Effect of Windmilling on Motor Starting

[Open Neutral]

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.

What you want is to sense the freewheel direction before starting; and if it's your direction, do not brake. Ponder....

And yes, WMATA is full of such blowers, some of which are BIG.

 

[GeorgeB]

Besoeker and Jraef didn't mention it, so my memory may be faulty, but it was once proposed that a large high inertia load could be gradually (multiple minutes) brought near sync speed via a "pony" motor before bring the large INDUCTION motor online. The rotor impedance is much lower at low slip, resulting in very very very large short duration current. The goal of reducing inrush was not achieved, rather inrush was increased.

And Jraef, occasionally large motors are started across the line; I've personally seen from less than 10 feet a 9000 HP 6900 volt motor started A-T-L ... in a power plant. This was in the 1975 time frame and may be different today.

 

[kwired]

Besoeker and Jraef didn't mention it, so my memory may be faulty, but it was once proposed that a large high inertia load could be gradually (multiple minutes) brought near sync speed via a "pony" motor before bring the large INDUCTION motor online. The rotor impedance is much lower at low slip, resulting in very very very large short duration current. The goal of reducing inrush was not achieved, rather inrush was increased.

And Jraef, occasionally large motors are started across the line; I've personally seen from less than 10 feet a 9000 HP 6900 volt motor started A-T-L ... in a power plant. This was in the 1975 time frame and may be different today.

In the power plant may be easier - not much line length and no other "customers" on several miles of line to be as effected by any voltage drop caused by this event. Isn't POCO idea when they prohibit across the line starting of large motors generally to help with power quality to other customers fed by the same lines more so then the fact their system may indeed be able to start the motor? Plus there was not so much for equipment around at that time that may have been as susceptible to any anomalies from such starting on the line as there was just 10 or 15 years later. So a few customer lights dimmed for a few seconds, not as big of a deal at that time as it would be today.

I could also see power plants in 1975 more likely to have extra capacity available, where today they may run at or near full capacity more often then they ever would have in 1975.

 

[don_resqcapt19]

...
And Jraef, occasionally large motors are started across the line; I've personally seen from less than 10 feet a 9000 HP 6900 volt motor started A-T-L ... in a power plant. This was in the 1975 time frame and may be different today.

I worked at a cement plant and hooked up a 2,500 hp crusher mill motor across the line. They also had a 4,500 one that was existing that was across the line too. Everything was fine when they were buying power from a utility. The plant was fed off a 138kV transmission line. To save some money the switched to a feed from the local municipal utility and the plant was from a 33.5kV distribution line. When they tried to start the big motor the voltage drop caused most of the magnetic starters in the plant to drop out and caused issues for residents in the area. They ended up switching back to the utility and their 138kV transmission line.

 

[kwired]

I worked at a cement plant and hooked up a 2,500 hp crusher mill motor across the line. They also had a 4,500 one that was existing that was across the line too. Everything was fine when they were buying power from a utility. The plant was fed off a 138kV transmission line. To save some money the switched to a feed from the local municipal utility and the plant was from a 33.5kV distribution line. When they tried to start the big motor the voltage drop caused most of the magnetic starters in the plant to drop out and caused issues for residents in the area. They ended up switching back to the utility and their 138kV transmission line.

Did the local municipality get any engineers, or even their more experienced operations people involved in this decision or was it just a greedy board of directors (or some other equivalent) thinking they may have a cash cow here and hadn't considered whether or not they could supply the demands?

 

[don_resqcapt19]

Did the local municipality get any engineers, or even their more experienced operations people involved in this decision or was it just a greedy board of directors (or some other equivalent) thinking they may have a cash cow here and hadn't considered whether or not they could supply the demands?

Not sure how the decision was made.

 

[hurk27]

Wonder if you could design a reversible variable pitch fan that could return to a neutral position until the motor started then pitch to the desired direction?

 

[kwired]

Wonder if you could design a reversible variable pitch fan that could return to a neutral position until the motor started then pitch to the desired direction?

I see no reason that wouldn't be a possibility.

Cost, maintenance, reliability, maybe added weight to the fan, are all going to be factors for doing so vs other options to deal with the issue.

 

[mgookin]

In the OP's case the project is a transportation tunnel. As jraef has pointed out, NTSB has all kinds of requirements which can be met with current technology but anything out of the box would require more time, cost & effort (not to mention project delay) than it's worth. You don't want a tunnel full of dead people on your resume. Do what works.

 

[kwired]

In the OP's case the project is a transportation tunnel. As jraef has pointed out, NTSB has all kinds of requirements which can be met with current technology but anything out of the box would require more time, cost & effort (not to mention project delay) than it's worth. You don't want a tunnel full of dead people on your resume. Do what works.

And the OP had valid questions concerning this. If the fan is windmilling in the wrong direction, and because of this there is starting issues - you very well may end up with ventilation having a tripped motor overload and no ventilation, especially with simple across the line starting methods.

