Fan anti-windmilling devices ??

[andyspark]

Has anyone had experience with anti-windmilling devices fitted to motor shafts ? We are having serious problems with standby fans running in reverse when duty fans are on. Then if standby fan is called for the starting / braking current created trips the crrent protection on overcurrent.
Any suggestions on a manufacturer of such mechanical devices ??

 

[Skogsgurra]

Yes. But that was not a mechanical device. We used a DC injection brake consisting of a low voltage transformer, a rectifier and a N.C. auxiliary contact on the motor contactor.

The technique was quite effective, in our case it was a large diameter axial fan that was tough to start from stand-still and impossible to start if driven backwards. Using about 20 percent nominal current as a braking current was enough to keep it almost at zero speed. The slow backwards movement we had was not a problem.

There is a caveat; when the motor contactor drops and the motor is spinning full speed, there is full voltage across the motor windings. You have to chose rectifier voltage to withstand that voltage. You may also have to include a series resistor to reduce current, but usually not necessary.

Gunnar Englund

http://www.gke.org

 

[macmil]

How about a one-way band brake ? Do you have enough room ? You would probably need a sheave / flywheel ; perhaps you could extend the motor shaft at the non drive end . The big advantage is no extra moving parts, controls etc . gravity on a weight or a spring would provide the necesaary braking effort.

 

[electricpete]

In the air system you can install a "backdraft damper". It is the air-system equivalent of a check valve.

Also, we have a variety of vertical pump motors fitted with fairly simple anti-rotation ratchets at the top of the motor above the thrust bearing. They all use the principle of gravity to pull the pawls down as the motor slows. I haven't see anything like that on a horizontal motor or a fan.



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[waross]

n the air system you can install a "backdraft damper". It is the air-system equivalent of a check valve.

You are wasting service fan capacity by allowing air to bypass back where it came from. Consider gravity or air pressure operated one way air dampers. You may want to install one way dampers on your main service fans also. It seems counterproductive to let the air from the emergency fans to return through the main service fans if they are stopped.
respectfully

 

[electricpete]

Maybe you already have backdraft dampers and they're just not working right? In that case look at repair of the dampers or replacing with a better design.

Maybe you have manual dampers intended to be manually closed on the idle fan? (not likely if there is a standby start feature). In this case make sure it is correctly positioned.

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[jraef]

A damper is definitely the way to go if at all possible as others have said. Assuming you already have investigated that angle and have come to this point out of necessity, any decent DC injection brake can work as long as it has Zero Speed sensing.

From an anti-windmill standpoint, you simply reverse the logic; brake BEFORE energizing the motor. Interrupt your existing Start/Stop circuit to control a relay. The relay energizes the DCIB, as well as enables the main motor contactor circuit, but not fully because the DCIB is braking (N.C. contact of the DCIB that opens during braking). When the DCIB is done (Zero Speed sensor engages), it releases its NC contact and allows the main contactor to close. If the fan was spinning solely because of reverse air movement, then there is no need to worry about voltage on the motor windings, there will be none. The thing that you must do however is to make sure the DC injection brake circuit is interlocked such that it can never come on at the same time as the main run contactor, so put a NC contact of that contactor into the DCIB enable circuit too.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[Skogsgurra]

Yes, Jeff. But a zero speed sensing device means a new cable and you know how difficult that can be. A timer that makes sure the rotor field has collapsed before you apply the braking DC is a way around the sensing device/cable nuisance.

Gunnar Englund

http://www.gke.org

 

[jraef]

Gunnar,
Now most packaged DCIBs sense Zero Speed without another wire. They either look at regenerated voltage level on the unused leg, or count the rotor bars passing through the field created by the DC by looking at current distortions on that unused leg.

 

[Skogsgurra]

Wow! That's good news. Where can I read about them?

Gunnar Englund

http://www.gke.org

 

[electricpete]

"From an anti-windmill standpoint, you simply reverse the logic; brake BEFORE energizing the motor."

I would say that is better than starting with the motor rotating backwards, but still has a downside:
1 - Braking will deposit all the kinetic energy of the rotor into the rotor to bring it to a stop. At that point we're going to start a high inertia fan with a rotor that's already hot...each and every time we start it.

Some other negatives of the braking solution:
2 - complexity
3 - even if you stop the fan from rotating, there would still be air flowing past it.

But I agree it's always good to some options if for some reason you can't make the backdraft dampers work.

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[Skogsgurra]

That's an interesting point of view, electricpete.

It never occurred to me that one wanted to start the fan soon after stopping it. A high-inertia fan isn't normally run that fashion. But, of course, if that is a problem - don't do it.

Otherwise, I have found that DC injection braking is very easy to use and even if it cannot make the fan stand absolutely still, it can reduce reverse speed to neglible values (a few RPM) that means that the fan can be started without tripping. The air flowing past the fan may be a problem in this case since it is a real "flow thief". I guess dampers are the best choice - even if I would be very temted to test those new DCIB units. Good to know they exist.

Gunnar Englund

http://www.gke.org

 

[jraef]

Electricpete,
You make a valid point, however I can say from experience that in most windmilling applications like this, it takes only a few (10-15) seconds of DCIB to stop the blades. Not a lot of reverse torque created by the draft, mostly just the blade inertia. What I always tell people is that when using DCIB, regardless of stoping a load or anti-windmilling, always count the braking cycle as if it were a starting cycle when determining your number of starts-per-hour.

 

[davidbeach]

There are also drives that can capture a reverse spinning motor, bring it down to zero speed and then accelerate it in the proper direction.

 

[Skogsgurra]

I would say that all drives, at least all flux vector drives, do that. But this is DOL. Isn't it?

Gunnar Englund

http://www.gke.org

 

[andyspark]

Thanks guys - all the feed back is greatly appreciated.

ElectricPete - the mechanical device of pawl /ratchet was the idea that I was considering myself Any manufacturers names ? Cannot see why we cannot try them on a fan motor shaft if I can find a maker !!
We are on a massive petrochem site in Kazakhstan with several hundred fans so a quick easy solution is needed ! The DC injection idea is a none starter, due to space within my mcc cubicles (buckets - as our US readers call them !!)

 

[itsmoked]

Careful andyspark... "quick easy" times 300 could end up being nightmarish and expensive. I wouldn't jump at any solution without some thorough research and testing if I was going to apply it more than 10 or 20 times let alone hundreds. This I have learned from the school of hard-knocks.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Skogsgurra]

Yes, I guess Keith has had as many knocks as I had. What concerns me is "serious problems with standby fans running in reverse when duty fans are on".

Doesn't that mean that you are losing flow through the stand-by fans? And isn't that also a problem. The ratchet solution will not address that.

Gunnar Englund

http://www.gke.org

 

[waross]

In the air system you can install a "backdraft damper". It is the air-system equivalent of a check

You are wasting service fan capacity by allowing air to bypass back where it came from. Consider gravity or air pressure operated one way air dampers.

A damper  is definitely the way to go if at all possible as others have said.

Doesn't that mean that you are losing flow through the stand-by fans? And isn't that also a problem. The ratchet solution will not address that.

The consensus of advice may be tending towards stopping the air flow instead of stopping the fan.
respectfully

 

[Barry1961]

Are you sure you can not replace you motor starters with VFD's? Some VFD's are about the same size as a motor starter.

I have seen open frame VFD's but don't know if they had windmill parameters.

Barry1961