motor is accelerating slightly over the course of a few hours on its own

[Alfont1120]

Without getting into too much detail about the process we are running over here, what could cause a motor to accelerate without manually changing the line speed? We are finding that we will set the speed to 17 FPM and come back several hours later to find that its running at 21 FPM. I know that doesn't sound like much, but when you are trying to plate gold onto a strip, it needs to be constant. I have attached a picture of a graph showing the speed over time. Any thoughts as to why this would be happening?

Thank you

 

[Alfont1120]

no attachment

no attachment

Sorry, it won't let me upload the attachment like I said in the post.

 

[junkhound]

With the paucity of info supplied, there are literally 100's of causes, from thermal drift of a voltage reference to line voltage variations to control loop instability.

What type speed control, VFD or an old Ward Leonard system, etc......??
DC motors or 3 phase, etc... etc...????
Supply...???
Load torque variations, etc. ....

 

[steve66]

What kind of motor is it, and how are you adjusting the speed? Does it have a VFD? Is there a dial on the VFD, or are you controlling the speed with an input? Any kind of feedback from the belt speed to the VFD for closed loop control?

And what's the maximum speed of the belt?

Edit: I type too slow

 

[GoldDigger]

Maybe a little farfetched, but if the speed control is not closed loop but instead just regulates voltage to the motor, possibly the frictional resistance of motor, drive train and belt decrease over time as the bearings, etc. warm up.
Experimentally, if you stop the line and then restart it does it drop back to 17 immediately or does that only happen after everything has had a chance to cool down?

I have worked with record players where the fine speed control was just an aluminum disk and a permanent magnet which could be moved to change the resistance to motion seen by the motor.

 

[Ingenieur]

As others have said
too little information
vfd?
reduction gearbox?
motor rpm? Motor type?
drive roller diameter?
constant load?
open loop or feedback control?
what is v and i at 17 and 21 ft/min?
encoder? Potentiometer? How is speed measured? Perhaps the feedback device slips Over time?

17 foot/min is slow, 3.4 in/sec
variation is >20%
not sure an ac induction motor could slip anywhere near that
1350 to 1750 for example

more info needed

 

[Sahib]

Without getting into too much detail about the process we are running over here, what could cause a motor to accelerate without manually changing the line speed? We are finding that we will set the speed to 17 FPM and come back several hours later to find that its running at 21 FPM. I know that doesn't sound like much, but when you are trying to plate gold onto a strip, it needs to be constant. I have attached a picture of a graph showing the speed over time. Any thoughts as to why this would be happening?

Thank you

It is a control loop problem. Check the specifications of the original setup.

 

[GoldDigger]

Sorry, it won't let me upload the attachment like I said in the post.

Try reducing the size of the image file by cropping the picture and/or reducing the resolution.
The other alternative is to save the file on a photo hosting site (like Photobucket or Shutterfly). Some offer limited free service. Then your post only needs to have a link to the photo(s) on the sharing site. Just make sure that we don't have to login to see the picture(s).

 

[Besoeker]

It is a control loop problem. Check the specifications of the original setup.

We don't even know if it's a motor problem at all.
We are given process linear speeds but nothing (yet) about how the motor is coupled to the process.

 

[Sahib]

Bes: I urged OP to take action instead of trying to confuse him with many possibilities.

 

[iwire]

Bes: I urged OP to take action instead of trying to confuse him with many possibilities.

Worried someone is taking your job away from you?

 

[Sahib]

Worried someone is taking your job away from you?

How do you know?

 

[Besoeker]

Bes: I urged OP to take action instead of trying to confuse him with many possibilities.

You stated to him that it is a control problem. We/you don't know that so you can't really assert, as you did, that it IS a control problem.

We need more details.
Junkhound hit the nail on the head with his comment about the paucity of info.

 

[Sahib]

You stated to him that it is a control problem. We/you don't know that so you can't really assert, as you did, that it IS a control problem.

We need more details.
Junkhound hit the nail on the head with his comment about the paucity of info.

It may or may not be control problem. But I believe it is a control problem. So nothing wrong in asserting to exhaust that possibility first.

 

[Besoeker]

It may or may not be control problem. But I believe it is a control problem.

That's better.

 

[Sahib]

That's better.

Adopt it.

 

[gadfly56]

It is a control loop problem. Check the specifications of the original setup.

You should adopt one of the tenets from the Hippocratic Oath: First, do no harm. Your assertion has no basis in fact, given the lack of data. Unless you are claiming some sidebar conversation with the OP we don't know about. I had a boss with your attitude. Just jumped in changing things without getting all the facts. Didn't work for him, either.

 

[Sahib]

You should adopt one of the tenets from the Hippocratic Oath: First, do no harm. Your assertion has no basis in fact, given the lack of data. Unless you are claiming some sidebar conversation with the OP we don't know about. I had a boss with your attitude. Just jumped in changing things without getting all the facts. Didn't work for him, either.

"Getting all the facts....." Do you think it is really possible? You have to always make decisions with limited availability of facts. By the way, do not blame your boss. He may be an expert in his chosen field and experts frequently behave that way.

 

[Besoeker]

Adopt it.

A wee tale for you....

We were commissioning some VSDs in a cement works. One was on an inclined conveyer, quite a common piece of kit in such plants. The conveyer was visibly varying in speed. Of course the drive and motor were immediatly blamed. Nothing unusual in that. So we checked the VFD frequency and motor speed. Both were as steady as a limestone rock............... Those were the bits we supplied and were responsible for. Job's a good 'un as they would say there - English Midlands

But there was no denying that the conveyer speed was varying. It turned out that there was a fluid coupling between the gearbox output and the conveyor drive shaft. Nobody seemed to know about and hidden inside machine guarding. Not a drive or motor problem.

The moral?
Don't jump to conclusions - sometimes they'll jump back at you.

 

[Sahib]

A wee tale for you....

We were commissioning some VSDs in a cement works. One was on an inclined conveyer, quite a common piece of kit in such plants. The conveyer was visibly varying in speed. Of course the drive and motor were immediatly blamed. Nothing unusual in that. So we checked the VFD frequency and motor speed. Both were as steady as a limestone rock............... Those were the bits we supplied and were responsible for. Job's a good 'un as they would say there - English Midlands

But there was no denying that the conveyer speed was varying. It turned out that there was a fluid coupling between the gearbox output and the conveyor drive shaft. Nobody seemed to know about and hidden inside machine guarding. Not a drive or motor problem.

The moral?
Don't jump to conclusions - sometimes they'll jump back at you.

See post #18. The real moral of your story: Engage an expert to solve a technical problem. He is more likely to pinpoint the problem immediately.