# VFD Controlled conveyor motor "cogging"



## frenchelectrican (Mar 15, 2007)

Hey MD .,

What kind of feedback you are running to the Powerflex unit ? 

V/Hz or others ?


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## MDShunk (Jan 7, 2007)

Sensorless vector, which is default and how all the others are running.


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## Cow (Jan 16, 2008)

I can't say I've seen the problem you're experiencing and I'm around VFD's a fair amount. Maybe Jraef will see your post.

Did you reset the VFD to factory defaults, I think it's P41 from memory, before "squirting" the same parameters back into it?

Have you connected this VFD to another conveyor/motor for testing? In the past I've laid THHN across the floor and into adjacent MCC buckets to rule out either the load or VFD.


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## MDShunk (Jan 7, 2007)

I have been being bull-headed about running autotune simply because, like I say, this is functionally identical to 80-odd other drives doing the same duty with "out of the box" parameters other than the motor data and the little bit to make it take stop/start/speed commands from the 20-COMM-E card.


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## emtnut (Mar 1, 2015)

Since it seems everything else has been changed out ....

Any difference in how the wiring for speed control is ran ?

given the drive change out, maybe a chassis or analog ground loose/overlooked ?


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## MDShunk (Jan 7, 2007)

emtnut said:


> Since it seems everything else has been changed out ....
> 
> Any difference in how the wiring for speed control is ran ?
> 
> given the drive change out, maybe a chassis or analog ground loose/overlooked ?


I wish. The only thing hooked to the drive, quite literally, is the 480 in, 480 out to the motor, the enable wire from an aux contact on the local disconnect (which I recorded once upon a time and it's steady), and the ethernet cable. No analog. Everything done over the ethernet. I can trend the speed command in RSLogix, and it's dead stable. It's done this since brand new, both before and after the drive swap.


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## frenchelectrican (Mar 15, 2007)

Hey MD.,

How clean the 480 input is ? if that is super clean and what about the distance from the VSD to the motor ?


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## emtnut (Mar 1, 2015)

I like Cows idea to temp hook up to another conveyor.

Something load related would make sense ... especially once a second on a conveyor.


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## splatz (May 23, 2015)

I agree, swap with another drive with identical programming in another location and see if the problem follows the drive or stays with the location. Then you've really ruled out mechanical problems or issues with the supply.


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## paulengr (Oct 8, 2017)

Put an independent ground wire between motor and drive. You should ground on the line side but it’s not necessary but must ground on the motor side to the motor. If it doesn’t you get common mode noise big time.

Also with such a small motor it is highly sensitive to tuning issues. Autotune it with the motor uncoupled and you’ll be surprised. Not just the motor but the inductance and capacitance in the cable matters, too,

Disconnect leads at drive and low ohm bridge. Ideally check inductance too if you can. I’d use a pdma but if you have a surge tester or even a multimeter with inductance that works.


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## Rora (Jan 31, 2017)

MDShunk said:


> The drive display does give a steady hz display at the set run speed while it is surging.


Is this based on the output, or the feedback? If not, what does the feedback indicate? Might be a red flag.

A constant, consistent surge almost sounds like a control loop oscillation, which might explain why everything is the same and working "correctly". Did the feedback components get replaced with everything else?


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## Westward (Mar 3, 2018)

whats your motor current doing? I'd go online and see what parameters are under program control. If your motor, VFD, and gearbox have been replaced, that leaves mechanical load and program control.


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## paulengr (Oct 8, 2017)

Is your motor C face or standard? If it's standard is the coupling hitting? Not just if it's touching...is the clearance meeting spec?


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## John Valdes (May 17, 2007)

If it has an autotune function I sure would try it. You got nothing to loose.
If its not mechanical and your certain the mechanical end is good than there is not much more you can do except try a drive from one of the identical set ups. This will for sure tell you if its a drive problem.
Like said above. Is the motor mounted to the gear box with a C or D flange? Coupling or direct insert? If its a coupled motor and gear box, you could have a coupling issue.
I have seen this before. The set screw for one side of the coupling was loose and the it backed away. The only way we knew was when we separated the two.


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## MDShunk (Jan 7, 2007)

It's a 56C motor, directly coupled to the hollow shaft gearbox and conveyor drive roller is similarly inserted in to the gearbox output hollow shaft. No couplings, just slip fit and shaft keys, which are in known good condition. 

Next time I've got an extra couple minutes, I'll switch the drive output conductors with it's neighbor drive a few inches over in the control panel and see what I get. 

I'm still resistant to run autotune, even though that may well solve the issue, because the same setup works for the 80 some other conveyor drives. There is something peculiar about this particular setup but I just haven't been able to put my finger on it.


