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Smooth motion at low rpm with VFD
7

Smooth motion at low rpm with VFD

Smooth motion at low rpm with VFD

(OP)
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!

RE: Smooth motion at low rpm with VFD

Run the motor at a higher speed and couple it to your low-speed load using a reduction gearbox.

A motor with a greater number of poles would have a similar effect.

RE: Smooth motion at low rpm with VFD

Hi nadica.

Simple answer is, I think - To run low speed you will need an encoder with large number of pulses per rev fitted to the motor and the drive in closed-loop vector control. You might try a little voltage boost (fixed or variable depending on what your drive has to offer) but I don't think you will be 100% successful.

At low Hz the applied motor volts is also very low and now the small voltage drops in the cables and stator are a much larger proportion of the volts applied so the motor flux is compromised. With an encoder on the motor shaft the drive 'knows' exactly where it is in relation to the applied stator frequency and will/should adjust it's output to achieve the required speed and 'smoothness'.

This ignores any other 'problems'.

RE: Smooth motion at low rpm with VFD

Motors are discrete devices. They have a couple of poles which are the sources of the torque or twisting effort. Because of that, they do not have a smooth continuous way of moving degree by degree. As they speed up the effort becomes smooth via the momentum of the spinning rotor. You are essentially trying to use a hammer as a screwdriver. It can be done but you must spend a lot of money to misapply everything effectively.

As was suggested above using a gear ratio gets you back to place where the motor can work effectively. You have gear boxes or belt drives to help you with this.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Smooth motion at low rpm with VFD

Although I completely agree that the best route to smooth operation is to gear down a faster motor, I'll also toss in here that not all "vector control" algorithms are created equal. There are more than a few bottom feeder VFD mfrs who use the term "vector control" in a manner "unbecoming an engineer". For Vector Control to properly function, the VFD must measure and create a mathematical model of the motor equivalent circuit. This can be a data set you can enter manually, but honestly, trying to get all the necessary info from a commercial-off-the-shelf motor supplier is like trying to get a government official to remember a campaign promise. So most good VFD mfrs provide whats called an 'Auto-tune" feature (or called something similar) in which the VFD does a test run of the motor to gather the necessary info. Some only do a "static tune" which means the motor is energized, but not spun, but the best data set will come from a rotating auto-tune procedure, which generally means disconnecting the load to do it. How you can tell when a bottom feeder is selling you a bill of goods on what they want to call a "Vector" drive is to read the manual and discover that there is NO auto-tune procedure at all and no way to enter motor data at all, so they can only be using a set of nebulous average data points. In my opinion, that belies the entire concept of a true vector control algorithm, but unfortunately there are no "spec police" keeping them honest, and people buy them because they are cheap. So before you judge all vector control drives as being incapable of smooth low speed operation, read your manuals carefully. If there is no auto-tune option, there is no true vector control. If they only offer a static tune, it could do better, but they chose not to tell you that. If they DO give you an option for a rotating tune, try that if you used the static tuning, it's better. If you DID in fact do a rotating auto-tune and you still don't like the outcome, but don't want to put an encoder on your motor, then use the gear box.


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

RE: Smooth motion at low rpm with VFD

(OP)
Many many thanks to all of you for your advise!

Jraef pointed out that not all VFD Vectrol Controls are equal. What about motors, could be a difference between motors running from identical VFDs?

I definitely will follow Jraef’s advise as I have to replace the VFD so I will look into more what I am going to get. The current one does static auto-tune.

Closed-loop control, Drivesrock, I will go down that route later but first I will see what is the best I can get out from a VFD with rotating auto-tune.

Gearing – it is partly done 1/9, couldn’t do more because there is a high rpm requirement, too. But the current motor is 4-pole, maybe use a 2-p motor and 1/18 ratio?

RE: Smooth motion at low rpm with VFD

Different motors will typically respond the same and the main factor will be the vfd capability.

Your research on different model vfds should include comparing min sensorless vector speed control speed or hertz. For instance, Hitachi says 0.5hz min control, which is typically much lower than most cheap ones. Your 'couple hertz' speed on the 1800rpm motor is only 60rpm. Many sensorless vector vfds will not be able to give any decent speed regulation here; this is about the slip of the motor itself and the 'fake' algorithms Jraef mentions, used by many low cost units, cannot operate here.

If changes to hardware is the future, I would definitely go to a real closed loop encoder feedback vector drive instead of messing with gearing and 2 pole motor. Think of it this way: change all that hardware and keep lowly vfd and get possible 2x improvement vs just change vfd to closed loop, add encoder, and get 1000x improvement.

