Effect of Windmilling on Motor Starting

[Besoeker]

The expansion of the acronym and the surface definition is not really as revealing as is the underlying definition and intent. The acronym is used to caste distain upon those, who in the eyes of the user, "don't get it" and can't rise up to the user's perceived level of enlightenment.

Well, I was under no illusion that it was anything other than a deprecating or disparaging comment.

Rather than interacting on an intellectual level the user labels the target and avoids engaging in something they consider to be beneath them. This allows the user to maintain their perceived level of enlightenment without actually having to test or improve said level. Its frequent use makes one prone to intellectual and social blindness to some degree.

I think it's fairly obvious why he chose not to engage in further discussion about his proposed "solution".

Back on topic. The OP states that the design assumes staring from stationary thus ignoring the possibility of windmilling in either direction.
What we don't know is either the direction of windmilling or the possible speed. If it's the forward direction (wrt the desired operation) , then it isn't likely to be problem.
If it's the reverse, I'm inclined to think it might not be very fast. What is causing it to run in reverse? How is the air movement generated? Vehicles or maybe external prevailing winds?
In either case, I think it's not altogether likely that it will be presented to the fan at anything like the air flow it is designed to produce. On that basis, reverse rotation would not likely be very fast.
The worst aspect is probably a longer run up period. As I said before, setting the overcurrent protection might be a challenge. In terms of time rather than magnitude.

 

[Open Neutral]

What is causing it to run in reverse? How is the air movement generated? Vehicles or maybe external prevailing winds?

That one I can address. The WMATA tunnels are just big enough to pass the trains. There is in fact a "feeler car" made from a wreck; it
is used to confirm there are no obstructions. So the arriving train pushes a wall of air ahead of it, and sucks in behind after passing. In most cases, there are fan galleries on both sides of the station to sink/suck the air. And usually, the station has open escalators allowing further equalization, but one station, Forest Glen, is elevator-only access. [It's 220 ft down...]

One can imagine that the power consumption on those fans varies dramatically as a train passes.

There are also drafts. Best example would be the trans-Potomac tube, about 7000 ft long. The pressure difference between those two stations often creates a draft when no trains are in the way.

 

[Besoeker]

That one I can address. The WMATA tunnels are just big enough to pass the trains. There is in fact a "feeler car" made from a wreck; it
is used to confirm there are no obstructions. So the arriving train pushes a wall of air ahead of it, and sucks in behind after passing. In most cases, there are fan galleries on both sides of the station to sink/suck the air. And usually, the station has open escalators allowing further equalization, but one station, Forest Glen, is elevator-only access. [It's 220 ft down...]

One can imagine that the power consumption on those fans varies dramatically as a train passes.

There are also drafts. Best example would be the trans-Potomac tube, about 7000 ft long. The pressure difference between those two stations often creates a draft when no trains are in the way.

OK.
Do the trains move the fans in the right direction or the wrong direction.
And at what speed?

 

[GoldDigger]

OK.
Do the trains move the fans in the right direction or the wrong direction.
And at what speed?

The right direction depends on what the fans are trying to accomplish at the time.
The required fan direction for a tunnel fire or smoke event may be totally unrelated to the travel direction of the most recent train.

Tapatalk!

 

[Besoeker]

The right direction depends on what the fans are trying to accomplish at the time.
The required fan direction for a tunnel fire or smoke event may be totally unrelated to the travel direction of the most recent train.

Tapatalk!

But how fast do the fans rotate even if in the wrong direction when not powered?

 

[GoldDigger]

Based on the volume and velocity of the train induced air flow, along with no braking on the fan shaft, I would not be surprised if it was over 1/2 of the powered run speed.

Tapatalk!

 

[Smart $]

Based on the volume and velocity of the train induced air flow, along with no braking on the fan shaft, I would not be surprised if it was over 1/2 of the powered run speed.

Without more information, I assume train-induced air flow would cause the unpowered fan to rotate in the 'desired' direction, that is, if pressure equalization is the only purpose at that time.

 

[gadfly56]

Without more information, I assume train-induced air flow would cause the unpowered fan to rotate in the 'desired' direction, that is, if pressure equalization is the only purpose at that time.

You don't really need more information. As the train approaches the fan, you have a high pressure pulse. As it goes away from the fan, you have a low pressure area. Regardless of the desired air flow direction in the fan duct, 50% of the time it's seeing induced flow in the "wrong" direction.

 

[Smart $]

You don't really need more information. As the train approaches the fan, you have a high pressure pulse. As it goes away from the fan, you have a low pressure area. Regardless of the desired air flow direction in the fan duct, 50% of the time it's seeing induced flow in the "wrong" direction.

Your 'logic' does not follow mine...

Approaching train creates positive pressure, forcing air out through duct, causing fan to rotate in the same direction as when powered to alleviate positive pressure.

Departing train creates negative pressure, pulling air in through duct, causing fan to rotate in the same direction as when powered to alleviate negative pressure.

 

[gadfly56]

Your 'logic' does not follow mine...

Approaching train creates positive pressure, forcing air out through duct, causing fan to rotate in the same direction as when powered to alleviate positive pressure.

Departing train creates negative pressure, pulling air in through duct, causing fan to rotate in the same direction as when powered to alleviate negative pressure.

Maybe I misunderstand the actual function of these fans. For tunnels of this sort, I was under the impression that it's strictly ventilation. If you have a non-stop passing through at 40mph, those fans are NOT going to be able to reverse rotation fast enough to do as you claim.

 

[GoldDigger]

Nor are they necessarily needed unless a train is stopped in the tunnel or workers are inside.
For an automobile/truck tunnel ventilation is needed for air quality.
For an electrified train the fans are primarily for abnormal situations, not to help the trains move.
Also think of long tunnels....


