480V 3 phase moter with VFD 2

[mrcheese]

I have a 480V 3 phase fan motor (<5HP) that i want to keep running for 14 seconds once the power it cut off. is it possible to connect a string of batteries to the DC bus of the VFD that will keep the DC bus energized in the event of a power outage? the DC bus would also have to keep the batteries charged.

 

[mikekilroy]

mrcheesy, what is your budget?

$ 1?
$ 10?
$ 100?
$ 1,000?
$ 10,000

This will really effect the answers you get. Sorta important info you have not yet given.

http://www.KilroyWasHere<dot>com

 

[Skogsgurra]

Unless I'm seriously mistaken, a flywheel should work very well for you, cheesy.

I did an Excel exercise:

http://gke.org/pub/files/Flywheel mrcheese engtips 1.png

You say that you had checked the energy storage possibilties in flywheels. How much energy do you really need? From what speed to what speed is the fan allowed to drop in those 14 seconds?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[mrcheese]

ScottyUK the UPS option was remove because of cost. A 480V 3/P UPS that is large enough to start the motors is about 26K. had about 3 manufactures price this for me


mikekilroy: great point! I am looking at a price point of between 1k-and 2k each for the first 500 units.


Skogsgurra: Fly wheels are great! and designed to do this very thing. I spec them all the time for the data center projects i do. Unfortunately, the low load requirement of this project does not allow for efficient sizing if a fly wheel. the rotating mass and the vacuum chamber does not allow for manufactures to build these things to match the load requirement so it would have to be very over-sized and/or custom. again i contacted 3 manufactures price this for me and we are looking well above the ups cost.

the other option i am looking at is DC motors feed by battery bank and charger. this might be the ticket but still waiting in charger pricing. because the the torque requirements i need a motor that will be 540VDC. I can get the batteries that i need for about $400 per string to get me to the 550VDC. the issue here is the charger. The charger would have to carry the motors in normal operation while charging the batteries. there is no charger i found that will do this. battery chargers are not designed to run motors! have calls into several manufacture on this as well

the reason i came to the VFD is that the 480V motors are already on a VFD. If i could simply connect batteries to the DC bus then i could solve this problem for about $500 per unit.

The VFD solution has 2 issues

1. all the VFD's that i have looked at require 480V input to run the controls for the unit ( i have a work around that uses and inverter to power the controls via the batteries but again this is pricy and fairly complex)
2. the DC bus on a VFD is not designed to charge batteries. yes they would charge but there is no way to prevent over charging with out addition components connected between the DC bus and the batteries.

I have also looked at capacitors and in fact there are many VFD's that have that ride through function by way of capacitors, unfortunately this is GREAT for 2 seconds but could not be able to run 14.

there are about 100 way to skin this cat if cost was no option or i wanted to have an overly complex solution. ALL good engineering solutions are simple! I know there is a solution i just have to find it.

 

[Marke]

IF you used 220V fan motors off a 480 V VFD with the output set back to 220V, you could get away with round 250 V DC batteries coupled back into the DC bus via a decent rectifier. Probably much easier to charge and less batteries etc.
The drive would adjust the output modulation to enable it to run on the higher DC bus voltage while on mains supply.

Mark Empson
Advanced Motor Control Ltd

 

[Skogsgurra]

I think that you overspec things quite a lot.

It is only high-speed flywheels that need to run in vacuum. You havent said anything about the speed from which you need to cover a power outage and down to which speed the fan is allowed to reduce its speed.

I assumed, very arbitrarily, 3450 and 2700 RPM for those limits and also that the fan would be fully loaded across that speed range, which it will not be.

Under that assumption, you need a two feet diameter disk that is less than an inch thick, coupled to the fan shaft. Compared to your battery arrangement, that you actually did consider, such a disk is neither big nor expensive. Life is next to infinite, if you align and couple it well and the extra losses (windage, mostly) will be very small if you compare it to the battery solution.


Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[sibeen]

ScottyUK the UPS option was remove because of cost. A 480V 3/P UPS that is large enough to start the motors is about 26K. had about 3 manufactures price this for me

I don't think you asked the right question. The UPS salesman would have been looking at the start up currents of the motor and going from there, and then specifying a UPS which is way, way to large for what you require.

The UPS doesn't need to be able to handle the start up currents of the motor. Let the bypass line of the UPS do that function. You're going to have UPS input voltage available when you start up, so use that. The UPS would only need to be 4 or 5 kVA, and a line interactive model would probably suit your needs, and a 4kVA model could probably be picked up for less than $2K. Now unfortunately I don't think there are many (any) three phase UPSs made at that size.

Can you source a single phase input / three phase output VFD, and perhaps use 220V fans as Marke suggested, and run the input via a UPS?

 

[ScottyUK]

sibeen beat me to it - the UPS only needs to handle the VFD input rectifier, not support a DOL start from a pair of motors. A 10kVA UPS with a small battery should be way less than $26k. I agree that very small 3-phase types are hard to come by, but 10kVA types are easy to find.

 

[sibeen]

mrcheese, we're all working here from limited information, so a follow up question.

Why are you using a VFD int his application?

Ga,

Sibeen stated:

and then specifying a UPS which is way, way to large for what you require.

Oh, for an edit button...which is way, way too large for what you require.

 

[ScottyUK]

OK, I'll put a rough price to it: 3000 GBP without any serious looking around.

You need a new UPS salesman.

 

[Skogsgurra]

Yes, a better UPS specialist. But, hey - aren't we that, too?

Sorry, didn't really mean to be mean.

I did some work in a spread-sheat, which can be found at

http://gke.org/presentationer/files/EngTips flywheel for fan 1.xlsx

I think it is correct. But would appreciate if someone checked the results against known-good data.


Re: starting an induction motor from a VFD. You can start fom zero RPM with even less than rated current. So, I see no reason to make the UPS very large. Then again, some UPSes arent ready to see a VFD input rectifier and its not-so-nive current waveform. So, then you may need a PFC/AFE in the VFD. Which can be a problem to run from an UPS.

I think that you should look very seriously into the flywheel solution.

But, first of all: Please give us (and the specialist you are going to consult with) complete data. The green cells in the spreadsheet say what those data are.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Skogsgurra]

There is one thing that I cannot understand. You say that "had about 3 manufactures price this for me"

What does "about 3 manufacturers" mean? Is it 2.7 - 3.3 or are the margins wider? 2 manufacturers? or 4? 5, perhaps? And all these manufacturers didn't care to understand your needs? Are you sure you talked to the right people? Or any people at all?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[iop995]

More than 20-25 battery in a string is not recommanded and equalization circuit increase cost. I think you may use 20 battery (240V) and a buck/boost DC-DC converter to 480Vdc connected to VFD DC bus. Same converter, configured in buck topology may be used as battery charger, feed from VFD bus. Regarding power of converter, for 14s working time (no time to warm up), need to check firstly current requested, not power of converter. If there are not such converter on market, I think may be made or may request to a specialized company and maybe don't overpass 1-1.5k$.

 

[LionelHutz]

More than 500 motors? You can't run the VFDs from a common backup system or run multiple motors from a single large VFD with a dedicated backup system?

 

[2dye4]

There are issues with attaching a long series string of batteries to the DC bus.

I once worked at a place that had a 'battery room' of large cells series connected for
emergency back up. The emergency reared its head one day and the battery bank was
called up quickly to pick up the load. Unfortunately one cell in the string was in bad shape
and it became the load for the whole system exploding violently.

The take away is that some kind of monitoring or other assurance is needed to verify the
cells are all good and post an alert if there is a bad one.

Now if you can tolerate a failure and the batteries are located in a place where they can fail
safely then just rely on quarterly maintenance checks.

Also 650 volts of battery can be quite dangerous from an electrical hazard point of view making a home
brew setup a bit of a worry for future maintenance. Make sure the bank is fused with a fuse that will
interrupt 650 VDC at short circuit current levels. DC is harder to interrupt than AC.

Personally I would make the company buy a solution for the above reasons so I could sleep easier.