VFD to 2 motors. Alternating run time

TConnolly said:

These are all engineering issues that should be addressed. It really seems like yet another case of the controls guy being tasked with compensating for someone's poor engineering.

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And as normal, they don't want to spend any money doing it!

All,

First thank you for all of the ideas.

A little background on this. The equipment is a nanofilter (thats what it keeps getting called atleast). It takes in city water (that has gone through a softner) and pumps the water at high pressure (the good pump is around 290psi before we run out of water) through a bunch of filter media. This filter media magically makes the water better. That's basically how much I know about it. The cleaned water is what we will use to refill our cold well.
As it stands right now, the good pump on this device has too much flow for our building water supply to keep up. We draw down the supply to 0psi. In the future, we can potentially get the supply upsized from the street all the way to where this equipment sits. This obviously is not going to happen in the next month.
This is why my immediate need is to slow down the pump. I need a vfd to do that. The idea of using contactors on the outlet of the vfd is a complete secondary. The contactors are already in the panel and will be coming out. Instead of benching them, I though I could put them to use in this way.
The nanofilter system was bought used for roughly $5000. New it is over $100,000.
What I am dealing with is cold water for dyno and heat exchanger cooling. We are building test cells. I'm sure if I was inthe manufacturing world, I would be more eager to include the redundancy of a second vfd.
I am actually putting a large enough enclosure that I can put a second vfd in. Once we get the bad pump/motor back from rebuilt, and the nanofilter proves it is not going to fall apart, we may go ahead and spend the money on a second vfd.

So, thank you all for all of the help. Please don't feel that I was an *** or ignoring good advice.

I have also posed the question of just putting on a manual bypass valve as bce123 suggested. I will try to let everyone know how it all works out.


Thanks

This is why my immediate need is to slow down the pump. I need a vfd to do that. The idea of using contactors on the outlet of the vfd is a complete secondary. The contactors are already in the panel and will be coming out. Instead of benching them, I though I could put them to use in this way.

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So the good running pump will automatically have many usage hours more that the motor and pump that is being rebuilt. If the good pump has at least as much run time as it will take to get the other pump rebuilt, then alternating the two units makes a lot of sense. That way the current pump will fail first with at least enough assumed time left on the other pump to get the failed unit rebuilt and reinstalled.

Lancie1 said:

So the good running pump will automatically have many usage hours more that the motor and pump that is being rebuilt. If the good pump has at least as much run time as it will take to get the other pump rebuilt, then alternating the two units makes a lot of sense. That way the current pump will fail first with at least enough assumed time left on the other pump to get the failed unit rebuilt and reinstalled.

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Yes.
Although now we are going to try reworking bypass for the pump. The VFD is on hold for now. I'm sure it will be on hold until the day before we need to refill the well.

I'm interested in knowing more about this nanofilter you are using. I'm facing the need to place an isolated cooling loop between a cooling tower and a process heat exchanger that requires super clean water but cannot use deionized water.

TConnolly said:

I'm interested in knowing more about this nanofilter you are using. I'm facing the need to place an isolated cooling loop between a cooling tower and a process heat exchanger that requires super clean water but cannot use deionized water.

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The information I have
Manufacturer: Osmonics
Mfg Part Number:
Model: 43 B HR(PA) 216K/DLX
Description: Brackish RO Rated at 150 GPM

Its called a RO system, we've been calling it a nanofilter. I don't know.

I attached a flyer from a very similiar GE unit.
I can't get the flyer to attach. Its too large.

Here is a link to the GE unit that seems very similar to what we have.

http://www.gewater.com/products/pro-series-reverse-osmosis.html

Why modify the already packaged system at all? Who knows what other problems you'll introduce by running the system at reduced pressure/flow.

What about adding a surge tank between the utility and the system? 500 gallons is not that large a tank and would provide 3 minutes of volume. Can the system be cycled in this manner?

OpZed said:

Why modify the already packaged system at all? Who knows what other problems you'll introduce by running the system at reduced pressure/flow.

What about adding a surge tank between the utility and the system? 500 gallons is not that large a tank and would provide 3 minutes of volume. Can the system be cycled in this manner?

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As far as I know, we are wanting to run it for a relatively long time when we need it. Supposedly the device does not work well when you cycle it on/off quickly. We will not fill until we need to fill several thousands of gallons.

Update.
We did some replumbing to recycle more of the waste water. This has fixed the issue for now. The supply flow is now decreased enough that our water supply can keep up, the water coming out is "clean enough".

I have another potential use for multimotor 1 vfd. I am starting a new thread for that.

Again, suggest you look into ABB's Pump/Fan Macro. You get, with the timed rotation option, equal running hours on each pump with continuously variable control with a single VFD.

DickDV said:

Again, suggest you look into ABB's Pump/Fan Macro. You get, with the timed rotation option, equal running hours on each pump with continuously variable control with a single VFD.

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For now we are not planning on vfd at all. The "replumbing" of the system has gotten it to work. Thanks for the info though.