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Fan anti-windmilling devices ??

Fan anti-windmilling devices ??

Fan anti-windmilling devices ??

(OP)

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 ??

RE: Fan anti-windmilling devices ??

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
www.gke.org

RE: Fan anti-windmilling devices ??

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.

RE: Fan anti-windmilling devices ??

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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Eng-tips forums: The best place on the web for engineering discussions.

RE: Fan anti-windmilling devices ??

Quote (electricpete):

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

RE: Fan anti-windmilling devices ??

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.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Fan anti-windmilling devices ??

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

RE: Fan anti-windmilling devices ??

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
www.gke.org

RE: Fan anti-windmilling devices ??

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.

RE: Fan anti-windmilling devices ??

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

Gunnar Englund
www.gke.org

RE: Fan anti-windmilling devices ??

"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.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Fan anti-windmilling devices ??

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
www.gke.org

RE: Fan anti-windmilling devices ??

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.

RE: Fan anti-windmilling devices ??

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.

RE: Fan anti-windmilling devices ??

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

Gunnar Englund
www.gke.org

RE: Fan anti-windmilling devices ??

(OP)
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 !!)

RE: Fan anti-windmilling devices ??

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

RE: Fan anti-windmilling devices ??

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
www.gke.org

RE: Fan anti-windmilling devices ??

Quote (electricpete):

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

Quote (waross):

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.

Quote (jraef):

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

Quote (skogsgurra):

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

RE: Fan anti-windmilling devices ??

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

RE: Fan anti-windmilling devices ??

Why not orient the blades and shafts so that duty and standby fans spin in the same direction for the same direction of airflow? No braking needed, you are simply starting a blade that is already part way up to speed.

RE: Fan anti-windmilling devices ??

That assumes he has variable pitch blades stevenal.

andyspark,
I appears that damper options are out since you are continuing on the ratchet concept. Backspin ratchets (a.k.a. Non-Reverse Ratchets or NRRs) are designed for use with vertical pump motors. If your fans motors are vertically oriented it may work, but you must check with the mfr because they rely upon centrifugal force to operate, so  horizontal orientation may not work at all. Also, a fan may accelerate too slowly to make that mechanism work properly. I know that some brands have trouble working when soft starters or VFDs are used on the pumps because of this issue. I don't know of anyone who makes an aftermarket device, they are usually sold with the motors because they must bolt directly into the end bell of the motor housing. Perhaps you can contact the motor mfr on your fans to see if they have anything available. Here's an example: US Motors NRR link

Beyond that your only other option as I see it is an electrically operated spring loaded brake assembly. Some are designed to be applied to an existing motor where you use the motor power from the pecker-head to disengage the brake so no additional wiring is necessary. The drawback to that would be having a wear part in your fans, the brake will engage to stop the fan every time you turn the motor power off. There are also what are called "static brakes" that are designed to only engage at zero speed, but that involves external controls and wiring out to the fan in your case.

Keep in mind that you asked this question in an Electrical forum, so most of us will think of solutions associated with the electrical side of this. You may want to post on one of the mechanical forums and reference this thread to see what other suggestions they may have.

Eng-Tips: Help for your job, not for your homework  Read FAQ731-376

RE: Fan anti-windmilling devices ??

The problem I read is that the breaker is tripping when you try to start the motor while the fan is spinning backwards. Is this an instantaneous trip or a time/current trip?

A good current regulating soft-starter that is properly programmed would be able to start the motor while it is running backwards without drawing too much current and tripping the breaker on instantaneous. This would count as more than one motor start though - possibly 1.5 or so depending on how fast the motor is spinning in reverse.

If the breaker is tripping on the time/current then you need to find a way to lower either the current or the time that the motor current exists. A soft-starter likely won't help because it will lower the current while increasing the time.

RE: Fan anti-windmilling devices ??

Don't some cooling tower fan gearboxes come with ratchet systems to prevent fans from backspinning?  The system is typically a 90° gearbox with a hoizontally mounted motor and vertically mounted fan.

RE: Fan anti-windmilling devices ??

Variable pitch is not needed, but he might need to replace some blades with the opposite pitch.

RE: Fan anti-windmilling devices ??

