V/Hz vs Vector control VFD 2

[MikiBg]

Hi all!
This thread may be considered as extension of one previous:

http://www.eng-tips.com/viewthread.cfm?qid=248018&page=8

After receiving request for offer and required techical data, one potential vendor asked: "Is it neccessary to provide VFDs with vector control, wich are much more expensive than ones with V/Hz control, since load is centrifugal pump and therefore suitable for V/Hz control?"
I tought that type of control can be selected as VFD's working parametar, but looks like I was wrong, at least for this manufacturer. Guess that the answer is "yes", but I'll appreciate if someone can explain to me why.

 

[ScottyUK]

V/Hz likely can be selected for most vector drives, but you will still be paying for a vector-capable drive even if you make it dumb down to a V/Hz mode.


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If we learn from our mistakes I'm getting a great education!

 

[MikiBg]

ok, but when can I say "I don't need vector control" for sure?

 

[Skogsgurra]

Centrifugal pump works very well with a square scalar (U/f) control. Vector control may actually be bad since it will usually maintain full excitation current at all speeds below base speed.

So, if you have a big price difference in favour of U/f, then buy U/f.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[DickDV]

A V/Hz or scalar or open loop drive will, at best, be able to control speed over the full torque range within about 1/2 of motor slip.

A vector drive, if sensorless (no encoder) will typically reduce the speed error under the same torque conditions to about 1/4 of motor slip. If an encoder is used, the error drops to one pulse per revolution plus or minus one pulse.

In addition, a V/Hz drive is not able to develop useful motor torque below about 2Hz. A sensorless vector drive gets down to about .5Hz and an encoder vector system will develop full rated torque at stall.

Finally, any drive without an encoder will have all negative speed error under motor loading conditions and all positive speed error when the motor is being overhauled. This error, however small, is cumulative. Systems using encoders tend to keep their error centered around zero which would be non-cumulative.

Therefore, given a specific load such as a pump or fan, what error or torque characteristic can you tolerate? You then choose the system that is equal or better than the requirement.

 

[itsmoked]

A centrifugal pump does not need a vector drive..

Keith Cress
kcress -

http://www.flaminsystems.com

 

[MikiBg]

Just a correction, I ment to say "no" (not neccessary) in first post. Looks like I got a confirmation, but with some explanations this time.
Thanks, guys.

 

[jraef]

There is another factor to consider here though.

GENERALLY, it has become a defacto standard for VFD manufacturers to offer at least open-loop (sensorless) vector capabilities in even their basic entry level drives now. The technology has been boiled down to a simple chip set they can all buy, and the difference in cost for one with or without that functionality is so negligible as to be insignificant. Couple that with the fact that most transistor mfrs are now ONLY offering smart IGBT packaged power modules that are capable of the high firing speed necessary for vector control. The combination could indirectly mean that if someone is offering a V/Hz ONLY drive for less, it is most likely a soon-to-be-obsolete product that they may be dumping onto the market. You should be careful when accepting that at face value, don't be a patsy who accepts the lowest cost item only to discover you bought something with a shortened support life. Granted, VFD designs have a relatively short product life cycle now anyway, but the major shift to all-vector-all-the-time happened 3 or 4 years ago, so if someone doesn't offer it still, this might be a sign of a REALLY old design.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[Skogsgurra]

THAT is a very important consideration, Jeff! Very important! OLS for you.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Skogsgurra]

PLS, of course.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[MikiBg]

jraef,
as I wrote in first post, I thought that vector control is usual technology for modern VFDs, and you've just confirmed that. But then the question from vendor came. It is a distributor of Allen Bradley, btw. Does the power matter, 'cause he said that price difference is specially big for large units? We're talking about 250kW and 160kW units. Anyway, thanks for letting me know, I'll see what support they will offer.

 

[jraef]

Wow, the A-B Powerflex 400 drives are apparently incapable of vector control. I find that unusual, given that it may have saved them $10 in the cost of the drive, if that.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[DickDV]

It may be that the PF400 involved is an HVAC variant. I'm not familiar enough with AB's model numbering system to know if that applies to the 400 level drives or not.