Centrifugal pump VSD question. 5

[justabitconfused]

Hi all, I am converting some centrifugsal pumps from fixed speed to variable speed using variable speed drives. The electrical engineer who is speccing the drives wishes to know if the drives are constant torque or constant power. My response is that a centrifugal pump is neither since both torque and power will change with the speed. The electrical guy insists he needs to know whether to spec the drive as constant torque or constant power. I don't know what else to say. What do the experts here think please? Thank you.

 

[GHartmann]

You are correct in your assessment. A centrifugal pump is both. See the attached article that supports this response:

http://www.emcsolutions.com/article_vfd_benefits.html

 

[GHartmann]

Sorry I meant a centrifugal pump has both variable torque and power. Neither are constant with variable speed. Reduced power at lower speeds is the whole purpose of the VFD.

Talk to your EE some more. Perhaps there is something in his "lingo" that is not clear to us non EE types.

 

[zdas04]

Do you want to be "right" or do you want a pump that performs properly? Your goal is to reduce power consumption, so if someone asks if you want "constant power" or "constant torque" the answer should not be difficult. It is easy to prove that required torque goes down as you lower required power, but so what? Call the guy and say "constant torque" and get on with your life.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[Oldhydroman]

I believe the issue is to do with the capacity of the "drive" when it is working at maximum output. It can be useful to compare the mechanical output of a motor which is fitted with a variable speed drive to the mechanical output of an identical motor connected to the mains without a "drive".

Suppose you had a fully loaded 11 kW motor running at its [full load] speed of 1450 rpm; the output torque would be 72.4 Nm. When you connect direct to the mains that's what you get. When you have the "drive" fitted and you dial in "1450 rpm" and the load is such that, at that speed, the torque is 72.4 Nm, then the mechanical power output will be exactly 11 kW. The drive will push enough current into the motor for you to get the torque you need to run the motor at the speed you asked for. As far as the motor is concerned (broadly speaking) nothing has changed; drive or no drive.

But what do you want the drive to be able to do when you dial in "725 rpm"? If you want the drive to be able to work at "constant power" then it must be able to push enough current into the windings in order for the motor be able to sustain that speed when the load requires 144.8 Nm. You have halved the speed and doubled the torque output so the POWER remains the same. You might actually need a bigger motor to be able to do this (because of the higher torque and current). And you certainly need a bigger “drive” because of the higher current output.

Conversely, if you specify a "constant torque" drive then the maximum torque capacity doesn't go up when you reduce the speed. This is a smaller drive than the "constant power" version and you don't have to up-rate the motor in order for it to be able to tolerate a higher current. If you halve the speed then you halve the maximum power output capability. As Mr Simpson says, you expect the torque to go down significantly when you reduce the speed (presumably that's why you are doing it) so the answer is “constant torque”.

But actually, all other things being equal, the power will go down as the CUBE of the speed. So if you halve the speed the power should go down to one eighth. Do you still need the same 11kW drive? If you know the characteristics of your pump and process then you might be able to specify a “constant torque” drive which is actually smaller than the original power output of the directly connected electric motor. I suspect you could do with talking to someone a little higher up the curve than your existing contact.

DOL

 

[justabitconfused]

Thanks a lot, some good advice there, I think it's going to be constant torque and I get on with my life!

 

[1gibson]

Don't think I agree with the motor not caring about the VSD/VFD drive. If motors are not rated for variable speed use, there can be issues with electrical noise in the windings. May likely affect temperature rise. Possibility for stray currents to run through the shaft bearings that may affect desired motor construction (bearing insulation/isolation.)

A lot of modern motors are acceptable for VFD use provided the service factor is reduced from 1.15 to 1.0. If you only have a 1.0 SF at constant speed and you are exceeding nameplate hp, and/or if you plan to overspeed with the VFD, you should take a closer look at things.

Assuming all above is addressed in terms of compatibility, you then need to review operating scenarios. If you want to turn down to 30% speed, does the pump still have enough downthrust to prevent the motor thrust bearing from skidding? If the motor has non-reverse ratchet, is it going to engage if speed is too low? If you overspeed, is the pump designed for this: increased NPSHr, increased fluid velocity, increased impeller vane tip speed, etc. How will the mechanical seal behave?

You can try to save some utility costs and roll the dice with maintenance issues (which might be just fine for the owner) but to really review this completely is quite an exercise. The bigger the machine, the higher the risk. You decide if it is worth checking it all out.

 

[bimr]

VFD's for centrifugal pumps are variable torque applications. See the Yaskawa product information:

http://www.yaskawa.com/site/products.nsf/products/Industrial AC Drives~P7.html

Motors should be inverter duty.

http://www.pdhengineer.com/courses/e/E-1007.pdf

 

[stanier]

Why do you want to use a VSD with a centrifugal pump? There are other options that can be more energy efficient. There has been great insight into this in other postings. Check out the attached presentation in particular the notes where you will find links to related papers.

