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VFD's causing chain stretch?

Vladdrac

Plastic
Joined
Jul 20, 2017
I’ll do my best to describe our sortation system in-order to better help you understand the issue. We basically have a drop tray sortation system. Tray’s are carried around horizontally by a drive chain and actuated by pneumatic cylinder stationed at each chute. Each chute is a potential drop for any of the carrier trays on this sorter. The chain is driven by two 7.5hp motor on each end. Each motor is controlled by a power flex 40 VFD which each have a set point of 40hz but no feed back loop. There is currently no feedback loop that allows the two VFD’s to keep tabs on each other and insure that they run in sync. Both VFD’s fire up at the same time and run at their 45hz set point. In terms of operating, this set up seems to work fine.

Our system is 5 years old now and the first time we had to tension the drive chain due to slack was about a year ago. A few weeks ago we had to tension the chain again by the same amount. Putting aside the fact that we weren’t sufficiently monitoring chain slack until it started causing us issues, our current feeling is that it seems like the chain might be stretching at an accelerated rate.

I checked the amperage for both VFD’s and each phase averaged about 2.7amps on one of them and 0.2amps on the other one. Theses results randomly alternate between each VFD after we restart the sorter and hold until the next restart. This suggest that the VFD’s aren’t perfectly in sync and only one of the two VFD’s is carrying the load. Also, when the chain is slack you can see the chain pushed outward as it comes of the sprocket at one end to the sorter, suggesting it’s driving the chain into the sprocket downstream (the manual say’s the chain should bend inwards off the sprocket when the chain is slack).
We’re trying to stay ahead of this issue as we would like to get another 5 years life from our sorter chain and of course avoid any potential hazards that could occur from excessive chain slack (we will continue to monitor though).

Here is my question; Is this sync issues were seeing with the VFD’s normal with type of set up? We’ve been exploring the idea of a mater/slave scenario between the two VFD’s by either upgrading the VFD’s or having it written into our PLC logic. One thing we don’t know is if expecting a situation where both motors share the load equally reasonable or even possible. I’d prefer not to spends a lot of resources trying to make our VFD’s perfectly in sync only to find out that those are unreasonable expectations for our system. Does anyone have any experience with this issue? Should we abandon further investigation of this issue and simply continue monitoring chain stretch? Thanks for reading this post. I’m curious to see if anyone has any opinions on this.
 
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I would say remove one motor and see how things go
Also see how much amps does the motor/gearbox draws freerunning
If that is more as 0.2 Amps one motor is driving the other
0.2 Amps freerunning with a gearbox sounds very low for a 7.5 HP motor
But wear on a chain speeds up over time
Look at the guiding of the chain If thats worn you even experience more wear
Replacing the sprockets is no good idea eighter
Because of wear the pitch of the chain gets bigger so the nominal diam of your prockets needs to be bigger in fact
Therefore the chain climbs on the sprocket and wears to a bigger nominal daimeter
With a new sprocket that gets more difficult for the chain to run to the bigger diameter resulting in even more wear

Letting run the 2 motors at the same amps will improve things IMHO
I would see the sollution in getting the Amps the same
More as in syncronising the speed
But I am no expert in VFD s


Peter
 
Restrict the current of each vfd?

If we knew which cheap Chinese vfd you were using...

If decent vfds, perhaps there’s a way to synchronize?
 
Thanks so much! That makes a lot of sense. If it got to the point where we had no choice but to replace the chain I assume you would recommend changing the sprockets at the same time?
 
They are powerflex 40 (Allen Bradley)22B-E1P7N104.I'm currently investigating way's to synchronize. I think the power flex 70's might have master/slave capabilities but I'm not 100% on that.
 
I just wanted to make a correction. I said in my original post the each VFD had a feedback loop using an encoder. That was incorrect. They simply have a set point of 40HZ and that's it. I've corrected it in my original post.
 
f you syncronise them with a encoder you still might end up with one motor doing all the labor
Perhaps if one motor is not powerfull enough to do it singlehand it might work
But that is not the case I think
Perhaps you can set output power on the VDF ???

See if there is difference in wear of the sprockets
That is difficult to detect for the untrained eye however
What is the pitch of the chain and lenht of the system
Only 2 wheels ???
Defenitly change the sprockets too
Or if big enough you can remachine them

Peter
 
To do this you either need an absolute position encoder, or you need to drive the motors with a torque control loop in addition to encoder feedback for velocity control. Most vfds can't do that.

Or just mechanically connect both shafts together?

Another solution is drive both motors from a single 15hp vfd. Slip variations will cause the motors to not share the load equally. But you will always have positive torque from both motors
 
And by the time you get all of this sorted, you could have bought an Allen Bradley Servo Drive and motor and been done with the issues in a matter of hours for probably about the same money.
 
I'm having a hard time visualizing this setup.

