Effect of Windmilling on Motor Starting

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Lee Bragg

Member
Location
Seattle,Wa,USA
Hi everyone, this is my first post to the forums! I have a question regarding the windmilling effect and fan motor starting... We are installing (4) 350HP reversible ventilation fans in a rail tunnel with full-voltage starters. Motor starting calculations have been done assuming that the fan blades are not moving at startup, however, there are no brakes on these fans and trains will be traveling through the tunnel at high speeds causing pressure changes and air flow throughout the tunnel. Because of this, the worst case fan startup scenario will occur when the fan blades are rotating in one direction and then the fans are started in the opposite direction. Does anyone have experience with this type of scenario? I assume the fans will take longer to reach full speed, but what about inrush current, do the magnitude and duration both increase? Is there a general rule of thumb or calculation method that can be used to predict the windmilling effect on motor startup?

Thanks in advance for your input!
 

iwire

Moderator
Staff member
Location
Massachusetts
It seems like brakes or a sprag clutch are in order. Could the motors be over sped by the trains air movement?


Sorry I can't really answer your question.
 

Smart $

Esteemed Member
Location
Ohio
It seems like brakes or a sprag clutch are in order. ...
Sorry I can't really answer your question.
A sprag clutch is a one-way clutch. Won't work on a reversible setup.

I agree with brakes or perhaps some other controlling means: soft starters, stepping controls, etc.

I too can't answer the question fully, but I do believe inrush duration will increase if the fan is rotating in the opposite direction of motor at startup. Magnitude may be slightly less until the reverse inertia is overcome, then same as starting from full stop.
 

GoldDigger

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Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Yes, but with a reversible fan, back spin can be either direction.
And in the case of subway/railway tunnels with traffic in both directions, it may actually be considered beneficial to allow the fan to freewheel in either direction when not powered. Hence the OP's idea of applying a brake a set time before starting the motor, rather than whenever the motor is off.
 

Smart $

Esteemed Member
Location
Ohio
And in the case of subway/railway tunnels with traffic in both directions, it may actually be considered beneficial to allow the fan to freewheel in either direction when not powered. Hence the OP's idea of applying a brake a set time before starting the motor, rather than whenever the motor is off.
You must have had a revelation beyond the confines of this thread... :blink:
 

Lee Bragg

Member
Location
Seattle,Wa,USA
A sprag clutch is a one-way clutch. Won't work on a reversible setup.

I agree with brakes or perhaps some other controlling means: soft starters, stepping controls, etc.

I too can't answer the question fully, but I do believe inrush duration will increase if the fan is rotating in the opposite direction of motor at startup. Magnitude may be slightly less until the reverse inertia is overcome, then same as starting from full stop.


Thank you for everyone's input. I agree that brakes or soft starters may be the best option, however, the project specifications do not allow for them (sorry, I did not share that before). This is why I'm trying to figure out how to calculate the windmilling effect on inrush current and startup duration. I have spoken with the fan motor manufacturer and with the motor starter manufacturer and they weren't sure how to calculate it either. Perhaps this is just not commonly done?

You said the inrush magnitude will be LESS until the reverse intertia is overcome and then it will start normally. Can you explain the logic there? In my head I thought the inrush magnitude might be higher while overcoming the opposite fan rotation (due to greater slip).
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Common problem usually addressed mechanically with gravity operated dampers or louvers to prevent back flow, but those too would be problematic on reversible fans.

The effect is that it does not actually change the magnitude of the starting current, but it does affect the acceleration time, often to the point where you exceed the thermal damage curve of the motor.

In general, transportation tunnel fan systems have some extra stringent requirements with regard to system reliability / redundancy, similar to Fire Pump controls, likely because of similar reasons. They are reversible not because of the train direction, but because in the event of a fire in the tunnel, the fans are operated to feed the fire, but isolate it in order to avoid having the entire tunnel become a blast furnace. So all of the fans on either side of an accident with incendiary possibilities are made to feed air TOWARD the area, and the fans nearest to it are made to exhaust out of the vents, which turns the local area into a small, but very controlled, burn zone. Side note: keep that in mind when driving in a long ventilated tunnel; if you are in or near an accident and fuel spills, GET THE HECK OUT!! They will sacrifice you to save everyone else.

OK, back to the controllers. I have done several of these systems, and besides the onerous process of NTSB investigation and certification, it is possible to do this with a VFD with Dynamic Braking, or a Soft Starter that can do DC Injection Braking. The control concept is in fact called an "Anti-Windmilling" start procedure. What you do is that whenever there is a "Start" command, in either direction, the first thing that happens is that the electronic brake is turned on and the motor shaft is stopped. Most electronic braking options will include a way to detect that the shaft has stopped to turn off that feature, so when it is done, the signal telling you that is then used to start the motor in the selected direction. The reason why this is not done with mechanical brakes by the way is because if the possibility of the failure of a brake coil creating a situation wherein the fan does not start.

