Smooth motion at low rpm with VFD 7

[nadica]

Hi All,

I would like to pick your brains if I may.

I try to run a motor with FVD at very low speed, even just with a few Hz. I know this is not recommended and could lead to problems but ignoring that... I observed that the motor is not turning in a smooth manner rather like the second hand of a watch, stops momentarily and jumps, oscillating if that is the ride world.
I tested with two different VFD and the one with more sophisticated control (vector mode?) was better but what else I could do to improve the smoothness? The current motor is a 4-pole one. Does the number of poles influence this?

Thanks in advance!

 

[mikekilroy]

Obviously adding an encoder will help both the vector algorithm as well as allow you to increase system gain.

That said, there are too many unknown to us variables in your situation with the info you have provided to state absolutely that you need to go to true vector mode. For instance, how are you doing torque control? Is there a torque loop? What is its bandwidth? Is it adjusted properly or are your gains too low and that is the cause of your mushy-ness? Do you really have a speed control and think it is in torque mode? How do you know for sure? Is your drive even capable of going to true torque control mode? I will guess that your drive is maybe not so sophisticated and cannot do a true torque control algorithm, or if it does, you have gains in the loop set too low for the response you require.

http://www.KilroyWasHere<dot>com

 

[Ananym]

That's a good point - looking through the parameters, while "IM Motor Vector Torque Control with Speed Limit" is certainly a discreet control platform, it doesn't look like I can adjust the gains of the torque control loop - only the speed controller. Would this imply that there isn't a true torque control loop?

The descriptors for the speed controller gains reads like this -

Vector Speed Controller Proportional Gain
Sets the proportional gain value for the speed controller when operating in Vector Speed or Vector Torque motor control modes...​

My drive is a fenner qd:neo, a versatile little thing that allegedly does support closed loop vector speed & torque control. I've not had prior experience with VFDs though, so I don't know how it compares.

 

[mikekilroy]

I think this is as I suspected, not a true torque loop - or the velocity gain term would have NO effect in that mode. So, since you appear to have a semi-torque controlled gadget, I would suggest you play with this proportional gain term and see if that makes your response better. Normally increasing a gain value will make it respond better. Also there is normally an INTEGRAL gain term also so look for it as it may have more effect than the proportional based on your want. And I assume you ran some auto tune routine to match the sensorless vector routine in this drive to that motor? If not, do that first.

http://www.KilroyWasHere<dot>com

 

[cswilson]

Lionel:

The OP clearly stated that the more sophisticated algorithm in his drive, which he presumed (probably correctly) was sensorless vector, produced better performance than the simpler algorithm (likely V/Hz). Given that, your advice to him to return to V/Hz mystifies me.

 

[LionelHutz]

He tested 2 different drives and you're the one making the assumption the first one was V/Hz. Even if it was, it's ridiculous to make the assumption that the V/Hz performance of one drive is the same as the next, much like assuming any drive with a "sensorless vector" mode will perform the same.

At the end of the day, he probably needs a better sensorless vector drive but may also need a good vector drive as well as someone who can properly set it up. Just slapping a encoder into the system could just end up in another big disappointment if it's not properly done.

 

[jraef]

OK, so that is actually an Invertek Optidrive P2 (brand-labeled to Fenner), so the instructions should be the same. Did you perform the Auto-tune function in parameter P4-02? What is the setting in P4-01? Your choices should be Speed Vector with torque limit, Torque Vector with speed limit, or "enhanced" V/Hz. If you want smooth action at low speeds, I would put it in speed vector with torque limit, but no matter which you choose, you MUST perform the auto-tune function or nothing will work right.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[Ananym]

jraef:
Yeah, it's rebranded. I've given it its autotune. During primary operation it needs to be in torque vector, since the operation demands a variable torque. I recognize that this is perhaps not the ideal VFD for the job - but this is what the project owns and I'll have to make the best of it until I've proved it's unsuitable. I'll give Kilroy's words a go and play with the speed controller gains to see if I can coax more from it. The aim is to improve its low torque accuracy and consistency.

 

[jraef]

Oh, now this has become curiouser and curiouser.

"Variable torque" does NOT mean you want the drive to be in torque control mode. All that means is that your LOAD demands less torque from the motor as speed decreases, ie a centrifugal machine like a pump or fan. Vector control, whether torque or velocity, should be irrelevant, and in fact if it is a centrifugal machine, that is the SIMPLEST of control capabilities of a VFD. But why would you be so concerned for low speed operation at all if it is a centrifugal machine? Generally speaking at some point in the operating curve of a centrifugal machine, usually somewhere above 30% speed, it ceases to provide any useful work at all! So either you have a misunderstanding of what variable torque means (or I do!), or you are meaning something different and CALLING it variable torque.

