Can a VFD do magic with motors

Goody

Member
Join Date
Apr 2002
Location
Huddersfield W Yorks UK
Posts
1,081
The question I would like answering is;

Is it possible for a motor that is controlled by a VFD (rather than just a contactor, to run at considerably less current while at full nameplate speed and HZ.

The story (short version)
A customer of mine was visited by a third party and an ABB expert.
They told the customer that by fitting ABB drives on about 30 of their motors considerable yearly electricity bills savings would be made. They gave actual money amounts saved.

The customer jumped at this and ordered the 30 ABB invertors.

The first ambiguity, although not really part of this is that each VFD was listed with the yearly saving figure, plus a figure of zero for fitting and commissioning.
The customer, after seeing the zero figure, thought that VFD’s were of a negligible cost to fit.

Every one of the drives are for production machinery. Mainly circulation fans for hot air from gas burners. The customer treats cloth with fire-guard and resins and the like.

Enter me. I was not privy to any of the prior discussions. All I got was ‘please fit them for us’

The first machine needed four drives fitting. These were for 4, 11KW motors. I had to make a panel and do all the interface wiring etc (The drives are quite big in physical size) The customer was upset with me because I took 3 days to do this and with parts he got a sizable bill. (he expected the fitting to be very low)

Now on to the crux of the matter.

The only way I could think of to automatically speed up and slow down these invertors was to follow the low and high fire of the gas burners.
It soon transpired that this was not working.
The correct temperature was not made in the center of the chambers. I then wired an override switch to make the invertors run at full speed (In the UK this is 50HZ) So now, most of the time the motors are running at 50HZ.

Everything is back as it was before the invertors were fitted. The customer is now not happy because he cannot see the point of the invertors in the first place.

I set up all the standard parameters on the drives including everything from the nameplate of the motors.

The customer has been in touch with the third party and ABB and they say an ABB drives expert can set up the parameters so that the motors will run at their original speed but still give the massive stated yearly savings. They say this was there intention and they were not supposed to run slower.

Does anyone know if this is so. I am attending a meeting Friday with the customer, the third party and the ABB expert. I would like to be furnished with all the relevant facts. I will either learn something I did not know or realize that the wool has been pulled over the customer’s eyes.

Please give your opinions on this.
 
Goody I have ABB drives and many other types, they all promote energy savings in some applications by using their drives.

At http://www.abb.com if you goto the Drive & Motor section (this may take a little browsing/searching) under AC motors then Applications you will find 2 programs and manuals that define energy savings for fan or pumps. There is also information on setup etc to obtain the desired goals. The program you may want to look at is FanSave 3.1.
 
I hope DickDV gets to weigh in on this; he's forgotten more about AC drives than I'll ever know.

I'm mostly familiar with using VFD's to replace restrictive flow control methods on fans and pumps; centrifugal loads have an inverse cube relationship between speed and power (yes, I'm simplifying greatly).

This only pays off if you can often run the motor at lower than base speed, such as is the case of an air-conditioning fan on a cold day. They're sized for max load (August), but they run at reduced load (the other 11 months in Seattle) most of the time.

But running at full speed ? ABB advertises a "Flux Optimization" feature in their ACS600 drives that can lower the amount of current being used to magnetize the stator. This can save "between 1 and 10 %" of power, but only if the motor is lightly loaded.

I can find reams of documentation on traditional energy savings methods with VFD's, but I have not seen a writeup or calculations on Flux Optimization with the ACS600. Probably they'll bring in their expert who knows how to tune up that feature.
 
Hooooo Boy! I'm an ABB drive specialist here in the US as well and posts like the above make me shiver.

Let's cover some basics first and, I might add, nothing that isn't true for pretty much any inverter brand. First, on fans there are huge potential savings in energy costs due to the fan shaft energy being a function of the cube of the speed and the air volume being a function of the shaft speed itself. For example, if you can slow the fan by only 10% the volume drops 10% but the energy falls by 27% (.9 X .9 X .9 = .73).

Second, an inverter can accelerate a load from stationary to full speed with far fewer amps than an across-the-line starter or even a reduced-voltage starter. That is due to the inverter managing the motor so it is always running on the front face of its torque-speed curve. Starters and soft-starters throw 50 or 60Hz at a stationary motor and suffer huge inrushes of current because the motor is way off its synchronous point. Check out a typical NEMa or IEC torque-speed characteristic curve for motors and this will be clearer. By avoiding these huge inrushes, you reduce the peak currents that, at least here in the US, will get you surcharges on your electric bill. For example, using a mag starter will generally result in 6-8 times nameplate current to accelerate, a soft-start will rarely get below about 3 times nameplate, and a vfd properly programmed can do it without exceeding nameplate amps. Now note, this isn't so much an energy saving as an avoided cost on your electric bill from peak demand surcharges.