Now if you must do this with an Across-the-Line starter (really? 350HP A-T-L?), then I suppose you could do it with a stand-alone DCIB unit attached to the starter. The thing is, it would need some sort of brains to make it all work, which implies either a bunch of relays or a small PLC, and getting that all certified to the NTSB reliability requirements may be problematic for a one-off project. Good luck.
 
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Besoeker

Senior Member
Location
UK
Hi everyone, this is my first post to the forums! I have a question regarding the windmilling effect and fan motor starting... We are installing (4) 350HP reversible ventilation fans in a rail tunnel with full-voltage starters. Motor starting calculations have been done assuming that the fan blades are not moving at startup, however, there are no brakes on these fans and trains will be traveling through the tunnel at high speeds causing pressure changes and air flow throughout the tunnel. Because of this, the worst case fan startup scenario will occur when the fan blades are rotating in one direction and then the fans are started in the opposite direction. Does anyone have experience with this type of scenario? I assume the fans will take longer to reach full speed, but what about inrush current, do the magnitude and duration both increase? Is there a general rule of thumb or calculation method that can be used to predict the windmilling effect on motor startup?

Thanks in advance for your input!
First of all, welcome.
And for posting an interesting question.
And I don't know the answer.

If the motor is running in reverse when the voltage is applied, then it is at negative slip. The torque will be fairly low and the current high. Probably around the locked rotor value until the motor is running in the forward region at near rated speed.
Setting the over current protection could be a challenge....
 

electric_cal

Member
Location
California
Hi everyone, this is my first post to the forums! I have a question regarding the windmilling effect and fan motor starting... We are installing (4) 350HP reversible ventilation fans in a rail tunnel with full-voltage starters. Motor starting calculations have been done assuming that the fan blades are not moving at startup, however, there are no brakes on these fans and trains will be traveling through the tunnel at high speeds causing pressure changes and air flow throughout the tunnel. Because of this, the worst case fan startup scenario will occur when the fan blades are rotating in one direction and then the fans are started in the opposite direction. Does anyone have experience with this type of scenario? I assume the fans will take longer to reach full speed, but what about inrush current, do the magnitude and duration both increase? Is there a general rule of thumb or calculation method that can be used to predict the windmilling effect on motor startup?

Thanks in advance for your input!
Could you possibly use motors that are equipped with what is known as a "Non Reverse Ratchet"? This would prevent back spinning.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Could you possibly use motors that are equipped with what is known as a "Non Reverse Ratchet"? This would prevent back spinning.
And how would the ratchet know what direction the motor was going to be started in next??
It might not be the same direction as it was last operating in, assuming that the ratchet could take that into account.
 

electric_cal

Member
Location
California
And how would the ratchet know what direction the motor was going to be started in next??
It might not be the same direction as it was last operating in, assuming that the ratchet could take that into account.
Ouch!!! :ashamed1: You are absolutely right!!! :thumbsup: I completely missed the word reversible in the post... Stupid Stupid Stupid.. :dunce:
 

Phil Corso

Senior Member
Anti-Windmillin Solution

Anti-Windmillin Solution

Lee Bragg,

Look into DC-injection.

Although 1-phase instead of 3-phase, I "windmilled" one fan using a second one. Then DC-current (about 10%of the motors rated-current) was suppied to one-phase of the motor to be started. It was vey successful.

I allowed windmilling of the motor to be started. Then DC was injected until rotation stopped as detected by a zero-rotation switch. Then AC was applied!

Regards, Phil Corso
 
We are installing (4) 350HP reversible ventilation fans in a rail tunnel with full-voltage starters.

If you can't do something with brakes or install some other kind of starter, I think you're basically hosed. Sometimes, you'll get lucky and start the fans in the direction they're already spinning, but other times it might trip the overloads and get no fans at all. You'll want a time delay when changing directions.

With all politeness, whoever specified this needs, ahem, to rethink the application.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
If the motor is already free-spinning in the desired direction, do you really want to stop it before starting it?
It would not matter, but given that the "desired" direction is relative to any given circumstance, you would have to know in advance what direction the fan is windmilling in. That means adding sensors and wiring, then having a contingency for deciding what to do if the feedback is lost, which basically means defaulting to "brake first no matter what". That is the same place you are at without having to install sensors or the contingency logic.
 

mgookin

Senior Member
Location
Fort Myers, FL
It would not matter, but given that the "desired" direction is relative to any given circumstance, you would have to know in advance what direction the fan is windmilling in. That means adding sensors and wiring, then having a contingency for deciding what to do if the feedback is lost, which basically means defaulting to "brake first no matter what". That is the same place you are at without having to install sensors or the contingency logic.

Makes sense. Thanks. :thumbsup:
 
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