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]

Sorry to hijack this thread. I don't understand the purpose of a VFD that maintains constant speed control. Maybe I am misuderstanding what is meant. I thought you wanted the VFD to vary the speed of the pump to achieve your desired flow and head conditions. Now once that speed is selected your try to control the pump at that point.

 

[Oldhydroman]

I've got a feeling the previous contributor meant "speed control": you dial in the speed you want and the VFD does its utmost to keep that particular speed constant in the face of varying shaft loads (positive or negative), fluctuating supply voltage/frequency, and changing motor temperature (which changes the resistance of the windings).

DOL

 

[BigInch]

yes, please delete "constant".
Thanks

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

 

[micalbrch]

I read all the posts twice and my impression is (I might be wrong) that the electrical engineer's world and terminology is different from the one of all the pump experts here. I find the following Danfoss training paper very helpful to understand the terms constant torque, constant speed, variable torque etc.

http://www.danfoss.com/NR/rdonlyres/1FDE93BE-95CF-46F9-B2AE-6F76C69594CB/0/vfdlesson4.pdf

 

[BigInch]

Too VFD technical past curve #2 VT on page 3.
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

 

[bimr]

The Danfoss paper does not mention "constant speed".

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

 

[BigInch]

Neither does Schneider's website. I erroneously picked up the speed term.
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

 

[justabitconfused]

Thanks again, much appreciated!

 

[ozmosis]

From a VFD manufacturer's perspective, the question about the type of pump is really about the overload requirement.
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.

 

[stanier]

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.

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/