Two pumps, two circuits, one VFD

[jayaredee]

Just found this gem on a site in L.A. (see picture)

They only have one chilled water pump, so they set up an extra condenser water pump to be re-routed (by manually setting four isolation valves) and used as a back-up chilled water pump.

Each pump has it's own feed (from different panels) but are both 200HP pumps.

The primary pump circuit is fed direct to the VFD, then from the VFD it goes to the LOAD side connection of a transfer swtich. The transfer switch 'emergency terminals' go to a safety switch and then on to the primary pump. The transfer switch 'normal power' terminals go to the LOAD side (double lugged) of the secondary pump safety switch.

The secondary pump circuit is fed directly to a combination motor starter and then to the line side of the secondary pump safety switch. The load side of the secondary switch then feeds to the secondary pump (again, double lugged).

This obviously has some major concerns for possible back feeding not to mention the fact that the load side terminals of the secondary switch are live when the switch is off (which it was, off and locked out...with live voltage on the load terminals!)

We were being asked to replace the VFD (with a 250HP VFD, run at 80% to provide more reliability !!!! :?:lol: ) (yes, I am confused about that one as you are).

Anyway, I am thinking of purposing to provide a transfer switch (select line A or line B) with a double throw safety switch after that to select VFD A or VFD B (going to propose 2 VFDs) and then have the VFDs each connect directly to their respective motors.

Thoughts?

 

[petersonra]

I would not want to make the mess just a little better. it should be cleaned up completely or left alone.

i don't understand why there are both starters and VFDs. What is the purpose of that? it does not serve any real purpose the way it is shown. i could understand maybe if the starter was a bypass contactor for use in case the VFD failed.

I think the best thing to do is probably to start out by trying to get them to define what it is they are trying to accomplish here. If it is a redundant pump, personally I would just start it across the line. probably best to use a soft start with that size of pump, but as a backup pump you can go a day or two without the VFD while the main pump is being fixed.

Or you could have a transfer switch downstream of the VFD that switches between the two pumps and get rid of the starters.

 

[Russs57]

Hard to follow along right now.

Quick and dirty. I’ll call chill water pump CHWP and other CWP. Keep power to VFD and CWP starter however it was. After VFD and after CWP MS add the equivalent of a pair of 3-way switches. One one common to VFD, one traveler to CHWP, other traveler to the CWP switch. On CWP switch common goes to CWP. Remaining traveler to it’s MS.

 

[drktmplr12]

We were being asked to replace the VFD (with a 250HP VFD, run at 80% to provide more reliability !!!! :?:lol: ) (yes, I am confused about that one as you are).

a warning-be careful with grossly over sized drives, you might find the feeder conductors do not have the ampacity to comply with 430.122. ampacity must be 125% of the rated input current of the drive.

Anyway, I am thinking of purposing to provide a transfer switch (select line A or line B) with a double throw safety switch after that to select VFD A or VFD B (going to propose 2 VFDs) and then have the VFDs each connect directly to their respective motors.

Thoughts?

i think what you propose makes the most sense. two feeds that can supply either drive. we do something similar for redundant standby generators. im sure others on this forum can provide insights or an alternate solution.

if they don't like that concept of two VFD's they can use one VFD and put the transfer switch after it. the only problem will be coordinating the drive settings depending on which motor is connected. i think some drives can have their settings changed based on a dry contact input. connect a contact from the 'load select' MTS to each drive to trigger the settings change. I would check that capability with whatever drive mfg is being proposed.

i would also make sure the 'line select' MTS can be locked off.