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 ??
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 ??
RE: Fan anti-windmilling devices ??
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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RE: Fan anti-windmilling devices ??
respectfully
RE: Fan anti-windmilling devices ??
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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RE: Fan anti-windmilling devices ??
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 ??
Gunnar Englund
www.gke.org
RE: Fan anti-windmilling devices ??
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 ??
Gunnar Englund
www.gke.org
RE: Fan anti-windmilling devices ??
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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RE: Fan anti-windmilling devices ??
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 ??
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 ??
RE: Fan anti-windmilling devices ??
Gunnar Englund
www.gke.org
RE: Fan anti-windmilling devices ??
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 ??
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Fan anti-windmilling devices ??
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 ??
The consensus of advice may be tending towards stopping the air flow instead of stopping the fan.
respectfully
RE: Fan anti-windmilling devices ??
I have seen open frame VFD's but don't know if they had windmill parameters.
Barry1961
RE: Fan anti-windmilling devices ??
RE: Fan anti-windmilling devices ??
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 ??
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 ??
RE: Fan anti-windmilling devices ??
RE: Fan anti-windmilling devices ??
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 ??
RE: Fan anti-windmilling devices ??
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 ??
RE: Fan anti-windmilling devices ??
RE: Fan anti-windmilling devices ??
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 ??
RE: Fan anti-windmilling devices ??
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 ??
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.
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RE: Fan anti-windmilling devices ??
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 ??
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.