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## RePhase277 (Feb 5, 2008)

Silly question perhaps, but have you uncoupled the motor and checked the mechanics of the conveyor? Run the motor at low speed, uncoupled? Run the motor on direct line power?


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## telsa (May 22, 2015)

No way that the spider in the coupling is going sideways ?

Elastomeric 'Slop' would produce the 'cogging' described in the OP.

This is possible even when the spider looks fine to the human eye.

Remember that the power is transmitted THROUGH the spider... inside the gear coupling... gear tooth to gear tooth.

The 'cogging' is created during the squeeze and release of the elastomer. 

Test this possibility by swapping it out.


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## MDShunk (Jan 7, 2007)

telsa said:


> No way that the spider in the coupling is going sideways ?


No. Not possible, as there is none. It's all keyed to a hollow shaft gearbox.

I even tried to test the eleasomeric phenomenon some time back, in a different way, when I pulled the gearbox off the drive roller and moved the conveyor by hand. I wanted to make sure the belting wasn't sticking on the bed material, loading up like a rubber band, breaking free to move some, then repeating that cycle. I could wind the drive roller by hand with no perceptible stiction.


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## MDShunk (Jan 7, 2007)

RePhase277 said:


> Silly question perhaps, but have you uncoupled the motor and checked the mechanics of the conveyor? Run the motor at low speed, uncoupled? Run the motor on direct line power?


I did uncouple the motor at one point and moved the conveyor by hand and there's nothing perceptible. I did run the motor uncoupled, but since the cogging is only slightly perceptible on the output side of a 40:1 gearbox, I was not able to perceive whether or not there was cogging on the unloaded motor shaft itself. I have not run the system across the line, but running it at 60hz with the drive, the cogging is just barely perceptible.


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## RePhase277 (Feb 5, 2008)

Put a scope on it. The uncoupled voltage waveform should reflect the cogging if it is a drive issue. The current waveform of the coupled motor will reflect the cogging if it's mechanical.

Or maybe just spend the 5 seconds and use auto tune and see what happens:vs_laugh:


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## MDShunk (Jan 7, 2007)

RePhase277 said:


> Or maybe just spend the 5 seconds and use auto tune and see what happens:vs_laugh:


I'm resistant for several reasons. One, nothing else needed auto tuned. Two, it would require the motor be uncoupled, which would involved planned downtime and at least two different types of safety plans filled out. Three, I need to send it a start signal from the PLC to start the autotune without turning on a bunch of other stuff, which would involve forcing a bit in the PLC, which requires another form be approved. (if I put the drive in local mode and run it from the faceplate, it will fault the comm monitor in the plc). Between you and me, I really think autotune is the answer, but I'm stymied why this one is different than the rest- particularly after a new drive, motor and gearbox. I'm hoping to get a better understanding of what's different about this one.


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## Bird dog (Oct 27, 2015)

Is it handling different material?


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## MDShunk (Jan 7, 2007)

Bird dog said:


> Is it handling different material?


No. Same stuff, day after day. Even when loaded, its essentially the same weight as unloaded. The whole 40' run might get 20 extra pounds on it when it's fully "loaded". The belt material is acetal and the bed guideways are UHMW, so it's pretty slick running.


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## RePhase277 (Feb 5, 2008)

There's a dead spider across the IGBT terminals.


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## Rora (Jan 31, 2017)

Hairbrained idea here... what influence does the sensorless vector feedback have on the output? Because if there is any, it's possible that everything is working exactly as intended, but the feedback is introducing oscillations... i.e. the VFD believes it is maintaining a smooth 30 Hz, when in reality it's responding to an imaginary error/some kind of modulation in the feedback to do so, and in doing so actually introduces surges.

edit: heh, _sensorless_... nevermind, I just need some coffee.


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## MDShunk (Jan 7, 2007)

RePhase277 said:


> There's a dead spider across the IGBT terminals.


Yeah, a 1Hz spider, no doubt. lain:


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## paulengr (Oct 8, 2017)

MDShunk said:


> I'm resistant for several reasons. One, nothing else needed auto tuned. Two, it would require the motor be uncoupled, which would involved planned downtime and at least two different types of safety plans filled out. Three, I need to send it a start signal from the PLC to start the autotune without turning on a bunch of other stuff, which would involve forcing a bit in the PLC, which requires another form be approved. (if I put the drive in local mode and run it from the faceplate, it will fault the comm monitor in the plc). Between you and me, I really think autotune is the answer, but I'm stymied why this one is different than the rest- particularly after a new drive, motor and gearbox. I'm hoping to get a better understanding of what's different about this one.