“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King

http://waterhammer.hopout.com.au/

 

[BigInch]

I agree with bimer. Cent Pumps are constant speed.
Normally cent pump's torque drops with speed as it comes up to operating rpm.
If you order constant torque, the drive will keep on turning up the dial to keep the torque setting constant, even though operating rpm has been reached.

I believe cutting and milling machines are constant torque, but that's out of my area.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[GHartmann]

I can now imagine how confused the OP is after some of these dissenting posts.

For my two cents:

It is not uncommon to use VFD's to adjust the speed of centifugal pumps to maintain the proper flow and head conditions.

Stainer's post about the necessicity of the VFD's should be considered.

Almost all the literature that has been cited suggests that pumps and fans are variable torque machines.

 

[zdas04]

Cent pumps are NOT constant speed. They do have a reasonably narrow operating band, but it is other than zero. If I can chase a sweet spot in the system operating curve with a VFD instead of with a backpressure valve then I'll use less energy.

A multi-stage centrifugal pump I recently evaluated had a maximum speed that was 1.6 times its minimum speed. So speeding up from minimum speed to maximum speed raised power consumption by a factor of 4, still within the operating envelope of the pump. There are operations where that is a meaningful number. There are operations where the power consumption is absolutely overruled by process considerations. There is not a universal truth here, just Engineering.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[BigInch]

David we know that centrifugal pumps are not constant speed in the sense that they can operate at any speed > 0 and less than fly-apart. That is not what this question is about.


What is the difference between Variable Torque and Constant Torque for AC Drive ratings?

VARIABLE TORQUE LOADS:Many types of loads require reduced torque when driven at speeds less than the base speed of the load. Conversely, such loads may require increased torque when driven at speeds greater than the base speed of the load. These are classified as variable torque loads. Many variable torque loads decrease with the square of the speed. This is characteristic of centrifugal pumps, and certain types of fans and blowers. Typically, as the speed decreases, the torque decreases with the square of the speed and the horsepower decreases with the cube of the speed.

That certainly looks like centrifugal pumps are indeed constant speed, as opposed to constant torque for AC Drive specification purposes.


CONSTANT TORQUE LOADS
With constant torque loads, the torque loading is not a function of speed. Typical applications are: Traction drives, Conveyors, Positive displacement pumps and Hoists. As the speed changes, the load torque remains constant and the horsepower changes linearly with speed. Constant torque loads cause motors to draw relatively high current at low speeds when compared to variable torque applications. This is why the same size drive may have a lower HP rating for Constant Torque applications.


Above was patched from,

http://www.schneider-electric.us/si...&country=US&lang=EN&id=FA105387&redirect=true

The question of whether a VFD is needed here, or not, isn't relevant to the OP's question. Even I know that VFD's sometimes even have their uses on pipeline work, however rarely that might be, or however much I do not like to think about that subject, but those reasons and many more have been throughly beaten to death on this site a couple of years ago. If that is of interest to the OP, I'd suggest that he do a search first and then ask for answers to what few questions were not discussed in those series.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[justabitconfused]

Thanks for all the thoughts, it is appreciated.

 

[TenPenny]

What threw me off was the way the question was posed. I have never heard anyone dealing with drives say 'constant torque or constant power', the choice has always been presented as 'constant torque or variable torque'.

 

[BigInch]

I've always heard variable torque or variable speed. Maybe it has to do with what equipment you're used to working with.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[stanier]

If the purpose of the VSD is to overspeed the pump then the electrical engineer may be concerned about the power drop off as the frequency increases above that of motor design. Without knowing why the VSD is being considered it is difficult to answer this posting in other than a general manner which has been done admirably above.

Biginch is right in that the topic of the misuse of VSDs has been battered to death in previous postings.

“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King

http://waterhammer.hopout.com.au/

 

[Compositepro]

This question might be better asked in the electric motors forum. The reason for asking has to do with the what features will be specified for the VFD. I've never quite heard the question stated as "is load constant torque or constant horsepower". In this case it is neither. However, VFD's can be set-up to control speed or torque. It is an important difference, and I've seen the consequences of a poor choice. Justabitconfused needs to get with the electrical engineer and reach an understanding of which is the correct choice. It seems the electrical engineer may be a bit confused. Two confused engineers is not a good thing.

 

[justabitconfused]

Thanks again, I agree, I need a discussion to further understand what the EE is actually after from me.

 

[BigInch]

The EE is not confused, although it seems that he does not know that centrifugal pumps need a VSD setup for constant speed control.
He's on the right track asking that question. I think he will be very happy with the answer.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek