VFD confusion 11

[Mysterrose]

Trying to understand how VFD's save you money, and I've seen people throw around that a x% reduction in speed is equal x% cubed in energy reduction. From what I can tell this is only true for pumping or air handling type applications where speed and flow are roughly a 1:1 ratio but speed to energy consumption has a cubed factor.

What about applications like a conveyor system? I'm thinking processing plants, shipping plants, heck even an escalator. Right now say they use a gearbox or belts to reduce the speed of the conveyor. Going to a VFD would eliminate the gearbox/belts and allow you to direct couple motor to conveyor just driving the motor at required speed. What I don't understand is if in a situation as described does the VFD save any money?

To me a gearbox/belts is a zero energy device in theory. I'm assuming no losses due to friction, heat, etc. So removing the gearbox/belts does not reduce amount of energy used. If you're not reducing the amount of energy used, how do you save money?

In the above assuming that nothing changes in terms of speed of conveyor or loading on conveyor. You're still processing or moving same amount of "stuff". So aren't you still doing the same amount of work?

 

[ScottyUK]

Mike,

What you appear to be overlooking is some real fundamental stuff. I'm sure you understand it, you are just not registering it. Do you agree that if you need to do a certain amount of work then it requires a certain amount of energy input? I'll assume that you agree with that, because it is conservation of energy and if that's wrong then a whole lot of other things also start to fall apart. Can we further agree that the required energy to to do the work can be delivered at a fast rate, or at a slow rate, but it is nonetheless the same amount of energy for the same work?

Where I think you are going wrong is that your assumptions result in the the moving mass on the conveyor increasing, but you don't account for that in your maths. If you slow the conveyor to 50% speed then you must slow the loading rate to 50% speed, otherwise the mass on the conveyor will increase and the reduction in speed no longer has a linear relationship with the work being done, and you can not assume that the power consumed by the motor is reduced to 50% by reducing the speed to 50%.

 

[LionelHutz]

Geeze, when the process requires 50% of the maximum material the conveyer can supply then you have to find the most efficient way to deliver that amount of material. You can't compare this power usage to the full 100% material delivery case. You only need 50% of the conveyer capacity. What are you going to do if you don't slow the conveyer, dump 50% of the material on the floor? No, you will cut back the feed onto the conveyer if you don't slow it down. So, you must compare 100% with a reduced fill vs a slower speed and 100% fill.

 

[mikekilroy]

guys, you are mixing your metaphores. You realize that to so a certain amount of work requires a certain amount of energy. What you miss is why a vfd is used: it is not used to do the same amount of work in most applications, it is used to vary speed on a motor shaft to help a process. See my examples above; none use a vfd to put out the same energy at the end of the day. If you believe a vfd is only used for your narrow minded application of a conveyor that does not need different speeds for different amounts of product then so be it.
everyone realizes you all understand conservation of energy, no sense beating it; I see you will not accept that the most common use of a vfd is to vary the speed of a moor shaft, not to conserve someone's energy at the end of the day, so that is that. thanks.

 

[mikekilroy]

by the way, as another example of why folks use a vfd on the conveyor, see two posts down in this forum:

http://www.eng-tips.com/viewthread.cfm?qid=327706

they have need for varying speed due to, as I suggested earlier in my example, varying product based on what they run that day. this is another example of why vfds are used.

 

[mikekilroy]

they could have just left the vfd off the motor and ran it full speed all the time. but instead, they will reduce their energy bill each month by the amount of days they run it slower than full speed. energy savings by vfd.

 

[ScottyUK]

Mike,

Your example is the one where we got into conservation of energy, with the assumption that halving the speed halves the current from the line. It's not impossible that this will happen, but it is highly improbable because most motors in industry drive either pumps or fans, neither of which would result in the change in line current which you seem to expect in your example.

You're right that the most common use of a VFD is to vary the speed of a shaft. One of the great misconceptions - and one for which the VFD manufacturers are largely to blame - is that using a VFD will inherently save energy. It won't: work is work, done quickly or slowly. A VFD can allow an over-sized motor to drive its load more efficiently in the right circumstances. For the addition of a VFD to allow energy to be saved, the constant speed application first has to be wasting energy. This is by no means a given.

 

[mikekilroy]

ScottyUK ....the assumption that halving the speed halves the current from the line. It's not impossible that this will happen, but it is highly improbable because most motors in industry drive either pumps or fans, neither of which would result in the change in line current which you seem to expect in your example.

Scotty, we did seem to get off on an energy vs power tangent. I respectfully suggest that the real issue was the disbelief in the above fact, which is so, even for fans and pumps as well any and all applications where torque at the lower speed does not INCREASE. I would be happy to show the math again why this is 100% true if you'd like.

 

[LionelHutz]

OK, I'm not arguing that reducing the speed of the motor won't show a reduction in the input power. I'm telling you that the reduction in input power doesn't necessarily mean you're saving money.

