VFD Ride-through Power for 15 Seconds

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natfuelbill

Senior Member
I have a building cooling water loop pump system is controlled with a VFD. The pump is 480V, 40 HP. Power is through a ATS (main/diesel). When the main fails, the VFD go down on loss of AC, the pump stops, the loop pressure drops, the diesel starts, the ATS transfers, the VFD may then restart.

The problem is that certain large HVAC equipment shuts down an low flow/high head pressure, and requires manual resets.

One method to solve this is to keep the vfd/pump running during the say 15 seconds while waiting for the generator to come online.

How can I keep the VFD in power?


edit: (corrected shutdown criteria)
 
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boater bill

Senior Member
Location
Cape Coral, Fl.
It sounds as if the VFD is programmed to ramp up from a power loss. If the power is out for 15 seconds, there may be enough ride through time to keep the pump operating, but probably not.

Thought, can check valves be installed in the chilled water lines? That way there wouldn't a loss of pressure as quickly on the lines. Reduced flow, but good pressure for enough time to keep the HVAC equipment on line.
 

Bob NH

Senior Member
natfuelbill said:
I have a building cooling water loop pump system is controlled with a VFD. The pump is 480V, 40 HP. Power is through a ATS (main/diesel). When the main fails, the VFD go down on loss of AC, the pump stops, the loop pressure drops, the diesel starts, the ATS transfers, the VFD may then restart.

The problem is that certain large HVAC equipment shuts down an loss of head pressure, and requires manual resets.

One method to solve this is to keep the vfd/pump running during the say 15 seconds while waiting for the generator to come online.

How can I keep the VFD in power?
Let's try to understand the magnitude of the problem.

40 HP for 15 seconds is 600 HP-seconds or about 125 watt-hours. A single 12 Volt lead-acid battery with 80 ampere hours capacity will store the total energy you need, so the problem is not storage of energy.

The problem is to deliver that energy quickly. Special rapid discharge NiCd batteries are used in electric model airplanes because they can deliver a lot of Amps for a short time. http://loke.as.arizona.edu/~ckulesa/nicads.html

A 1.2 Volt NiCd AA cell at 1500 ma-hours of capacity will deliver 1.8 watt hours, so you could get 125 Watt-hours of energy from about 70 rechargable AA cells. The issue is, how fast can such a battery deliver that power? What is the current limit?

But we aren't designing it here; we are just trying to understand the scope of the problem.

Now if you can get cooperation of the VFD supplier, you could probably find where in the controller you could deliver a DC supply into the controller so it can deliver that power to the motor in the normal course of its operation.

I suspect that VFD manufacturers have faced this problem before and have solved it.

If all else fails, I suspect that a good electrical engineer with knowledge of power supplies could put together a system to do the job.
 

Bob NH

Senior Member
Another solution would be to use hydropneumatic water storage or small water tower to supply the water needed until the auxiliary power comes up. You probably don't need a lot of pressure in the heat exchangers.

I don't know the head/flow requirements but hydropneumatic or tower storage for the 15 second capacity of a 40 HP pump is a pretty straight-forward propostion.

If you tell us the pressure and GPM capacity of the pump, I can give you a quick answer on what you would need for 15, 30, and 60 seconds of water storage, and an idea of the controls that would be required to automate it.
 
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coulter

Senior Member
natfuelbill said:
...large HVAC equipment shuts down an loss of head pressure, and requires manual resets. ...
Or possibly look at changing the equipment so it doesn't need a manual reset. If you don't have a process reason to keep the HVAC running, maybe this is a control problem consisting of orderly shutdown on power loss, and orderly startup on power restore.

carl
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Check with your VSD supplier. They may have the option of providing DC battery power to the VSD, or a replacement VSD with such capability. I don't know of any specific products, but I know that the concept of doing this has been floating around the VDS world for years, specifically for the elevator folk. (I come at the VSD market from the technology development side, and one of our big projects is VSD systems for battery powered vehicles.)

In fact, a quick google search turns up the Hitachi SJ300EL, customized for elevator operation. They say 'Battery Backup', but don't give any details. http://www.hitachi.us/supportingdocs/forbus/inverters/Support/SJ300EL_f.pdf

-Jon
 
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