Third, inverters and some soft-starts have a programmable feature which have a lot of meaningless names but, at least for ABB inverters, is called motor flux optimization. In those drives I am familiar with, it would be a parameter in Group 26. When a motor is lightly loaded, the drive or soft-start reduces the voltage to it. This cuts the amount of current that simply maintains the field in the motor and reduces motor heating and noise. It also can save some energy if the motor is lightly loaded for long periods of time. The motor becomes rather soft in speed regulation but at light loads it isn't noticeable. As soon as the load increases, the voltage is returned to normal levels so speed stability returns. Unfortunately, this can't happen quickly so, on applications where fast response is needed, flux optimization isn't a good idea.

Now some observations and speculations on the specific situation posted above. Since there is huge potential savings in my first item and only minor savings in the second and third, I would assume that there would be the basis of claims for huge savings. But how is the end user going to get them if the drive is set to run only at full speed! A control system which provides some basis for slowing the fan when demand falls is essential. Where is it?

It may be that the air flow is currently being controlled with supply side dampers. The installation of vfd's on fans with dampers is normally aimed at eliminating those dampers. A drop in maintenance costs and better efficiency results.

Maybe, the aim here was simply to reduce peak demand charges as in my second point. I can hardle imagine that paying for itself but, maybe these charges are larger in other areas than here in the US. In that case, the drive could be set up for accel ramp to full speed with no external controls.

Maybe the plan was to leave the dampers in place and take advantage of flux optimization when the fan unloads at low volume. This would be possible but stupid, it seems to me, since greater savings lie within easy reach by eliminating the dampers entirely. Further, the same thing could have been acheived thru soft-starters if the dampers must stay, and for much less money.

And, finally, any claim that drives have no installation costs comes straight out of Ripley's Believe it or Not!! How ignorant! How absurd!! Doesn't anybody check this stuff out? The unfortunate thing here is that ABB will get a bad rap over this when, in fact, the product is probably excellent. Instead, at least two people, by the account given, have failed to exercise any due-diligence before committing to this project.

Definitely, sit down with the parties involved and find out what each was thinking. Maybe some weren't thinking! In that case, get an outside expert in the application of inverters to render an opinion. It is quite safe to say that significant energy savings are within reach here but, it won't happen automatically. Someone is going to have to do some thinking!

(Sorry, I didn't mean to get shrill, but I can't seem to stop shivering!)
 
to Ken Roach

Sorry for the duplication, Ken. And thanks for the compliment.

You slipped in there while I was writing my rather lengthy reply.

A mark of real genius is the ability to say a lot in few words. On that basis, you've got the honors!

And I'm way back there in the dust! Boo Hoo!

Happy Thanksgiving everyone.
 
Thank you all for your responses and I too was hoping DickDV would give his much appreciated and valued opinion.

First the fan motors. I already knew that big savings could be made by reducing the speed 10%. This is why I set the drives to run at 90% top speed and 80% low speed.
These speeds were following the gas burners at High and low fire.
As I said, it did not work, the cloth was coming out wet through lack of air flow.

There are no dampers on the gas burners, they fire directly into chambers within the machine. They are controlled by temperature controllers.

The fans are never lightly loaded, they are either on or off. Any reduction in speed causes bad circulation. They had to slow the machine to a crawl to get the cloth dry and treated correctly.

The savings are purely energy savings. Not power factor savings.
The savings quoted had a payback time of less than 1 year

I was intrigued by the magical tuning of parameters that could run the motors at full speed and lower the currents.

So far I have only fitted these four, there are another 26 in box’s waiting for the outcome of this meeting.

Has someone been over zealous in selling these or will the savings quoted be met.

In my mind, these Tenter machines were designed long ago with all the air flow factors taken into account. But I am always prepared to be taught something.

Incidentally, I had never used these ABB VFD’s before and I liked them. Easy to set up with plenty macro’s and functions and esthetically pleasing too.

One last thing, I also thought the salesman did overkill on the programming plug in units. (30 of them) Some of the other machines are going to have 8 VFD’s side by side inside a panel that only me is ever going to go in.
 
Well, Goody, now that I've quit shivering (yes, its snowing outside too) I've thought some more about your situation.

I would give the ABB people a chance to explain their energy saving calculations and let them explain just how the drive and motor have to be set up to acheive the savings. It could be that there is just a lack of communication here about what's supposed to happen.

Regarding the keypads, all ABB drives that I deal with come with their own keypads standard. That is because a supprising number of drives are actually controlled directly from the keypad by the machine operator.