Torque Vector Control in the VFD is for when Torque must be maintained at the EXPENSE of speed accuracy. A classic example is a winder application, something like a toilet paper winder or wire drawing machine. As the take up roll changes diameter, the surface speed at which is pulls the product through would change if the shaft speed of the roll remained constant, increasing the likelihood of breaking or stretching the product. So the shaft speed must be varied to keep the surface speed of the product relatively constant. But the real culprit in risking breaking or stretching the load is the tension applied to the product regardless of speed; that must remain much more closely controlled. So Vector Torque Control will maintain a more precise CONSTANT torque on the shaft as a form of tension control, and the actual speed accuracy becomes secondary to that torque control. If that's what you meant, that is not VARIABLE torque, that is CONSTANT torque at variable speed.

So if that is the nature of your application, and you have properly set up the drive yet continue to have low speed instability, now you might be experiencing the differences in quality that exists out there with regard to vector control algorithms and the feedback mechanisms inside of the VFD, especially when it comes to "sensorless" vector control; not all are created equal and as a gross general rule, the less expensive the drive, the more compromises were made in accomplishing the finer points of that task. For 99% of applications out there, what you are experiencing is uncommon, so the ability to keep it from happening suffers from the laws of diminishing returns for a VFD mfr, meaning if they chase it too religiously, they add cost faster than they increase market share, which makes them lose even more market share.

But all that said, it also might be that if indeed this is a VT application on a centrifugal machine, and you have been chasing accuracy trying to use a torque vector control loop, then I might be inclined to side with Mike Kilroy on this: you have been chasing your tail here when a simpler form of control, such as V/Hz would serve you better.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[Ananym]

Uncanny! I see what you're getting at.
So yes, my project is fundamentally a winding application where the tension in the feed must be kept as constant as possible while speed accuracy is fairly inconsequential. However, the pattern that is to be wound is geometrically complicated, and these changing conditions mean that the actual torque output required to maintain a constant tension follows an arbitrary oscillation as the wind progresses. So, the torque accuracy is important, and the set point will also be changing constantly. I recognize that this is an unusual situation in automation.

Ultimately, the bounding range of allowable tension won't be prohibitively small, so this should be achievable. During the early stages of the wind, with the feed beginning mounted to the centre of rotation, the small radii involved are what cause the low torque demand that the motor is struggling to provide accurately.

I have plenty to play around with - thanks for taking the time to reply to this out-of-depth intern.

 

[mikekilroy]

AnAnym, I am not sure if it is correct to keep posting on your needs since you sorta hijacked the OP thread here, but since others are and the OP seems to have gotten the answers he needed, I will reply again.

Know that your application is NOT uncommon; most of us who use variable speed motors on here have many done similar applications - yes, even with non linear per revolution torque control. I believe cswilson works 99% of his time in torque control mode with his projects, and he is one of the worlds best motor control loop designers. Ditto a lot of the others on here.

I went back in posts to look at the size of your motor again to see if perhaps you should consider going "the right way" with the redesign - if you choose that route, but alas, no details. I assume it is small?

What is the bandwidth of your response requirement? Since you want snappier response than you presently have, perhaps letting us in on this would help? Your induction motor is great for control, but does have a very high inertia compared to a brushless PM motor, so it will limit your response time. If that time required is well under what you can achieve with your motor then no problem. Otherwise, you may be pushed to a servo style lo inertia motor to get the response you wish.

http://www.KilroyWasHere<dot>com

 

[racookpe1978]

If you are going to be operating at that low speed for any length of time above a few seconds, you definitely need a motor capable of that. The way to attain that is going to be indicated by the "turn down ratio" of the motor, meaning the range of speed in which the motor is DESIGNED to operate without sacrificing performance and/or service life. You will want to look for a motor with a turn down ratio of 1000:1, meaning it can SAFELY operate at frequencies down to fractions of a hertz.

So, what would the original poster use if he needed a "1 per day" motor output - like to track the sun or drive a calendar-type clock? Here, he "seems" to be trying for a "1 per second" (sub 1 hz) or less rate, but how would he go much slower? A gear drive would become too heavy to be effective, would it not, for a 1/2 horsepower or smaller motor?

 

[Compositepro]

Clock motors have gear boxes.