MissedApproach
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28 VDC Ground Power & Soft Start

Thu Nov 17, 2005 8:21 am

We have some 28.5 VDC ground power units where I work, & as well as ground power they have a soft start feature. When set to soft start the pilot initiates the starting procedure, & the output from the power unit starts at low voltage & ramps up to 28.5 volts. The idea is to reduce shock on aircraft electrical & mechanical components.
My understanding is that only some aircraft make use of this (I was told the T-33 is one of them). Does anyone know what other aircraft can make use of soft start?
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WrenchBender
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RE: 28 VDC Ground Power & Soft Start

Thu Nov 17, 2005 8:46 am

The T Bag was the last of the soft starts in our inventory, the reason was the starter quill shaft could shear on a "hard" start (too much torque)

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Airgypsy
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RE: 28 VDC Ground Power & Soft Start

Thu Nov 17, 2005 10:52 am

The BAe-146 (RJ-85, etc...) has soft start to prevent the starter from breaking the starter pad off the gearbox (which they did). When I first worked on them, starting an engine sounded like a car crash immediately followed by a very fast crank up. Soft start saved the engines and the starters. The aircraft does the "soft start" voltage control. I think it's a stepped 6/12/28. The low voltage allows the geartrain to takeup slack like a freight train does. This prevents the crashing the gears event and then controls the torque application as voltage steps up. (It was a LONG time ago for me.)
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HAWK21M
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RE: 28 VDC Ground Power & Soft Start

Thu Nov 17, 2005 8:04 pm

Quoting MissedApproach (Thread starter):
We have some 28.5 VDC ground power units where I work

Why 28.5vdc,not 28vdc.
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MEL
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Okie
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RE: 28 VDC Ground Power & Soft Start

Fri Nov 18, 2005 12:44 am

Quoting HAWK21M (Reply 3):
Why 28.5vdc,not 28vdc.

The quiescent charge of a lead acid "24volt" battery is 25.2-25.4 volts.
Normal charging using alternator/ground power/cart is 26.4 to 28.8 volts.


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HAWK21M
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RE: 28 VDC Ground Power & Soft Start

Fri Nov 18, 2005 2:52 pm

Quoting Okie (Reply 4):
The quiescent charge of a lead acid "24volt" battery is 25.2-25.4 volts.
Normal charging using alternator/ground power/cart is 26.4 to 28.8 volts.

Agreed.But why the Decimal.
regds
MEL
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erj-145mech
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RE: 28 VDC Ground Power & Soft Start

Sun Nov 20, 2005 12:20 am

We used soft start on the Garrett powered airplanes as well, the Miti's, the King Air B100's and Turbo Commanders.
 
Okie
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RE: 28 VDC Ground Power & Soft Start

Sun Nov 20, 2005 2:20 am

Quoting HAWK21M (Reply 5):
Agreed.But why the Decimal.

I can only assume that the manufacturer of the equipment has the exact output of the ground power unit listed.
The closer the output of the charger to the maximum amount of voltage the battery can safely accept the faster the charge, this also has the effect of letting the battery float in the system while the ground power unit supplies most of the current for starting.
A fully charged 24 volt lead acid battery will show close to the "impressed voltage", for lack of a better term, immediately after charging and will slowly drop back to the quiescent voltage over time or if a load is placed on the battery.



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