It seems to me the easiest option, assuming the motors are relatively small and of a standard design would be to consider the fitment of sprag-bearings in each motor. These are special bearings designed to rotate in one direction and lock up in the other. They are often seen in fan applications where windmilling is a problem and in certain gearboxes driving inclined conveyors where roll-back is a problem due to the weight of the product and the effect of gravity. We have in the past had unknowing HVAC Engineers bring fan motors into the shop complaining of bearing problems as they are apparently siezed up in one direction. Sprag bearings are available in most of the smaller 62 and 63 series sizes and are sometimes known as mono-directional or one-way bearings. One thing to be aware of is that they will not take the same amount of loading as the standard design equivilent single row ball-bearing. You need to identify the size(s) required and talk to a good bearing supplier. If its a feasible option you could even overhaul each motor, check balance the fan and fit the bearing at the same time.
I have just done a quick search but owing to time this is all I can find.
http://www.zxz-bearings.com/onewaybrg.htm

RE: Fan anti-windmilling devices ??

See, I knew the mechanical guys would know more wink

RE: Fan anti-windmilling devices ??

(OP)
Ok guys your suggestions have been taken onboard.

Most of the fans that are causing the problems are fin-fan units of various sizes, and I agree that the air wastage is ridiculous - but the plant had been designed and built when I arrived, so its try and improve whats here.
Having reviewed our options here on site (with a mechie type person) we are to experiment with a spring loaded ratchet type device on the fan shaft (belt driven)as opposed to touching the EX rated motors- as that obviously opens a new can of worms !! We can but try and see what happens.

Thanks again guys - your replies are very much appreciated and keep up the good work on this site.
Best Regards
Andyspark
(ABZ - Scotland)

RE: Fan anti-windmilling devices ??

Be aware that ratchet units will work long enough to get you ( and the supplier ) through the warranty period;  afterwards,  in my experience there can be problems so keep spares or get good guarantees from the supplier.  they are items that do need some maintenance.

RE: Fan anti-windmilling devices ??

You never said Ex before. Make sure your ratchet assembly is made from non-sparking materials!

RE: Fan anti-windmilling devices ??

Having the fan and motor rotating slowly is much better than holding them stopped dead IF they have ball or roller bearings and IF there is any detectable vibration transmitted to them from other equipment.  False brinneling.

If when backdraft dampers are fitted and the fan stops completely then I'd put a wedge in the damper to hold it open enough to make the fan creep.

RE: Fan anti-windmilling devices ??

Methinks false brinelling is a problem on applications where the motors are off line for long periods of time, no? I mean months at a time, not a few hours per day. I'd like to know your thoughts and / or opinions however. I once wrote an application note on this suggesting that people used VFDs or Soft Starters on timers to periodically rotate motors slowly once per day to avoid false brinelling in moderate vibration applications. I was shot down somewhat harshly by motor people who said I was exagerating the problem.

RE: Fan anti-windmilling devices ??

After rereading, I now see where I may have misunderstood the problem. Most are assuming that duty and standby fans are oriented parallel to each other, while I imagined a series arrangement with standbys moving air in the opposite direction from the duty fans. (Seems unlikely)

So going back to the original question: Is OC tripping the only problem? Can the motors withstand this treatment? If so, the problem might possibly be solved by delaying the protection.

RE: Fan anti-windmilling devices ??

I'd say that at my plant evidence of false brinneling is pretty common in the bearings that I get an opportunity to inspect.

Of the 15 or so pairs of motor bearings I've examined after being removed due to vib symptoms, I'd say at least a third of those had a characteristic false brinneling appearance - Axial-directed marks spaced at ball spacing... multiple sets at different intervals but the same appearance within a set. For some reason I see it more on outer races than inner races but not sure why.

In none of those cases was the visible damage anywhere close to something that would cause failure.  But at least two the false brinneling was the only thing wrong and was the reason that the vib analysis indicated a bearing fault.  The other three or four there were worse things going on in the bearing... ball defects and large spalls... not sure what was the original problem.

That's my personal experience. I also know our SKF rep says in his experience lubrication is the #1 cause of bearing problems followed a close second by both false brinneling and skidding due to light load.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Fan anti-windmilling devices ??

BTDT.  The problem I worked on was almost the same.  After looking at it I decided I was fixing a symptom not a problem.  The system I was working on was leaking enought to compromise the function of the system ( containment purge on a PWR).  I turned the problem back to MEs.  I went through the above solutions and didn't like any of them.  
We even considered a homemade ratchet made of a piece of flexible plastic.  No matter the system would have still been broken.

RE: Fan anti-windmilling devices ??

re:false brinneling
Probably should start a new thread, maybe even in the bearing group.
I suspect for a particular application the time-to-damage depends on severity of vibration and time.  Big vibration = short time, and vice versa.  The last time I was researching it seriously there was maybe 10X difference in the amplitude among bearing mfg engineering groups how much vibration is too much.  ALmost understandable, since the definition of "failure" runs from bearing smoke to subtle variations in workpiece surface finish, depending on industry and application.

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