Dynamic Brake "Blisters"

[Erik_Mag]

"Air is pulled in the side grilles, then goes thru the fan where it's pushed upward thru the grids."

Does this have anything to do with pushing a given mass of cold air through the grids takes less energy than pulling the same mass of hot air from the grids? I'd also gues that from the tunnel motor experience, that the hot air needs to go up as opposed to going out the sides.

[bogie_engineer]

Erik_Mag

"Air is pulled in the side grilles, then goes thru the fan where it's pushed upward thru the grids."

Does this have anything to do with pushing a given mass of cold air through the grids takes less energy than pulling the same mass of hot air from the grids? I'd also gues that from the tunnel motor experience, that the hot air needs to go up as opposed to going out the sides.

 

I think that's part of it. The development of the radial grids on top of the fan actually began with a "combined cooling system" concept started in the early 1970's as a way to get the grids off the engine and save the cost of the grid fans. In that system, the grids were much less in height and mounted on top of the 48" AC cooling fans for the engine cooling water radiators. It never left the drawing board because of the hit to fuel efficiency - the cooling fan speed was proportional to engine speed (this was long before cost-effective variable speed AC drives) so the engine would have to be run at a higher speed in dynamic braking to cool the grids. When the SD50 was being conceptualized the desire to get the grids out of the space above the engine led to the development of a bigger 54" fan with sufficient air mass flow to cool grids where the two 48" fans were previously used. It also puts the grid fan blades and motor in a nice cooler enviroment. 

 

If the fan pulled the air from the roof, it would blast the walkway on both sides with 900 degF air, to duct it to one side as GE does would be a significant flow restriction and bring back the problem of high temperature air on the fan blades and motor bearing.

 

Dave

[Front Range Trains]

bogie_engineer

 Thanks Dave!  Very useful info.  

 

Front Range Trains

Interesting thread. Thank-you to those who have shared knowledge.

On the EMD SD50, is the dynamic brake fan visible from above or does the dynamic brake grid block the view of the fan?  Also, how many blades does the dynamic brake fan have? 

I haven't yet found any photos or videos of the EMD SD50 (or similar) which depict the fan itself.

 

 

 

 

The DB fans sits below the grids; the grids bolt to the fan frame. The grids are about 20 inches in height with a fairly dense ribbon array that you can't see through. On the roof, all you can see is the grille on top of the grids to prevent debris from falling into the grids. Air is pulled in the side grilles, then goes thru the fan where it's pushed upward thru the grids. IIRC, the fan, which was new with the radial grid development is 54" in diameter with space for 12 blades, which are all present. There is a mix of short and long blades done to get the airflow distributed evenly thru the grid ribbons. Looking up thru the intake air grilles on the sides of the carbody should let you see the fan blades.

Dave

 

[SD60MAC9500]

 

When GE first introduced the DASH-8 series, early models had the DB heat exhausted through two horizontal vents on top of the long hood. IIRC there was an issue with moisture leaking into the hotside causing DB issues. Later DASH-8's had the hotside of the DB vent configured into a vertical configuration.

 

 

 

 

 

 

[Erik_Mag]

bogie_engineer

If the fan pulled the air from the roof, it would blast the walkway on both sides with 900 degF air, to duct it to one side as GE does would be a significant flow restriction and bring back the problem of high temperature air on the fan blades and motor bearing.

900ºF is 1360ºR, which is more than double than the 570ºR that would be found on a very hot summer day in the southwestern deserts. The volume of air coming off the grids would be more than double the volume going into the grids. This would imply a doubling of power to move the air by pulling from the top, but as you pointed out, the temperature would be too mcuh for the motor and bearings.

With respect variable speed AC drives, power electronics has come a long way from when I took a course in it in 1976. At that time, the easiest way of making a variable speed drive was with cycloconverters, but those need a source frequency of at least 3X the desired output frequency.