MissedApproach From Canada, joined Oct 2004, 713 posts, RR: 2 Posted (8 years 10 months 1 week 2 days 5 hours ago) and read 4159 times:
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?
Airgypsy From United States of America, joined Nov 1999, 130 posts, RR: 2
Reply 2, posted (8 years 10 months 1 week 2 days 3 hours ago) and read 4129 times:
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.)
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.