www.KilroyWasHere<dot>com

RE: Smooth motion at low rpm with VFD

Here's one possible way to approach the selection; look for a drive that offers Sensorless Vector Control cability, but can ALSO provide full blown Flux Vector Control (FVC) or Field Oriented Control (FOC) by adding an encoder to the motor (and often an accessory card to the drive). If the DRIVE is capable of being upgraded to FVC/FOC, that means it has the brain power to accomplish it, which will factor into the performance of the SVC algorithm. That way you can try their SVC and if it works for you, you are done but if not, then get the accessory card and an encoder and I know for sure it will work. The "holy grail" of FVC /FOC performance is the classic "100% torque at zero speed" capability which was only attainable with DC drives for years. That takes a lot of brain power to accomplish in an induction motor drive, which does not come cheap. But it is attatinable for sure. In fact, I know of a couple of high-end drive products available that are capable of FVC without the encoder feedback now, because of improvements in the mP technology as well as the way current is sensed on the motor output (an integral part of the accuracy when not using an external feedback device). I wouldn't use that for servo-like positioning or for hoist control where you need torque proving, but it is excellent for torque or velocity control accuracy. And by the way, another way of determining the capability of a drive to get you there will be to see if it is capable of running servo motors instead of induction motors. I'm not saying you need to use a servo, but a VFD CAPABLE of running a servo will be one that is capable of giving you what you need from your motor.

What you need is out there, one way or another. The main point is that if you are operating at that low of a speed, you are at the margins of the typical working envelope for induction motor drives. The drive mfrs or products who lead with price rather than performance are the ones which fall off at the margins.


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

RE: Smooth motion at low rpm with VFD

Oops, I forgot the motor.

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. You never mentioned the size, but at small power ratings you may be able to get that with a TENV motor, but larger sizes will usually mean the motor is going to be "blown", meaning it will have a separate blower for cooling air that is NOT powered by the VFD output, so it stays at full capacity even as the motor is barely rotating. Those motors will also come with better winding insulation and better bearing designs to thwart other potential issues of running on VFDs.


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

RE: Smooth motion at low rpm with VFD

Jeff, are you thinking NFO?

It is now available in the US. UL listing and all.

But it is not, to my knowledge, available separately as a drive. If you really want to test one, then talk to DeLaval, the dairy and farm equipment manufacturers. They may be able to let you test one.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Smooth motion at low rpm with VFD

(OP)
I managed to improve the smoothness greatly with the current hardware. The story is long why but I changed the "Motor Rated Voltage" setting to 170V from 200V and the rotation at 5Hz VFD output is much much smoother. The VFD is plugged in a laptop and I had the following figures on screen with the two different settings:

"Motor Rated Voltage"
230V 170V
Output freq. 5Hz 5Hz
Output volt. 28.7V 18.9V
Output amp. 5.2A 2.4A
Output power 149.2W 45.4W
Output torque 27.1% 11.5%
Bus volt 315V 314V

I am wondering if these figures could be correct? Why I have a smoother motion at lower figures. There was no load on the rig just some inertia and friction from the mechanism.

Should I try to play with the settings in V/f mode? (i.e. not vector mode)

PS: I am in the UK but the NFO website is not working.

Many thanks again to all of you!

RE: Smooth motion at low rpm with VFD

What is the nameplate voltage of the motor? 200V? If so, what you have done is lower the V/Hz ratio, which makes the motor create less torque. If you don't need much torque at this low speed, the lower saturation of your windings may make it appear smoother, but it would be at the cost of increased slip, which means more current for the work being performed. If you are monitoring current and it's still low however, there should be nothing wrong with that, other than the cooling issues as mentioned before.


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

RE: Smooth motion at low rpm with VFD

Yes, the power output would drop because the motor isn't constantly accelerating and decelerating. I would try V/Hz mode. There typically is a voltage boost parameter that increases motor voltage at low speed with V/Hz that you'll have to adjust lower I would expect.

RE: Smooth motion at low rpm with VFD

Get a Smart Motor from Moog-Animatics in Milpitas Ca

RE: Smooth motion at low rpm with VFD

If you can change the gear ratio, it should be possible to run a 4 pole motor at speeds up to 120 Hz to meet the high speed requirement. This will let you run the low speed at a higher frequency. This worked for us in a special treadmill to train wheelchair athletes and cyclists up to 40 km/h but still be stable with 110 kg footballers at walking speed

RE: Smooth motion at low rpm with VFD

Quote (notnats)

If you can change the gear ratio, it should be possible to run a 4 pole motor at speeds up to 120 Hz to meet the high speed requirement. This will let you run the low speed at a higher frequency. This worked for us in a special treadmill to train wheelchair athletes and cyclists up to 40 km/h but still be stable with 110 kg footballers at walking speed

That's fraught with a LOT of pitfalls, the least of which is the dramatic loss of torque as you go above base speed. There are tricks of the trade to come up with work-arounds, but those are not for initiates in the drives world and should never be done without consulting the motor mfr.


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

RE: Smooth motion at low rpm with VFD

Quote (itsmoked)

Motors are discrete devices. They have a couple of poles which are the sources of the torque or twisting effort. Because of that, they do not have a smooth continuous way of moving degree by degree. As they speed up the effort becomes smooth via the momentum of the spinning rotor.

I don't agree with this. AC motors are not the same as stepper motors. The magnetic field will rotate smoothly if you apply a sinewave current.

The root problem is that the vector control algorithm is not applying an AC current waveform to the motor, but rather a modified waveform that is supposed to create more torque. But without a sinewave current, the motor can begin to operate similar to a stepper motor at low speeds.