Here's a different example. You have to feed process water. You have 2 choices. Install a tank and use on-off control for the pump or install a VFD and vary the speed of the pump. The VFD will use less power during the low water demand times compared to the high water demand times, but it will use more energy at the end of the year compared to the tank. Sure, you can argue that the VFD saves energy compared to running the pump full-out all the time but the process doesn't require running the pump full-out all the time so it's a useless comparison.

The thread you linked is about changing the speed to suit a process. If the poster get paid by the pound for the cucumbers then you have to look at the energy used to move the cucumbers and the amount he gets paid for them. Sure, the conveyer will use less power and energy when running slower, but he likely will get paid less due to running less product weight per period of time. In this case, the $$ paid for the product vs the energy cost to run the product must be examined to see if money is being saved or not. I personally hope he's getting paid more per pound for the smaller cucumbers because he's also paying people to inspect X number of cucumbers per day and running smaller cucumbers means more manpower cost per pound of cucumber.

 

[mikekilroy]

Lionel,

Of course we all can agree that if one has a process where they slow the conveyor down TOO slow to keep up with the cucumbers, they will loose money. OK. But, reality is that the numbers of cucumbers is not a constant: If it were, farmers could guarantee their income: they can't. Farmers who rent my land pay farmers insurance for this reason; if the crop is really bad, they get paid back the missing income. We rent approx. 300 acres to farmers of corn and soy beans here in Ohio..... these poor farmers are hurting this year due to the water drought. Reality is no one can predict exactly how many cucumbers will be made per unit time. The result is that farmers may produce LESS or MORE for a given crop per year. So, the conveyor moving them CAN CHANGE SPEED TO ACCOMODATE THE VARIABLE VOLUME. That means that yes, they don't have to have the conveyor running at base speed if this year is less cucumbers than last year. They can save money by reducing the speed of their conveyor. Simple math.

OK, off cucumbers. Onto widgets. See 2 posts down as I suggested: I propose that if you do a study and ask ALL VFD USERS WHY THEY BOUGHT A VFD you would find a high majority bought them to slow a process down - or speed it up - based on the product required. Yes, I give you there are a FEW processes that that will require the same amount of work (energy) to get the job done as you keep insisting. The village will use the same amount of water as you suggest per week whether it is sent slowly to the tank or fast to the tank. But I suggest to you that THAT is the EXCEPTION, not the rule.

If there is a way on this forum to make a survey, I will accept the majority answer: is a process using a VFD most likely to require constant work or constant power? Let the games begin!


Assuming you too, along with Scotty (beam me up Captain), require the math for why a VFD DOES 100% save power if turned down, I recorded my barn fan for you at 15hz, 30hz, and 60 hz. Rememember as you watch my video that a fan torque goes up by the CUBE of the speed; be that as it may, the video below clearly shows that turning a VFD down in speed SAVES POWER. I do not require the high air movement rate on a cooler day; hence I am allowed to SAVE MONEY BY TURNING IT DOWN! You cannot deny the imperical data, sorry. If you wish to post a similar real world test to show I am wrong, please do so. Here is the video I posted to something called U tOBe I think..... I am sorry, I am not an accountant, but an electrical engineer so do not know how some of this stuff works.

BTW, if someone is young enough to want to go for a patent on saving energy with a slight VFD modification that is patentable, let me know: I am old enough & well off enough after 40 years at this that I don't need it, but it would be a shame to leave it for another 10 years to be discovered...... back to topic, here ya go my friend. Actual data recorded off MY BARN FAN TODAY for you.

I will summarize: IT CLEARLY SHOWS THAT THE POWER USED ON A VFD DRIVEN MOTOR GOES DOWN LINEARLY WITH SPEED IT IS RUNNING: THAT TRANSLATES DIRECTLY TO $$ ENERGY SAVINGS IN YOUR ELECTRIC BILL.

http://www.youtube.com/watch?v=reo7CZLyHLg&feature=youtube_gdata

]MOTOR LOAD SIDE OF VFD on faN
SPEED AMPS WATTS
60....... 5.3..... 805
30....... 4.6..... 427
15....... 4.5..... 284

INPUT SINGLE PHASE INPUT SIDE OF VFD (1.73x higher than 3phase)
SPEED AMPS WATTS
60...... 7.9...... 968
30...... 3.1...... 521
15...... 2.5...... 223

INPUT *IF 3PH* INPUT SIDE OF VFD (1.73x higher than 3phase)
SPEED AMPS WATTS
60...... 4.6
30...... 1.8
15...... 1.4

 

[LionelHutz]

Your last post means nothing. You're stuck on this "a power reduction saves money" theory and haven't read or acknowledged anything about how you have to ensure the lower speed actually saves money. Saving power by running the motor slower doesn't automatically translate into saving money.

At this point, I've got nothing more to say. It's useless giving another example. The way you twist and change things and also the way you argue points I'm not even disagreeing with make it very clear you're not comprehending.

 

[mikekilroy]

agreed. you will not accept putting 1/2 the power into a motor saves money. ok.

 

[jraef]

OK, I'll take up the cause now, you've worn out the others...

