VFD design for centrifugal pumps 1

[pittengineer]

I am putting together a design for a VFD controlling two 40hp motors on centrifugal water pumps which are pumping water from a tank.

Only 1 motor ever runs at a time and the other motor is strictly sitting on standby as a backup. The idea is to control the speed of the motor and thus the flow of water from the tank depending on the water level in the tank. Water is continously being pumped into tank but tank is not that big so the level tends to change quickly.

A couple of questions regarding the design:

1) As mentioned was planning on using 1 VFD for running both motors since 1 is always a spare. I figured the best way to do this would be to have two contactors on the ouput of the drive which would be controlled to determine which motor should run. Anyone have any experinece with this type of setup?

2) For these 40 hp drives will there be a signifigant cost savings by only using 1 drive with contactors to control both motors or is there not enough of a savings with this setup to justify not purchasing two individual drives?

3) Any suggestions on things to look out for when sizing a vfd for a centrifugal pump application? Should I just match the hp requirement of the motor?

4) Whats a quick way to roughly gauge the cost savings by varying the speed of the motor as opposed to keeping the motor at constant speed and modulating a valve to control flow?

5) Planning on putting this drive on a devicenet network. I believe the drive should be able to distinguish commands on which of the two motors to run as well as provide feedback on which motor is running through this Dnet interface. Anyone have experience with this.

I'm working with the vendor on designing the control schematis, but just wanted to see what the experts thought about these questions.

Thanks in advance.

 

[ScottyUK]

At Rockwell UK prices and with the current exchange rate? You must be running a charity, not a business...


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If we learn from our mistakes I'm getting a great education!

 

[panelman]

It’s the euro/pound parity that’s killing me at the moment says he sitting with german beer in hand watching the 14th inch of today’s snow drifting down outside the window (and hoping it clears before I have to do the 500 miles back to the ferry)

 

[ScottyUK]

14"? Wow. Where are you - Germany / Austria? We are just seeing that last of about 7-8" melt away, with the promise of more soon. Of course if you guys on the continent could like to let it fall somewhere more deserving - like France - then so be it!


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If we learn from our mistakes I'm getting a great education!

 

[panelman]

Yep, was out in Germany, although out there everything doesn’t seem to grind to a halt at the first sight of white stuff, it’s winter, it snows, everybody puts the winter tyres on and life continues.

Anyways, back in the UK now and the scenery is the normal damp & green