Sensorless Vector VFD

I routinely use a "standard" VFD for controlling air flow of multi-stage centrifugal blowers. A prospective customer recently indicated "we were advised that the blower motors need to have vector drives if they are running on VFD’s " It is my understanding that vector drives are primarily used when accurate torque and speed control are required, particularly at low speeds. The blowers are a typical centrifugal variable torque loads, and never operate below 40 Hz.

1)Can anyone suggest a valid reason to use vector drives on this kind of application.

2) If the owner insists on vector drives, is it necessary to specify a tachometer or encoder on the motor?

I dont understand why you would need vector control for a fan/blower. The majority of VFDs used in the beginning were on fan/blower applications. I have a white paper, http://www.patchn.com/mtrwhtpaper.htm that starts with:

Until recently the majority of AC variable speed drives have been applied to variable torque, pump and fan applications.

Click to expand...

They are refering to standard V/hz units. I dont see why you would need a closed loop system for a blower. You can do sensorless vector control but normally that is used when speed regulation is needed.

See if this paper helps: http://www.ab.com/drives/Articles/sensorless_article1_phtml.html

Maybe Dick or Don will comment.

Totally agree with you Tom, for almost all centrifugal fan/pump applications I can think of a V/Hz drive would be the right choice. Tell you what...just charge them for the Vector drive and then set it up in V/Hz mode.

Even then most vector drives should have a "sensorless vector" mode that eliminates the need for an encoder. Encoders are really only needed for torque production at zero and close to zero speed.

Tom, I consider you a master, so this is my 2 cents.

I have run into this on very large projects. Vector control was not used on the motor itself, but rather air pressure.

The idea was to start standard 600V (100hp) 3Ph induction motors that where coupled to fans in a vacuum.

The starting torque was nill. We had 4-20mA louvers in front of the motor. The were closed at start up or basically zero air intake. Many have to think about that one.

Restrict air intake, less start up current.

John

Tom, many mainstream drives now have software-selectable V/Hz and sensorless vector operation. You do not need anything additional on the drive or motor to operate in sensorless vector.

Operating a fan in sensorless vector mode is certainly unneccessary but can bring about a couple of obscure benefits.

First, usually the catch-a-spinning-load feature works better in sensorless vector mode. This is particularly true if the fan is spinning backward and needs to be started forward.

Second, while I doubt that the difference is measureable, it is claimed that energy savings is enhanced because the motor is always magnetized only enough to develop the necessary torque and therefore, motor heat is reduced and supposedly, motor life is extended. Actually, sensorless vector operation in many drives is so marginally different from V/Hz that any improvement even in speed regulation is undetectable. Only the few best sensorless vector systems produce significantly better motor torque and speed management.

Finally, a really obscure benefit is that the drive can display and transmit on its analog output the torque in the motor shaft withing 1-2%. This can be useful for troubleshooting bearing problems etc. but, on fans, there isn't much more that torque will tell you.

The industry is trending towards both operating modes in the same drive with no real increase in cost. In view of this, asking for it needlessly in a spec is of small consequence. In my view, the real disadvantage of using vector mode is that you have to tune up the speed controller even on jobs that don't need tight speed regulation. Startup is faster and simpler using V/Hz.

Thanks, guys. It is nice to have my opinions confirmed. We've used drives that had sensorless vector available, but I've never seen enough advantage to make it worth the extra work, so I've never implemented it.