It seems like one motor is pulling the loaded side of the conveyor, and the other is pulling the unloaded side. That would just put your slack in weird places.

Also, doesn't chain wear begin to happen rapidly beyond 3% chain stretch? I thought at that point the pivot areas are so wallowed out you get a single sliding contact point instead of a smooth rotation on a large load bearing area.

Measure the pitch of your chain and see if it is out of spec by 3% or more. If so, replace the chain, it could be a hazard.
 
I think there is no need for any further synchronization. Induction motors have slip that varys with load. So they tend to synchronize themselves. If the load on one increases it slows down and the load transfers onto the underloaded one which is trying to go faster.

Chains stretch which increases the pitch length and then they don’t match the pitch of the sprocket and the chain climbs the teeth of the sprocket. So chain and sprocket wear begin to happen faster and faster. You’ll have to change out the chain and sorockets sooner than you might like.
 
I think there is no need for any further synchronization. Induction motors have slip that varys with load. So they tend to synchronize themselves. If the load on one increases it slows down and the load transfers onto the underloaded one which is trying to go faster..

Unless the motors are really overkill Thats the case here I think
A 7.5 HP motor draws about 10 Amps with full load at 415volts
Here 1 motor is pulling 2.7 Amps and the other 0.2 Amps (even below idle amp??)
That is fishy

Peter
 
You can’t push a chain.
All the weak motor is doing is taking up slack.
Move it down the line by 1/4 to 1/3 the line length.
It’s possible the weak motor is leaving too much slack on the loaded side of the line and the strong motor is “jerking the slack out.
If that’s the case adjusting the weak motor speed down will keep a little tension in the loaded side of the line, same for adjusting the ramps to keep that slack to a minimum.

In any case I don’t think there’s a problem...
5 years is a reasonable maintenance interval.
 
After 5 mins of googlefu. The powerflex 70 drives comes with torque regulation options, and the 750 etc comes with positioning options too.

As per the specs for ac 40s communication capabilities.:

• Integral RS485 (Modbus RTU, Metasys N2, P1-FLN)
• Optional: *EtherNet/IP, *ControlNet,*DeviceNet,
BACnet, *Bluetooth®, *LonWorks®, *PROFIBUS DP.

You say you have plc logic available as id imagine with the system you describe.
If you want the to run in synch and have control of the vfd loads, etc. Running them as slaves on a profibus dp network, fx lets you do that.


But those drives do not have the options to run with torque control, positioning feedback etc. So the powerflex 40 isn't really suited for that, it seems to be a very very basic vfd in terms of software.

Did the motors use to run with a equal load ca before these issues?

So you will want to upgrade drives, get the correct comm cards etc for your needs. And get it programmed into your plc logic, if thats what you find out is causing the issue in the first place.
 
"I'm having a hard time visualizing this setup.

It seems like one motor is pulling the loaded side of the conveyor, and the other is pulling the unloaded side. That would just put your slack in weird places.

Also, doesn't chain wear begin to happen rapidly beyond 3% chain stretch? I thought at that point the pivot areas are so wallowed out you get a single sliding contact point instead of a smooth rotation on a large load bearing area.

Measure the pitch of your chain and see if it is out of spec by 3% or more. If so, replace the chain, it could be a hazard."


Thanks for that. The good news is I've measured the chain stretch recently and it was something like 0.02% so that part seems ok. I'm still a little concerned about the poor syncing as it seems like it's putting stress on the chain and can cause timing issues on the slack side in terms of tray drops. It's basically like a 400 foot horizontal carousel. The chain carry's trays around and axis which actuate at designated chutes.
 
Unless the motors are really overkill Thats the case here I think
A 7.5 HP motor draws about 10 Amps with full load at 415volts
Here 1 motor is pulling 2.7 Amps and the other 0.2 Amps (even below idle amp??)
That is fishy

Peter

I'll admit it's strange but those are my amp readings. You can see the chain slack one side and tight on the other from the poor synchronization. It's switch's sides seemingly randomly but holds until the system is stopped and started again.
 
I cannot recall seeing two drive motors on one chain/belt/whatever.

Most times one would have a drive motor and a tensioner



As noted, one is really not going to be doing much
 
You can’t push a chain.
All the weak motor is doing is taking up slack.
Move it down the line by 1/4 to 1/3 the line length.
It’s possible the weak motor is leaving too much slack on the loaded side of the line and the strong motor is “jerking the slack out.
If that’s the case adjusting the weak motor speed down will keep a little tension in the loaded side of the line, same for adjusting the ramps to keep that slack to a minimum.

In any case I don’t think there’s a problem...
5 years is a reasonable maintenance interval.

Thanks, unfortunately the two motors alternate between which one carry's the load so adjusting one of them isn't an option. Based on my findings so far I'm guessing monitoring the chain and swapping it out when it gets to worn is our best option. There doesn't seem to be a quick fix for this issue.
 








 
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