V/Hz mode should give a better sinewave to operate the motor more smoothly.

RE: Smooth motion at low rpm with VFD

Lionel:

I agree with you in part, but disagree in part as well. There are several interesting issues, so it is worth breaking them apart.

I concur that AC motors are fundamentally continuous, not discrete. This includes both asynchronous induction motors and synchronous motors like brushless servo motors and even stepper (yes!) motors. The windings on these motors produce at least approximately sinusoidal torque functions as the motor (or just rotor field) spins. With multiple phases, you can get very low torque variations if you put in sinusoidal waveforms.

Of course, higher harmonics from the windings, and reluctance/cogging torque in the magnetics will limit how low you can get your torque variations, but many AC motors are designed to minimize these effects, and it is possible to compensate for these in the controls.

You may be surprised that I included stepper motors in this class. But most stepper motors (I will exclude cheap VR steppers) are AC synchronous motors. With either open-loop microstepping control or closed-loop control (treating it as a high-pole-count brushless servo motor), you can get very smooth control with sinusoidal command waveforms to the phases.

But I fundamentally disagree with your assertion that "the root problem is that the vector control algorithm is not applying an AC current waveform to the motor." We've been doing vector control for almost 30 years now, and we have always applied AC current waveforms to the motor, which are sinusoidal functions of time in the steady state. The power stage for a vector control drive is really the same as for an open-loop VFD. Both synthesize AC waveforms from a DC bus by a modulation scheme, usually PWM. The differences are in the control schemes for synthesizing the waveforms.

Roughly speaking, there are three classes of control schemes (focusing now on induction motors):

1. Open-loop VFDs ("Volts per Hertz" drives): These command AC waveforms with no knowledge of what the motor is actually doing, relying on the electromagnetic feedbacks in the motor to make the motor (roughly) follow the command signals. As their nickname suggests, the output frequency and magnitude are at least roughly proportional. These are cheap and simple, but have the lowest performance. They have a lot of trouble at low speeds, because they have no capability to resolve the magnetization and torque component interactions as a vector drive would.

2. "Sensorless vector" drives: In the technical literature, these are called "shaft-sensorless" drives, because they do rely on voltage and current sensors inside the drive to try to figure out what the rotor is doing. Fundamentally, they attempt to back out the back EMF voltage component on the phases to compute rotor velocity and angle. This works well at high speeds when the back EMF is large, but is much more difficult at low speeds, when it is a small component of the overall values, and the signal-to-noise ratio is horrible. Still, it is significantly superior to open-loop control.

3. Full vector drives: These employ a high-resolution feedback device on the drive that directly measures rotor angle. This provides excellent performance all the way down to zero speed. In fact, it has been common for over 20 years now to use this to make induction motors operate as positioning servo motors. If a machine tool has a spindle that is capable of "hard tapping", it is almost invariably an induction motor under full vector control. And smooth motion at low speed is vital for this function.

I know of no performance disadvantages of full vector control compared to sensorless vector or Volts/Hertz drives. Of course, they are more complex and expensive.

Curt Wilson
Delta Tau Data Systems

RE: Smooth motion at low rpm with VFD

"I know of no performance disadvantages of full vector control compared to sensorless vector or Volts/Hertz drives."

You forgot a VERY important part - if properly implemented. Your dissertation about how vector control is so superior doesn't change the fact that the sensorless vector control in THe OP's VFD on THE OP's application is providing a crappy waveform to the motor that is causing it to step.

I guess you missed the part that I'm addressing the OP's question. I would have thought that was obvious since I'm responding in the thread where he posted his question.

RE: Smooth motion at low rpm with VFD

I have a similar problem.
My motor is highly rated for my application, so there are times at which I need to be able to run both at relatively low speed and low torque. However, this is a torque controlled application, controlled at the moment with sensorless vector control from a VFD. What I find is that the stuttering mentioned by the OP is present, but also that when torque reference is low the motor will not gain speed as needed, and I lose the tension in my load.

In sensorless torque control, the motor should speed up until the torque output is matched, so this implies that the system losses are enough to match the output torque. However, this seems unlikely given the numbers involved. Does sensorless vector torque control struggle to provide accurate torque output at low torque references? Will adding a motor shaft encoder for closed loop feedback help this issue? Are there any other obvious solutions? Cheers.

RE: Smooth motion at low rpm with VFD

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.

www.KilroyWasHere<dot>com

RE: Smooth motion at low rpm with VFD

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.

RE: Smooth motion at low rpm with VFD

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.

www.KilroyWasHere<dot>com

RE: Smooth motion at low rpm with VFD

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.

RE: Smooth motion at low rpm with VFD

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.

RE: Smooth motion at low rpm with VFD

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

RE: Smooth motion at low rpm with VFD

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.

RE: Smooth motion at low rpm with VFD

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

RE: Smooth motion at low rpm with VFD

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.

RE: Smooth motion at low rpm with VFD

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.

www.KilroyWasHere<dot>com

RE: Smooth motion at low rpm with VFD


Quote (jraef)

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?

RE: Smooth motion at low rpm with VFD

Clock motors have gear boxes.

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