No. You switch back to keypad control and press start. Although the risks are decreased with an uncoupled motor remote control is always a no no when autotuning. PF40 also has a hardware jumper and/or software enable input which is normally used for an E-Stop circuit so you will have to check for that and either software or hardware jumper it out.

There is also static autotune. Not as effective as dynamic but may do the trick here. It basically does the IR drop test but none of the others. Quite often when I'm setting up motors for various reasons dynamic autotune just isn't practical. So first I use V/Hz and autotune (static) and get things set up manually that way then go to sensorless vector and autotune statically again.

Might want to try V/Hz mode too. Advantage is that it's "stupid". It does current feedback out except current limiting. Disadvantages are lower efficiency and looser speed control. Since its not a feedback loop after that though if the problem disappears it shows you it's either a tuning issue or something in the conveyor catching.

By the way, vibration and infrared are not good ways to test conveyor idlers diagnostically. They are ok for PM purposes if done often enough but if the idler bearings wear out and it either jams or sits down on the housing and doesn't engage anymore, you see nothing. The best test unfortunately is visual inspection WITH those tests. If one isn't turning you have to check it by hand. IR and vibration is just fooling yourself into thinking tests that work great on gearboxes, couplings, and electrical connections work just as well on idlers. They do early on in failures sometimes but not after that, and definitely never on low speed conveyors. I'd do a full manual and visual inspection.

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## MDShunk (Jan 7, 2007)

Yes, the enable is already jumpered. 

I think I will change to V/Hz mode and see how that effects the observations.


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## paulengr (Oct 8, 2017)

One final possibility. Some motors naturally cog or slur. There are “bad” slot and rotor bar ratios that have this problem. It shows up particularly on lightly loaded motors at slow speeds then disappears under higher speeds or loads. There are a myriad of winding patterns and the bad combinations are widely publicized in the motor industry so designers and winders have no excuse but we see them all the time, particularly for anything other than 2 or 4 pole motors because designers and winders are creatures of habit.


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## MDShunk (Jan 7, 2007)

paulengr said:


> One final possibility. Some motors naturally cog or slur. There are “bad” slot and rotor bar ratios that have this problem. It shows up particularly on lightly loaded motors at slow speeds then disappears under higher speeds or loads. There are a myriad of winding patterns and the bad combinations are widely publicized in the motor industry so designers and winders have no excuse but we see them all the time, particularly for anything other than 2 or 4 pole motors because designers and winders are creatures of habit.
> 
> 
> Sent from my iPhone using Tapatalk


Possible, but the installation actually uses the same catalog number motor as the 80-odd others that work, and the one that's on it presently isn't a rewind.


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## varmit (Apr 19, 2009)

Are you sure that the other motor/VFDs have never been tuned? If the motor or other components on this one have been changed, that would throw off the tuning.

Have you observed the output current? Is it cyclic?

If the output Hz are constant, and the current is oscillating, it would seem to indicate a mechanical problem. 

Look at the commanded Hz and see if this is stable.

In todays world a component being new does not mean that it is any better than the one that you threw away. It is not uncommon to have a new VFD (or anything else) to have problems out of the box.


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## varmit (Apr 19, 2009)

If this is a location with a bunch of conveyors, often there will be multiple motors VFD leads in the same conduit. This can create problems. 

Another common problem that I find: wirenuts. Wirenuts seem to be a common failure on motor connections from a VFD. 

Check the power input line frequency to the VFD. If there are a bunch of drives on the same buss, with no line reactors, the frequency can be way more than 60 HZ. I have found drives that were giving problems that were caused by another drive imposing additional frequency on the power distribution system. I found one location with 93 Hz. on the 230 VAC distribution. It was an old DC drive causing problems with other VFDs.


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## Peewee0413 (Oct 18, 2012)

Run motor/gearbox disconnected in keypad control. If it runs fine its probably the conveyor since you said comm signals were good.

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## JRaef (Mar 23, 2009)

When you run in SVC, and you don’t use Autotune, the drive uses some default values for the motor equivalent circuit. Sometimes you can get lucky and those default settings work fine, more often that’s not the case. The fact that a lot of your units run just fine is extremely lucky. But honestly, what you are describing is classic for an untuned motor application. 

The other more remote possibility is a problem in your motor. Did you swap that out? If so, that eliminates that possibility. If not, your symptom could be indication of a broken rotor bar or it’s connection to a collector ring in the rotor. That’s hard to see but is usually diagnosed by eliminating other causes, then disassembling the motor.


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## MDShunk (Jan 7, 2007)

That settles it. I'm running autotune tonight provided I can get the safety plans signed off on and I'll report the results.

Yeah, the motor's new. At least once. I'd have to look back through the records, but possibly more than once.