According to your argument then, this will equivocate to any process in which less work is done as long as less energy is consumed. So let's say I need to drive from San Francisco to Los Angeles with 50 people in a bus that gets 10 miles per gallon. If your logic holds true, I can get a smaller bus that only holds 25 people but gets 20 miles per gallon and I have saved energy! Forget the fact that 25 people stayed behind and didn't get to LA, that's irrelevant according to you. The fact is, I used a bus that consumed less fuel, ergo energy was saved. Right?

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[mikekilroy]

if you too agree that most vfd's are used in order to do the same amount of work as before they were applied, then ok. Truth be told, I think a survey would show that most vfd's are applied to help a process, not to do the same amount of work in a given time, so that argument is not valid. If I don't need the fan running 1800rpm, I can turn it down and be cool and save power, energy, and money. It is just sad this point is not understood. so be it.

 

[jraef]

Huh? You are thinking we are saying it will do the SAME amount of work as before the VFD is supplied???? That's exactly the opposite of what we are saying. If you turn down the speed, LESS work is being done. So the energy you "save" is invalid, because you are not doing the same work. Less energy with less work is just less energy USED, but is not energy SAVED; i.e. I think maybe you are equating energy REDUCTION with energy SAVINGS. There is a difference.

There is some sort of comprehension disconnect here that makes me think we are not saying different things, just approaching it in different ways.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[LionelHutz]

THE AMOUNT OF WORK BEING DONE DOES NOT MATTER TO THE ARGUMENT. I have even given at least 2 examples (I actually think there were 3) where the amount of work being done per day varies. Yet, you can't seem to comrehend these examples because you misquote and mis-reply to them.

With a process, you can either control the process by using a VFD or you could vary the process by some other means. BUT, YOU MUST CONTROL THE PROCESS BY SOME MEANS. Examples of other means to control a process;
Fan - damper
Pump - throttling valve
Conveyer - sliding gate on the feed hopper
Milling machine - gearbox
etc
etc

THE ENERGY SAVED (or the extra energy wasted) IS THE DIFFERENCE BETWEEN THE ENERGY USED BY THE VFD AND THE ENERGY USED BY THE OTHER PROCESS CONTROL MEANS. IT IS NOT THE DIFFERENCE BETWEEN THE ENERGY USED AT 100% THRUPUT AND THE ENERGY USED AT THE REDUCED SPEED.

If you can't comprehend the above 2 sentences then you shouldn't be telling anyone how a VFD will save energy. You're simply not qualified for that task.

 

[Mysterrose]

My very simplified understanding of the answer to my question is that you must change the amount of Power used. You can do this by either reducing the work done or by reducing the speed you are doing it at. In the end the process requirements for the conveyor line dictate the amount of material you need to move (i.e. the amount of work you need to do) and what speed you need to do it at. Simply using a VFD instead of gearbox won't save you energy, or very little if you want to look at VFD vs. gearbox energy losses.

 

[Kapil1982]

Is it possible to replace gearbox with VFD for speed reduction in liquid ring vacuum pump application.

 

[mikekilroy]

jraef (Electrical)
12 Aug 12 21:51
Huh? You are thinking we are saying it will do the SAME amount of work as before the VFD is supplied???? That's exactly the opposite of what we are saying. If you turn down the speed, LESS work is being done. So the energy you "save" is invalid, because you are not doing the same work. Less energy with less work is just less energy USED, but is not energy SAVED; i.e. I think maybe you are equating energy REDUCTION with energy SAVINGS. There is a difference.

that is exactly what I have been saying. Never have I said reducing motor speed by itself will reduce energy to do a given task; a given task needing x amount of energy will need x amount of energy to complete irregardless of speed.

these 2 points are the crux of it: if no vfd is in the picture, there is no chance to slow a motor down and save power and hence energy - assuming the process can benefit from it - or said in your words, assuming the process can benefit from the energy savings.

again, most processes where a vfd is used can benefit from, in your words, this energy savings, by turning the speed down. we can and have all given examples of this as well as processes where the energy required is a constant and a vfd will not save energy.

 

[mikekilroy]

LionelHutz (Electrical)
12 Aug 12 22:38
THE ENERGY SAVED (or the extra energy wasted) IS THE DIFFERENCE BETWEEN THE ENERGY USED BY THE VFD AND THE ENERGY USED BY THE OTHER PROCESS CONTROL MEANS. IT IS NOT THE DIFFERENCE BETWEEN THE ENERGY USED AT 100% THRUPUT AND THE ENERGY USED AT THE REDUCED SPEED.

nonsense. energy saved by a vfd is certainly saved energy if you can turn the motor down and use less energy. You feel the extra money in your pocket at the end of the day is not real money?

 

[LionelHutz]

if no vfd is in the picture, there is no chance to slow a motor down and save power and hence energy

Is this a craftfully written statement to try and prove your point. Without a VFD you may not be able to slow the motor down, but there are still lots of ways to save power and energy.

What do you think happens to the conveyer power usage if you partially close the gate on the feed hopper?
What do you think happens to the power usage of a pump if you partially close a throttling valve on a pump?
What do you think happens to the power usage of a fan if you partially close the dampers or you lower the pitch of the blades?

Can you answer these 3 questions?