one vendor is there, it shouldn't be surprising if other industry leaders are as well.
 
Hello all...

Thanks for your posts... They had opened my brain for other possibilities...

I am writing again to update about this application...

So, as you know, nowadays the customers doesn't want wasting time with tests. They want to produce. After a lot of meetings and conversation, the customer gave us one day to change the system to AC motors.

Today the system is working with one motor in Speed control and the others as Torque follower mode.

BUT...... The conveyor is too old and has a lot of vertical curves that make the things a little worse...

I think that due the above facts the chain behavior isn't so stable. Sometimes we have the chain too tightened and sometimes we can see it loosing.

For the next steps, I am thinking in configuring one drive as speed controlled and the others as speed followers with a fixed torque limitation due the current difference between the motors...

If you can suggest any other control method I will be very, very glad...

Cheers,
Mauricio
 
in my previous posts, i did suggest a speed follower with a torque limit. that should fix your bag and pull bag and pull you get. I did leave out one small detail though... when you implement this mode the followers need to be "overspeed" to about X% (i.e. 5%) faster than the master, then the torque limit be set to keep a proper tension in the chain... bam, bag and pull goes away.
 
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