One other factor that I left out of the earlier discussion is the conduction losses inherent in any drive. In any modern AC drive you can figure 2% of the load hp or kw is lost in heat in the drive. If you think about that, it says that you start 2% in the hole regarding energy savings and then have to save more than 2% to start building any net savings. That pretty much wipes out any potential savings from flux optimization and puts a pretty good dent in peak demand charges you might avoid. (Note: this is not power factor. This is peak demand which, in the US is what you get charged for--not actual demand)

Unless the people who did the energy analysis and sales are completely off in the ozone somewhere, they must have some plan for reducing fan speed at least some of the time. You mentioned two speeds on the existing motors. How is that done?

The bottom line, I guess, is to let them talk. Who knows what will come out. We always learn more by listening than by talking anyway. At least, I do.
 
My hat is off to DickDV for putting such a complex issue into clear and rational perspective. I've been on both sides of the fence when it comes to selling/buying technology, and as with everything else, it can often boil down to buyer beware. Often we are dealing with well meaning people on both sides of the buy/sell equation who are just hoping to somehow use the latest gizmo or technology to magically increase their sales/savings. Most of us however can't put numbers to some of these equations, but obviously DickDV has the knowledge in this area to really narrow the numbers down.

What I really like most about your response, is that you didn't attack either of the two parties involved in this issue. You didn't condemn the salesman for claims he likely could not meet, nor the buyer for expectations that may have been unrealistic. In both cases, they obviously didn't have your knowledge on the subject, and in the end it was likely more a matter of the blind leading the blind. It just goes to highlight how important it is to find someone you have confidence in, and when it comes to drives, I'd buy from this dickDV guy any day.

Bill
 
Thanks Dick DV for that information. I shall report back after the meeting.

Originally the motors were one speed only. Direct on line.

The job was given to me with no information whatsoever. Apart from these experts came - gave us these figures - go fit them.

There are four gas burners and four motors (one for each burner)

The gas burners run on high fire until temp is reached then go to low fire until heat is called for again.

The cycle when up to temp is approx - 1 minute high fire, 30 sec low fire.

I thought of using the signals for low and high fire for controlling the speed of the invertors. I used one of the multi speed macro's

Of course, as previously said, it did not work as thought. I came under fire (high fire ha ha) for the cost of the installation and it not working.
A bit like shooting the messenger I thought.


And now (from a telephone call from the customer) I am informed that they were not meant to vary the speed, they would save energy without speed changes.


That is the part I did not understand properly.

The heat loss calculation had already crossed my mind because I put a cooling fan in the panel and hot air is constantly streaming out of the escape vent. The only bonus was an extra fan heater to warm the factory :p
 
Let's speculate a little on what the VFD proponent might have been thinking. He sees a fan running at a fixed speed and a gas burner cycling between high and low fire to maintain temperature. He thinks to himself,
"There is some lower fan speed at which I can maintain temperature without cycling between low fire and high fire. Running a fan at lower speed generates significant electricity savings. This is a good application for a VFD."

He then might have calculated the potential electricity savings by looking at the high fire/low fire duty cycle to calculate how much slower the motors could turn.

Two factors that would offset the electricity savings come immediately to mind. First, the system would be using more gas than previously, since the burner would be running continuously at high fire. Second, and the one you seem to have already discovered, is that you need the higher flow rate to carry away the moisture extracted from the fabric.

If the intent of the system was simply to maintain the temperature in the burner chamber, reducing the fan speed might have generated a savings, but the savings would have depended upon the cost differential between gas and electricity.

As I mentioned at the outset, this is nothing more than idle speculation on my part. Terry is fond of advising us to 'be the computer'. I though it might be fun to 'be the VFD salesman'.
 
The only thing left to do here, it seems, is to get the sales people and the customer together and engage in some real world conversation about how the energy savings was done and how the process really works. Unfortunately, someone likely is going to take a hit here when the facts are all out.

The shameful thing is that this should have been done before the deal was done. I know from my own experience just how difficult it is for a conscientious drive salesman to walk into a completely unfamiliar facility and be expected, in less than an hour, to have a detailed understanding of how to modify a process beneficially. I've also observed many times just how gullible some customers are, simply taking everything they hear at face value, no questions asked. It's a dangerous business on both sides and years of experience have taught me a good bit of humility and skepticism regarding this work. I ask a lot of questions initially and even that doesn't cover all the risks.

In Goody's case, though, there was apparently a written energy savings analysis done. One would think that even a minimally curious customer would at least brush thru the analysis to see if it had any relevence to his operations. And, I've never seen an energy analysis that tried to take into account the customer's installation costs. To include those would require a knowledge of how the customer expenses his maintenance and engineering personnel hours or what kind of commercial relationship the customer has with a contractor, if he uses one. Normally, this addition cost is left to the customer to add. It sounds like this was not done. Seems incredibly naive, doesn't it.

