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?

 

[jraef]

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. ...

Sigh...

Almost, but I'm afraid that this side track has confused you a little.

... 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 reducing the wasted energy in the system. Reducing the speed WILL reduce the work done in many cases. If it does not, then there is no change in the energy consumed. Energy = Work. But reducing the work done results in energy reduction, not energy savings, because your process produces less of whatever it is intended to accomplish.

"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

 

[John2025]

I'll bite. It seems mikekilroy is making an argument for when VFDs do save energy by slowing down the speed and Jraef et al are making the argument that they don't ALWAYS save energy by slowing down the speed. I think we all agree that there are apps that work. Conveyors can fall into that category, but not always. I agree the hype is misleading, slowing the motor speed does not necessarily save energy/money at the end of the day.

 

[jraef]

How about this:
If you subscribe to the notion that REDUCING energy is the same as SAVING energy, then isn't the ultimate goal going to be to turn it off completely? You cannot save any more enegy than when the machine is off.

But of course, it is doing no work at all. So what have you "saved"?

"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]

John2025 you are right on, yes. we all are saying the same thing now, except when carried to extreme as Jraef shows can be done by turning it off.... the anal issues being thrown out here are that some folks think there is no inbetween full on or full off work....

If I am working on the tractor in the hot barn and turn my fan on w/o a vfd, I will use .75hp (1kw) of power running my fan across the line full speed for the whole 1 hour it takes me to fix it, thus using 1kwh of energy.

Or, if I am working on the tractor in the hot barn and turn my fan on with a vfd at 1/2 speed (because it ain't as hot as it was yesterday and 1/2 speed is enough to cool me today), I will use .375hp (.5kw) of power for the whole 1 hour it takes me to fix it, thus using .5kwh of energy.

Lastly, If I am working on the tractor in the hot barn and turn my fan off to use no kwh of energy, I may not save a penny as the hospital bill for heat stroke will likely be much higher than the energy saved.

The moral of the story is there are processes as I have been saying that do not require the same work all the time, and thus can save money by turning the motor speed down.

 

[John2025]

Sorry guys, I should have admitted the horse was dead. I do wonder though, does beating the carcass slowly for a long time use less energy than beating it quickly for a short time?

 

[LionelHutz]

John2025 - No. mikekilroy is arguing that you use a VFD for process control and you will save the energy difference between full output and the reduced output. He's totally 100% ignoring the fact that without the VFD that another means to achieve process control is required. In fact, I now believe he thinks a 75kW motor will draw 75kW when running at full speed regardless of the load. Believing this fallacy would explain why he thinks the only way to reduce the power drawn by a motor is to reduce the motor speed and also would explain how to justify this 50% speed = 50% energy crap.

Personally, I think he's completely full of it. In his 11 Aug 12 7:42 post he claims that he designs motors and then in his 12 Aug 12 15:17 he claims he rents land to farmers and has also hinted that he is a farmer (fan in his barn, working on tractors). And to boot, he's got this great patentable VFD energy saving idea which he can't be bothered to persue because he's so well off. So, a motor designing land baron farmer who's to rich to be bothered to persue a new money making idea? Seriously?

It's easy to give an example of energy saving by using a VFD on a fan that was pumping excess waste air into the barn. The old system had no process control and the new system has process control. Using a waste energy application example is an easy way to cheat when proving that a VFD will always save huge amounts of energy. This is exactly the same as comparing a VFD controlled conveyer with a conveyer which is dumping the excess material onto the floor. This is exactly the same as comparing a VFD controlled pump with a pump which is dumping the excess water back to the inlet.

Try comparing a system with mechanical process control means vs updating the system with a VFD for process control. Do this - measure a the conveyer poer at 1/2 fill and full speed vs the conveyer power at 100% fill and 1/2 speed. You'll quickly find your 1/2 speed = 1/2 power & energy rule is complete crap.

 

[electricpete]

I may be speaking from a glass house of past experience, but I agree. Horse is dead.

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[mikekilroy]

ha! that's funny loin! I'm glad you find my life experiences funny sometimes one can open the minds of closed minded folks, and sometimes one cannot.

yes, the horse died many days ago

 

[ScottyUK]

My head is making mushy sounds.

.

 

[LionelHutz]

Well, sadly I fully expected you to ignore the 3 questions I posed. Looks like the horse is very dead.

John - sounds like we should apply for a research grant to study that one

 

[mikekilroy]

LionelHutz (Electrical)
13 Aug 12 16:03
Well, sadly I fully expected you to ignore the 3 questions I posed.

Sorry Loin, didn't think beating the dead horse anymore made any sense, but since you insist.... if one does something else to reduce the energy usage, guess what? energy usage goes down? Unfortunately, in my barn, I have none of those things on my fan; just a vfd. I get to reduce my energy usage by simply turning speed pot down. Someday you may open your mind enough to see the whole picture; there is more than one way to skin a cat. Saving energy dollars can be done many ways, including turning down a vfd speed pot if the excess motor energy usage is not required. Someday you may accept that, or not. If you want to continue this, for instance, your nonsense 75kw comment, off line, feel free to pm me and I will try to help open your mind to more ideas from the real world.

 

[LionelHutz]

Geeze, you just must keep using your little barn fan example so lets pick on it a bit.

You keep going on about how your barn fan with the VFD uses 50% power at 50% speed. What would you think of your precious VFD if I presented a cheap easy to install product which would give you the same airflow but use 25% of the power? Would you still think your VFD is so wonderful? I bet not. Sure, the VFD is saving you 50% of the power compared to running at full speed, but it's now costing you 25% more power compared to my solution.

Now think on a grander scale. Lets scale up from a dinky little 1kW fan to a real fan, say 1000kW. I bet you'd want to buy my solution and throw the VFD in the scrap pile. After all, my solution is saving you 250kWh MORE energy every hour it runs at the reduced airflow and that amount of savings would add up to some serious cash at the end of the year.

 

[electricpete]

mikekilroy - You're a new guy. Most people don't make such an entrance as you did here.

I think you should know that the three guys you've been conversing with (Lionel, Scotty and jraef) are among the most knowledgeable in this area (as in many others) on the forum. I'm not saying that means you have to agree with everything they're saying, but it's something you should know so you should consider before statements like "feel free to pm me and I will try to help open your mind".

And Lionel's name is Lionel.

Respect is a two way street. Give a little bit and you may get a little bit.

This thread is not heading anywhere productive, especially if you think the only thing left is to explain your point to the others (they understand it, believe me).

I'm not anyone different than anyone else here, and I don't really have a right to lecture anyone. That is not my intent, just trying to help out. If it were in-person vs forum I would have a private side conversation with you. We don't have PM's here, that's not an option. Really no other way to do it.

Personally, I hope you'll stick around and keep contributing in other threads.
Don't be put off by this thread, others will be different. I think you will grow to enjoy it.


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