Tapatalk!

 

[Smart $]

Maybe I misunderstand the actual function of these fans. For tunnels of this sort, I was under the impression that it's strictly ventilation. If you have a non-stop passing through at 40mph, those fans are NOT going to be able to reverse rotation fast enough to do as you claim.

As I said in my post, it was an assumption based solely on pressure equalization... nothing else!

 

[mgookin]

It's my understanding:

that this thread is about a train tunnell. Fans need to be able to windmill because if they did not then it would slow down the train. This windmilling also provides for air exchange in the tunnell.

The critical nature of the fans being powered is in case of a fire in the tunnell. The fans will create negative air pressure at the duct closest to the fire and create positive air pressure at the ducts each side of the fire. This allows firefighting forces to enter the tunnell and fight the fire.

Jraef, who is exceptionally qualified in this situation, said that if you tried to use a sensor to determine which direction the fan is turning at the time you want to start it up, you'd get into a hornets nest with NTSB on what fail safe/ redundancy systems you have to alleviate failure of any of that. The industry standard seems to be a simple "Is it moving?" and if it's moving, brake it before you apply power to put it in the desired direction.

I'm not an expert on it. I've just been reading the thread and that's my take from it. I hope this clears up some of the confusion. And if I'm misunderstanding, by all means, please correct me.

 

[zbang]

If we're going to mix facts into the topic-
http://dslweb.nwnexus.com/tawhite/CASCADE TUNNEL.html

or
http://en.wikipedia.org/wiki/Cascade_Tunnel
"The fans are powered by two 800-horsepower electric motors, clearing the air through the seven miles of tunnel within 20 minutes."

The Moffit tunnel also has some Honkin' Big fans.
http://www.trainsim.com/vbts/showthread.php?253669-Moffat-Tunnel-question

 

[Open Neutral]

Nor are they necessarily needed unless a train is stopped in the tunnel or workers are inside.

While vital in a fire; the fans do everyday work as well.

WMATA moves LOT of people every day. They need air to breath.
A car may hold 150 people at "crush" hour; there are 6-8 cars in a consist.

Further, the train consume/shed a lot of power. Each car draws ~0.7MW.
Plus there is the HVAC load of each car, non-trivial.

 

[kwired]

Regardless of whether a train is present or not when not "running" the fan propeller with no positive brake at the time will turn depending on atmospheric pressure changes. When outside pressure is rising air will be entering the tunnel, when outside pressure is dropping air will be leaving the tunnel and only has tunnel openings to get there.

I would think instead of sensing "windmilling" and braking regardless of direction it would be even simpler control wise to just apply a brake whenever the motor is not running, and then there would be no windmilling.

 

[Open Neutral]

I would think instead of sensing "windmilling" and braking regardless of direction it would be even simpler control wise to just apply a brake whenever the motor is not running, and then there would be no windmilling.

I'd think that is contrary to the goal; getting ventilation needed at lowest power cost. If you "push" with the fans at Rosslyn but get enough flow without also sucking at Foggy Bottom.....

I'm still wondering about unbraked starting.....

 

[kwired]

I'd think that is contrary to the goal; getting ventilation needed at lowest power cost. If you "push" with the fans at Rosslyn but get enough flow without also sucking at Foggy Bottom.....

I'm still wondering about unbraked starting.....

If ventilation is needed the fan would be running and no braking, I meant brake is on whenever motor is not running, maybe delayed after turning off if that helps with life of the brake.

 

[Open Neutral]

If ventilation is needed the fan would be running and no braking, I meant brake is on whenever motor is not running, maybe delayed after turning off if that helps with life of the brake.

You miss my point. The Metro, to paraphrase the Late Senator from Alaska, is a series of tubes.

You push air in at Foggy Bottom; to exhaust at Rosslyn. Windmilling the fans at the exhaust means less drag/more flow than locking them stationary. Of course, if more CFM is needed, Rosslyn will be powered in the exhaust direction.

 

[Open Neutral]

Since it seemed to be of interest, I asked a Metro guru about the fan/shafts on WMATA. The Metrorail varies from well overheard to surface trackage to a few feet below the street to 200+ ft down in other places; Forest Glen being the deepest.

Here's what he laid out, including things I didn't know/have wrong.

1. Shaft at the ends of station are always vent shafts, blow out shafts, tunnel walls and or roofs tapered to direct the air pushed by trains out the shaft instead of into the station.
2. First shaft beyond a station is always a fan shaft.
3. Second shaft beyond a station is always a vent shaft provided there are 3 or more shafts between station pairs.
4. CFM and the number of fans varies depending on the shaft. 2 or 3 fans is typical some have 4. In the older parts of the system, pre 2000, the fans are 4' and are rated at 4,000 CFM.
5. Post 2000 sections of the system have 2' fans (fan jets) I believe they have the same CFM rating as the older larger fans.
6. As far as I know all of the fans only blow air out of the tunnels.
7. There are dome relief shafts in the crown of the station vaults, typically 4 spaced evenly along the length of the station, the dome relief shafts are used to allow the hot air to escape from the crown of the station vault. In cut and cover stations the dome relief shaft openings are typically behind the curb along the street the station is under, if the cut and cover station is not under the street they are along the center line of the station. In station bored out of the bed rock the dome relief openings in the crown of the vault vents into a air plenum that runs the length station to shafts that are adjacent to blow out shaft openings.*
8. All shafts have automatically or remotely controlled dampers in them to control the direction and flow of air.

I was sure that the fans were reversible, but I'll see if I can get the details from someone with knowledge of the Control Center.