Pump Variable Drives

[nornrich]

All,

In the electrical control of pumps I've seen AC drives for soft starting pumps. Is this just done to bring up the pump slowly or can the AC drive be used to control flow as well? If you do run the pump at slower speeds how do you correlate this to actual flow on your pump curve? Is there a breakdown speed that if you are lower then this speed the pump won't function?

Regards,

Rich.....

Richard Nornhold, PE
ampdesign@earthlink.net

 

[cbrn]

Hi,
I used to design some pumps actioned by synchronous motors: in these cases, electronics allowed for two things:
- recognize the sense of rotation (synchro bipolar motor starts indifferently in one sense or the other)
- allows to dimension the motor for the service load and not for the initial synchronization, by making the frequency lower at startup and then increase in "ramp".

If the motor is asynchronous, its torque curve has a maximum which is not at the zero regime; by varying the frequency with a chopper, you translate the torque curve so that this torque maximum is virtually at zero speed, which is great for starting; using a chopper has, as far as I know, only one severe drawback: it generates a lot of harmonics, which is not good for the net. But I seem to know that there are lots of ways to filter these, so running a pump at lower speed by the means of an electronic regulator is THE BEST way to lower the pump's characteristic curve (much better than insert valves at outlet or so on...). There is no "breakdown", at least not for reasonable speed decreases (of course the runner must provide enough energy to the fluid to overcome the own internal losses inside the pump). Concept is: once you have your design flowrate and head, adjust the pump curve so that its efficiency is near its maximum for this working point. You can measure the overall efficiency simply by measuring flowrate, head (this will give you hydraulic energy increment), voltage and current intensity (these will give you the electric power absorbed by the pump, with some simplification as regards power factor "cos phi"). hydro power is density*flowrate*gravity*head, electric power is voltage*intensity, overall efficiency is hydroP/electricP.

Maybe someone else could give you more precise/detailed explanations.

Regards

 

[DesignerMike]

What type of a pump are you looking at?

Each type of pump will bahave differently.
If you are talking about Centrifugals, it will depend on impeller diameter etc. The pump manufacturer will be able to give you curves at different rpm settings. (not linear)

A tight PD pump with somewhat viscous product should be close to a linear relationship between frequency and flow (the pump running @ 60hz should pump close to twice as much as running @ 30 hz)...but again it will depend on the pump.

Most of the time when I have used a VFD on a centrifugal I will have a flow meter and pressure transmitters on the line and I have a specific flow rate or velocity that I am trying to attain. By changing the speed of the pump I can keep my other parameters where I need them.

 

[nornrich]

DesignerMike,

Sorry. It is a standard Centrifugal Pump.

Regards,

Rich...

Richard Nornhold, PE
ampdesign@earthlink.net

 

[Stresfree1]

VFAC drives can be used to "soft start" pumps if needed. This is more common in the smaller size pumps, often the drive is less $ than a soft start. As they get bigger, the drives get more expensive than the soft start.

Mikes right - flow curves should come from the vendor at the different rpm's. This is commonly used to control flow, more common in Europe due to the energy savings compared to a valve in the outlet.

There will be a breakdown speed depending on the motor that is selected. As the motor slows down, the HP available from the motor decreases. At some point, the available motor HP will become lower than the hydraulic HP required to drive the pump.

Just confirm with the motor vendor that the HP needed for the lowest pump curve you need is available at that speed as well. If not, consider a bigger motor, or higher rpm motor and pump to get your turndown. Then set a minimum in your control system.

Hope this helps.

 

[stanier]

I am working on a project that has 4x1500kW pumps with VSDs. The soft starting and stopping is used to prevent waterhamme fatigue damage.

refer

http://www.pipingdesign.com

for risks on surges in systems.