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 ??

 

[stevenal]

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.

 

[jraef]

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.

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[LionelHutz]

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.

 

[Laplacian]

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.

 

[stevenal]

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

 

[stardelta]

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

 

[jraef]

See, I knew the mechanical guys would know more

 

[andyspark]

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)

 

[macmil]

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.

 

[jraef]

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

 

[Tmoose]

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.

 

[jraef]

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.

 

[stevenal]

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.

 

[electricpete]

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.

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[BJC]

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.

 

[Tmoose]

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.