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## MTW (Aug 28, 2013)

MDShunk said:


> That settles it. I'm running autotune tonight provided I can get the safety plans signed off on and I'll report the results.


Great news, I think you have a much better chance of getting a signed recording contract now.


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## RePhase277 (Feb 5, 2008)

MTW said:


> Great news, I think you have a much better chance of getting a signed recording contract now.


He has a beautiful voice, I'll have you know.


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## Bird dog (Oct 27, 2015)

....


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## MDShunk (Jan 7, 2007)

Wasn't able to get the downtime approved this week to try autotune, but I did switch the mode to V/Hz during a brief coupole minute shutdown, and the cogging stopped. Essentially pointing that it's a tuning issue. I put it back to sensorless vector and intend to try autotune this weekend assuming the creek don't rise.


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## oliquir (Jan 13, 2011)

If it is running ok in V/hz mode i would leave it like that. Most application don't need vector control


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## MDShunk (Jan 7, 2007)

oliquir said:


> If it is running ok in V/hz mode i would leave it like that. Most application don't need vector control


There you go talking common sense. :wink: It's personal at this point. I will not be defeated. :vs_mad:


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## Peewee0413 (Oct 18, 2012)

MDShunk said:


> Wasn't able to get the downtime approved this week to try autotune, but I did switch the mode to V/Hz during a brief coupole minute shutdown, and the cogging stopped. Essentially pointing that it's a tuning issue. I put it back to sensorless vector and intend to try autotune this weekend assuming the creek don't rise.


What frequency are you running at?

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## MDShunk (Jan 7, 2007)

Peewee0413 said:


> What frequency are you running at?
> 
> Sent from my VS995 using Tapatalk


No more than necessary. :smile:

35Hz- presently.


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## Rora (Jan 31, 2017)

Initially I thought this sounded like a feedback oscillation issue... you've checked everything extensively and replaced physical components, but still gets modulation--tells me there is a distinct possibility that the VFD thinks everything is working "as intended" but there is a disconnect between what the VFD thinks is happening and what you can sense is happening in reality. The fact that running V/Hz mode (which is open loop/no feedback) seems to agree with this theory.

Except, you're using sensorless vector. Doesn't really change this theory except to say the "feedback" is emulated from a model. I thought perhaps with your systems the model would have been generated from the parameters you copy-pasta from other VFDs, but seems likely that it's generated during auto-tune. Perhaps a rough model is assumed from parameters, and a real-world model is measured during auto-tune.

Now, why the others didn't need auto-tune? In my experience tuning feedback loops, the difference between tuning systems which stabilize on their own and those that oscillate continually or go out of control can be very small, and that would be the case especially with a fast reacting loop like a VFD. Still doesn't really answer why this one deviates enough from the norm to need auto-tune, though, and that is the crux of your frustration. I might say you may need to scrutinize any difference between the values you measured on this compared to other drives... slight differences that seem "close enough" may what's pushing this over the limit of remaining stable.

If auto-tune resolves the issue. That also suggests that the other drives could benefit from auto-tune as well, although they stabilize well enough that it's not noticeable.


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## MDShunk (Jan 7, 2007)

Rora said:


> If auto-tune resolves the issue. That also suggests that the other drives could benefit from auto-tune as well, although they stabilize well enough that it's not noticeable.


I suspect you're 100% right. If I bothered to measure the actual speed on some of the others I suspect I'd find measureable oscillation but not visual oscillation. The one I'm puzzling on is the only one that's visually observable. There are appurtenances on some of these conveyors that are speed sensitive (data coding, rejects, etc.) that use their own encoder feedback to count pulses between when they're triggered and when they have to do something, so precise speed control by the conveyor prime mover isn't really a true requirement.


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## just the cowboy (Sep 4, 2013)

*A thought*

Marc
Have you put a tach on it to see if it is really cogging?
The reason I ask is does it have LED lighting above it? 
I have a cooling fan motor on a motor at work and a heater motor at home that look like they are cogging due to the strobe light affect of a LED flashlight for the cooling fan, and a 4' strip light above my heater it looks like it stops sometime.

Just a thought from
Just the cowboy


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## MDShunk (Jan 7, 2007)

just the cowboy said:


> Marc
> Have you put a tach on it to see if it is really cogging?
> The reason I ask is does it have LED lighting above it?
> I have a cooling fan motor on a motor at work and a heater motor at home that look like they are cogging due to the strobe light affect of a LED flashlight for the cooling fan, and a 4' strip light above my heater it looks like it stops sometime.
> ...


 Quite certain.


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## mitch65 (Mar 26, 2015)

Simplest thing to do seems to be swap 2 drives and see if the problem goes with the drive or stays with the conveyor


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