rnjcurtis(at)charter.net
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 Posted: Sun Aug 30, 2015 4:39 pm Post subject: AeroElectric-List: Hydraulic Pump Start-Up Amperage |
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My Lancair Legacy has a nominal 12 volt electrically powered hydraulic pump that raises and lowers the landing gear. When the airplane has sat in the hangar overnight, the hydraulic pressure bleeds down enough that the pump turns on momentarily when I turn on the master switch. Master relays aren’t designed to make or break large electrical currents and I always wondered if I was damaging it by hitting it with the large surge current from the hydraulic pump turning on as the master relay closes.
A friend has a fancy little electronic box that measures electrical current, sampling many, many times per second. Here is a trace of the current flow through a shunt between the battery and master relay. The blue trace shows an 80 amp surge as the master relay closes and the hydraulic pump almost simultaneously turns on. In this case, with the airplane on jacks, I had the gear down and the landing gear selector switch in the “up” position when I turned on the master switch. That means that as soon as the master relay closed, the hydraulic pump also turned on to raise the gear.
The 80 amp surge lasts for about one or two milliseconds. About ten milliseconds later, the hydraulic pump obviously starts and the current surges to 155 amps. Landing gear extension and retraction each take about six seconds. In order to focus on the very brief surges, this graph shows only a fraction of the first second.
The other two traces show the current draw from raising and lowering the landing gear in the customary manner, after the master relay has already closed. Their two traces were overlayed with the first one.
I’m not sure what this means. However, years of trouble free operation for my airplane and many similar ones show that the master relay isn’t damaged by the hydraulic pump turn on surge.
Dennis
Lancair Legacy, 860 hours
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This is not a problem! In your example the contacts have completely closed before the heavy load is applied. Most T/C aircraft put the starter load through the Master contactor, after it has closed, with no ill effects. Where others have run into problems is when there is a heavy load that has been applied prior to contact closure. This situation will cause arcing across the contacts resulting in possibly welding the contacts or at least shortening the life of the contactor.
Roger |
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