Now I suppose it is possible the motor is oversized intentionally to be able to withstand such conditions better.

 

[Besoeker]

And the OP had valid questions concerning this. If the fan is windmilling in the wrong direction, and because of this there is starting issues - you very well may end up with ventilation having a tripped motor overload and no ventilation, especially with simple across the line starting methods.

Now I suppose it is possible the motor is oversized intentionally to be able to withstand such conditions better.

Or design the motors for the application. Make the rotor characteristics suit the requirements.

 

[kwired]

Or design the motors for the application. Make the rotor characteristics suit the requirements.

I was probably searching for that terminology more so then "oversized" just wasn't coming to me at the time - thanks.

A motor that large isn't exactly an "off the shelf" general purpose motor, it is likely designed specifically for the application it will be used for.

 

[mgookin]

And the OP had valid questions concerning this. If the fan is windmilling in the wrong direction, and because of this there is starting issues - you very well may end up with ventilation having a tripped motor overload and no ventilation, especially with simple across the line starting methods.

Now I suppose it is possible the motor is oversized intentionally to be able to withstand such conditions better.

I thought jreaf addressed it with the industry standard of having a sensor determine if it's windmilling, regardless of direction, and braking it prior to putting power on it to turn in the desired direction.

If you get too complicated with it (such as trying to determine whether it's turning in the direction you want) then you have to go through countless redundancy scenarios which are not going to be favorable to NTSB which can get you in bigtime trouble, according again to what jreaf said.

 

[kwired]

I thought jreaf addressed it with the industry standard of having a sensor determine if it's windmilling, regardless of direction, and braking it prior to putting power on it to turn in the desired direction.

If you get too complicated with it (such as trying to determine whether it's turning in the direction you want) then you have to go through countless redundancy scenarios which are not going to be favorable to NTSB which can get you in bigtime trouble, according again to what jreaf said.

OP also mentioned there are no brakes on these fans. I think his concerns are still valid. Don't know if the design he described is favorable or approved by NTSB, or if OP is the designer, installer or other.

 

[Jraef]

Or design the motors for the application. Make the rotor characteristics suit the requirements.

If it were almost any other industry, that would would be the case. But for transportation tunnels, if the fans don't work, the tunnel is closed. So they want standard standard standard motors that can be purchased anywhere in an emergency.

Like I said, there is a LOT that goes into the decision making process that is not normal, everything is approached as a "belt and suspenders" process. I used soft starters on a big tunnel project in NY State once, lucky for me we were not the first to propose it, otherwise it would have been a nightmare. But to give you an idea, the soft starters had to be rated to deliver locked rotor current for 60 seconds, then they had to have bypass starters cable of across-the-line emergency starting, then of course the reversing contactors, then the DC injection brake. I think those were 400HP, but by the time we got done with everything, each fan controller was in an MCC style cabinet that was 8ft. wide.

The big issue that came up was the across-the-line bypass capability, because if there was a power failure, the backup generators would be incapable of starting enough of the fans simultaneously to avoid creating the wind tunnel effect if there were to be a crash in the tunnel WHILE the generators were on-line. The solution, larger generators and then the space, fuel storage, ventilation etc. that came with that, was too costly. So they settled on acceptance of it being statistically unlikely that there would be a crash AND a loss of utility power for very long at the same time. When Super Storm Sandy hit, I thought about that exact situation at that tunnel ...

It occurs to me now that with this being a TRAIN tunnel, the rules might be slightly less stringent because the tunnel is not occupied all the time, like a highway tunnel is. Plus each car is potentially its own incendiary bomb, whereas a train, not so much. So it might very well be that these are custom motors and starting them X-Line is fine.

 

[kwired]

I guess 350 hp would be somewhat of an off the shelf standard motor in some places, but not in my shop

 

[Besoeker]

If it were almost any other industry, that would would be the case. But for transportation tunnels, if the fans don't work, the tunnel is closed. So they want standard standard standard motors that can be purchased anywhere in an emergency.

Don't you think they just might carry a few spares in their inventory?

 

[Lee Bragg]

Wow, thanks for all of the great input! After reading through your comments I feel like perhaps I need to have a talk with my client and push for them to allow for braking before the ventilation fans startup. That sounds like the most straighforward way to do it while allowing for the lowest possibility of failure.

 

[Besoeker]

I thought jreaf addressed it with the industry standard of having a sensor determine if it's windmilling, regardless of direction, and braking it prior to putting power on it to turn in the desired direction.

Yes....but....
More parts. More complexity. Without addressing the issue of where to fit a mechanical brake or even whether it's practicable/possible.
DC injection braking? Many more parts. More complexity.

If you get too complicated with it .......

See above.

The DOL starter with the right motor has to be about as uncomplicated as you can get.

 

[Phil Corso]

Lee Bragg,

The method I proposed requires 3 diodes, a 2-pole DC contactor, and a zero-speed switch!

Phil Corso