Finally, repeating what I said earlier, it is a rare case where fans running with across-the-line motors cannot be coaxed to yield up substantial energy savings. It takes some creativity usually and a willingness to do some open thinking.

Hopefully, something good will come out of this for both parties. At minimum, at least we've had a good time with it, don't you think?

Thanks, Goody, for sharing this with us.
 
I'm not feeling as kind as Dick - in my opinion the VFD salesman is either ignorant or deceptive. The customer was either naive or negligent.

One potential savings with VFDs that hasn't been mentioned is reduced power factor. Because the VFDs convert AC to DC and back to AC, they more or less run at a high power factor across their range. Unfortnately, most users don't pay power factor penalties, and for those that do power factor accounts for a small percentage of billing.

I have had great success in many projects reducing demand charges using VFDs, but this was the result of reducing power consumption, not the result of reduced starting current. The impact of starting on demand is generally exaggerated. Most utilities use a 15 minute "window" for averaging kW when they determine demand charges. Even the high inrush current from an accross the line start isn't going to add much to the demand for that total period, although there is obviously some impact.

The odds are the salesman isn't purposely deceptive. ABB is an honest firm with good products. The salesman probably looked at some standard or average savings data for centrifugal fans, which can be significant. Then, presumably through inexperience and forgetting totally the first law of thermodynmics, he applied these typical values to your customer's application. I have seen this happen before. I hope that in your meeting you get a chance to see the calculations, and ask for their basis, underlying assumptions, methodolgy, etc. In my opinion the only proper way to do the analysis is to look at the load profile, the system air flow requirements, the fan curves (Q vs P and Q vs hp), the system curve (Q vs P) and come up with a weighted average power requirement. Several of the VFD manufcturers show how to do this in their application guides.

(Daddy always told me, "If it sounds to good to be true, it probably is!")
 
Last edited:
Hi everyone,

I'm a maintenance tech in the UK and have had some experience of VFD's. I would like to state how I would adjust the VFD's in this problem just to see if my theory stands up to scrutiny from an expert (which I am definatly not). I am not offering this as a solution its a question.

In the Uk our 3phase supply is 415v-440v. If the VFD parameter for max output voltage is limited to a value below this the current output also falls. This in effect allows the power of the motor/drive to be fine tuned to that required to do the job (reduce the voltage to much and the motor stalls/slows down). The torque output of the motor is reduced proportionatly to the voltage. In some cases when this has been done motors that were overheating have started to run cool (power Input reduced).

Is this a sensible thing to do and what are the problems associated with it. What is causing the drop in current is it a limiting function of the VFD or is it a natural response from the motor due to the reduced voltage supplied.

Be kind to me I'm only an electrician!
ian
 
If I was a sceptical man, I might think that someone was now running scared.
The meeting for last friday was cancelled due to them being unable to attend. It was re-arranged for today, Monday. Nobody showed and no telephone call.

Is this a case of 'head in the sand' and hope it goes away?

I have visited the company twice now on a chargeable visit,its getting good innit
 
Ian, what you are describing is a manual means of accomplishing the same thing as we discussed earlier in this thread--flux optimization. The benefit of doing it automatically in the flux opt. software parameters is that full excitation and full torque is available if needed for short-term things like acceleration, etc. You may want to check earlier in this thread for a more complete explanation of how this works.

And, yes, it can be overdone. If you starve the motor for voltage too much, the rotor will start to slip excessively due to a weak field and current will go up dramatically. So will motor heat.

Best to leave this to the brains in the drive and use flux opt parameters, if available, in my opinion.

Either way, you won't hardly get enough energy savings in most applications to cover the drive overhead losses.
 

Similar Topics

Hello, I'm trying to delve a little into rs-485 communications for a couple projects of mine. Until now I've been using a delta vfd and a delta...
Replies
2
Views
54
I have an issue with Power Flex 525 during running processing, the VFD stopped suddenly while the PLC and VFD connection ok, VFD does not have any...
Replies
1
Views
92
Has anyone setup communications with Red Lion 3.0 MODBUS to Baker Hughes Centrilift GCS VFD? Thanks
Replies
0
Views
70
Hello, I've been trying to learn this a while now and still have not found out how this works. I have an Omron CJ2M PLC and an ABB ACS 355 VFD...
Replies
1
Views
191
Looking for some assistance. I am very familiar with Yaskawa VFDs, but not so much with AB VFDs. I am working on some hardwired AB PowerFlex 700...
Replies
2
Views
89
Back
Top Bottom