Author Topic: Another VFD is best argument  (Read 3850 times)

Cary Austin

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Another VFD is best argument
« on: March 03, 2018, 10:56:30 AM »
Hey Cary, I'm a new guy to the forum and am going to be installing a well pump on my property in the next month or so and have taken an interest in your CSV's. I've been trying to comb through the forums and website to try and get as much details on it as I could but decided to just ask you about it personally. I only have a couple local pump guys that I could have install my pump for me, and both of them pushed pretty hard about installing a VFD. The problem is their quotes were a lot steeper than I wanted to pay and discovered they had the VFD's at MSRP price after I found the same ones on my own online for $600-$700 cheaper. So they pissed me off and needless to say I've decided I'm going to just install the pump myself. I work in the power generation industry and discussed installing a CSV instead of going with a VFD with some mechanics I work with because i'm trying to save money, and don't have a good knack for electrical work. After discussing it with them they are under the impression that a CSV will be hard on the pump from excessive back pressure, whether it is blowing seals or ruining the bearings. I have a general understanding of how pumps work and that if you stay within the manufacturers pump curve then you shouldn't be damaging a pump, so I guess my question is could you explain to me why the CSV isn't hard on the pump even though it is throttling the discharge of the pump whenever there is a low flow demand? This is both for my own knowledge and to have some ammunition on why my co workers shouldn't have wasted their money on VFD's. Thanks for any info.


Hello
You are exactly right!  They are MADE to pump against a restriction.  If there was no restriction, you wouldn't need a pump to start with.  I will attach a pump curve you can print out to show them.  A curve shows what a pump is designed to do at different amounts of "restriction".  Any pump will produce less GPM in a deep well (more restriction) and more GPM in a shallower well (less restriction). 

The attached curve shows a 2HP pump at 28 GPM, with a 1.2 service factor it is pulling a 2.3HP load with only 200' (86 PSI) restriction.  But when you restrict it further to 320' of head (138 PSI) it is only pumping 2 GPM and only drawing 0.8HP load.  That is just the way pumps work.  There is a BIG myth-understanding about this.  Everybody thinks restricting a pump makes it work harder, when just the opposite is true. 

That pump producing 28 GPM thinks it is in a 200' deep well and draws up to the maximum service factor load, which is 13.8 amps.  But when you restrict it more and make it think it is in a 320' deep well, it can only produce 2 GPM and only drawing 0.8HP load, which is about 5 amps.  The maximum amps or heat that motor can handle is 13.8 amps, so when it is only drawing 5.0 amps, it is just loping along, running cooler, needed less flow for cooling, and will ultimately last longer because of it.  Not to mention that the restriction from the CSV is what keeps the pump from producing more water than is being used, which is how it eliminates the cycling. 

Running cooler at reduced amps and eliminating the cycling will make this pump last several times MORE than it was designed to do, not shorten its life in anyway.

The only restriction that will hurt a pump is one that make the water heat up.  And since the CSV can never completely close, that cannot happen.  As a matter of fact the minimum flow built into the CSV (1 GPM) is designed to be several times more than the pump actually needs to stay cool anyway.  It just takes very little cool water to keep a motor/pump happy when it is only drawing 40-50% of max amps and not actually producing any heat.

I was doing VFD's 30+ years ago when I figured this out.  A VFD is just trying to trick a pump into doing something it already does naturally.

Hope this helps.

Thanks
Cary
« Last Edit: March 03, 2018, 11:02:03 AM by Cary Austin »

DRoads9

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Re: Another VFD is best argument
« Reply #1 on: March 04, 2018, 10:44:05 PM »
Thanks for the reply Cary that does help clarify. It's kind of common sense but you don't really think about the fact that a deep well pump is being restricted the deeper it is. So for example adding a CSV and only needing 1 gpm flow from a sink is the same as putting the pump deeper in the well. It shouldn't hurt the pump if it is sized right. I'll probably be coming back for some more information on a few things after I get the well drilled and know for sure what I have to work with. Thanks again.

Cary Austin

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Re: Another VFD is best argument
« Reply #2 on: March 05, 2018, 07:41:37 AM »
Exactly!

Pongsrits

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Re: Another VFD is best argument
« Reply #3 on: October 04, 2018, 03:02:23 AM »
Estoy aprendiendo sobre VFD.

Cary Austin

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Re: Another VFD is best argument
« Reply #4 on: October 04, 2018, 07:27:28 AM »
Espero que no aprendas de la manera más difícil como la mayoría de las personas. Los VFD tienen muchos problemas, por lo que ganan mucho dinero con ellos.

mwolf00

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Re: Another VFD is best argument
« Reply #5 on: May 17, 2019, 09:58:14 AM »
I was reading that whole thread, which is now locked so I'm replying here.  I have nowhere near the knowledge of either of you two but I have this question about pump cavitation.  If there were truly cavitation happening at the well pump, wouldn't that result in air in your pipes?  It seems that any type of cavitation would result in air bubbles that would flow up the well pipe and into your home system.  You'd know if it were happening pretty darn quickly.  Am I missing something?

Cary Austin

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Re: Another VFD is best argument
« Reply #6 on: May 17, 2019, 10:34:24 AM »
Not really.  Cavitation is the act of the air bubbles imploding.  They have usually imploded before they even get out of the pump, so you don't see air in the lines.