Centrifugal pump VSD question.
Centrifugal pump VSD question.
(OP)
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.





RE: Centrifugal pump VSD question.
http://www.emcsolutions.com/article_vfd_benefits.h...
RE: Centrifugal pump VSD question.
Talk to your EE some more. Perhaps there is something in his "lingo" that is not clear to us non EE types.
RE: Centrifugal pump VSD question.
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.
RE: Centrifugal pump VSD question.
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
RE: Centrifugal pump VSD question.
RE: Centrifugal pump VSD question.
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.
RE: Centrifugal pump VSD question.
http://www.yaskawa.com/site/products.nsf/products/...
Motors should be inverter duty.
http://www.pdhengineer.com/courses/e/E-1007.pdf
RE: Centrifugal pump VSD question.
“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King
http://waterhammer.hopout.com.au/
RE: Centrifugal pump VSD question.
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
RE: Centrifugal pump VSD question.
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.
RE: Centrifugal pump VSD question.
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.
RE: Centrifugal pump VSD question.
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/sites/us/en/suppo...
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
RE: Centrifugal pump VSD question.
RE: Centrifugal pump VSD question.
RE: Centrifugal pump VSD question.
"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek
RE: Centrifugal pump VSD question.
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/
RE: Centrifugal pump VSD question.
RE: Centrifugal pump VSD question.
RE: Centrifugal pump VSD question.
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
RE: Centrifugal pump VSD question.
RE: Centrifugal pump VSD question.
DOL
RE: Centrifugal pump VSD question.
Thanks
"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek
RE: Centrifugal pump VSD question.
http://www.danfoss.com/NR/rdonlyres/1FDE93BE-95CF-...
RE: Centrifugal pump VSD question.
That one is a perfect matche to a centrifugal pump.
"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek
RE: Centrifugal pump VSD question.
The VFD choice is between variable torque and constant power. Few VFD applications use constant power.
VFD's for centrifugal pumps are variable torque applications. See the Yaskawa product information:
http://www.yaskawa.com/site/products.nsf/products/...
Motors should be inverter duty.
http://www.pdhengineer.com/courses/e/E-1007.pdf
RE: Centrifugal pump VSD question.
What was in my mind at the time was that it was definitely not constant torque.
"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek
RE: Centrifugal pump VSD question.
RE: Centrifugal pump VSD question.
There are always exceptions but typically, a centrifugal pump will require a lower overload during start-up than a PD pump. It was common to call them "variable Torque" (centrifugal pump) and "Constant Torque" (PD Pump). From the drive point of view, it is about 'getting the pump started'.
You will see a more meaningful description these days talking about "normal overload" and "high overload".
The Voltage/Frequency characteristic is something used to try and align the benefits of energy saving towards the characteristics of the pump load (pressure) and this is where 'variable torque' terminology comes in. The square load effect of a centrifugal pump is often better served by not applying a linear volts/frequency as the frequency(speed) reduces. This has the benefit of saving more energy.
It is often thought that the VFD 'saves the energy' on a centrifugal pump but in reality all it is doing is following the load demand required by the pump.If you can match that with a volt/frequency curve that is closer to the load on the motor, then all the better.
RE: Centrifugal pump VSD question.
Never a truer word spoken. The energy use is related to the hydraulic demand, efficiency of the pump and the efficiency of the drive and its attendant equipment (transformers, VSD, cable etc etc). There is no magic fairy working for Danfoss, Schneider, ABB, Siemens or other VSD supplier that gives you energy for nothing!
“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King
http://waterhammer.hopout.com.au/